Bigpower

Just trying to expand my knowledge of this subject... I see a lot of people saying that they tune for AFR of 11.2:1 for power but others vary in the opinion.

I am not sure whether this varies by car because when I was engine tuning on my mini, the AFR was a hit and miss affair to say the least :roll:

Is there a definitive answer or is it another case of run it slightly richer than optimum at peak torque to lubricate the cylinders?

The afr will depend on the amount of boost you run and also at what rpm you make it.

AFR is a bit of a meaning less expression. In effect it is Air fuel ratio. Air fuel ratio is dependant on you knowing what fuel the engine uses and also is harder to use in terms of understanding what exactly is going on in the burn.

If we are to look at it as Lambda (L) it is a decimal version.

on a petrol scale

1 L = 14.7:1
0.9 L = 13.2:1
0.8 L = 11.7:1

What Lambda ( L ) tells us that afr doesnt is that regardless of fuel type or any other variable that it is a certain percentage rich or lean.
ie 1 L is a complete burn or stoich. 0.9 L is 10 % rich and 0.8 L is 20 % rich etc etc.

What you are trying to achieve in the fuel map is the correct percentage rich over stoich to cool the combustion process to enable the correct conditions at PPP ( peak pressure point ) which gives us the correct bmep for the fuel.

If the mixture is to rich then you will wash power down the exhaust and if it is too lean it will create excess heat in the combustion chamber. this could lead to melting of piston crowns etc and melting of spark plug electrodes.

a Good but by no means fully accurate view of the combustion heat is viewed as EGT ( exhaust gas temp ) but this data is only useful when the engine is at a constant load.

Scott

does this help

helps, but still a little bewildered...

EGT - Am i right in thinking these need to be quite high to create the speed of exit gases (or something along those lines) but again a balance between too high and not having the velocity of gases or too low ?

Boost is at 18.5psi from 3000 upwards tapering down to 14 at 6500 if that helps on a 2.5 Litre

I havent got a lambda sensor at the moment but one will be installed next year I believe if it will help get the best from the engine and/or protect it from the worst

AFR as shown by the ECU drops to 11.13 WOT in the power band

The egt needs to rise up peak rpm where it will be as close to optimum as you dare. on a track car or race cars ie my car peak egt is at my gear change. High egts will help spool the turbo in the low rpms.

My car tends to do the following

idle badly at 12:1 @ 300 deg C ish
idle badly after a good thack at 11.5 afr @ 400 deg C ish

Cruise at 16.5:1 at 722 deg C

Peak at 805 deg C at 14.7 psi at 12.3 AFR at 7500 rpm

The egt measurments will vary by as much as 300 deg C when taken post turbo. They will also vary as per thermocouple position. Ideal postion is pre turbo and one in each runner. The danger of going to the real engine peak is that you could have a hot cylinder and not know about it so always best to be safe. these can also vary with you breakfast on hot / cold days etc

TBH you need to know the egt values to start with . What make is the Lambda sensor ?

11.1 at 18 psi which i asume on a scoob it will be at 5000 rpm ish and dieing off to 14 at say 6000 ish. Scoobies are odd like that.

I would say you are a tad rich but nothing i would worry about. eg from 11.5 afr back to 12.3 i would guess there is about 30 ish hp. Thats what u tend to find ( howeva each engine has its own variables )
to go back from 11.1 to 11.3 i cant see you being able to notice anything at all its gonna be in the 10 or sub 10 hp region. After a good thack take the plugs out and post up a pic.

How critical you can go will depend on how well the package works as a whole unit.

TBH i would get water injection and lean that puppy out and add some timing to unlock those horsies...

Scott

Re egt..

Egt can do some very odd things. i have in the past through gut fealing leaned a car out and the egt goes down.

High egt could mean the following

too advanced timing
too retarded timing
too lean afr
+ many other things like a lean cylinder etc


low egts could do similar and also lie to you if there is a exhaust leak like a lambda will

Very informative. Water injection rears its head once again. Will have to look into this next year.