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Old 09-24-2002, 04:44 PM   #1
annointed
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Default overseas dyno differences: 3rd vs. 4th gear pulls

I came accross this thread on the MRT forums:

http://www.mrtrally.com.au/forums/to...?TOPIC_ID=1645

One guy suggests that getting the truest readings requires doing dyno pulls in the gear closest to 1:1, which is apparently 4th gear in the WRX.

AFAIK, most tuners in the US do 3rd gear pulls...could this partially explain why our numbers seem *inflated* compared to the boys overseas? I'd always assumed in the US that we must run different dyno programs that gives higher readings than OZ and UK.

Of course, the type of dyno no doubt plays a role in this as well, but I'm wondering how much of a difference the gear actually makes.

Mark
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Old 09-24-2002, 04:55 PM   #2
Jon [in CT]
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Who really cares? Well, maybe the folks in the Proven Power Bragging might have an interest. This thread should have been posted there.
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Old 09-24-2002, 05:53 PM   #3
annointed
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Quote:
Originally posted by Jon [in CT]
Who really cares? Well, maybe the folks in the Proven Power Bragging might have an interest. This thread should have been posted there.
Thanks for that contribution.

I'm hoping to learn something here...how does gearing affect wheel hp figures? And is that effect real (ie, do you actually have less hp in 4th than in 3rd), or is the difference only related to the dyno's program? An engine dyno wouldn't even involve gearing, so I suspect that the peak hp difference is built into the dyno's program?

Anyway, I'm just brainstorming, so I'd just like to learn more about this whole issue.
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Old 09-24-2002, 06:18 PM   #4
nmyeti
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Default Re: overseas dyno differences: 3rd vs. 4th gear pulls

Quote:
Originally posted by annointed
I came accross this thread on the MRT forums:

http://www.mrtrally.com.au/forums/to...?TOPIC_ID=1645


AFAIK, most tuners in the US do 3rd gear pulls...
Mark
Mark,
I can't speak for the other shops, but we NEVER do 3rd gear pulls when we are tuning. We run 4th gear pulls with a 15 second ramp time between 2000 and 7000rpm.


-Nathan
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Old 09-24-2002, 06:28 PM   #5
annointed
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Default Re: Re: overseas dyno differences: 3rd vs. 4th gear pulls

Quote:
Originally posted by nmyeti


Mark,
I can't speak for the other shops, but we NEVER do 3rd gear pulls when we are tuning. We run 4th gear pulls with a 15 second ramp time between 2000 and 7000rpm.


-Nathan
Technically, what is your reason for 4th gear pulls?
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Old 09-24-2002, 07:30 PM   #6
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Quote:
Originally posted by annointed


Thanks for that contribution.

I'm hoping to learn something here...
My contribution was to direct you to the correct freaking forum. Comparisons among diferent types of dynos and how to (mis)use them has zero Factory Forced Induction relevance, whereas it's a common and germane topic in the Proven Performance Bragging forum.
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Old 09-25-2002, 08:36 AM   #7
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and perhaps that was best posted in the Self righteous Idiot forum.
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Old 09-25-2002, 08:37 AM   #8
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Old 09-25-2002, 09:05 AM   #9
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OK... If you are looking for engine hp...

Power is power. Your engine will produce the same power from 1000 rpm to 7000 rpm no matter what gear you are in. You just have to convert the torque values to engine torque values based on the gear ratio of the gear you are in.

How you get a power curve on a dyno...

The dyno actually controls the speed. You place the car (engine) on the dyno, select the gear you want. The dyno is then set for the correct idle speed. Then, the throttle is set at WOT. The dyno will then keep the speed at idle speed. The force of the wheels (driveshaft) on the dyno will create a load (the car wants to go faster, the dyno is stopping it). This is measured with an in-line torque meter or load cell. This will give you torque. The dyno will then ramp up the speed at a certain rate and constantly record the torque. For emission testing, the Fed. usually specifies 8 rpm per second. At redline, you stop.

What you then get is a speed vs. torque graph. You then need to correct for gear ratios, humidity, temperature, and drivetrain losses to get your actual engine hp.
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Old 09-25-2002, 10:33 AM   #10
Mach V Dan
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Quote:
Originally posted by 2000vfr800
The dyno actually controls the speed.
This is true of a resistance (braking) dyno like a Dynapack, but not of an inertial dyno like a Dynojet. The inertial dyno has heavy rollers that the car has to accelerate, and the rate of acceleration of the rollers translates into how much torque the car makes.

Dynojet, at least, does recommend using the closest gear ratio to 1:1, although since it knows both engine RPM's and wheel speed, the dyno software SHOULD be able to figure out torque in any gear. (It could calculate the effective gear ratio by dividing wheel RPM's by engine RPM's. I don't know that it actually does this, though.)

Assuming there's no fancy calculations on the part of the software, a lower-gear pull on an inertial dyno will make it appear that the car has more torque, since the torque multiplication of the gears will allow the car to accelerate the heavy rollers faster. I haven't use this software extensively, so I don't know if there is provision to tell it what gear/what gear ratios you are using.

Nathan, does the Dynapack software do the math so the computer will figure out what gear you are in, or not?

--Dan
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Old 09-25-2002, 11:22 AM   #11
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Quote:
Originally posted by Mach V Dan



Nathan, does the Dynapack software do the math so the computer will figure out what gear you are in, or not?

--Dan
Mach V
FastWRX.com
Dan,
We setup the pull by telling the computer what our gear ratios are. It will show actual torque (gear multiplied) or what it calls "flywheel" torque which is without the gearing factor in it.

There is also a "torque correction factor" which will allow you to guess at drive train loss, but we leave that as a "1" so that what you get at the wheels is what you see...



-Nathan
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Old 09-25-2002, 02:27 PM   #12
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I would think that dynos that measure torque as the engine is accelerating would show lower torque values in lower gears (corrected for gear ratio of course) due to inertia of the wheels, flywheel and everything else you're trying to accelerate. This is the same reason why some people think lightened flywheels and pulleys give additional horsepower when they actually just let the engine accelerate faster.

-my 0.02
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Old 09-25-2002, 03:53 PM   #13
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Quote:
This is true of a resistance (braking) dyno like a Dynapack, but not of an inertial dyno like a Dynojet.
You're right. I am just used to working with higher end active dynos and such.
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