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Afr selection on boosted engine

Understanding AFR

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Hi Zac/Andre,

Just a quick question here regarding air fuel ratio on a boosted application. In the course, Andre gives general guidelines for afr depending on boost pressure, say lamda 0.80 for 100kpa of positive boost pressure.

The question I have is what if you're tuning a 4 cylinder evo at 100kpa positive boost vs say a Nissan RB26DETT at the same boost level. Both cars may be making significantly different amounts of power at the same boost.

Is this a consideration when selecting an Afr to run the engines at ?

To expand on my point, I have a buddy who has a R34 GTR and the engine is developong in excess of 500whp at 20psi boost.....I am sure on an Evo you may never see that sort of power running similar boost....basically my question is would you run the Nissan richer because it's making more power ?


Okay, so I'm going to use some pretty dodgy assumptions here, but it'll get my point across I think... ;-).

Assuming stock engine sizes, the GTR's 2.6L RB26 has an extra 0.6L of displacement over the Evo's 2.0L 4G63. Call it an extra 30%. 30% of 500HP is 150HP, so if both engines were making the same power per litre of displacement, the evo would be making 350HP... Which is certainly in the ballpark for an Evo running 20psi of boost :-).

So if you look at it in terms of HP per litre, they're pretty similar.

To answer your original question though, no, 0.8 Lambda would be a good starting point for either engine, and then tune things to what it's happy with from there :-).

Understood Zac, I get the point you are making.

For this specific example though, I know you're probably referencing those lamda figures on what you call pump fuel in your parts which is 98 RON, but here in Trinidad the best we have is 95 RON....or so they claim it is....we don't really know for sure lol.

At 20psi on our fuel I'd probably be starting at lamda 0.78, do you agree?

Also in the course Andre goes on to say you won't richen the afr indefinetly as power rises and on a piston engine on pump fuel the richest he'd go is lamda 0.75. Do you agree with this statement ?


I'd probably still start at 0.8 Lambda, as while adding extra fuel can help to control detonation, the octane rating of the fuel has a much larger effect. The limits you're going to strike running 95 RON fuel might be improved slightly by running things a bit richer, but you hit a point of diminishing returns, where as you richen it up further, the extra boost and timing you can run decreases. 0.8 is still a pretty rich mixture in the scheme of things.

Going any richer than 0.75 lambda is likely to be well into the point of diminishing returns, and going any further than that is likely to have more negative effects than positive, so yeap, I agree with Andre for sure on that one :-).

Thank you for the responses !

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