A/F Ratio

[Rosco]

Another question in my ongoing “educate the dummyâ€￾ self help program

What is the optimum A/F ratio to aim for?

I have an engineering book that says max power ratio for gasoline powered piston engines is between 11:1(max power rich) and 13.8:1(max power lean).

While these figures are in the ballpark they are for aero engines that have large sloppy pistons and turn relatively slow. Not really precise enough for the high speed advanced engines that we are trying to get every reliable HP from.

Also, how is A/F determined on the dyno?

If an exhaust probe is used down the end can, will the readings be accurate if the pair valve is bleeding air into the exhaust flow?

While these might be simple enough questions for an experienced tuner, they might be good to know when talking to your local dyno dude. Lets face it, there are good ones and bad ones.

Any advice would be appreciated

Cheers
Ross

 

[shenoyp]

Id like to know about this too...

 

[K Wolf]

Under ideal circumstances, you would want to run at stoichiometric (the chemically correct ratio, at which, every molecule of fuel has exactly the right amount of oxygen to combine with: approximately 14.5:1) all of the time. Unfortunately, ideal conditions rarely happen in our engines.
Less than perfect fuel atomization/distribution and flame propagation causes the need to run richer due to some of the fuel not being ignited. Even more important than that is the use of a richer mixture to help fend off detonation. The excess fuel helps cool combustion temperatures and slows down the violent burning that can become detonation.
Tuners with the monetary means use sophisticated "knock" sensors, oxygen sensors, and exhaust gas temperature sensors to find the optimal combination of A/F ratio and ignition timing.
Average people like me have to stick with the old school rule of going slightly leaner than stoichio at no engine load/high vacuum and getting richer as load increases to about 12:1 at full engine load/wide open throttle (richer under forced induction) .

I hope this helps.

 

[Lloyd]

12 is what we usually go for on the ZRX`s..

 

[frisbee]

 

[BLT_GSX]

You can run leaner with Race Gas, I would like to stay around 12 -12.5:1 AFR. With Pump Gas I would try to tune to around 11 - 11.5:1 AFR. Running higher octane fuel helps preven detonation especially in forced induction engines.

 

[manatrader]

This is when I take my bike to a shop I can trust, I can put things together, take them apart but tunning I always seem to have problems.

 

[BusaFever]

I BELIEVE AROUND 12 UNTIL ABOUT 8500 TO 9000, THEN GOOD TUNERS RICHEN IT UP A BIT TO COMPENSATE FOR THE RAM AIR. IT WILL SHOW LESS HP ON A DYNO, BUT WILL BE FASTER ON THE TRACK. BUT HEY THATS JUST WHAT I READ ON THIS AND OTHER SITES, I AM NO MECH.