Optimum Air/fuel ratios

[EbloomPonycar]

Had my car dynoed a few weeks ago (03 Cobra vert). 415/448 rw. The A/f was 11.5 to 1 which I thought was a little rich. Evidentally on the centrifigal dyno they run about .5 richer that actual street driving which puts me at about 12 to 1. Should I be looking to get closer to 12.7 to 1 to optimize the tuning? The A/F was steady all the way across. I am using an SCT 2 and wondering if I can tinker with some settings to optimize or just play it safe and do it on the dyno.

[Greg@SVTOA]

12.7:1 on a blower car??? YIKES!! Keep it in the 11's.

[dwtalso]

Some folks have run that kind of AFR. I'm at altitude (5300 ft ASL) and way back when I was running Eaton on low/medium boost (12 psi peak) I did that for a while. A little less timing, the lower static compression in the motor, and reading the plugs regularly will keep things together just fine.

However, I've watched a lot of tuning on these cars and while the optimum AFR on the nomrally aspirated motors is in the neighborhood of 12.5:1, I think that the gains from running that lean on these blower cars are, imho, not worth the risk. You'll make more power and be safer just running a pound or two more psi with a rich tune. You'll be able to run more timing as well which, imho, has much more impact than AFR tweaks.

Personally, if I did it again, I'd just run between 11 and 11.5:1 on the dyno :).

[EbloomPonycar]

Thanks for the answers! I quess I am in the right neighborhood with my setup.

[EbloomPonycar]

If my mind from automotive school serves me correctly. 12.7 to 1 is the ratio for optimum combustion at sea level. So if you have everthing dialed in correctly why shouldn't you shoot for that number? Boosted or not. Isn't the end result to make it optimum? What does safe mean? I guess in my quest to supertune I maybe getting lost in the reality of the situation.

[dwtalso]

Well, define "optimum" :). For optimum fuel efficiency you want to stay near stoichiometric (14.7:1 for most pump gas). For optimum power production the value ranges from 12 to 13:1 depending on a lot of factors, including the flow characteristics in the motor. Increasing the pressure in the manifold above ambient has some impact on this, I would think. Also, timing and intake charge temperature have to be factored in as these will impact the combustion process.

The issue with the blower is the heat generated by the pressurization. Detonation breaks motors and heat increases the risk of detonation. Running lean increases the combustion temperatures so the chances of experiencing detonation increases. The idea of running rich is to cool the intake charge, buffer the combustion, and reduce the detonation risk. Again, typically, the power difference between 11:1 and 12.7:1 is relatively small while the cost of a mistake (a new motor) is pretty dramatic. At the same time the relative heat increase from a boost change is much less and is further reduced by the intercooler.

So, "safe" means giving up a little bit of power in order improve the reliability of an engine that's already working pretty hard. If you simply were not worried about costs and rebuilds and wanted to squeeze every last ounce of power you could, then, yeah, you'd push the AFR and timing right to whatever it took :). But most of us are anticipating a long term relationship with our engines so we don't do that ;).

[EbloomPonycar]

14.7 to 1 is what I was thinking about. It has been 20 years since that class. The bottom line is safe from detonation. If you optimize and all of a sudden through humidity (or lack therefo) or other outside environmental factors (elevation, temp, etc.) you may create the detonation scenario and then your done. Thanks for the refresher.