AFR for nitro

Received this email from a knowledgeable racer:

Crew of an A-Fuel car (nitro)
Bought all your books. Excellent reading.
However, my calculated AFR number gives unreal value. How can I make this applicable to nitro?


I expect you are seeing AFR umbers much less than one to one for AFD. I think one set of calculations that I did showed an AFR of 0.2 to 0.3 I was surprised as you are.

The interesting thing about unblown nitro is the differences in Nostalgia AF and NHRA AF. Both are similar engine sizes, car weight, and RPMs. But different fuel pump sizes and dramatically different performances, and AFR numbers.

In gas or alcohol, engine power goes up for enrichment, then down with further enrichment. For nitro, engine power goes up for enrichment. Then up more with further enrichment until other limits occur. One of the limits for very rich nitro mixtures is slow flame speed that causes a reduction in power at higher engine speeds. Another is misfire.

So the tuning task is pushing AFR up until power is reduced at high engine speed or more misfire occurs. I wonder if the AFR numbers calculated from the engine size and air density are a repeatable indication to reach the maximum performance before either high speed power loss or misfire.

It may be that AFRs may need to be different for different outside temperatures.

Don Jackson, of DJE, said he to me a few years ago that he uses AFR to tune blown nitro cars. That indicator was very repeatable for determining the top fuel tuneup. Although he said at that time that no one else in the top fuel ranks was using AFR. He said that he could qualify a top fueler with his tuneup anywhere without burning a spark plug.

I calculated AFR numbers for several nostalgia blown nitro setups and always got the same numbers. I ran the numbers for AFD only once so I do not have a feel for that repeatability. In addition I found that some of the experienced racers use other adjustments to tune unblown nitro engines, changing the nitro percentage (from experience) or changing a high speed shim (from experience) for air density changes.

We just started our experience with nitro ourselves. Last week, I richened our blown alcohol engine 12 percent, added only a few percent nitro, and burned some plugs. There were other issues but the experience was humbling. The engine idled 30% faster with the small amount of nitro and was much more responsive. I was told that smaller percentages may be more difficult to manage than higher percentages. So we will see. We will be creeping up on it and hope to publish our results. But nitro was a different animal in the engine.