Race Fuel question (specific gravity/density)

4 replies to this topic
  • Wyatt

Posted January 30, 2002 - 06:42 AM


If a race fuel has a lower specific gravity than pump gas and a lower lbs per gallon weight. Should it cause overall settings to be richer or leaner? The fuel is non oxygenated.

6.15-6.11 lbs per gallon (pump)
6.03 lbs per gallon (race fuel)

.74-.73 specific gravity (pump)
.725 specific gravity (race fuel)

The fuel is phillips b32. The reason I ask is that I have had different people tell me one or the other. A phillips rep told me that it would cause a lean mixture, but he seemed unsure.

I am trying to fine tune the jetting on my '01 426. I use 100% b32. The bike pulls hard through the midrange and on top, and it always has a nice tan plug, but it is lacking down low.

I have the stock pilot #42, stock needle in stock position #4, 168 main. I have done the BK mod and have the spray set at .3 to .4. The spray is timed so that it just misses the slide without hitting it. (stock exhaust system)

The problem I have is down low. It never hicups or hesitates, but it simply does not pull as hard as it should. I never realized this until I had a chance to ride a fellow TTer's bike, which was completely stock. (no BK mod) His bike simply had more snap than mine down low. (immediate opening of the throttle)

I feel that I don't have the jetting dialed in for the fuel that I'm running. I want to keep running this fuel because I believe these bikes need a high quality fuel and the bike is very easy to start with it. I know many of you are running race fuels with excellent results. I would appreciate any advice you could give me on this subject.

  • freestyle111

Posted January 30, 2002 - 02:34 PM


race gas needs to be ran leaner than regular gas.higher octane gas is more stable and harder to detonate than cheap fuel.it takes more air and compression to get fuel to vaporize and detonate.liquid gas will not detonate in cylinder it will only burn.air being compress into gas causes fuel to vaporize reach detonation.the more air[oxygen]you have mixed in with fuel during detonation the greater the explosion

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  • JohnnyG

Posted January 31, 2002 - 10:22 PM


I wouldnt think specific gravity has anything to do with it. If you filled your tank with water (Sp=1.0) would it make your bike run too rich?

  • skthom2320

Posted February 01, 2002 - 09:04 AM



Check out this from Eric Gorr's web page. Concerning Avgas he says that there is potential for a weak bottom end. Just as you are experiencing..

Click here

I'm probably going to Splendora again tomorrow. I'm going to wait on Three Palms until after the STC race next week. Let me know if you are going. I'll be there at opening.

Steve T

  • Boit

Posted February 01, 2002 - 10:16 PM


There are huge tuning requirement differences between race fuels. For instance, in the VP fuels, MR2 is oxygenated and usually requires richer jetting while C-12(freestyle111 is correct with this fuel) is non-oxygenated and requires typically leaner jetting. These general jetting changes are based on a assumption that pump gas is being replaced with race fuels. With the 426 and 250F, what is called for is a fuel that not only has adequate octane for the compression ratio, but also has the distillation curve that allows these engines to rev cleanly to their redline and still allows crisp throttle response. If you just dump a tank full of one of these fuels into your bike and expect it to suddenly perform better, you will be sadly disappointed. Most likely, it will run worse. You must jet accordingly to reap the benefits.

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