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Diesel EGT vs AFR

Practical Diesel Tuning

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Good Evening All

I know the following question is very general and vehicle specific logs would be required; but what is a general afr that one can tune to to keep egt in a safe range of 650’c to 720’c.

i have a newish 4n15 and the egt in stock form reaches 800’c pre turbo(18.5:1 afr with lm2). A 4JK1 isuzu 2.5 reaches 650’c pre turbo( 21:1 afr with lm2) on the Isuzu at 18.5:1 we run 780’c. I know different turbo’s will give different back pressure ingluencing this. 99% of people will not allow me to dril the manifold to measure egt when tuning due to warranty still in place at time of tuning. Hence the reason I want know if there is any rule of thumb on afr. I also understand fuel pressure and soi timing will have an influence on this. But if you only look at afr and egt what correlation is there?


I think the only rule of thumb you can use in these scenarios is limited to engine families of similar size and design. It seems like you've got pretty consistent data supporting ~800C (1400F) for 1.25 lambda on your engine family of choice. In my experience running lambda over 1.20 is usually very safe (EGT wise) and 1.15 is usually close the limit of long term reliability. You might also consider looking at the factory lambda limits and EGT as a second data point in your consideration of reliability. Do they regularly run lower than 1.25 lambda sustained, or EGT over 1400?

Of course this all hinges on your timing and fuel pressure being consistent like you said. I think you're on the right page.

Hope this is useful,


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