.035" is the minimum quench. Check out Speed-O-Motive's site for lots of info on quench. www.speedomotive.com then click on "engine building tips."
I have 76cc SR Torquers on my 66 (355). I used a .018 shim gasket and with the .025" deck I get 8.98: CR. With this gasket my quench is .043". I run a lot of advance with no detonation problems. I use the middle grade gas. The car will run OK with regular, no audible ping, but power is down. A tight quench reduces the propensity for detonation and allows higher compression or more advance or a combination of both.
With less than .035" quench you run the risk of a piston hitting the head due to rod and piston stretch. You need to know how far the piston is down the hole then add the gasket thickness to that.
66 El Camino, daily driver
67 El Camino, STRIP/street
[This message has been edited by Pat Kelley (edited 11-26-99).]
Quench, for those who don't know what it is, is the distance from the piston deck to the flat areas of the cylinder head when the piston is at TDC on the compression cycle.
There are three different dimensions that quench can affect the combustion process, too much, proper, not enough.
The universally accepted quench distance is between .040 to .045. Experts and OEM performance divisions have agreed on this subject since the late '60s. If the quench is not within the correct distance range, detonation and excessive piston top heating/erosion starts to occur. An improper distance also makes the engine respond with being timing and/or fuel/air mixture over-sensitive.
What does proper quench actually do? It makes sure the fuel/air mixture is retained in the critical detonation/heat areas for cooling of the piston tops and detonation control, and octane requirement of the fuel.
Too much. When the quench distance is too much, above .045, the air/fuel mixture is allowed to migrate away from the critical areas it is needed to cool the piston tops and upper rings, and both detonation and heat are produced because there isn't any cooling fuel retained within the quench area, and can cause serious piston erosion and damage.
Too little. A too small area has the same effect as too much area, but the lack of area causes the mixture to be blown out of the area, and the heat and detonation is the result.
Proper quench distance. This allows the fuel/air mixture to be retained within the quench area and upper ring land are, for proper cooling of the piston top and upper ring. The benefit of keeping these areas properly cool is that the static and dynamic compression ratios may be raised.
How to get proper quench area on your engine. When the whole quench thing started back in the sixties, the proper area was found, and at that time, engines came with a .020 compressed thickness shim head gasket, and a deck height in the .020 to .025 range. Adding the two dimensions, we get the desired .040 to .045.
When the pack type head gasket became popular, fuel was still of good quality and octane, and the transition in gaskets was not very noticeable. The quench went up. Most pack type head gaskets have a compressed thickness of .039/.040, added to the .020/.025 deck of the piston, we get a dimension of .060 to .065, detonation, compression and octane sensitivity of the largest concern.
If the deck is set to zero or just below the block deck, say .003 or so, we get the magic .040 to .045 quench, the compression can be raised about a half to three-quarters point and still use pump gas. Timing becomes less promne to cause detonation, and the engine runs a whole lot harded all around. All this assuming a static compression ratio of 9.50:1 and pump premium fuel as the starting point for a street daily driven engine.
Getting zero deck area means either decking the block or getting the proper compression height piston.
With the proper deck (zero) and aluminum heads, the compression can be raised another three-quarters of a point over iron heads and the same deck height. I run a bored and stroked 1963 Buick 215 aluminum V8 in my Vega, with .042 quench, and 11.00:1 c/r, and a cam with 233/[email protected] cam, without detonation, on pump premium fuels. I have built both GM and Ford engines with iron and aluminum heads, and found the quench dimensions first used and discussed here to give the best results.
Just what I have found to work over the last 30 years of my playing around with lots of gasoline automotive and motorcycle engines.
The new Kawasaki ZX900 motorcycle engine is aluminum, has a .040 quench and 12.60:1 c/r.
It also has a better digital ignition, but the quench area is a major reason it can use that compression ratio.
[This message has been edited by IgnitionMan (edited 11-27-99).]
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