How thick of a head gasket, and what do you think!!

[pparaska]

Quick discussion of Quench area (actually a volume, I guess):

It's the volume above the piston where the cylinder head surface is flat and flush with the top of the surrounding head gasket. As the piston approaches this flat head surface and ALMOST touches it, it causes the mixture to churn and homogenize for better combustion. Probably some more benefits too.

 

If you have too much distance between this flat part of the head and the piston at TDC, the churning is too weak and there are rich and lean areas of mixture in the chamber that burn at different rates or can preignite before other areas. When that happens, two or more flame fronts can be set up and when they collide you get knock.

 

This is probably not a great explanation.

 

Anyway, lowering compression by using a thicker gasket or more deck height means you increase the quench height. Once that gets very far beyond .045 or .050" or so, the reduced quenching action actually can cause more of a tendency to ping than higher compression with better quench (.035 to .045, etc.) Too little quench height and the piston hits the head. After all winging a piston and rod up and down at high speed causes the rod to stretch and compress. It stretches at TDC when it tells the piston to turn around and come back down the bore. If it stretches more than the quench height, bang, the piston kisses the head. Some racers that run really light pistons and light/stiff rods run less than .035".

 

If they don't understand quench height and it's importance, I'd be looking for another shop. Even HOTROD magazine covers that!

[grumpyvette]

I did not write this but took it from my notes,

 

Gas Burn Rate

 

Several factors affect the burn rate (flame speed) of the gas. The air-fuel ratio (a/f/r) affects burn rate. Mixtures with a/f/r of less than 11:1 have little chance of burning (to rich), and a/f/r greater than 20:1 have little chance of burning (too lean). The fastest burn rate is at 17:1 but that is far to lean for reduced emissions, and way to lean for maximum power. Best power is achieved at an a/f/r of 12.6:1.

 

Homogeny of the gas affects the gas burn rate. Homogeny refers to the uniform distribution of air and fuel molecules within the gas mixture. As we mentioned earlier, the a/f/r affects burn rate, so homogeny also affects burn rate. Homogeny also introduces another issue concerning failure of ignition. If the localized a/f/r where the spark plug is located is to lean or to rich due to poor homogeny, then the spark plug will fail to ignite the gas, and that power stroke will be missed. This concept is referred to as the probability of ignition. The better the homogeny, the greater the probability of consistent ignition for each power stroke.

 

Because poor homogeny can cause ignition failure, a longer duration spark discharge into the spark plug is better than a shorter discharge duration. The turbulence and swirling actions due to the intake port shape and piston motion, may very well replace that lean mixture with a normal mixture while the spark is still arcing. When this happens, then the probability of ignition is improved.

 

Multiple sparks can help to overcome the failed sparks (due to homogeny problems) but multiple sparks will not make the combustion gas burn any faster. Dual spark plugs could make the resultant gas burn time shorter because of two burn sources. Sort of like burning a candle from both ends. The candle will burn faster this way, and so will the combustion gases. But each end of the candle still burns at the same rate. A rotary engine is the exception, and uses multi-spark dual spark plugs to compensate for poor homogeny due to the abnormally long combustion shape of the rotor in conjunction with the ported intake gas flow.

 

Turbulence, Squish & Quench

 

As mentioned earlier, the shape of the combustion chamber can help to prevent detonation in two ways. The shape of the piston crown as it approaches the shape of the cylinder head, can create tremendous turbulence in the gas. This squishing of the gas mixture causes swirling and tumbling actions which causes shear tearing of the air & fuel molecules, which results in better homogenization. This improved mixing of the gas makes the gas burn faster. The same gas when burned faster has less time for spontaneous combustion. The faster the burn, the less time that is available for detonation to take place.

 

Another advantage of a faster burn is that ignition spark doesn't need as much advance. With less ignition advance, there is less time to build burn pressures before reaching TDC. This reduces braking action to the piston compression pressure, which increases pumping efficiency of the engine. This results in less power wasted to pump the engine cylinders.

 

Quench is quite another story. It is reasonable to expect that the gas in direct contact with the metal cylinder walls, piston crown, and the cylinder head surface; would be cooler because the metal absorbs heat from the gas (the metal is cool as compared to the burn flame temperature which can reach 5000F degrees). Because this thin layer is cooler, it does not burn and results in what is called a boundary layer of gas attached to the metal surfaces. This boundary layer is only a few molecules thick, but acts as an insulator which keeps the burning gas temperature from direct contact with the metal engine parts. This contains the gas burn temperature and prevents imparting excessive heat directly into the metal engine parts, which could melt aluminum parts. Like all insulators, it leaks some combustion heat into the metal parts and the engine cooling system must absorb that heat.

 

At TDC, portions of the piston crown get within about .040 inch from the cylinder head (squish region), and the close proximity of boundary layers quenches any attempt for gas in that region to burn. The .040 inch gap is hundreds of times thicker than the boundary layers, but the cooling effect quenches any gas trapped there. When that gas cannot burn, it reduces the chamber temperature which results in less heat available to cause detonation during the time from TDC to 16 degrees ATDC (after the squish time). This cooling effect is referred to as virtual octane because the cooler gas escaping the squish area as we leave TDC, steals heat from the burning gas, which reduces the chances of spontaneous combustion. This quenching effect results in a virtual octane increase. It has been found that the squish region has little effect if the piston to head squish clearance is 0.060 inch or greater. The optimum quench clearance is 0.040 inch.

[deMideon]

Thank you very much!!! It explained it very well. I am very glad to have this resource available, it will keep me from making mistakes!

 

[pparaska]

Thanks Grumpy. Now I see why it's called a "quench height". Quench, as in cooling, i.e., like when you temper a metal, etc.

 

Great post! It's in my bookmarks! I actually learned something cool and useful today!