Guest Fast Frog Posted April 6, 2001 Share Posted April 6, 2001 Does anybody have any info, graph, sliding scale, etc. regarding the octane rating of gas vs compression ratio? I've got access to 110 oct racing gas by the 55 gal drum and I can cut it with pump premium (91 oct) to get a needed oct to run 10.5 to 1 or even 11 to 1 CR when I re do my engine. Thanx!! Quote Link to comment Share on other sites More sharing options...
Evan Purple240zt Posted April 7, 2001 Share Posted April 7, 2001 Hey fast, i dont know if your motor is anything like a nissan but you can most likely run 10.3 on pump fuel. I would be concerned about finding this 110 on a regular basis if my supply dried up or i went out of town.... its just a thought dude. Otherwise i cannot help you, sorry. Evan Quote Link to comment Share on other sites More sharing options...
John Scott Posted April 7, 2001 Share Posted April 7, 2001 I'd bey there are more variables to the equation. Aluminum, no doubt, combustion chamber design, rod length, cam specs, altitude etc. What the #%^& did you decide on anyway Frog? Remind me to inhale before you start it up. Might be my last breath JS [This message has been edited by John Scott (edited April 06, 2001).] Quote Link to comment Share on other sites More sharing options...
blueovalz Posted April 7, 2001 Share Posted April 7, 2001 No tech pages here, but my 10:1 runs easily on 92. When I had 12:1 domed pistons, I ran combination avgas and premium which by my calcs was a tad bit above 96 with no problems. But John brings up a good point. Combustion chamber shape, plug heat range, camshaft variables that affecting dynamic compression ratio, any hot spots, timing, etc, etc, etc. This one has no simple answers. Quote Link to comment Share on other sites More sharing options...
Guest Fast Frog Posted April 9, 2001 Share Posted April 9, 2001 Thanx guys!! John: I almost fell out of my chair LOLMAO reading your response! Terry: are you still running 12 to 1? If not, why did you stop? You guys have reconfirmed that one can run high CR with blends of Av gas, or whatever, with pump premium with no probs!! My engine builder is a great guy, but he's old school! He builds a great engine but his application is still old school carburation. He thinks the only way to run anything above 9.5 to 1 CR is with racing gas or 115 oct WWII av gas that powered B-25s!! When I was running my big turbo sys a few yrs back, I'd get Av gas at the airport and mix it with pump prem with no probs. BTW, John, I'm just checking out the idea of building an engine( for the blue beast) with a CR of 11 to 1 or even 12 to 1. Pls note that this car has never been a daily driver. For a daily driver, I'd probably keep it under 10 to 1. Thanx again Quote Link to comment Share on other sites More sharing options...
Andrew Bayley Posted April 9, 2001 Share Posted April 9, 2001 Rick, I know alot of the Pro Rally guys run about 25-30 psi of boost with a 9:1 C.R. and run a mix of 50% 93 octane and 50% 110 octane. These guys run ballz to the wall for up to 12 hours and their motor's seem to hold up just fine. Either way, you can just send your engine down here to my place and I'll do all the octane testing you need. -Andy ------------------ "But I don't sit idly by, I'm planning a big surprise." Andrew's Land of Z and Honey Quote Link to comment Share on other sites More sharing options...
Guest Fast Frog Posted April 9, 2001 Share Posted April 9, 2001 Hey! Thanx Andy! You've all been helpful! Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted April 10, 2001 Share Posted April 10, 2001 Hey Frog, We're all interested in what eng.build you've chosen!!! Regarding Fuel Octanes; David Vizard in one of his books wrote a fine 6 page article on Fuels & Octanes/how they were derived. He also built a 14.7:1 comp.ratio engine to perform a test on 8 brand name service station fuels. I'll attempt to make this short. He claims even tho trends toward better performing fuels have improved/there still exists a 25% difference between the best fuels & the poorest performing fuels; so choice is an issue & that all equally rated Octanes where not equal in performance. The octane values of a fuel is normally tested on what is known as a C.F.R. engine. This is a large single cylinder engine whose comp.ratio. can be varied while the engine is running. Likewise; the fuels abilty to resist detonation is confirmed by changing the comp.ratio until the engine detonates. This value is compared w/the detonation level of a mixture of two basic fuels....Normal Heptane (N-Heptane) which detonates very easily and has an arbitrary rating of "0"; and Iso-Octane which doesnt detonate easily & has been assigned a detonation resistance rating of "100". Therefore; if a fuel blend detonates at the same level of a blend of 92% Iso-Octane & 8% N-Heptane (which is a merely a point of reference)...then it is given a 92octane rating. If it detonates at the same level as 100% Iso-Octane-its then given a 100 octane rating. If that isnt confusing enough-it gets more complicated. He says Europe uses the RON-Research Octane Number while the US uses a combination of the RON & MON-Motor Octane Number; the formula for obtaining octane in the states is the one seen on the pumps "R + M/2 = Octain Rating". The RON rating is arrived by running the C.F.R. engine at 600 rpms w/an intake temperature of 150*F. The MON rating is obtained by running the C.F.R. engine at 900 rpms w/intake temp's at 300*F. So, some fuel fractions may resist detonation very well when induced into the eng.at low temps but not show very good results at high temp's. Likewise; some fractions may be okay at low temps, yet not degrade significantly as temp's rise. A typical service station usually indicates its RON about 8points higher than its MON. The difference between the two is known as the fuel's temperature sensitivity. A high sensitive fuel may show "10" points difference whereas an incensitive fuel may only show "6" point difference. The immediate paragraph above indicates why some lower grade fuels perform better than some higher grade fuels. The old school technique of blending fuels was to add lead to the blend until the requried octain was reached/lead is gone so now fuels w/a natural higher rating must be used; such as Tolulene. Simply adding a naturally higher rated fuel sometimes changes the RON but barely effects the MON. The net result is the "R + M/2" value increases, but the MON, which is the level at which most steet vehic's run, changes little if any. In his test engine while testing the 8 different fuels; he tested 3 identical intake manifolds were tested-but prep'd differently: 1)Heated w/Exhaust Heat Riser 2)Exhuast Heat Riser Blocked 3)Water Cooled Heat Riser. This test gave three distincly different Intake Manifold Temp's for each fuel tested; at which time the Dyno would register the power output. The results indicated were that the "Daeco Race Gas 110 Octane" the highest rated fuel, didnt show its true value in terms of power delivered when using the heated intake manifold and not till the cooler manifolds were used did its higher rated octane manifest its full potential. His graph indicates that Texaco, even w/a lessor grade was a close match to the race fuel. The results are obvious-with the trend among modern fuels is that they do not tolerate high intake temp's and become detonation limited sooner; therefore, a concentration of keeping the intake temp's cooler will aid in utilizing the existing octaine values listed. He also claimed in another article in that same book; the largest gain in HP from the use of racing fuels comes from an engines ability to utilize comp.ratio's of 11:1 to 17:1. Sorry to ramble; just thought it was interesting & informative. Kevin, (Yea,Still an Inliner) [This message has been edited by Kevin Shasteen (edited April 10, 2001).] Quote Link to comment Share on other sites More sharing options...
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