Compression Ratio vs Today's Gas
Thread Starter
Team Owner
Joined: Mar 2002
Posts: 44,653
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From: Huntsville AL
Compression Ratio vs Today's Gas
How many BB Vette owners out there are running High Compression stock setup on pump gas without problems? I ask because I'm still back and forth on which direction to go on my L71 build-up. I seem to keep going back to a Blueprinted Crane Cam and 11:1 Pistons. :chevy
Le Mans Master
Joined: Sep 2000
Posts: 5,635
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From: Orlando Floriduh
Re: Compression Ratio vs Today's Gas (Mr D.)
I have a stock L-36 (10.25:1) I have limited the total advance to 28 deg and do not use a vacuum advance. The ONLY fuel I can run is Amoco 93 without pinging and even then I have to be careful in the summer when the temp outside is over 90F. I experimented quite a lot and that's the best I could get without adding some 110 racing fuel.
Thread Starter
Team Owner
Joined: Mar 2002
Posts: 44,653
Likes: 1,827
From: Huntsville AL
Re: Compression Ratio vs Today's Gas (bluevetteman)
I have a stock L-36 (10.25:1) I have limited the total advance to 28 deg and do not use a vacuum advance. The ONLY fuel I can run is Amoco 93 without pinging and even then I have to be careful in the summer when the temp outside is over 90F. I experimented quite a lot and that's the best I could get without adding some 110 racing fuel.
Racer
Joined: Dec 2003
Posts: 257
Likes: 0
From: Livermore CA
Re: Compression Ratio vs Today's Gas (Mr D.)
I have 70 Nova with a 396 that was rebuilt with 11 to 1 pistons and a Comp Cam 282 solid lifter cam. It runs GREAT on 92 pump gas!!!! It never pings. I think if you run higher compression you need to run a cam with more overlap to bleed off some of the compression at lower RPM's so it won't ping (thats the way it was explained to me). I'm not sure I understand it, but I do know this setup works GREAT for me!!
Thread Starter
Team Owner
Joined: Mar 2002
Posts: 44,653
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From: Huntsville AL
Re: Compression Ratio vs Today's Gas (JJMURFS)
I have 70 Nova with a 396 that was rebuilt with 11 to 1 pistons and a Comp Cam 282 solid lifter cam. It runs GREAT on 92 pump gas!!!! It never pings. I think if you run higher compression you need to run a cam with more overlap to bleed off some of the compression at lower RPM's so it won't ping (thats the way it was explained to me). I'm not sure I understand it, but I do know this setup works GREAT for me!!
Melting Slicks
Joined: Mar 2001
Posts: 3,365
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From: Lake Villa IL
Cruise-In IV Veteran
Cruise-In V Veteran
St. Jude Contributor
Re: Compression Ratio vs Today's Gas (Mr D.)
I'm running 9.7:1 (approx, we had to juggle head gaskets) with iron heads (Vortecs) and 34 degrees advance, on my small block, with no troubles. I typically run 91 octane, but can go lower without issues.
[Modified by joeveto, 8:55 PM 12/12/2003]
[Modified by joeveto, 8:55 PM 12/12/2003]
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Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Compression Ratio vs Today's Gas (GDaina)
I am running at least 10:1 now with iron heads, my cranking compression is 230psi in all cyliners. I don't have any detonation problems and a runnig 40 deg advance. My new 406 is going to be 11:1 with aluminum heads, I don't think I will have a problem with pump 93 octane., :D
Melting Slicks
Joined: Dec 2000
Posts: 2,041
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From: Wichita, KS
Re: Compression Ratio vs Today's Gas (MotorHead)
You might want to read this article and download the program on dynamic compression ratios to see what you can do about bleeding off compression with a long duration camshaft. http://cochise.uia.net/pkelley2/DynamicCR.html
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Re: Compression Ratio vs Today's Gas (Mr D.)
You cannot compare BB's and SB's as for CR. Also the early head design is very different. There are several variations and comdustion chamber shapes and piston designs that all have the same C/R. There were more changes made over the years than just reducing compression. In the case of my stock 69 there is a huge difference in brands of fuel (all with the same octane rating) AND the outside air temp makes a big difference too. It all boils down to whatever works for you.
Instructor
Joined: Oct 2000
Posts: 137
Likes: 0
From: Flemington, nj 08822 usa
Re: Compression Ratio vs Today's Gas (bluevetteman)
It's an issue of your specific situation and atmospheric conditions. The engine sees only cylinder pressure as influenced by the cam profile, CR is a theoretical number that is a reference only. The relationship of the chambers to the pistontop profile etc also is a factor. Cold damp air helps also.
My 427 has custom JE pistons, CR set to 10.6 and a Lunati hydraulic roller, it runs fine at 38 degrees total advance, no vacuum advance with Sunoco Ultra and 5 Gals of VP C12 per tank. Never tried to use the pump gas alone.
Larry
My 427 has custom JE pistons, CR set to 10.6 and a Lunati hydraulic roller, it runs fine at 38 degrees total advance, no vacuum advance with Sunoco Ultra and 5 Gals of VP C12 per tank. Never tried to use the pump gas alone.
Larry
Drifting
Joined: Nov 2002
Posts: 1,689
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From: Loveland, CO
Re: Compression Ratio vs Today's Gas (Mr D.)
Don't forget altitude. But, I guess that will not help you much in Florida. Here above 5000', I have gotten away with 11:1 on pump gas. Good luck. :cheers:
Thread Starter
Team Owner
Joined: Mar 2002
Posts: 44,653
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From: Huntsville AL
Re: Compression Ratio vs Today's Gas (wallyknoch)
Thanks everyone for the input. I have decided to build this engine, 427 / 435hp Tri-power using onginal GM specs. Between TRW and Comp Cams, all the correct parts are out there.
