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Old 09-29-2004, 10:02 PM   #1
snookem
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Default How do you target A/F's to fuel and timing

Could someone direct me in how you road tune A/F's on a Subaru motor that optimises A/F's for different octanes and boost when your in the 50% and above load zones. I understand the 10% to 50% spool the turbo A/F's and ST/LT trims etc. I also understand timing though load zones and max torque plus EGT's.

Let me explain some more.
If you have engine management ie Link, UTEC, Hydra etc and you start with a base MAP on a modified engine with say 9.0 compression that is 2.4 liters. The A/F's are a conservative 10.0.

What determines the optimum A/F's for octane, timing and boost on any Subaru turbo motor. What are the steps need to get there.

Most say on a WRX that 10.8 A/F's on 93 octane is OK. STI tuners I've talked with say you can have 11.2 A/F's to 5k and 11.6 A/F's from 5K up on 93.

I have seen maps built for 91 octane with 22 psi and maximum 10.5 A/F's for STI's.

I wish to build a track day(circuit reliable) tune.
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Old 09-30-2004, 02:06 AM   #2
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Let me start by saying that this is just what I've learned, and I'm no expert. Anywhoo, most cars make max power at an a/f ratio of 12.5-13.5 to 1. Having more fuel doesn't optomize anything, it just hurts performance. The reason we add fuel on top of what makes the max power is because of heat. The extra fuel is used to cool down the cylinders and combustion process by absorbing some of the heat from them and getting expelled out of the exhaust. This is to make our engine's run safely. If we didn't need the extra cooling that fueling beyond 12.5 to 1 gives, we wouldn't make the a/f ratio any richer then that. The optimal a/f ratio is where you can get it so the engine/egt's aren't too hot and you're makin the power you want.

The lower the boost/rpm the cooler the engine will run. As the rpms/boost go up, the engine will generally run hotter often creating a need for richer a/f ratios. Usually higher octane fuel makes it easier for the engine to run cooler which allows more boost and a leaner a/f ratio.........

peace

Last edited by hippy; 09-30-2004 at 02:25 AM.
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Old 09-30-2004, 02:46 AM   #3
White 2.5rs
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wouldnt they make the most power out of stoich or even a little lean of stoich?
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Old 09-30-2004, 02:54 AM   #4
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Stoich is the a/f ratio at which the ratio is perfect for all the a/f to be used when lit(chemically). Course, if you don't wanna use all the air going into the engine, you could use less fuel making the car a bit more fuel efficient. Also think about how not all the air/fuel is used, and how combustion is never perfect. Having 10-15% more fuel then stoich is generally what people say makes the most power. It's could also be fuel specific since some fuels have o2 in them and what not, but......

peace
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Old 09-30-2004, 02:55 AM   #5
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No Im pretty sure you just get the best mileage at stoich.

edit: hippy beat me to it
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Old 09-30-2004, 02:58 AM   #6
White 2.5rs
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but wouldnt best mileage be the most efficent, or most power for least amount of gas used?
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Old 09-30-2004, 03:00 AM   #7
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Using the amount of fuel where the engine makes the most power per fuel used probably translates into the best mileage also.
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Old 09-30-2004, 03:11 AM   #8
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Part of a post by Jon [in CT] in another thread:

Quote:
For most fuels used in spark ignition, max torque is achieved with an air/fuel mixture that is about 15% richer than stoichiometric. For gasoline with a stoichiometric ratio of 14.7:1, that works out to 12.5:1. Running an even richer gasoline mixture is "safe" because the evaporation of the extra fuel cools the intake charge to stave off knock; but some torque is sacrificed in the process.
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Old 09-30-2004, 08:47 PM   #9
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Thanks guys but its not what I'm after.
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Old 09-30-2004, 10:27 PM   #10
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snookem

Do yourself a favour and buy this book.
http://www.amazon.com/exec/obidos/tg...glance&s=books

Without a doubt the best book I have ever seen on tuning. Corky bell's book looks like a 3rd grader wrote it compared to this. HOK who's a member here put me onto it. He's got a Hydra EMS and it helped him solve alot of his problems.

Page 135 will answer your question in more ways than you thought possible.
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Old 10-01-2004, 12:15 AM   #11
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I agree with Wombat, substantial reading is in order. It's not so simple as reaching the optimal AFR. The piston is connected to the rod and acts as lever, there is a angle to the rod that corresponds to the point of greatest torque potential, known as maximum brake torque (MBT). MBT moves around with RPM, not for any physical reason (MBT should be constant if all revolutions are the same), but because at different RPM the time for combustion/exhaust varies and the heat generated varies (hotter cylinder means faster burning fuel, that needs less ignition advance). How the MBT varies is something that you'll have to determine with testing, but there are some general rules for the subarus. It helps to remember that the fuel is ignited while the piston is moving up toward the head, slower burning fuel (i.e. cooler cylinder or higher octane) gives more power with more advanced ignition (the burn starts earlier, but pushes with the most torque at the same instance (MBT) that a hotter cylinder or lower octane fuel would).

Enter knock: knock more so than power often determines what you can achieve in terms of power. In the mid-RPM range you will not be able to achieve either best fuel or best timing from a theoretical stand point. It will be a trade off between adding fuel to prevent knock, or retarding ignition timing.

Early in the RPM range under full load you can run AFR up to stochiometric, one to save yourself fuel under cruise conditions, but also to use the heat generated to spool up the turbo. Once you are at stochiometric, varying the timing of the ignition will give you a feel for the MBT ignition timing, ~20 degrees advance. Using this ignition advance as a standard, you are going to try and match it as RPM increases, adding fuel if knock occurs.

Once you get into high RPMs, you can start to remove some fuel, and advance timing. At this point your car is barely able to complete the burn of fuel and get all the combustion products out of the cylinder. Plus it's hotter than hell. You are holding on to what ever power you can get above about 6000 RPM.

Lot's of other things can alter the mix, and timing, of the tune in your favor and these are the source of many modifications, most of which cool the air prior to the intake (larger intercoolers, water injection), but also improve flow of the exhaust (headers), or help to expel exhaust more completely (cams). And the list goes on....
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Old 10-01-2004, 11:01 AM   #12
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I remember when someone asked me this a long time ago... I told them that they were seeking the meaning of life...
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Old 10-01-2004, 02:34 PM   #13
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Quote:
Originally Posted by HOK
I remember when someone asked me this a long time ago... I told them that they were seeking the meaning of life...
You know it.
I tune NA and boosted Hondas locally, and street WB02 tuning is by far the hardest to do. I learned that reading the plugs, along with knowing what A/F you're running is the best one can do.

Great book recommendation, btw.
"How to Tune and Modify Engine Managements Systems"
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Old 10-01-2004, 01:30 PM   #14
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Quote:
I told them that they were seeking the meaning of life...
That from a guy who tuned with the Hydra completely on his own for a long time.....that's balls right there, fearless.

Yeah it's a tough one, it's like asking Van Gogh "How do you choose your colors?" There is science to it, but it's pretty touchy feelly.
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Old 10-01-2004, 03:01 PM   #15
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so if the most power is made at ~12.5 could you tune staight c16 at this ratio or would you still need to add extra fuel for cooling?
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Old 10-01-2004, 03:56 PM   #16
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the issue with this type of street tuning is that all of it is highly theoretical... if someone is successful at a certain AFR at a certain boost it has little bearing what I can get to. 12.2 AFR is the theoretical max power stated in that book.... (for forced induction) but practically speaking that statement means nothing to me as a guy on the street tuning my own car. many people also don't consider where they are in the turbo's efficiency range. you maybe able to get max power at a certain AFR advance combo but how long is the question... no matter how one is tuning you are shortening the lifespan of the engine and drivetrain... its a given... how much shorter is the question... its the balance between power and how long the engine will last.

For c16 I wouldn't necessarily lean out the mixture. I wouldn't go beyond 11's. What I would do is keep advancing to where you feel good about the power and having no knock. I would never live with knock in any circumstances, not going up steep hills, having a lot of people in may car, boosting for 10 seconds in a gear etc. I believe its unacceptable in a street car... but to each their own. Also sideways for where you live you need to tune for the larger changes in weather... it really makes a big difference...

so the only certainty one should remember is to do things in baby steps...
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