Afr

[1795]

Greetings everyone,

I thought that I would start a discussion about what an appropriate AFR is for a car based upon it's use and look for opinions (of which I image there will be plenty )

I had my race car on a chassis dyno recently tuning the carbs in. The person running the dyno was knew Weber's and we were able to get them balanced and worked on the jetting. He ended up tuning the carbs to run in the 12.7-13.1 range at WOT for maximum HP. A lot of the work that they do is for drag racers and high performance street cars.

When I discussed the AFR numbers with my engine builder later, he said that those numbers are fine for a drag race car, but not for a road race car. He wanted to see a high of 12.4 for an AFR to avoid being lean. I tended to agree with him, but wanted more advice.

I sought advice from John Garuti, who is in charge of the engine shop at Cobra Automotive and asked him what he tended to shoot for. John had some nice advice. He said that an engine dyno or chassis dyno operate in optimum situations that do not exist in driving situations. AFR's in the high 12's-low 13's will produce maximum HP, but in a road race situation they could lead to engine failure. One of the greatest contributing factors is the g force that the car is encountering. In high g corners the fuel will be pushed to one side causing that side to be rich and the other side lean. If you are running at the edge with being lean that could lead to a significant lean condition. When they are tuning at the track they change spark plugs after every session to check for whether the car is running lean or rich. In his opinion, you cannot get a good read on plugs after they have been used once.

John further suggested that to truly operate at the safe end of the lean vs rich AFR range for maximum performance that people usually sacrifice a couple of engines to find that point. If you are not at the price point to be able to afford that, then tuning to be in the low 12's is a safer bet.

Thoughts? I will be increasing my jetting from where it was just set to, back to where it was running slightly rich. Does this concept apply to EFI as well, or just carbureted engines? I would think that EFI would be less affected by g force, but I could be wrong. How often do we consider the way the car will be driven outside of the controlled environment of the dyno session?

Jim