Brent 930

I know there are alot of factors when race gas should be used or not. So here is my scenario, 11.5:1 A/F (fall weather), 15-16psi of boost, 25 deg. timing, single plug. Do I still need to mix race gas street or track, winter or summer???

Thanks,

Brent
83' 930 CIS 450rwhp

Dan89_930

Brent, here's my two cents - 1.0 bar = 14.7 psi. You're running more than 1.0 bar (although not much). You are single plugged. You're in CO. I'd say in the cold of winter you probably don't need race gas (assume you mean 100 octane at the pump). However, the price of race gas is always a good insurance premium to pay to prevent detonation.

As for summertime, yep, I'd add race fuel. Again, if nothing else, it serves as insurance against unforseen detonation.

In my (former) '89 930 3.4 (single plug, larger intercooler, fuel enrichment), I'd run between 25%-33% 100 octance in the winter. In the summer, it was never less than 50-50. Also, I never ran above 1.0 bar in the summertime. In the cold of San Jose winters :-) I'd run 1.1 bar on occasion.

Never had a problem with detonation. At $4.299/gallon, the 100 octane was the best insurance against detonation I could buy (for the price!).

Dan

Steve Weiner-Rennsport Systems

Well said, Dan.

New P/C's and the labor to change them is not cheap.

Petevb

FYI Dan, 1 bar = 14.5 psi. 1 atmosphere= 14.7 psi. They are not quite the same thing.
What’s CO premium- something like 91 octane? And you're making 450 RWHP, which means your torque and V.E. are both far above a stock car's. Depends a bit on the other mods, but you are almost certainly much closer to detonation than your slightly higher than stock boost would suggest (what’s your peak torque?). In general I agree- octane is cheap insurance, and the smart move.

Brent 930

Let me ask this question another way, with my setup does it matter what the A/F ratio is??? My thought was it did matter ALOT. Like I said I'm 11.5:1 up top, not 13.5:1. Let's say I was 9:1 up top, then what? There must be weight a rich A/F at max power has to what octane level is needed or not. Yes, here in CO 91 is the best and it's more like 90 yet I'm about 5K feet above sea level.

Thanks

Petevb

I'll give you the rules of thumb I have read:

*1 PSI of boost is roughly equivalent to 2 points of octane.
*Changing the air/ fuel ratio from 13:1 to 12:1 is roughly equivalent to 1 point of octane or slightly more.

The book that listed these said in the same sentence that these are very dangerous approximations to use, and the author prefers a much more sophisticated method.

Based on the above we can make some rough estimates, though. You’re at 5000 ft, so ambient pressure is close to 12 psi. If you’re making 16 psi of boost on top of this, you’re at a total pressure of about 28 psi. This is roughly similar to a stock car close to see level running 13.5 psi of boost (though you are generating more heat because of the higher boost). You, however, are doing this with perhaps 90 octane gas, which is 3 points down from what most of the country can run. On the other hand you are running significantly richer, so we’ll give you a point or so back. So you’re about -2 points octane, equivalent to roughly 1 psi, so you’re in the ballpark of a stock car at sea level running 1 bar of boost on 93 octane.

Now we get to the wildcard- your mods. The fact that you are making 450 rwhp at anything close to these boost levels means you are getting a lot more air into the cylinders than a stock car. This means your effective compression ratio is higher, and that means a higher chance of detonation regardless of how the air gets there. The only real way I know to do this calculation is with a good engine simulator, with can take all of the variables into account- intercooler efficiency, ambient temp, VE, etc. Back of the envelope, though, it looks like you would be close to the detonation threshold if your car was stock- any significant increase in your VE would seem very likely to push you over the edge. You don’t want to push the A/F ratio richer from where you are now- you are about as far as you want to go with that. From here on reducing the boost will get you more overall power. You also want to make sure your are richest at your VE peak, roughly your torque peak, as this will be where you have the greatest chance of detonation.

Brent 930

Thanks Pete that was the kind of info I like to read about. This will help me understand my situation better and what I need to do. I was guessing I would still need to mix at high boost but wanted more info. I have been running stock boost with just pump gas for several days now since my A/F will be around 11:1. Will pull some plugs just to take a look, I know you really can't get a true reading like this but if anything really bad has happened it will show.

Brent