How much compression safely on pump gas?
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How much compression safely on pump gas? Ok a co worker has a 95 Z28 M6 LT1 & wants to build a 383. What's the highest compression ratio that can safely run on 92-93 octane? Also a good set of headers for this setup. I messaged Lloyd & we went over cams, heads & fuel system upgrades. Just wanna hear what you guys think. Were probably gonna go with the milder setup with the better low end since its a street car.
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From: Flushing, Michigan
Many claim to have run 12:1 without any problems. I shot for 11:1 because its a pure street car and didn't want to have to worry about gas quality, etc. I run Kooks headers because I wanted to ensure a proper fit though there are more economical options such as xs power, pacesetter, etc.
Did Lloyd recommend a CR with the cars intended usage in mind?
Did Lloyd recommend a CR with the cars intended usage in mind?
Last edited by LT1383Z; Jul 26, 2017 at 01:18 AM.
I ran 93 octane on the 12:1 heads/cam motor, but only for easy street driving and drag racing (motor never got hot in these circumstances). I ran half race fuel for roadracing where things get really hot. 228/234 HR cam.
The new stroker for road racing will be 11.5:1 and will run pump fuel. 238/246 SR cam.
The new stroker for road racing will be 11.5:1 and will run pump fuel. 238/246 SR cam.
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The highest static comp you can run for 93 octane depends more on your dynamic compression ratio. You could be running over 12:1 safely with the right cam, rod length, stroke, etc. Generally speaking, a larger cam will allow you to run a higher SCR while maintaining a safe DCR. Check out this calculator:
https://www.jeepstrokers.com/forum/calculator/
https://www.jeepstrokers.com/forum/calculator/
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Many claim to have run 12:1 without any problems. I shot for 11:1 because its a pure street car and didn't want to have to worry about gas quality, etc. I run Kooks headers because I wanted to ensure a proper fit though there are more economical options such as xs power, pacesetter, etc.
Did Lloyd recommend a CR with the cars intended usage in mind?
Did Lloyd recommend a CR with the cars intended usage in mind?
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From: Tucker, GA
Detonation is what kills engines in regards to compression ratios. As Catmaigne said, DCR is more important than SCR, and quench plays a key role here as well. I had a relatively mild cam with a low SCR (Spec'd by Lloyd Elliot), but a higher DCR and a tight quench. Even then, I had a lot of ping on very hot days, and that was with 93 gas - despite Lloyd and Karl telling me I'd "probably" be ok with the DCR/SCR on the engine as-built. Not faulting either of them in the slightest - nobody can tell for sure what an engine is gonna do on the street until its actually on the street.
I opened up the quench a bit and decreased my compression a smidge with a slightly thicker HG and haven't had a ping since.
I opened up the quench a bit and decreased my compression a smidge with a slightly thicker HG and haven't had a ping since.
Stay with 11.5-1 and under and not worry about anything from 91-93 octane. I am running 12.15-1 and have been told I should not run pump gas on my setup. My dynamic is at 9.3-1 which I was told was too high to run pump gas safely (stay under 8.5-1 DCR for pump gas). I could fatten it up and run a lot less timing and run fine on pump gas but do not drive my car enough to take away power to run pump gas. I can run pump gas putting around town and cruising no problem but would not want to take it up under load in 3rd gear on pump gas. I currently run Sunoco std 110 octane but am planning a change to 260 gt plus soon. If I had it to do over I would have shot for either 11.5-1 or go all the way up to 13.5-1 (either sacrifice some power and not worry about gas or make more power and still run race gas).
Detonation is what kills engines in regards to compression ratios. As Catmaigne said, DCR is more important than SCR, and quench plays a key role here as well. I had a relatively mild cam with a low SCR (Spec'd by Lloyd Elliot), but a higher DCR and a tight quench. Even then, I had a lot of ping on very hot days, and that was with 93 gas - despite Lloyd and Karl telling me I'd "probably" be ok with the DCR/SCR on the engine as-built. Not faulting either of them in the slightest - nobody can tell for sure what an engine is gonna do on the street until its actually on the street.
I opened up the quench a bit and decreased my compression a smidge with a slightly thicker HG and haven't had a ping since.
I opened up the quench a bit and decreased my compression a smidge with a slightly thicker HG and haven't had a ping since.
I thought 9:1 was the limit for older SBCs? Guess it depends who you ask. Anyone know the DCR of a stock LT1? Can't find accurate adv duration for a stock cam.
Last edited by Catmaigne; Jul 26, 2017 at 10:32 AM.
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From: Tucker, GA
Finally got Chris (Rocko) involved and that guy really took his time and dialed it in using all the tricks in the book. I think we went back and forth with 6-8 logs over the course of 3 months and he had to adjust fueling and timing to such an extent that it became obvious we were just dealing with too much compression.
Popped in the thicker HG and rolled back in the lost timing and haven't had a knock since.
Last edited by atlantadan; Jul 26, 2017 at 10:51 AM.
A very good read...lengthy but has tons of information.
http://garage.grumpysperformance.com...mpression.727/
I was told that reverse cooling is not significant from std sbc cooling as far as compression ratio limits are concerned. I always thought that the reverse cooling and aluminum cylinder heads with efficient chambers helped with running more compression on the lt1's but was informed otherwise. The research I have done and folks I talked to wouldn't chance pump gas on anything over 11.5-1 on a sbc or lt1. I would love to try it out as I know the LSX engines are running 11.8-1 on pump gas but I also know they are running a safer tune with safeties put in the tune for timing retard and fueling if a bad batch of gas is run.
BTW, I am running a 4.060 head gasket and a .033 quench on 12.15-1 flat tops. I built the combo to be on the ragged edge of pump gas but that was in 2007-2008 time when I put the parts together. I was informed that gas has changed a lot since then and not to risk running 93. I do not run a knock sensor due to it being a solid roller cam. YMMV.
http://garage.grumpysperformance.com...mpression.727/
I was told that reverse cooling is not significant from std sbc cooling as far as compression ratio limits are concerned. I always thought that the reverse cooling and aluminum cylinder heads with efficient chambers helped with running more compression on the lt1's but was informed otherwise. The research I have done and folks I talked to wouldn't chance pump gas on anything over 11.5-1 on a sbc or lt1. I would love to try it out as I know the LSX engines are running 11.8-1 on pump gas but I also know they are running a safer tune with safeties put in the tune for timing retard and fueling if a bad batch of gas is run.
BTW, I am running a 4.060 head gasket and a .033 quench on 12.15-1 flat tops. I built the combo to be on the ragged edge of pump gas but that was in 2007-2008 time when I put the parts together. I was informed that gas has changed a lot since then and not to risk running 93. I do not run a knock sensor due to it being a solid roller cam. YMMV.
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From: Flushing, Michigan
My car is 100% street also so I went 11:1 but I would have been comfortable going 11.5:1 but wouldn't go any higher myself.
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Same here 11.5 on my LS engine. He doesn't wanna LS swap. He's got a spare engine so we"ll build this & he can drive till its done. Well, I might have to go through his T56, stuck in 4th gear atm. If we build this 383 were staying 11:1 area. This kid is 20 yrs old it"ll be a rocket for him!
Depends on where you quench is, your cam specs, and the quality of your tune.
The highest static comp you can run for 93 octane depends more on your dynamic compression ratio. You could be running over 12:1 safely with the right cam, rod length, stroke, etc. Generally speaking, a larger cam will allow you to run a higher SCR while maintaining a safe DCR. Check out this calculator:
https://www.jeepstrokers.com/forum/calculator/
https://www.jeepstrokers.com/forum/calculator/
KW