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Old 10-30-2005, 02:37 PM   #1
sponaugle
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Default Proper use of dyno correction factors (a must for those professional tuners)

I would like to start a discussion that is of great interest to those here on PPB: The dyno. Thru the years of discussion and analysis on forums like these we have all grown our collective view of what the dyno means as a tool.

In the beginning everyone compared their dyno results without delay. Shortly after that, people realized that the same car on one dyno somehow had different results on other dynos. Some time later, people figured that all dynos of brand X at least read the same, so we could assign some time of scaling factor to move from one brand to another. (recall the Vishnu dyno spreadsheet). Then at yet another date, we discovered that even the same brand dyno doesn’t always read the same. The next thing you know, the same dyno would read the same car differently on different days. Oh hell, it must just be futile.

No, not really futile just difficult and challenging. There are two significant areas in relation to the use and comparison of dynamometer results that deserves some further discussion. Let us start with the ever persistent ‘correction factor’.

The notion of the use of some kind of ‘correction factor’ is based on original work by many different standards bodies to inject some amount of reproducibility in power measurements. In the US, SAE is certainly the most well known, but among the rest of the world, standards from ISO and DIN have equal penetration into the engineering mindset. Lest we Americans think not the world revolves around us.

The concept of having a correction factor is derived from both the changes in engine operation due to atmospheric differences, but also testing procedures, steady state and dynamic efficiencies, and of course repeatability. Most people referring to “SAE Corrections” in the US are talking about SAE1349, which is the current and most updated and recognized industry standard. Like all standards, it is VERY easy to talk about with an air of authority without actually TALKING about it. I took the time to read the standard and much to my surprise it was VERY easy reading. This document is nothing like the usual standards body work in both obfuscation and terminology. I highly recommend it for any professional tuner.

While SAE prevents publication of the document itself, US Copyright fair use provisions do allow quotation and analysis of the document. There are several interesting factors used in the SAE correction, but the “Power Correction Factor” is what most people here would be interested in. Let is take a quick look at the following text:



This section describes the corrections used for non standard atmospheric conditions. There are three critical things to get from this text.

(1) Input conditions to the engine MUST be the same as the calibration is configured for. This practically means you must use an inlet air temperature sensor that measures the inlet conditions, not just the room temperature. (of course air inlet in not intended to be the actual vehicle air inlet, but the effective vehicle air inlet in normal operation. This allow changes the use of fans directed at the air inlet path, if such a path would not see this additional flow in normal operation. SUMMARY: Most dyno’s measure the room temperature, which while better then nothing, is still not a very precise method.
(2) Turbocharged cars need not apply. The use of the standard SAE air density corrections for turbocharged applications is plain wrong. As stated in the standard, without any need for additional clarity, “For example, boosted engines with absolute pressure controls shall not be corrected for ambient barometric pressure”. It need not be any clearer. You will notice by the way that all of the PDXTuning posted data is non corrected. Further in the document, the use of intercoolers is discussed, and there are changes to the air temperature corrections based on the efficiency of these intercoolers.
(3) Using a correction factor of more then 6% total (3% air and 3% fuel) is considered non standard. This is VERY important. Consider that every line of text in this standard has undergone significant review and discussion. This section is very clear for a reason. To help understand this issue, I’ll make the case for it below.


Of the three above, the first two are pretty obvious. While denser air in a normally aspirated car has a predicable change based on the models used in SAE1349, the variable conditions in a boost controlled turbocharged or supercharged engine make such corrections very difficult to model. The rapid slope of the efficiency plots of a turbo charger make changes in inlet pressure (and thus changes in the turbocharger pressure ratio) very non-linear. A small change on some pressure points will make a much greater difference then the same change at a different part of the curve. Since the boost control system will adjust the output pressure, the actual air pressure delivered to the motor will be more dependant on that system then the differences in air density (SIGNIFICANTLY)

The third factor, the limit to 3% air correction is critical. The notion that an engine made 300whp at 6000ft in Colorado, but would make 400whp at sea level is absurd WITHOUT even the simplest analysis. The basis of the correction factor assumes several very important things. It assume the fuel and ignition system, and volumetric efficiency, the exhaust flow limits, and the knock thresholds of the motor scale in the same linear fashion. It also assumes the engineer performing the tests is proficient. It assumes the test parameters are known, and the engineer can intelligently ascertain that at the corrected level, the engine would accurately perform as the scaling indicates. Translation: This means you have actually tested the engine at the other location, and there is plenty of sensor, fuel flow, knock limit, and well as all of the other mechanical and electrical limits that are not railed. To make this more simple, consider the following:

“I made 300whp on my 550cc injector based WRX at 6000 ft. My fuel injectors were at 100% duty cycle, and the boost was 18 psi.. My dyno said the correction factor was 1.22%, so I really made 366whp. Of course when I went down to sea level I ran the same boost with the denser air, my injectors were again at 100%, my AFR was 13:1 on pump gas and the car blew up. Why didn’t anyone tell me injectors can’t run at 122%!!!’

The same story above can be said for so many different parts of the car. If you hit the MAF limit at 6000 ft, do you think somehow at sea level the MAF would magically have more headroom for all this additional magic power?

If you see someone using a correction factor of more then perhaps 6% for temperature corrections (a 2x jump over the official standard view), you should discount at a rate of 2:1 just for the stupidity penalty.

Last but not least, consider section 5.6 of SAE 1439:



Yes, that does say “These correction formulas are not intended for altitude de-rating”.

As I mentioned at the beginning, “There are two significant areas in relation to the use and comparison of dynamometer results that deserves some further discussion”. The second area is what interests me the most. This is the area that I am very much looking for contribution and discussion from other parties. As there are several real SAE engineers (unlike myself, who is only an engineer pretending to play an SAE engineer on TV) present on this forum, as well as countless others who have experiences to contribute.

Let us look at the effects of measured power output based on the type and usage model of the dyno itself. Consider first the simpliest case, the dynojet. The dynojet dyno uses a simple inertial method of measuring power output, and uses the engine rpm to down calculate the torque produced. It does not directly measure torque output, and does not allow for variability of load to the car. When using the dynojet, we are measuring the transient power output by sweeping the car thru a range of engine speeds. This is not a measurement of steady state output.

The SAE standard does have something to say about this:



The most interesting part of the above paragraph is the following: “ The method for determining test conditions used for rating engines from light duty vehicles is to obtain and record time synchronized data on all engine control parameters from an engine installed in a vehicle during a transient maneuver and then duplicate these control settings during steady state operation on a dynometer.” This leads us to a very important conclusion. The rate of acceleration on the dyno must match the rate of acceleration on the road. This means the rate must include effects of aerodynamic drag, wheel drag, angular inertia drag, as well as the effects these loads have on the power produced. Not only must the rate (not just in total, but also in shape of delivery) be the same, but the vehicle operating parameters must match the real transient maneuver. That means the same boost, timing, fueling, etc.

It is instructive to look at how the three most common dyno’s handle the above requirements:

Dynojet: Load is a static load based on the weight of the rollers. (actually the angular inertia). If your car happens to weight the same, your closer the if it doesn’t.

Dynapak: The Dynapak has a dynamic load cell that can provide any possible load curve. However as the software is currently written, it appears to offer only a static sweep rate. The static rate allows you to make a RPM range (say 2000-7000 rpm) to take 12 seconds, in a linear slope. This does have the benefit of allow the operator to make the dyno load more or less, but however it is very flawed in simulating real world conditions. If you car produces 500whp vs 200whp, the same time sweep means that the higher hp car is seeing much more load. In the turbocharged application, this can results is abnormally high initial torque readings as the dyno applies reverse torque to match to specified sweep rate. To properly use this dyno, one would have the test the vehicle on the road, and make the sweep time the same as the road test vets.

Mustang: The Mustang, in theory, has the best possible simulation. It does both a static load (weight of the rollers), a dynamic weight (based on the vehicle weight, as set by the user), and a second dynamic weight based on the aerodynamic drag (specified as hp@50mph). This allows the dyno to provide a load that is similar to what would be seen on the street. However, in my experience, this method works only as well as the user has loaded the right calibration files. These calibration files can make s HUGE difference in shape and size of the curves produced. This is evident by comparing results from the Mustang dyno here, the Gruupe-S, FIS, and many others.

Not only does the load (how fast the pull goes) change the engine loading and time for turbine spool, but it also effects the amount of energy load in the flywheel to wheel conversion.

This conversion from flywheel to wheel is often of the most interesting debate. Before looking in that aspect of dyno tuning, let us first look at the effect of sweep rate on energy used to accelerate the drivetrain components.

As we all know from high school physics, it is easy to model the effects of drum accelerating using only a few simple calculations. I’ll save the space for now and only show the abbreviated approach. The goal is to determine the amount of power used to accelerate the drivetrain and wheels(called wheels below), given a fixed inertial weight dyno drum (no dynamic load).

Terms:
Twa = Torque accelerating the wheels
Tds = Torque applied to accelerate the drum
Ti = Torque input to the system (from the engine)
Fds = Force at the drum-wheel surface(drum)
Fws = Force at the drum-wheel surface (wheel)
Rw = radius of the wheel
Rd = radius of the drum

Fds=Fws (unless the wheels break loose)
Fds=Tds/Rw
Tds = Ti – Twa
Twa = Iw*Aw
Fds = (Ad*Id)/Rd

Since Fds = Fws

(Ad*Id)/Rd = (Ti – IwAw)/Rw

Simplification yields what we are looking for:

Twa = Ti * ( 1 / ( [Rw^2*Id]/[Rd^2*Iw] ))

In less mathematic form, this suggests the loss to accelerate the wheels is based on the ratio of the inertial load of the wheels vs the load of the drum. To put some quick calculations in place: A 3200lb equivalent drum, used on a car with 4 32lb wheels, results in the loss of torque used to accelerate the wheels at about 2%. Keep in mind this is using a simple filled cylinder approximation for angular inertia for the wheels (the wheels are not a solid cylinder of course). The wheel loss might actually be a bit higher, and of course the transmission and shaft load is something non zero as well.

None the less, it is clear that if you change the load weight from 3200 lbs to 1600 lbs, you get 4% loss. This difference in loss is more then the typical SAE correction used. This figure is probably under rated, given the real weight and inertial mass of the components.

Translation: The rate of acceleration on the dyno (controlled or uncontrolled) will directly effect the measured power. On a dyno like the dynojet the load is consistent so the same car with the same wheels and driveline should experience the same comparative load. This comparative loss is not ties to the power produced. In this way, the dynojet model does produce results that can be more consistent across the same kind of dyno. (given the same weight drums, which is not true of course).

On a dyno like the dynapak, the above equations are useless because the torque load delivered to the engine is not a static value. It varies based on the power output of the car, which makes even run to run comparisons very difficult.
This does not make the results on the Dynapak useless, or any less useful for tuning. It does however make the results VERY VERY difficult to compare to results from other kinds of dynos. It unfortunately also makes comparisons of different cars on the same dyno also a bit difficult. The comparisons can still be made, but differences in the 2-5% range should not be considered significant.

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Old 10-30-2005, 03:20 PM   #2
Rick Schu
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Some interesting stuff, some of it a little over my head. I guess the real question is, would it be possible for dyno operators, even with different brand dynos, to adopt the SAE procedures and would this result in comparable results? If so, then maybe there could be a coalition that dyno operators could join and then results from those dynos could be used for comparison?
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Old 10-30-2005, 03:42 PM   #3
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Great info!!! I would still rather tune on a DynaPak any day of the week over any roller dyno. Much more safe, especially when tuning higher power cars, and alot less factors to deal with.

-Matt
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Old 10-30-2005, 04:40 PM   #4
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So one gets the impression (that I have always had), that the dyno could potentially be used for 2 seperate functions:

1. Simulate operating conditions for an automobile in particular, thereby allowing tuning to be most thorough, safe, and repeatable.

2. A standard communication device. i.e. a method for communcating the potential output of a vehicle.

For each of these two purposes, a different tool would be optimal, yes?

If we are speaking of purpose #2... then it seems to me that on the surface we are merely lacking the coordination to organize and standardize the details. There are many variables at play beyond simple air density correction. My car, yesterday, was at no risk for heat soak. 55 degree air was blowing at a hell of a rate over the intercooler, the engine, etc. The same engine, in Houston, in August, would see heat soak much faster. Net out... if we are to make a dyno curve a standard cumminication mechanism... several variables need to be pegged.

If the interest in standardizing the variables in order to communicate engine output is purely scientific, it's going to be difficult to do. There is business at play here... marketing, sales, promotion, grass roots innovation, etc. Only a portion of forum members would have an interest in standardizing variables for the better of alot of us. Many would probably rather stick to a deceptive game of dyno queening and unsupportable showboating.

If we could standardize, would "we"?
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Old 10-30-2005, 05:16 PM   #5
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Quote:
Originally Posted by Rick Schu
Some interesting stuff, some of it a little over my head. I guess the real question is, would it be possible for dyno operators, even with different brand dynos, to adopt the SAE procedures and would this result in comparable results? If so, then maybe there could be a coalition that dyno operators could join and then results from those dynos could be used for comparison?
Unfortunatly, the hardest part is that the standards themselves don't apply for this kind of operation. While new standards could be developed, the difficulty (and the primary contention in the second half on the original post) is that the dyno itself makes the car make different power based on how it operates.

It is not a the end of the world of course, but do consideration is needed when comparing the information from different sources. (and from the same source, in the case of a dyno that uses a fixed sweep time).

Cheers,

Jeff
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Old 10-30-2005, 05:17 PM   #6
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Quote:
Originally Posted by ShaggyGT
Great info!!! I would still rather tune on a DynaPak any day of the week over any roller dyno. Much more safe, especially when tuning higher power cars, and alot less factors to deal with.

-Matt
Agreed, as the Dynapaks are very quiet, and they work very well. After spending time tuning on a collection of different dynos, I can see the advantages of each. The Dyno Dynamics is one of the best, albeit a very low reading dyno.

Jeff
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Old 10-30-2005, 05:25 PM   #7
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Quote:
Originally Posted by RainMaker
So one gets the impression (that I have always had), that the dyno could potentially be used for 2 seperate functions:

1. Simulate operating conditions for an automobile in particular, thereby allowing tuning to be most thorough, safe, and repeatable.

2. A standard communication device. i.e. a method for communcating the potential output of a vehicle.

For each of these two purposes, a different tool would be optimal, yes?

If we are speaking of purpose #2... then it seems to me that on the surface we are merely lacking the coordination to organize and standardize the details. There are many variables at play beyond simple air density correction. My car, yesterday, was at no risk for heat soak. 55 degree air was blowing at a hell of a rate over the intercooler, the engine, etc. The same engine, in Houston, in August, would see heat soak much faster. Net out... if we are to make a dyno curve a standard cumminication mechanism... several variables need to be pegged.

If the interest in standardizing the variables in order to communicate engine output is purely scientific, it's going to be difficult to do. There is business at play here... marketing, sales, promotion, grass roots innovation, etc. Only a portion of forum members would have an interest in standardizing variables for the better of alot of us. Many would probably rather stick to a deceptive game of dyno queening and unsupportable showboating.

If we could standardize, would "we"?
On #2, that is a very tall order.. As you suggested, the best tool for #1 might not be the best for #2. If you ignore the atmoospheric variables, in a non corrected mode the dyno tells you what power you made.

I think it is a critical point that corrections are not to correct to what you really made. You made what the dyno measured uncorrected, period. The correction is intended to help you understand what you MIGHT have made in different conditions, assuming you follow logical and well established procedures, and your not an idiot.

IMHO, all results should be pubished uncorrected, with data logs. Then again I believe in the tooth fary, so better discount what I say a bit.

As for the showboating, it occurs in so many ways. Even thou 1/4 mile times are a display of power they cover VERY narrow portion of the equation. My opinion of correcting 1/4 mile times (not done here, but the car magazines do it) is the same. Flawed. Very flawed with forced induction vehicles. As well, lap times at your local track are very dependable in the driver, tires, suspension, etc.

In the end, I think it it GREAT that so many people publish the results they get. I very much welcome the information, and it is the intrepretation that needs more thought.

Cheers,

Jeff Sponaugle
PDXTuning.com
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Old 10-30-2005, 07:25 PM   #8
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I think this information pulls layers of obfuscation away. I've been blasted before in threads when people where claiming outrageous numbers on 91 at high altitude with insane correction factors.

Secondly, I think there's some confusion about the use of the mustang dyno to record hp when dyno tuning vs inertial dynos like dynojets. For tuning sessions it is appropriate to enter the hp@50 mph to simulate the load of real world driving. I've been told these numbers affect torque curves more than outright hp, but nonetheless should be considered less important when measuring outright hp.

What's good everybody is now more aware about the vagaries of interpreting dyno numbers. The question is how to make them more meaningful for interpretation. Some increasingly report a low stock reading car as evidence of the validity of power output. Providing as much information as possible, including dynos from difference turbos, logs and 1/4 mile trapspeeds all demonstrate what a car can put down. I'm a fan of including ¼ mile trapspeeds because it's the validation of everything that has gone into the tune and system of parts.
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Old 10-30-2005, 07:32 PM   #9
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Quote:
Originally Posted by mnavarro
Secondly, I think there's some confusion about the use of the mustang dyno to record hp when dyno tuning vs inertial dynos like dynojets. For tuning sessions it is appropriate to enter the hp@50 mph to simulate the load of real world driving. I've been told these numbers affect torque curves more than outright hp, but nonetheless should be considered less important when measuring outright hp.
Excellent point. The HP@50 (as I have experimented with), causes the Mustang dyno to add additional load as speed increases. In my experimentation, changes to thge HP@50 make a greater change in measured high end hp, while changes to the base weight changes low end torque more. If I set the HP@50 to something like 50, (it is usually something more like 13), the car runs the pull fine until about 5000rpm, then it starts to slow to a crawl and eventually can't even finish the run.

Equally, if you set the weight to 8000 lbs, the car can bearly get above 2500 rpm as it tries to fight against the load.

The mustang also has a rpm hold mode, which provides an equal force against the car to keep the RPM consant. That works very well for a steady state measurement of HP, and of course for tuning.

Cheers,

Jeff
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Old 10-30-2005, 07:42 PM   #10
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Well, in reference to the Mustang hp@50 (air resistance in HP to maintain 50mph... rises in resistance as a cube to speed) and vehicle weight, we *could* datalog a car in a gear (say 4th, since thats what Martix dynos in generally) on the road, and then try and repeat that curve by varying the dyno parameters. Then everyone could agree (heh... I know I know... I said "agree") to use those Mustang parameters in PPB.
Then everyone could then agree to forgo "flywheel estimate" numbers, and leave correction factors out other than to mention the temperature in the room.

OR, one tuner wiht alot of marketplace weight could begin by stating the hp@50 and vehicle weight (as a constant) and making all of their promotional "output" the same. Then maybe they could drag the community/marketplace along with them?

Wouldnt that be... hypothetically great?
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Old 10-31-2005, 02:52 AM   #11
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Quote:
(2) Turbocharged cars need not apply. The use of the standard SAE air density corrections for turbocharged applications is plain wrong. As stated in the standard, without any need for additional clarity, “For example, boosted engines with absolute pressure controls shall not be corrected for ambient barometric pressure”.
Ok, so first this just further reinforces that the dyno is just a tool in my opinion, and that comparisons should be made to the same dyno using baselines for comparison to get a somewhat accurate view of power increase and then use that information along with quarter mile times.

What I quoted above though has me confused, are they saying any MAP based system, that has compensation tables for barometeric pressure changes does not need correct? In which case, would something like a Link plus need corrections since it does not have correction tables, and the Autronic would not because it does have correction, and using a UTEC would since it does not have those tables, but EcuTek would not because it has the stock ecu correction?

great info as usual jeff.

PS, how about those pistons

Ben
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Old 10-31-2005, 11:22 AM   #12
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Well said Jeff.

That's why, honestly. All cars get a baseline. What matters is the differential between when the come in, to when they left.

Very interesting though Jeff!

I think I'll play around with some of the different correction factors after SEMA.

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Old 10-31-2005, 04:13 PM   #13
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Sponaugle,

Re: the Mustang Dyno, what constants do you use for WRXs and STis. We are having a tuning session in about a week and I would like to compare my numbers to PDX numbers. I am specifically asking for weight, hp@50 mph, and other details. I guess temperature and barometric pressure will bary based on location. I will be at sea level. This information will help us to compare your numbers from any given set-up to our own.

I am most concerned with the drag (hp@50) constants for the STi with a stock wing.

Thanks
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Old 10-31-2005, 04:47 PM   #14
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Very good read, a few parts went over my head, but I think I got the gist of it. Keep it up man!
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Old 10-31-2005, 06:31 PM   #15
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please email to I-Speed
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Old 10-31-2005, 06:41 PM   #16
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Quote:
Originally Posted by RainMaker
Well, in reference to the Mustang hp@50 (air resistance in HP to maintain 50mph... rises in resistance as a cube to speed) and vehicle weight, we *could* datalog a car in a gear (say 4th, since thats what Martix dynos in generally) on the road, and then try and repeat that curve by varying the dyno parameters. Then everyone could agree (heh... I know I know... I said "agree") to use those Mustang parameters in PPB.
Then everyone could then agree to forgo "flywheel estimate" numbers, and leave correction factors out other than to mention the temperature in the room.

OR, one tuner wiht alot of marketplace weight could begin by stating the hp@50 and vehicle weight (as a constant) and making all of their promotional "output" the same. Then maybe they could drag the community/marketplace along with them?

Wouldnt that be... hypothetically great?
Interesting.. This same approach is used on the EcuTek Deltadash Dyno software, as it uses the frontal area and Cd of the car to add back power lost to aero drag. I'm not sure if it also adds a constant amount of power for rolling drag or not.

None the less, the right constants would make things a bit eaiser. As far as the constants for the Mustang dynos, I would have to have Tim from Mustang chime in. From my experience, even the same constant on different dynos makes different lenght pulls. With our dyno set to 4500 lbs, it is still a faster pull the the Gruppe-S Mustang dyno at 3200lbs. ( and the Gruppe-S reads lower). The calibration files have a lot to do with this as well. Perhaps if we all used the same file, we might get much closer results!

I was going to ask Jarrad to talk to Steve at FIS, as their Mustang calibration file looked much better or more like the road dyno software shows.

Jeff
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Old 10-31-2005, 06:44 PM   #17
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Quote:
Originally Posted by RiftsWRX
Well said Jeff.
That's why, honestly. All cars get a baseline. What matters is the differential between when the come in, to when they left.
Very interesting though Jeff!
I think I'll play around with some of the different correction factors after SEMA.
Jorge (RiftsWRX)
www.ProjectWRX.com
Yep, that is very true. I suppose I'll see you at Sema tomorrow then!

Quote:
Originally Posted by cpturbo
Sponaugle,

Re: the Mustang Dyno, what constants do you use for WRXs and STis. We are having a tuning session in about a week and I would like to compare my numbers to PDX numbers. I am specifically asking for weight, hp@50 mph, and other details. I guess temperature and barometric pressure will bary based on location. I will be at sea level. This information will help us to compare your numbers from any given set-up to our own.

I am most concerned with the drag (hp@50) constants for the STi with a stock wing.

Thanks

I believe we use 13.5 for the HP @ 50, and 3400 for the weight.. However sometimes we run the weight at 5000lbs, and it still is less then the load on the street. As a results, I suspect something else is wrong in the software config in terms of static weight applied to the rollers.

The calibration file is what really counts? Do you know which one you will be using?

Jeff
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Old 10-31-2005, 07:26 PM   #18
hotrod
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Very good post!

You are correct that altitude correction is difficult under the best of conditions, and for turbocharged cars doubly so. Here in Coloroado it is common for SAE altitude correction factors to compute out in the 1.25 - 1.27 range because of the way the SAE standard is setup. (most dynos do the calculation automatically so the user has no direct control over the calculation only the data inputs like air temp).

In practice we find that the proper altituded correction factor is typically about 1/2 the SAE value. One shop here in the Denver area has recognized this and uses a fixed 1.15 correction value that gives more reasonable results when compared to sea level dyno numbers. Unfortunately by doing so, they lose all day to day correction for different air temp/ barometer readings so to make comparable tests on the same car you would need to make those corrections by hand calculation.

The altitude correction factor for turbocharged cars can never be a simple mulitiple of the NA correction factor because the change in altitude directly effects the effeciency of the turbocharger itself. By the same token how much boost you are running also changes the proper correction factor.



Below is a discussion thread I posted over on awdpirates on this same topic with a few minor editorial notes.

=================================

There have been many long and bloody discussions on several forums regarding a fair and meaningful correction factor for Dyno runs at high altitude. The current attempts to compare high altitude dyno numbers for supercharged and turbocharged cars to near sea level runs indicates there is a problem.

It is clear to most of the performance community that the current dyno correction formula based on the SAE J1349 ( Jun 90 ) procedure is broken for high altitude turbocharged cars. It causes a huge over correction for sea level conditions.

Over the last few years, my own research and many on-line dialogues have led me to the following analysis of why the current formula is broken.

I propose a simple modification that appears to fix the problem and give more reasonable corrections.

Any comments or discussion please post to the motor forum!

Larry

Discussion :

The SAE J1349 (Jun 90) power correction formula is:

Metric units millibar / deg C
CF = 1.180 [ ( 990/Pd) x (( Tc + 273)/(298)) ^ 0.5 ] - 0.18

in hg / deg F units
CF = 1.180 [ ( 29.23/Pd) x (( Tc + 460)/(505)) ^ 0.5 ] - 0.18


Pd = dry air pressure ( ie absolute air pressure minus the contribution of the partial pressure of the water vapor in the air)

Tc = Ambient air temp ( ie temp outside the car, not the intake air temp)

The formula is supposed to give you the hp the car would make if it was at an absolute air pressure of 990 mb (29.92 in hg) 0% humidity and 25 deg Centigrade (77 deg F) outside air temp.

My understanding is that the ambient air temp measurement is supposed (according to the dyno manufacturers as they specify that their temperature sensor be placed some distance from the car) be the free air temp well away from the car, not the air temp under the hood where the intake is.

If you measure it under the hood, at the air filter, a car that was dyno tested with the hood closed, and has crappy heat management and pulls in very hot air would be corrected to a higher hp number than a identical car with the hood open and cool air flooding the engine compartment from the dyno cooling fans.

One way to cook the dyno numbers produced by a chassis dyno is to artificially increase the dyno correction number by giving it false air temp data. If the air temp sensor is positioned so it reads heated air, like one dyno tune plot I saw, where the intake air temp was listed as 101 deg F.

This is likely the actual intake air temp during the test, not the ambient dyno cell air temperature.

For accurate adjustment for air temp, it should be measured at the air filter!!
Not at some fixed location in the dyno cell.

If you make x hp at an outside air temp of 101 deg F, then you would make significantly more if the outside air temp was 77 deg F. This is what the correction factor formula attempts to do.

By telling the dyno the outside air temp is that hot, it makes a bigger correction to get back to what the hp would be under standard conditions for the formula which are, 990 mb dry air pressure and 25 deg C air temp.


Basically the formula uses the long standing principal that an engines power output varies at the inverse of the square root of the intake air temp, and directly with the local air pressure.

If your dyno conditions are exactly 990 mb dry air pressure and 25 deg C ambient air temp, the correction goes to zero or 1.00.


SAE appears to have tried to come up with a correction formula that comes out to a very user friendly correction factor of the form 1.xx , and attempts to correct for an assumed drive line power loss of 18%.
(I'm useing the term drive line loss to account for all drive train related power losses including rolling friction of the tires)

The way they structured the formula when they did that, broke the formula for test conditons that fall well outside their "norm".

The formula in essence assumes all cars are NA, and have an 18% driveline power loss to the wheels. This causes it to underestimate drive line losses on AWD cars which appear to be in the mid to high 20% range.

If you discard the first and last terms of the formula, which appear to be a drive line loss correction, and add a factor for absolute manifold pressure, you still get a correction in the form of a decimal number near 1.00 as your correction of power compared to their standard conditions.

You would need to compensate for drive train loss independently if you wanted to correct back to flywheel hp.


For example if you throw out the first and last terms, you have an equation of:

(sorry I like english units -- to convert to metric simply replace the 460 value with 273, and the 29.23 value with 990 and so on for the other inputs)

in hg / deg F units
Larry'sCF = ( 29.23/Pd) x (( Tc + 460)/(505)) ^ 0.5

For the test conditions of this dyno report which was taken at an altitude of about 5500 ft, of 101.41 deg F, and 24.49 in/hg ( we will ignore the humidity to simplify things)

Larry'sCF = ( 29.23/24.49) x ((101.41 +460)/505) ^ 0.5

Larry'sCF = (1.19396) x (1.05437) = 1.25887 <---- this would be the correction factor appropriate for a NA engine. For a turbo charged or supercharged engine you would have to also include a correction for the boost pressure, because the difference between sea level absolute manifold pressure and high altitude manifold pressure when expressed as a ratio would change the higher your boost pressure became.


For example = 20 psi boost at 29.24 in-hg, vs 20 psi boost at 24.41 in-hg air pressure

would be 40.7 in-hg boost, plus 29.24 in-hg atmospheric pressure = total manifold absolute pressure of 69.947 in-hg.

20 psi boost at 24.41 in hg air pressure = 65.11 in-hg abs

the ratio is 69.95/65.11 = 1.074 correcton at 20 psig manifold pressure

if the boost is only 10 psi, the ratio becomes 49.59/44.76 = 1.108 correcton at 10 psig manifold pressure

So the higher the manifold boost, the lower the effect of altitude. Compare the two boosted values above to the 1.194 correction factor due to air pressure in the NA case.

This all assumes there are no effeciency changes in the turbocharger or suprecharger due to altitude which we all know is not the case, but much better than the standard SAE formula.

So my final modified correcton formula for the example should be:

Larry'sCF = ( [(boost)+29.23]/[(boost)+24.49]) x ((101.41 +460)/505) ^ 0.5

metric
Larry'sCF = ( [(boost)+990]/[(boost)+829]) x ((38.56 +273)/298) ^ 0.5

boost = manifold gauge pressure in millibars.
millibar = 0.1 kpa
in-hg= 3.386389 kilopascal (kPa)
1 atmosphere= 101.325 kilopascal (kPa)
1 psi= 6.894757 kilopascal (kPa) = 68.94757 mb

Boost expressed as in-hg = 2.036 x psi
Boost expressed in mb = 68.9116 x psi


***********************

boost = manifold gauge pressure in millibars.
millibar = 0.1 kpa
in-hg= 3.386389 kilopascal (kPa)
1 atmosphere= 101.325 kilopascal (kPa)
1 psi= 6.894757 kilopascal (kPa) = 68.94757 mb

Boost expressed as in-hg = 2.036 x psi
Boost expressed in mb = 68.9116 x psi


=================

In general form the modified factor would be:

Metric
Larry'sCF = ( [(boost)+990]/[(boost)+Pd]) x ((Tc +273)/298) ^ 0.5

english
Larry'sCF = ( [(boost)+29.23]/[(boost)+Pd]) x ((Tc +460)/505) ^ 0.5

Boost= manifold gauge pressure
Pd = local dry absolute air pressure
Tc = local ambient air temp

=======================

Now the above also assumes (unspoken) that all other variables are controlled, such as tire pressure, tie-down tension, oil temp and weight, coolant temp etc. etc.

Fact is, very few dyno operators even have the means,(to measure oil temps, and coolant temps in real time) let alone the time and economic interest in controlling all those variables. Transmission gear lube temps (for example) can make a difference of over 20 ft/lb of torque between a warm gear box and a gear box at full operating temperature. The same sort of effect is seen in turbo spool up with header temps. An engine that is started and run on the dyno with relatively cool exhaust headers will spool noticably slower than the same car allowed to stabilize at normal operational temps before the actual pull is made.



Larry
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Old 10-31-2005, 07:34 PM   #19
cpturbo
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Quote:
Originally Posted by sponaugle
I believe we use 13.5 for the HP @ 50, and 3400 for the weight.. However sometimes we run the weight at 5000lbs, and it still is less then the load on the street. As a results, I suspect something else is wrong in the software config in terms of static weight applied to the rollers.

The calibration file is what really counts? Do you know which one you will be using?

Jeff
I will find out and get back to you!

CP
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Old 10-31-2005, 07:47 PM   #20
mpj_becks
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Something else that comes into play with the Mustang dyno outside of which Parasitics cal file is used is whether the load cell span was setup correctly during installation.
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Old 10-31-2005, 08:09 PM   #21
caraz
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As a dynopack operator I thought I would throw out a little information about it.

You all know it can do a sweep, this is used for power pulls. The reason everybody uses this is because this is what people are used to seeing. You are able to vary the time to whatever you wish.

You can also do a stepped pull where at every 1k RPMs it will hold the motor for a given time.

It also will hold the motor at whatever RPM and at whatever load you wish, this is what you would normally use to tune EM one cell at a time. Very similar to a Dyno Dymanics unit (which I very much like, although I love my hearing)

I don't know if this has been discussed since I have just skimmed over the post.

HTH
Brian
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Old 10-31-2005, 09:48 PM   #22
Christian.
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caraz,

This is not to bash you or that dyno...this is just part of the open discussion. This is just my observation. One thing I have noticed with the hub adaptive dynos is that most shops have them set-up to accelerate at linear rates...when I have never seen a Subaru generate a linear torque curve thus that dyno tends to overload and underload the vehicle depending on the RPM range of the test.

I have also noticed that the holding feature is not as quick to react as the Mustang or Dyno Dynamic Dynos which makes MAF calibrations a little more difficult.

Regardless of which dyno a company chooses...a chassis dyno is a tool, it can have all of the bell and whistles in the world. But the operator needs to be intimately familiar with the equipment and how to properly operate it to get an optimal tune.

Take care,
Christian.
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Old 10-31-2005, 10:33 PM   #23
RiftsWRX
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Quote:
Originally Posted by sponaugle
Yep, that is very true. I suppose I'll see you at Sema tomorrow then!
Next year bud. The shop is closed this week and the guys are all going to be there. It's been a HECTIC last 6 weeks on the dyno (3 nights have I gotten home before midnight). I'm taking the week to spend time with the family. So don't have too much fun out there heh.

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 11-02-2005, 03:34 AM   #24
patr
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well, for what its worth, its interesting jeff that you didn't throw in the DD into your comments

a) we have a Dyno Dynamics unit - stock sti is around 215

b) it reads "low" (only when you have people reading this forum saying "how come I was told my car puts out 80whp more") - but you _could_ correct that (but we dont). it doesn't matter really except for proven power bragging because its all relative from beginning to end, but in any case, you can still correct to 'known' values to publish fancy graphs if you really wanted to. We _could_ correct the 215 to 300 and go from there but of course we all realize where that gets everyone (nowhere). but the point is you have complete control over it. I know for a fact one shop here on Nasioc is using a correction factor on _everything_ !!

c) we try not to use atmospheric/ambient correction - which lowers our numbers even more (but we publish them and the dyno automatically stores what the correction factors would be if used) - and we always run an ambient + an ambient intake temp probe. basically, sae is useless for turbo cars IMHO except for getting higher numbers than you should. But when operating its good to know the ambient intake when tuning and making your own comparos etc..

d) the load holding and ramping is the best I have used for what we need to do (soobs) (I have used DP, Must, DJet and that sweedish thing)

e) this forum annoys me that people dont publish the cor factors and the ambient data. I've seen waaay too many "cheater correction graphs" its not even funny. I too can post a "600 whp" graph, very easily of course !
but the biggest 'game' is the over-fan game... anyways we try to simulate 60 mph for averages, and do a few 'no fan' pulls for god measure, but some guys are running 120mph worth of CFM into those scoops !

f) even though we run uncorrected, people always want to get a "corrected to mustang" or "corrected to dynapak" number because of certain post-whores on nasioc :-)

g) but most importantly, with the DD, it always keeps the 'raw' data - you can show corrected, or uncorrected, etc. so yo can compare things but it still keeps the raw stuff regardless of correction. In other words, you can show it a certain way, but it doesn't mangle the data.

h) The "shootout" mode actually makes it difficult to 'fake' any data unless you trick the ambient probes, but still, I have been discussing this with a number of DD users here in the us about coming up with a "american shootout mode" with no ATC correction, more realistic intertia values and ramp rate of 100. This makes it 'level' for all. but no one wants to use it, it seems, which means most people are getting more out of their correction/parameter values than maybe they want to admit ?

i) what I especially like is you can change tire size, you can change gears, you can run at any speed, the graph is pretty much the same (if you have the right ramp rate and the fans to support it) and

j) that last issue, the ramp rate, is what you are talking about. With fine control over the ramp rate, you can actually do a lot of different kinds of things. And it DOES directly affect your graphs. The DD has awesome ramp rate control, you can ramp down at the controlled rate and do a graph "backwards" if you want - and it does it well (again, if you have the fans and balls to do it on your map, that is). Want more boost ? lower the ramp rate. Want to tune that wastegate right up to the edge of spool ? Ramp rate and hold again. But this directly affects your dyno graphs, the rate you use. And if you try to use a correction factor, ramp rate affects it as well (yet another reason not to). So anyways, a common 'rate' is also important when comparing the same car. I actually do several pulls at different rates on baselines because it affects different things. the subtle point is the DP torque numbers are "difficult to refute".

anyways, other dyno drivers (or others) flame away, I think what we are doing is the "right" thing to do, but I know for a fact the noobs are reading the forums saying "well, they told me its supposed to be 60 whp more" - it makes you think all the published stuff should be corrected sometimes

-pat
www.rocketrally.com

Last edited by patr; 11-02-2005 at 05:12 AM.
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Old 11-02-2005, 05:52 AM   #25
hotrod
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Unfortunately due to all the interactions, as you say a dyno operator/tuner can get just about any number they want from a car. Some do it with full knowledge and fore thought in order to sell a specific product or service. Others not so well educated on how all these inputs effect the output, do it out of simple ignorance. They don't even have a clue they are cooking the numbers. Last are the ones that due to pressures from miss-educated consumers bow to the pressure of the market place, and tweak the setup to give numbers comparable to what the consumer expects. Not because he wants to, but because he must in order to stay in business. Especially in the case of a tuner who's work is frequently judged on peak power not percentage of change in peak power, its a question of survival to produce "commonly expected" results on the dyno.

It would be interesting for someone to take a car and publish a comparison showing the wide range of numbers that they can get from an unchanged car with simple and not so obvious changes in the dyno setup. Ie tie down tensions, tire pressures, tire types, oil temps, gear box temps, changes in lube oil and gear lube viscosity, over cooling the intercooler, under cooling the intercooler, pressurizing the dyno cell with ventilation fans, etc. etc.

The real problem here is that the buying public is for the most part totally ignorant about dyno variablity and actually believe a dyno printout is a hard and fast "proof" of a specific power level. Until the average consumer quits looking at a dyno output as a hard and fast number, and starts seeing it as a relative number I suspect the problem will not go away.

Larry
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