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Subject: Compression ratio on pump gas?

Original Message
Name: Kevin
Date: July 27, 1998 at 12:49:46
Subject: Compression ratio on pump gas?
I have heard a lot of different specs from many different people on the maximum compression ratio to run in a street engineon pump gas. Some say 9.0:1 max while others say 10.5:1. I thought the new Corvette runs 10.5:1. Most 4 cyl. engines run at least 9.5:1. What is the real maximum you should run on 92 octane pump gas with an older big block. What are some of the variables that can or do change this specification, example- timing, air fuel mixture, cam selection? Is there anybody out there who will end this argument for me?

Response Number 1
Name: Bill M
Date: July 27, 1998 at 14:51:06
Subject: Compression ratio on pump gas?
I'll try. Most experts recommend 9-9.5 to 1 for Iron head carburated engines. 10 to1 max for aluminum headed carb engines. My figures are for small blocks but it may apply to both.
The reason many late model engines can run higher compression is due to computer controlled timing and fuel injection. Timing changes are made to guard against detonation by sensing spark knock and retarding the timing accordingly. Earlier engines had no provision for this adjustment.
You can run higher compression in an old style motor by having enough cam overlap to bleed off excessive cylinder pressure. But big cams are not really the way to gain power on the street.

Response Number 2
Name: Ron D.
Date: July 27, 1998 at 15:00:17
Subject: Compression ratio on pump gas?
Hi Kevin,
I have a '70 Chevelle 454. It has 10-1 compression and does not spark knock as long as I keep high test in it. I believe the key to running higher compression is cylinder pressure
rather than compression ratio, though the two are inter-related. My cam (540 lift - 296 dur.)
bleeds cylinder pressure, hence no spark-knock.
Idle is rough and power band starts a little higher but its just right for me (18 mpg @ 60
mph with air on (has 700-R4)).
I hope that this is at least a partial answer to your question.

Response Number 3
Name: COPO
Date: July 27, 1998 at 20:14:37
Subject: Compression ratio on pump gas?
Back in '85 when I had my engine rebuilt the fuel then was only good for 9:1. That's why I opted not to reinstall the 11:1 pistons. With my 11:1 I got tired making trips to the Airport. We had the Green 110 octane fuel in the 80's. Not sure if it's still around today. With 9:1 I ran on regular fuel, but years later the octane dropped so I'm now using Sunoco 94 octane. If I use anything else I ping.
Unless you have todays technology with computers or the right cam in your 60's-70's muscle car your pinging and retarding the timing.


Response Number 4
Name: Jason67Beau
Date: July 27, 1998 at 22:37:45
Subject: Compression ratio on pump gas?
In the latest SUMMIT magazine I noticed MSD has a aftermarket knock sensor with a dash mount LED indicator for you guys that are right on the line of detonation. Its pretty hard to actually hear the pinging with the throttle wide open. I wish I could go higher with compression, but I think 9.5 is what I'll stay at as long as I drive the car on the street.

Response Number 5
Name: Nick
Date: July 28, 1998 at 11:18:52
Subject: Compression ratio on pump gas?
As stated in an earlier reply, I think cylinder pressure is definitely the issue rather than compression ratio (which, as was said, would have an effect on cylinder pressure). For example, before being pulled from the street to be dismantled and go racing, I was running about 11.5:1 compression in the 350 in my El Camino on 92-93 octane pump gas, with no detonation. The two keys to that were 1. aluminum heads, which, due to better cooling/heat transfer characteristics allow a higher compression ratio; I've seen estimates anywhere from 1/4 to 3/4 of a point to quantify it, but I would suspect it would depend on the combination; and 2. A camshaft with lots of overlap to bleed off low-lift cylinder pressure. My particular cam ran only 106 degrees of lobe separation, and gave me a nice, evil-sounding lope to the idle. However, that much overlap would also result in low manifold vacuum, which could be detrimental to power brakes and other vacuum-operated accessories. I had no such accessories on my car, so in my case that wasn't a concern.


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