What compression ratio is the cut off for race fuel?

[Kintore]

Hey guys, im running a 18cc dome in my 265 Duncan PV motor right now. Id do a compression test but its such a tight fit I would have to pull the motor to do that.

I was wanting to know if anyone perhaps knew if say I could run a 19cc dome and 94 octane and be fine?

Im running a 18cc dome and 102-103 right now and its fine.

Price of race gas for all the riding I do is gettin a lil hairy! 3-3.50 a liter :(

Thanks guys! :w00t:

[Daddio]

I can't answer your question but we just bought cam2 110 octane leaded racing gas for $6.75 a gallon at a nearby Marathon station. In my opinion it is way better than that high priced oxygenated stuff you get in the cans from the cycle shops.

[jason85atc250r]

kintore a 20cc dome is border line okay for 94 octanr gas. you'll want a 21cc dome

[WIkid500]

Jet the beast up 40% and run E85. Alcohol motors rock and E85 is a great way to make awesome power with compression for cheap.

[Kintore]

Thanks Jason, exactly what i was lookin for!

[oscarmayer]

typicall yin autos when dealing with them, anything over 12.25:1 we turned to race fuel. not tons but like 100-104

but yea as someoen stated, you could jet up 40% and run e85, but keep in mind, it's limited where to get it, and i9t's very corossive expeically to rubber and plastics. the floats in these carbs are plastic so think about that. and there are many rubber seals in the carb. i get the feeling every 6 months you'll be doing a full carb rebuild and float replacement.

[Kintore]

Yah im just going to a 21cc dome and running 94!

[300rman]

it depends on many factors. the guy up at PR Racing had built engines that crank 240PSI and run fine on 94 pump gas........without a stupidly retarded timing.
there are more factors than just PSI, and really, the only way to tell would be to test.

i would thing running a 25/75 ratio of race fuel/94 would be ok for a 19 or 20cc dome.
at most, a 50/50 ratio.

[Derrick Adams]

Josh you really need to check your cylinder pressure with each dome, so you know where you are. I'm not exactly sure any more, but i'm thinking anything over 200lb. should have race fuel.

[SWIGIN]

there is no cut off or magic number for runing race fuel

when i raced i ran 220-230psi on pump 92 and it never pinged. i even tryed race fuel once and it ran the same.

if it dont ping you dont need it and it will run no better with high oct

[cr480r]

I have run 270psi on pump gas... and had a 210psi engine detonate its brains out on good fuel.... there is a alot of variables... If you have expensive parts on your bike why take a chance?

[Billy Golightly]

From what I've learned, cylinder compression #s (180-200-etc) are not really an accurate gauge for whether you need race fuel or not. The best way to do it is with a true compression ratio. I think we went through this bit a while back when DirtCrasher was asking about it. http://3wheelerworldforums.com/showthread.php?t=82500

Josh you said your running an 18cc head dome, what is your bore size on that motor? Run the math with your information from this post: http://3wheelerworldforums.com/showpost.php?p=611540&postcount=16 and then report back here with it. You'll have an abnormally high #, like 18:1 or something, don't think its wrong. That is just the un-corrected #. The Japanese like to measure with a "corrected" # that counts the volume of the cylinder AFTER all the ports closed but its a whole nother set of formulas that really arent necessary.

Figure out your true compression ratio and then we'll decide whether its to high or not.

[Daddio]

Race gas smells better.