Thread: Domed pistons and quench?

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Originally Posted by FAYLUR

Thanks Techinspector1. So quench would remain the same as long as compression height and chamber cc remain the same. So I guess what I am trying to figure out then is how do professional engine builders determine what dome size would be best for the application,,,,by cam selection? And cam selection is determined by RPM operating range (I think)?
I guess I don't understand why engine builders use a paticular size dome,where in the equation does this come into play? Just to provide a paticular static CR? If so,then why that paticular static CR?

Quench is set with the piston deck height and gasket thickness. Piston deck height is the measurement from the piston crown to the flat deck surface of the block with the piston at top dead center and has nothing to do with the chamber cc's. When building a motor, you would bore and hone the block for the oversize pistons, then mock up the crank in the block with main bearings and bolt up the rod/piston assemblies to the crank with rod bearings, but without piston rings. You would then turn the crank so that each piston would be at top dead center, then measure the distance from the piston crown to the block deck. This can normally be done with just four piston/rod assemblies, one at each corner of the block. On a small block Chevy, this would be cylinders 1,2,7 and 8. On a Ford, cylinders 1,4,5 and 8. Having these measurements, you would then cut the block decks to achieve the block deck height that you wanted to achieve. It would be very unusual to find a block that is square corner to corner and I have found some over the years that were off by 0.008". That is to say, for instance, that cylinder 2 might have the piston down in the bore at 0.025" and cylinder 8 (on a Chevy,passenger side of the block) might have the piston down in the bore 0.033". In order to square the block in this instance, that deck would be cut 0.008" more on the end of the block where cylinder 8 is. Let's say that in order to clean the entire deck on one side of the motor, you might take off 0.005" at cylinder 2 and 0.013" at cylinder 8. This would result in the measurement of each piston on the passenger side of the block having a piston deck height of 0.020" This process is repeated on the driver's side of the block. Actually, you wouldn't do any cutting until all 4 corners of the block were measured for deck height and all cut accordingly to the shortest piston deck height. Now, let's assume that all pistons measured out at 0.020" piston deck height after you have cut both block decks. In order to achieve the optimal quench (or squish, whatever you want to call it), we will have to use a gasket that will give us 0.035" to 0.040" clearance between the piston crown and the underside of the head with the gasket installed. With a piston deck height of 0.020", we could use a 0.016" shim head gasket to achieve a quench of 0.036". Again, this has absolutely nothing to do with the chamber size of the head. We would choose a chamber and piston crown configuration to achieve the desired static compression ratio. You NEVER use different gasket thicknesses to achieve your target static compression ratio. Different gasket thicknesses are used to achieve your target quench.

Some fellows will go ahead and cut both block decks so that the piston is exactly even with the deck at top dead center. This is called "zero decking" and you would then use a gasket with a compressed thickness of 0.035" to 0.040" to achieve your quench.

Different domes together with different chamber sizes would be used to achieve a target static compression ratio. Conventional wisdom says that with iron heads on a street motor having to use readily available pump gas, somewhere around 9.0:1 is the best choice. With aluminum heads, you can run another full point, up to 10.0:1. Now, as we've stated on this forum quite a few times, a fellow can get away with a lot higher than 9.0:1 on pump gas by choosing the proper quench and the proper camshaft. Running 11.0:1 on pump gas is doable with the right combination.

The reason for using different static compression ratios is to optimize the motor for power. Each full point increase from about 8:1 up to about 11:1 will result in a power increase of about 4% for each point increase. So if a guy is going racing and/or has access to some good high octane fuel, he would want to start with a higher static compression ratio.



I love explaining this stuff. If it isn't clear yet, keep asking questions.