proxlamus© Posted July 4, 2009 Share Posted July 4, 2009 So I've been thinking.. With my F54/P90 combo.. and a 2mm headgasket.. I have like a 6.7:1 compression ratio.. During a compression test at this altitude gives me readings around 130psi across all 6 cylinders... Since I live in Denver, CO with an altitude over 5,200 ft MSL.. I'm assuming it would be somewhat beneficial to increase the compression ratio to compensate for the altitude!! I realize turning up the boost will compensate plenty.. but I'm talking about off-boost response!! Chime in please!! Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 5, 2009 Share Posted July 5, 2009 I don't have a technical answer for you're solution, but as a Pilot, all my books tell me that Gasoline becomes more prone to knock at high altitude as well. This is why Cessna's run like 8.5 NA and still have 100LL gasoline. So higher altitude can be compensated with a higher CR, but it's probably important to think about knock too, and if you ever do take a trip off the mountain, it could be a problem. Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted July 5, 2009 Author Share Posted July 5, 2009 are ya a private pilot or commercial? I just finished up my degree in Aviation last year.. working on my CFI =) Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 5, 2009 Share Posted July 5, 2009 As a guy who takes trips to mountains, I can say that I've noticed that the available gas there usually has a lower octane rating. I assumed that was the barometric pressure is lower at altitude. Googled, found this: http://www.cartalk.com/content/columns/Archive/1997/June/09.html So yes, if you wanted to increase the compression ratio it should be fine until you go to lower elevations. Likewise I think we've had threads here about turning up the boost at higher elevations too and how 15 lbs of boost at xxxx altitude feels like 8 lbs of boost at sea level, etc. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 5, 2009 Share Posted July 5, 2009 private, i think i'm gonna work on my rotor before i get my commercial though, since the air is thinner, wouldn't it take more air to reach the same PSI at a lower altitude? so really the turbo has to work harder and ends up moving more air at the higher altitude to accomplish the same PSI? this could be 100% wrong, i'm just throwing it out there... Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 11, 2009 Share Posted July 11, 2009 Most of the crazy octane requirements in normally aspirated aero engines are there because the engines run lower rpm ranges which makes them more susceptible to knock. Also.....the mixture is in the hands of the pilot and a higher octane will resist detonation better than a lower octane fuel when someone manages the mixture badly and runs it too lean with high map at low rpms. That is why you commonly see avgas with ratings such as 100/130 in that the first number is the octane when run very lean and the second is the octane when run rich. AVgas is also more consistent in it's vapor pressure with regard to density altitude changes. All of this goes toward ensuring engines are harder to blow up which keeps airplanes from dropping out of the sky. Quote Link to comment Share on other sites More sharing options...
jeffer949 Posted July 11, 2009 Share Posted July 11, 2009 Hey prox. I think you have some bad math some where. Your compression ration should be 7.1:1 stock is 7.4:1 then the 2mm headgasket brings it down to 7.1:1 Quote Link to comment Share on other sites More sharing options...
slown280z Posted July 12, 2009 Share Posted July 12, 2009 So yes, if you wanted to increase the compression ratio it should be fine until you go to lower elevations. Likewise I think we've had threads here about turning up the boost at higher elevations too and how 15 lbs of boost at xxxx altitude feels like 8 lbs of boost at sea level, etc. Tuning material I've read (such as the Megasquirt tuning manual) claims that turbo car's aren't as susceptible to performance changes due to altitude because their volumetric efficiency stays pretty consistent due to the turbo's back pressure. I could see this being a problem on a turbo with a large exhaust turbine though. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted July 12, 2009 Share Posted July 12, 2009 For one Av gas is not formulated the same as auto fuel. LL by av standards is high lead by auto standards which is due to the air cooled nature and the need for lead to lubricate the valves, so let's not go down the av gas path in relation to cars. They are not the same. In an off boost situation all engines see a drop in HP, as much as 20%. IME you can run about a point higher in compression at altitude to compensate some, but you never get it all back, it just makes it snappier to drive. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 13, 2009 Share Posted July 13, 2009 AVgas is also formulated for a low piston speed. May people only look at the octane number which is only a very small part of the equation. If you want good high octane gas that will give best power, try VP racing fuel's SR1. It makes more power on cars with below 10 to 1 compression ratio N/A than most any other fuel that tests as legal........at least at this time.....might change soon. Quote Link to comment Share on other sites More sharing options...
MoNkEyT88 Posted July 14, 2009 Share Posted July 14, 2009 I just drove through Co on Saturday, I sure wish I had a turbo period. I think you'd be better off keeping a low compression ratio and just cranking up the boost. It is far easier to back off boost than compression. <-- my 2 cents.. lol coming from a guy who has a 11.1:1 V8.. Quote Link to comment Share on other sites More sharing options...
AK-Z Posted July 16, 2009 Share Posted July 16, 2009 Doesn't the stock l28 NA (factory matching) have a CR of like 8.X:1 ? Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted July 16, 2009 Share Posted July 16, 2009 I think the best option would be to have about 9:1 and a turbo for the l6. The v8 probably 10:1 and a turbo with low boost probably 12psi max making around 1000hp. Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted July 16, 2009 Author Share Posted July 16, 2009 Jeff, good point!! I dunno where I got that number from!! I should be at 7.1:1 after my 2mm headgasket. I was thinking of changing my compression to roughly 8.5:1 to make it a little snappier and allow adequate boost Quote Link to comment Share on other sites More sharing options...
jeffer949 Posted July 16, 2009 Share Posted July 16, 2009 Jeff, good point!! I dunno where I got that number from!! I should be at 7.1:1 after my 2mm headgasket. I was thinking of changing my compression to roughly 8.5:1 to make it a little snappier and allow adequate boost That is what Gabe is running (Austins old motor) and i believe that is what Speeders(rick) L-series was set at. If i was going to build a forged bottom end that is what i would go with. Quote Link to comment Share on other sites More sharing options...
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