I see some ppl running less timing with a leaner A/F and others that like to run more timing with more fuel. Both claim good results.
My main question is: Where is that balance between timing and fuel?
What is the optimum choice? Is it whatever works for your car? Personal preference?

 

The correct way is to go as lean as possible, while be safe and leaving a margin for error {ie. 11.5 AFRs in 4th} and then setting timing accordingly.

 

Try everything, just monitor real time knock and use a wideband.

Keep in mind that too much time (extended combustion duration) with excess fuel will give a scewed O2 reading and will show leaner than the actual burn ratio

 

Should 11.5 afr be in 3rd gear (six speed)???

 

shouldn't matter. AMS tuned my car in 3rd and we did a couple pulls in 4th just to check things out but no changes were made.

 

Gas produces the most power when it burns @ 12.5:1 AFR. Having said that, not all engines have a good enough combustion chamber design to run @ 12.5:1. I think the Evo 8 is one of them. The evo 9 has a better cylinder head and can run leaner. I have owned both a 9 and an 8 and my 9 can run leaner than my 8 with higher boost.

A lot of people in this sub-forum have claimed that there is no power to be exacted from the Evo beyond 11.5:1 AFR. So a lot of people tend to shoot for that AFR.

You also have to consider the octane of the gas that you are running. 93 octane gas has more knock resistence under boost/compression than 91 octane. Unfortunately, I only have 91 octane in CA and I tend to run my AFR in the mid to low 11 AFR under 21-22 psi and tapering to 20 psi by redline.

 

WTF, why are you rolling eyes on me....

 

lmao

 

Extra fuel is needed to help control combustion temperatures and control combustion rate. Pushing the tune to the leaner side (11.5+) takes away some of that cooling capacity and increases burn rate. In my experience, this results in a condition where knock tends to come on like an avalanche, vs with a richer tune, where in my experience knock seems to build more slowly. For a daily driver where strict control over fuel quality is impossible, I recommend tuning towards the richer side.

 

I dont disagree with anything said in this thread so far, however it all depends on the specific gravity and octane of the fuel we are dealing with. All the comments so far tend to be oriented toward 91-93 octane and SG of .76 or so. Throw some 94 with 10% (at a minimum, like Cenex fuel around here for instance) and 12:1 becomes very easy to tune for. I apologise if I clouded the issue, but the real question should be timing/AFR for xxx octane.

 

-Gas doesen't burn most efficiently at 12.5:1. It burns completely at 14.7:1. We run richer than stoich because homogeneous mixing is impossible so to make the leanest portions of the cylinder richer than stoich people shoot for numbers such as mid 12's. Most newer engines like mid 12's most older engines are in the 11's due to poor air/fuel mixing in the CC.

-Gasoline is poor at controlling combustion chamber temps. Reducing timing does a much better job of cooling the CC by burning later and thus moving some of the exhaust energy (heat) to the exhaust manifold instead of the combustion chamber. The heat capacity of gasoline is poor at best and if you are using it as a coolant your hurting yourself more than helping. Reduce timing to control knock not add fuel. To sufficiently quell knock you'd have to reduce cylinder temps by a few hundred degrees and thats not going to happen by adding a few cc of fuel.

-You don't add fuel to control combustion rate. As a matter of fact you want the burn rate to be as quick as possible to decrease the amount of time the fuel has to be lit off BEFORE TDC (where the piston is pushing the crank backwards). A quick burn means you can run less timing, burn the fuel later, and use more of the burn to push the crank FORWARD instead of BACKWARDS. The reason more timing can make more power (for a constant AFR) is it places the peak pressure of the burn at a good mechanical angle on the crank making more torque. The magic number is 12-20 degrees ATDC for peak burn pressure across the crank.

-Specific gravity is just a number used to designate how much fuel you will need for a given amount of oxygen. The AFR after being tuned is the same. You should run fuels that have the same SG as the fuel you tuned on or your AFR is subject to change.

-Octane has nothing to do with the AFR an engine likes to run at. Whether it be 91 octane or 118 octane I'll tune for mid 12's for max power. For street cars I tune for 12:1 to account for different SG's of fuel manufactures. For instance Sunoco runs lean whereas Exxon runs rich. Tuning for 12:1 on Exxon enables me to put in sunoco and not go too lean.

 

hmm, did not think you would find different afr based on brand alone. most brands differ only in add packs that cover detergents for injector maintenance.
that is another variable to add to the tuning process.

are you discussing oxygenated race fuels or street gas?

 

Pump gas changes a significant amount from brand to brand. Sunoco seems to put a lot more ethanol in their gas requiring you to dial in more injector. Near big cities winter blends change the AFR as well.

Your normal sunoco blue and purple runs richer than any of the pump gasses that I've used. I want to say VP import runs leaner than the sunoco blue and purple but its been a couple years since I've used it. If you look on a barrel of race gas it gives you a bunch of data including specific gravity.

 

Interesting....I have also heard that 12.5 af was a target. However, I thought that was for normally asperated cars and figured because of boost that the 11.5 number was used for the Evo..As for the effeciency of the engines, hey, Its the same 2ltr-- same bore and stroke same head and intake.....all hemi design with central plug so I doubt thats the reason in this case.. If 12.5 afr were perfect for power and 11.5 was the std evo afr target....I would think there was something in between that would work far better then 11.5afr. So, anyone here have the proof...dyno with Knock counts and power output for leaner ratios then 11.5...? Or are we just blowing up motors at that point..

 

We are talking about power, not efficiency. Gas produce most power when it burns @ 12.5:1. Gas produces the least emissions @ 14.7:1. that is why the narrowband o2, in part, keeps the AFR in the 14.5-14.7:1 during cruising. It is all about emissions these days.

Do you run 12:1 all the way to redline or only during spool up and mid range rpm? I tested my Evo with 12.xx:1 all the way through spool up and midrange and tapering to 11.5:1 to redline. Then I tested @ 11.5:1 and tapering to 11:1 by redline. (see attached chart). I kept the timing and boost the same. This was done under similar conditions. Temps were 25*C, baro was 101 kpa and humidity was 48-51% for both runs.

The power on DLL was almost the same.

The lean run made 281 hp and 271 lbft
Te richer run made 275 hp and 272 lbft.

Are 6 hp worth it to run that lean and risk detonation on a hot day? Not for me thanks. I would rather run 11.5:1 and not have these 6 hp and the risk that comes with them.