[Lynn]

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Can some of you engine builders answer a question for me? Reading this post made me think of a statement a friend of mine told me (he has been building motors for 30 years). He said that you could reduce or eliminate detonation by making custom pistons that will give you "zero deck height". Meaning that the piston at it's peak of rotation is perfectly flush with the deck of the block.
What I'd like to know is this, have any of you heard about this and why would it help?
Thanks, Rich.

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OK Rich, I'll take a stab at this one.
First, I know there are much more knowledgable engine builders out there. I am a (substitute derogatory adjective of your choice) lawyer by trade, but still a mechanic at heart. Worked as a mechanic at several independent shops and a GM dealership, then owned my own shop for a few years before going to law school.

Your friend is on the right track, although, proper quench can't cure detonation all by itself if the static compression is high enough. If it would, car makers would be putting out 14:1 compression engines.

First, it is important to understand detonation. While it has been studied for decades (Gulf introduced lead in the 30's, calling it "nonox" gas to deal with those 7:1 "high compression" engines of the day) the exact mechanics of normal gasoline burn and detonation are not fully understood. It just happens too fast.

1. "Normal" combustion.
After ignition, the burn progresses at a speed of some hundreds of feet per second, delivering power smoothly and relatively slowly. This "slow" burn pushes the piston down the bore creating the smooth power delivery we all want.

2. Detonation
Detonation occurs when, during the "slow" burn process, the unburned portion of the air fuel mixture is compressed by the expanding gases and warmed by radiation. When the pressure and temperature in the unburned portion pass a critical point, detnonation commences and progresses at a rate of some MILES per second (instead of the hundreds of FEET per second of the "slow" burn). Boom! The heat energy is released much more rapidly, and its conversiion to mechanical energy is less efficient than when "slow" burning occurs. Even mild detonation causes some damage and loss of power. More severe detonation can destroy an engine. That peculiar sound we call knock or ping is actually the cylinder walls ringing. You may as well hit the top of the piston with a hammer.

3. Contributing causes.

High compression. Engines with high compression are inherently more susceptible to detonation because the "slow" burn gases don't have to expand as far to put undue pressure on the unburned portion of the mixture.

Carbon build up. Not just because it obviously raises compression, but usually forms little peaks that get so hot they contribute to the radiant heat that pre-ignites the unburned mixture.

"Dead" cylinder head and piston design, meaning not enough turbulence in the combustion chamber.

Crappy gas, or low octane gas. The lower the octane, the less resistant to detonation. Hey, isn't that what started this thread?

4. Ways to combat detonation. Yeah, I am getting there.

Higher octane. Been discussed.

Make sure chambers and piston tops are clean.

Lower compression. Hurts performance.

Retard timing. Really hurts performance. And mileage.

Move spark closer to center of combustion chamber. Not much of an option on vintage cars.

Aluminum heads. Dissipate heat from the combustion chamber lowering the radiation factor and reduces the chance of detonation.

Increase turbulence. Aha!!! This is where quench comes in to play. Of course, combustion chamber science has progressed incredibly in the last 35 years. Notice all the great heads have heart shaped chambers? Look at the chambers on the 186 head or the 461 head (virtually identical). While they were great for their day, the straight line delineating the quench area doesn't create near the turbulence as the newer heads. However, that doesn't mean it can't create greater turbulence. By decreasing the clearance from piston to head, it creates greater turbulence.

Vintage small block Chevys have a factory deck height of 9.025", assuming you actually have a square block, which you don't. But on the average, that is the deck height from crank centerline. The top of the flat part of the piston (not including any dome, dish or reliefs) is 9", meaning the piston at TDC, is down the hole .025". Head gasket thickness varied, depending upon application, but even with a .030" thick (compressed) head gasket, that leaves a .055" piston to head clearance, or quench. Many of the Chevys got .040" head gaskets, meaning the quench clearance was .065".

Most guys when referring to "zero deck height" are talking about having the block surface decked to a true 9" height from crank centerline, which also makes the block perfectly square. This obviates the need for custom pistons. However, custom pistons accomplish the same thing, but only if the block is perfectly square to start with. Pretty rare. If the block is out of square, you could have a piston at one end a few thousanths below deck, and one at the other end a few thousanths above the deck. Hence the reason GM used a 9.025" deck height and a 9" piston height, to allow for tolerances and not have to worry about a piston hitting the head.

So, what is the "ideal" piston to valve clearance? Most manufactures warn against going less than .010" PER INCH OF BORE SIZE. That would be .040 on a 4 inch bore. Personally, if things are set up properly, I have no problem going .035" on a 4" bore. However, all pistons rock in the bore, and forged pistons rock even more when cold, because they take awhile to expand as the engine warms. That is why in my original post I stated you should go with the minimum clearance recommended by the piston manufacturer.

Sorry this is so long, but hope it is helpful.

Lynn