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Old 08-13-2012, 10:59 AM
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Gasket quench

Hello all

I have a stock 400 with stock pistons, I am putting 64 cc heads on the block. My question is what head gastet should I use, I've read here on this board that you have to have a certain quench. Could someone help me out

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Old 08-13-2012, 11:05 AM
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Basically, find out how far in the hole the pistons are and add the gasket thickness to that, look for .040 and about .060 max. FelPro and others make head gaskets in varying compressed thicknesses.
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Old 08-13-2012, 12:12 PM
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Originally Posted by 86chevytruck View Post
Hello all

I have a stock 400 with stock pistons, I am putting 64 cc heads on the block. My question is what head gastet should I use, I've read here on this board that you have to have a certain quench. Could someone help me out
It takes some math, but fortunately there is a lot of help on the web to work out what is the ideal compression ratio the fuel octane limit you want to burn combined with the best squish/quench distance that closes in on the compression ratio desired and the amount of squish/quench needed to maintain such a compression ratio.

The 400 with the stock deep dish pistons is going to be a less than optimum solution for squish/quench no matter what you do short of new pistons. This is a place where new pistons with a D shaped dish under the valve pocket with a flat opposite side that closes tightly with the heads squish/quench step is optimal.

Without the piston change this engine will have a weak squish/quench which is a way of building what's called mechanical octane into the engine (good for up to 5 octane numbers). Where flat surfaces on this side of the combustion chamber come to a tight closure, the optimal being about .040 inch, and .060 being about the upper limit of effectivity for a street engine. Racers run tighter than .040 for better effect but accept the higher risk of the piston hitting the head in the interest of winning and with frequent teardowns and maintenance of wearing parts this limits the part collision possibility where a street engine that's expected to go a long time between openings these tight dimensions become too risky.

So given the deep circular dish will greatly reduce the effectiveness of squish quench, and the factory piston being .025 inch in the hole already, the use of even a .019 shim gasket will have minimum affect to the much needed squish/quench. So you’re going to have to compromise on compression ratio through the thickness of the head gasket with a 64 cc chamber. You may very well find that an 8 or 8.5 will be as much as you can run and that on premium fuel without encountering detonation.

The events of squish and quench occur in this order as the piston traverses the region of Top Dead Center (TDC) of the end of the compression stroke and the beginning of the power stroke. The mechanical octane happens from these two events.

Squish happens as the piston closes on TDC the closeness of the far side chamber flat surfaces forces the mixture toward the spark plug with great force. This action breaks up the remaining globules of fuel while mixing the fuel and air together. By pushing this well stirred mixture into a small area in front of the spark plug it greatly increases the chances of it catching fire (minimizes miss or late fires) and the increased molecular density burns faster and more completely by out racing the chances of pre-ignition and unleashing more heat energy early in the piston position cycle where it's most effective in producing power.

Quench happens in the late part of the burn just where the temperatures and pressures far from the spark plug are becoming high enough for the mixture ahead of the flame front to spontaneously ignite. Under these forces and temperatures it goes off more like a bomb and you get the detonation explosion that can blast the piston apart. In quench, the close surfaces present a lot of area to the volume of mixture between them. This soaks the heat out which reduces the pressure and temperature build-up ahead of the flame front reducing the possibility of this mixture zone exploding before the flame front gets there.

When optimized the functions of squish/quench can make the fuel react as if it has about 5 more octane’s that it is rated at. It, also, reduces misfires at both low and high RPMs and it smoothes the combustion process eliminating another phenomenon known a combustion rumble, where the burn is herky-jerky and doesn't complete before the exhaust valve opens. This being mostly a cruise problem where the throttle is not open much, the mixture is a bit lean and of low molecular density and the RPMs low with high road speed. Port or cylinder injected EFI gets around this but if you run a carb or Throttle Body Injection it can be there.

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