AFR settings why the variance and does it have to be adjusted for different fuels
Turbo Mini Forums

Having used the search function my query is not yet answered.

Basically it’s the matter of fuel air or air fuel ratio. Part of my training/education had us looking into piston engines in all their glory. On the subject of fuelling we were told that the perfect ratio for best power development was 15:1 period.

On several threads I have seen this subject touched upon and ideal figures of 11:1, 12:1 and so on being suggested. We know the consequences of running too rich and too lean and I keep wondering why there seems to be such a variance in the fuel air ratios being used by forum users?

The only hypothesis I can think off is that the type of fuel has changed i.e. 4 star to unleaded and that fuelling has altered accordingly.

The other point of issue is the use of fuel additives. I use millers with normal pump unleaded fuel at times when I feel rich I will fill up with a tank of 4 star leaded. The performance change is instantly noticeable. Now not having a AFR gauge fitted I admit to not knowing what the charge mix is when swapping fuels, but will the use of different fuels and fuel additives have an adverse affect on an AFR sensor and what affect on AFR does an additive have? Does the AFR have to be adjusted for unleaded and leaded fuel?

Own the day

the theoretical and actual best ratios for best power are what change from engine to engine , combustion chamber shape, squish, valve sizes, tumble, swirl, sparkplug location, knock suppression, cylinder wall temperature are all things which will affect the air fuel ratio.


if for example your running right on the edge of knock, you may find you loose a few hp by running a richer mixture, but as the temperatures drop in the cylinder (lean burns hotter, just think of an oxy-acetaline torch) more boost may be allowed making the engine ultimately more powerfull, or just simply more likely to survive!

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

best power is not 15:1 not ever!

12:1 max power

14.7:1 only make your cat work (reduction and oxidisation needing slightly rich then slightly lean mixtures)

16:1 best fuel econo..

Only if you change the base fuel does that change.
ie lambda=1 for:
4 star 14.5:1
methanol 9:1

AlexF

Yep, best torque at 12.5: or 0.86 lambda

Anything more than this you are pretty much wasting fuel, and anything less you are loosing torque.

some engines actualy preffer less than lambda 0.86 for best torque, so this is strictly a rule of thumb. Thats why we have dynos

My race motors run best at 12:1 on In cylinders any Leader and as Colin said torque Falls off

Bottom line, as sproket says the rolling road session will let you know what setting your engine build works best at.

Cheers thanks for the AFR lesson 101.

Own the day

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

He was my lecturer when I was doing my LWTR training which was back in 1994 so no!

Own the day

He's either confused or talking about somthing else LOL

AlexF

Denis hit the nail on the head, you can have rough figs to start with, but all engines are different, i run mine with afrs that the skyline boys don't agree with, and they can't understand why i haven't blown it up.

Best 1/4 mile 13.2 seconds @116 mph
First 5 port miniturbo to make over 200 bhp on a carb?
First 5 port miniturbo to make over 200 bhp on Injection?

http://www.mattwoodsphotography.com

And don't forget that the AFR shown by a WBO2 sensor is the average of all cylinders so with the siamese ports what you see is not what you get.

Jean

http://www.jbperf.com/

When my car runs lean it runs warmer, you sure you have that the right way around sprocket?? think you'll find that more fuel cools the engine not having less.

Best 1/4 mile 13.2 seconds @116 mph
First 5 port miniturbo to make over 200 bhp on a carb?
First 5 port miniturbo to make over 200 bhp on Injection?

http://www.mattwoodsphotography.com

miniwilliams what kind of figures do the skyline boys run then? how different are they to yours?
lee

Speeding is like masturbating, everyone does it, but not all of us film it and put it on the internet

[quote=fab,8th Oct, 2010]fuck off

Edited by Sprocket on 23rd Jun, 2009.

Edited by evolotion on 23rd Jun, 2009.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

There is no need to run a skyline down that rich at all....

Alex

AlexF

sprocket is right - highest combustion temp is obviously going to be highest at the point of maxium power - thats when the most energy is being converted.

But the amount of energy absorbed by the engine is going to be a function of burn time, burn timing and rpm.

Typically as you lean off an engine so the cylinder head temp and hence coolant temps rise.

AlexF

Just to throw a spaner in to the works how does representative altitude affect AFR?

Own the day

If you are running a decent ECU with a MAP sensor then it won't affect it at all if the compensation table it set right.

On a carb it will basically overfuel at high altitude as there is less oxygen in the atmosphere.

So a barometric form of control would be required.

Own the day

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

I take back my last statement. Due to how the SU works I am of the thinking it wont over fuel that much so extra control may not be required.

Own the day

finding this one hard to work out, i've always been told that leaning off the mixture makes heat, so you say "Lean is not hotter, it is cooler" and then you say "Lean mixtures burn slower, therefore the relatively hot gasses are in the combustion chamber longer and the engine picks up more heat" why does it pick up more heat if lean is cooler?

I think we all better stick to what we know and what works for use lol

the skyline boys, and boy for insance (about 850 bhp @ wheels) runs with the afrs in the low 12s high 11s.

I have seen mine in the low 13s before and they have ablue fit lol.

Best 1/4 mile 13.2 seconds @116 mph
First 5 port miniturbo to make over 200 bhp on a carb?
First 5 port miniturbo to make over 200 bhp on Injection?

http://www.mattwoodsphotography.com

Compairing a mini and a skyline is always gonna throw up differences - the way the flame front moves in a modern design head is so different to what happens in a mini.

You can get away with very different AFRs and adv on a mini as the head is far more robust than a skyline.

Alex

AlexF