[psi425rdr]

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?

 

[ghost]

Your right on the money. 185/190 is the threshold for 50/50 mix.

 

[C-Leigh Racing]

I would say with 165 compression, you could probably use 87 pump gas low as that is. Any time you have compression over 220 you should use race gas. You should figure in things like, where the ignition timming is set, if its advanced alot you need more fuel to keep the piston cool.
Temps outside makes a difference as well. Maybe not as much for you guys because its so cold there, but we have very hot weather around here & just makes the engines run hotter.

One thing about race gas, & this might be the reason that guy is saying to use it, its the best grade you can get, not like the pump gas.
Pump gas, you realy dont know from one tank full to the other if its the same or not. Service stations dont care if you get quality fuel, why should they, you got to buy it from somewhere & your going to pay top price wheather it the best grade or the poorest.
Race gas, those vendors have to have a product thats up to spect or else they wont be selling it for very long.
Neil

 

[psi425rdr]

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.

 

[Michael88R]

If you ran 50/50 last year with no problems, I'd run the same or maybe cut it to 25% race gas. The consistency of race fuel and the lead in race fuel makes it especially good for 2 strokes.

 

[ghost]

plus nothing like the smell of race fuel buring through a bad ass R!!

 

[blasterman88]

you may take this into consideration as well......since you have stated your ice racing, im going to assume your elevation is above 4000-5000 ft???? when you are setting your dial a dome for 165, where are you basing this from, PSI web site??? Have you done an actual compression check. Depending what elevation PSI does there "testing" will scramble your #'s. Also, who ported the cylinder, because who knows if PSI tested the "basic" compression #'s with a stock cylinder, or the exhaust port rasied?? Do your self a favor and put a comression gauge on it, and test it that way, take the guess right out of it. i would bet you have less comression then 165.