Quote:
Originally Posted by kobrabytes
Can anyone give me some general guidance as to the best A/F ratio of an FE?
I have stage 3 heads on a 427 built by Keith.
len
|
Len,
This is a tricky question to answer as the the AFR selection is about more than just performance. Fuel is also a coolant so rich mixtures can be important in certain circumstances (this is why a supercharged or turbocharged engine is typically set to target 11.xx-1 AFRs for example). I would say that 12.7:1 or so would definitely be safe for most naturally aspirated street driven engines and this would be a good place to start for a WOT AFR. This would be on the side of "rich best torque". I think that the FE's that Keith builds seem to like a leaner mixture than this (Keith had mine tuned for a 13.6:1-13.8:1 AFR at WOT; I am running EFI with a 482 ci FE, stage 2 heads, and one of Keith cams). My exhaust system is not as good as Keith's dyno headers in terms of flow so I have adjusted my WOT AFR to be a little richer 13.1-13.3 while I retune the engine to better match the exhaust on the car. This is still on the lean side and would be considered "lean best torque". Also keep in mind that the choice of AFR will effect your ignition timing requirements at full throttle as well so you want to approach the lean limit VERY carefully. The only way to be ger these settings to their optimal values for a given motor is to dyno the motor with a load bearing dyno and an exhaust setup with per cylinder pyrometers so that you can see what is going on with both torque output, AFRs, and EGTs (Exhaust Gas Temperatures). Almost no one has this sort of setup so the next best approach is to use a load bearing dyno (best is to use a chassis dyno so you can calibrate with you actual exhaust system). At cruise levels (light throttle), I would aim for 14:1 - 14.7:1. In an EFI setup, you would typically blend WOT settings into the light throttle settings as throttle is rolled in for acceleration. The attached is my current AFR table that is in my EFI setup. NOTE THAT NO ONE SHOULD COPY THIS TABLE AND JUST USE IT - THESE SETTINGS ARE NOT APPROPRIATE FOR EVERY MOTOR AND WHAT WORKS FINE IN MINE MAY VERY LIKELY DAMAGE YOURS.
It should give you some idea of what the final curve should look like. AGAIN, START ON THE RICH SIDE (say 12.5:1) and only lean it out if you have the equiment to accurately measure what you are doing. Are you using a carb or EFI? Also, what does your motor have in terms of a cam and static compression ratio?
I hope this helps you,
- Fred