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Fuel requirements based on compression.

2881 Views 6 Replies 5 Participants Last post by  blasterman88
Hi Neil,

I have an 87 250r with a PSI 425 powervalve cylinder on it. I live in northern Alberta, Canada, and we use this set up for 6 hour endurance ice racing. I am having a debate with my riding partner about what type of fuel we should be running for this set up. We have set the dial-a-dome on the head for 165psi and he feels that we need to be running straight race gas. On the psi website, they say that for 175-190psi, you can run premium, for 195-220psi, 50/50 mix, and for 225-250psi, full race gas. The reason we are having this debate is that one of our competitors has been running this set up for a few years and he claims that you have to run straight race gas at 165psi. I'm saying he's spending money on race gas for nothing. What do you think?
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Thanks Neil,

We're using the stock ignition, so timing is stock. We normally run in temperatures between 0 and -20C ( 32 to -04F), and the speeds are high, so we definitely don't have the heat you guys would. Obviously, we have to jet richer for the cold, I'm running a 205 main, DEG needle in the middle position on a 40.5mm Keihin carb. Not sure if this matters, depends who you talk to, but the exhaust duration is approximately 190 degrees. This engine runs the stock 72mm stroke, 86mm bore.

I agree with your comment on quality of gas. That was my impression as well, if he ran race gas, then there wasn't going to be a debate on things like jetting on race day. Last year, we ran a 50/50 mix with the same engine with the compression set at 150psi, without detonation issues. Also, does it make sense that with race gas you could run a little leaner yet still not create detonation or heat issues. I'm not sure how much a change in jet size from a 205 to a 200 would make in the fuel economy of this engine, but I worked it out last year and we were getting a little better than 10 miles/gallon with this engine!!! When you are burning 35 gallons of gas per six hour race (we race 4 events), the money difference of running premium to 50/50 to race gas is huge!!! But I really don't want to burn down an engine because I'm cheap with fuel.
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