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tips, technology, tools and techniques related to vehicle driveline components

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Carl Floyd
Kingsport, TN
(4517 posts)

10/23/2007 11:32PM

Main British Car:
1979 MGB Buick 215

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Compression Ratio And Pump Gas
Posted by: MGBV8
Date: June 25, 2023 10:18AM

I am just about over paying a buck more for 93 octane. This is a great article on the subject.


Jim Stabe
Jim Stabe
San Diego, Ca
(829 posts)

02/28/2009 10:01AM

Main British Car:
1966 MGB Roadster 350 LT1 Chevy

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Re: Compression Ratio And Pump Gas
Posted by: Jim Stabe
Date: June 27, 2023 02:41AM

Good article but the subject of tight quench was only mentioned briefly and I think it plays a very key role in detonation reduction. In my engine (2nd gen 350 LT1 Chevrolet) I have 11:1 static compression and I run 10 lbs boost from a Vortech supercharger and I only have 91 octane premium here in California. I do run 50/50 water/methanol injection under boost but I feel that one of the major factors to the engine living is that I only run .033" quench. The engine has run on the chassis dyno without the water/meth injection turned on and registered no detonation but that is under ideal temperature conditions so I run the injection for extra safety margin. The .033" clearance was established by cutting the deck so the piston is only .005" down and using a .028" head gasket. The engine is also set up with fairly tight piston clearances to keep the piston from rocking in the bore. The engine response is much crisper than it would be with a 9:1 compression ratio that most people run with a supercharger.

This is a good article on high compression and the benefits of tight quench [www.motortrend.com]

Edited 1 time(s). Last edit at 06/27/2023 02:45AM by Jim Stabe.

dave cox

(183 posts)

04/30/2018 03:00PM

Main British Car:

Re: Compression Ratio And Pump Gas
Posted by: turbodave
Date: June 28, 2023 12:26PM

That is a very interesting article.
I agree with Jim - I feel like the quench (squish as I call it) is important, but it is referenced in the link posted by Carl:

"A tight quench is achieved by minimizing the piston-to-head clearance to less than 0.045-inch for 4.00-inch bore wedge head engines. So, for an engine with a 0.003-inch piston-to-deck, adding a 0.041-inch gasket would produce a piston-to-head clearance of 0.044-inch. A tight quench area improves mixture motion and will actually enhance combustion efficiency. Avoid wide piston-to-head clearances of over 0.050-inch."

From the MotorTrend link (Authored by David Vizard so well worth a read):

"On a small-block Chevy with a stock block height, a stock compression height piston is typically .025-inch down the bore. With a .040-inch gasket this makes the static quench clearance .065-inch, which is way too wide. By cutting the quench clearance the burn rate and quality improve to the point where the motor gains compression and is less likely to detonate even at the higher ratio involved.
So how closely can the pistons approach the head face? Although it comes under the heading of "don't do this at home" I have run the static piston/head clearance down to as little as .024-inch in a 350 with stock rods and close-fitting hypereutectic pistons. The pistons just kissed the head at about 7,000 rpm. As far as power is concerned, an associate of mine ran some tests in a nominally 450-horse 350 and found that each 10 thousandths of quench reduction was worth approximately 7hp. If you are building from scratch, make maximizing the quench your number one priority toward achieving compression and avoiding detonation."

The factory late 3.9 pistons sit approximately .035 down the deck (I think previous Rv8 pistons are similar). In my SD1 engine (and its sister backup) I have decked the block by .030 to reduce the squish as much as possible. Of course the later composite head gasket is like 50 thou compressed (I recall - need to check my notes), so there is still a thicker squish band than I would like, but all I am ok doing all i can while using the superior composite (elring) gaskets and leaving some deck thickness.

Decking the block this much brings up other complications, but most things work well - like using a bead of RTV instead of the silly valley end seals (this is needed as it pushes the gasket ports further up as the heads are sitting lower, removing the seals lets the valley gasket sit a little lower, matching the ports better).
I also feel like there is a significant benefit - especially with the later 10-bolt heads that have a reduced height 'hump' on the base of the port - to bias the airflow more towards the roof of the port anyway; especially when combined with the EFI manifold.

Edited 2 time(s). Last edit at 06/28/2023 12:31PM by turbodave.

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