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Re: Cylinder base gasket options
Posted by: 66vairman ()
Date: August 07, 2021 03:03PM

KenHenry Wrote:
> I guess the point would be that get the optimal squish dimension down to 0.040” or so. I would use anaerobic sealer, have had great luck with it in other applications.

The whole "squish" theory sounds good, but years ago I read an article on "squish" The purpose is to promote maximum mixture turbulence just before ignition to minimize "knock".

Years ago I read a good article about squish pads and surprisingly the effect was not much until you closed the gap to a value that risked piston to head strike at high RPM, especially with carbon build-up (One of these days I find that article).

Better ways have been developed using intake path design and piston and chamber shape.

I'll also say that both my 140's heads were done so the calculated compression ratio was down 0.5 vs. factory specs. by reducing the pad height slightly. At first I resisted the idea, but the fellow has been doing this for years and said it worked fine with today's pump gasoline and any loss of performance would be unnoticeable for a street engine. I did CC the heads to verify the lower compression.

I have to say I've driven 140's with stock heads and my engine "feels" as strong as any of them. A benefit for me --- I'VE NEVER HAD A DETONATION ISSUE!! And I have A/C and have driven in 90 degree heat.

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Re: Cylinder base gasket options
Posted by: 63turbo ()
Date: August 07, 2021 04:22PM

The improvement in turbulence keeps getting better until the piston hits, on some engines this dimension can be in the neighborhood of .020". Theres a couple of ways to reduce this distance on our engines, one is more common than the other. My engine was done the "other" way, which is to fill the squish pads by welding them up, then a relatively slight clean up pass to remove the bumps in the weld. Now the squish distance is set by the head gasket thickness. Some of the benefits of doing it like this are that the stock length pushods can be used, the spark plug hole is at a stock distance and the shroud holes dont have to enlarged. It of course is a lot more welding though. Initially, on my engine, I used .032" head gaskets for a quench distance of .032" but I found that the pistons were hitting the head frequently from carbon deposits. For a while, I was taking the heads off to clean off the carbon but finally went to .042" head gaskets and dealt with the carbon deposit problem by going to EFI and keeping the engine running at stoich or close to it in light loads. No more carbon deposits! When the squish distance gets into that .030-.050 range theres not a lot more to be had that cant be compensated for by better timing, better fuel air ratio for the load, reduction of exhaust backpressure via freer flowing exhaust/porting, a better cam, better spark plug heat range for your driving.


Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car
Test Start#2 [www.youtube.com]

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Re: Cylinder base gasket options
Posted by: jimc ()
Date: August 07, 2021 05:25PM

In my case I had not discovered this forum and wealth of information available and already had my heads rebuilt. The gasket surface was cleaned up, all new valves guides and Alice porting and had chambers cc’d.

My budget was expended and I did not have funds nor wanted to have my “fresh” gasket surface cut, as maybe I should have to promote more turbulence, but my CR goal was 9.5 and I believe we achieved a true 9.4. My best pump No/E premium gas around here is 92 and sometimes 93.

It was a matter of budget and stubbornness on my part to take my fresh heads back to the shop for more, albeit expensive, work. My heads are still almost full depth as this one owner still has less than 110,000 verifiable miles though now a total restoration.

Robert c:
With a thin coat of permatex #2 it squeezed out paper thin.

1965 Corsa Coupe, two owner history

HACOA Member
CORSA 2017 Concours Senior Gold Award
AACA 2015 Award

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Re: Cylinder base gasket options
Posted by: JimBrandberg ()
Date: August 07, 2021 08:59PM

I was chasing all the compression I could get for a race engine years ago but never went so far as leaving off cylinder base gaskets. I think Jon Brakke used to do it. Race engines for him were not usually together very long.

Jim Brandberg
Isanti, MN

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Re: Cylinder base gasket options
Posted by: wittsend ()
Date: August 07, 2021 09:14PM

Interestingly quench is often spoken about as a fix point in time (maximum upward piston travel) and yet with spark advance the point of ignition is before minimum quenched is reached.

I'm no expert but logically one would assume the higher the cylinder pressure the less turbulence there is as the air is more compressed? That there is more opportunity for air to swirl in a larger, lower pressure when the piston is deeper in the hole. Think of a bunch of balloons in a room bouncing around as opposed to them being shoved in a closet.

Is this the point where we talk about Singh Grooves? smiling smileyLOL

Edited 1 time(s). Last edit at 08/07/2021 09:15PM by wittsend.

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Re: Cylinder base gasket options
Posted by: joelsplace ()
Date: August 08, 2021 04:27AM

It is true that the spark has already happened but the pressure from combustion is rising and the pressure from compression is also rising so when the piston reaches TDC is when bad things tend to happen.
Two different things happen when you get the clearance tight. Squish and quench. Squish describes the turbulence and quench is the cooling the compressed mixture gets from a large surface area which is good for everything but emissions. This is one of the reasons modern combustion chamber designs don't have the large quench area that works in our engines.

Northlake, TX
5 Ultravans, 138 Corvairs and counting...

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Re: Cylinder base gasket options
Posted by: Wagon Master ()
Date: August 08, 2021 04:53AM

With all the discussion on deck height, ya'll must be running a replacement piston. I've never had a stock or about half of the aftermarket slugs that weren't at least .010 "In The Hole".

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Re: Cylinder base gasket options
Posted by: Jonathan Knapp ()
Date: August 11, 2021 10:06PM

I used 400 degree RTV on the base of my cylinders. Ran it that way for years in my autocross car with no leaks. The compression ratio was 10.1 to 1 with 0.025" clearance, a 95 HP distributor, and a 280 Isky. Ran on 91 octane pump premium gas with very little pinging (only in the hottest weather, heavy loads, low rpms). I did have water injection for insurance against the pinging and keeping the piston tops clean.

Jonathan Knapp
Wheeling, WV
'66 Corsa Autocrosser

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Re: Cylinder base gasket options
Posted by: Willt65 ()
Date: August 12, 2021 06:13AM

I removed the base gaskets, removed the step as well as .010" on the pad. My squish is .041" to .046", Removing the base gasket gave me close to 0 deck height. Passenger side is approx. .003" tighter that the drivers side. Chalk that up to excellent tolerances at the factory! Lol! I would need to blueprint the engine for better, this being a street engine I'm good with it. I would like to have a tighter squish, but hoping .043" to be of some benefit. Seems to run well so far. Not sure of my final compression ratio, I did not cc my chambers.
Anyone want to take a wild guess to what my compression might be close to from what I described? They are 140 heads. Thanks!
Also I used permatex gray sealant. Cant remember any more than that for description from my desk.

Will Teeter
Flintstone, MD
65 Corsa project

Edited 1 time(s). Last edit at 08/12/2021 06:17AM by Willt65.

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