Thread: Could use some help on CAM specs, Timing and idea please!

Tweet

With today's pump gas quality, most fellows will limit static compression ratio to somewhere around 9.5:1 for use with iron heads or 10.5:1 for use with aluminum heads.

Problems arise when a fellow goes to aluminum heads without cutting the block decks for zero deck height. To keep squish/quench shallow, like 0.035" to 0.045" to insure operation of the motor on pump gas without detonation, they will use stock steel shim head gaskets. Look at any and every manufacturer of aluminum heads and see what they recommend for head gaskets......a composition gasket somewhere around 0.040" in compressed thickness. None of them recommend a steel shim gasket because the difference in cooling and heating between the iron block and the aluminum head can cause the head to move around and "fret" itself against the steel gasket. Using a thicker composition gasket allows the head to move around a little without fretting. I don't know what will happen with an aluminum head and steel shim gasket, but would suspect that the head will fret until it removes enough material to cause rupture of the head gasket. There are fellows who are running this combination, but my question would be, for how long?

Problem arises when the builder fails to cut the block decks when using a composition gasket, so that the squish/quench ends up at roughly 0.065" (0.025" piston deck height plus 0.040" gasket thickness) ...... too wide to help effectively control detonation on pump gas. The builder needs to either find a piston that has a taller compression height that brings the piston crown up close to the stock height deck or disassemble the motor and cut the block decks on a mill for zero block deck height. At zero deck with a nominal 0.040" composition gasket, the squish/quench is 0.040" and the motor is happy on pump gas.

There is another reason to cut the block decks to zero and that has to do with errors at the factory. No block I ever saw measured for block deck height had the decks on all four corners at the same height from the crank centerline. In a mass production shop such as the Chevrolet machining line, it would be easy for a mill operator to miss a chip in the register when cutting the block decks, resulting in block decks that are not parallel with the main bearing bore. Then, when you attach cylinder heads to the "imperfect" block, the heads are askew and the intake manifold has little chance to sealing up properly.

I'm aware that no builder wants to hear this when he is eyeballing the motor running in the truck, but that is the truth of the matter.

At least one supplier on ebay (Skip White) has addressed the problem with the small block motor. He went to a piston manufacturer and specified a piston that has a 0.015" taller compression height, 1.575" instead of 1.560" on a stock stroke 350, 1.440" instead of 1.425" on a short rod stroker motor or 1.140" instead of 1.125" on a long rod stroker motor. This reduces the piston deck height and allows a composition gasket to be used with good squish/quench when using aluminum heads. It will also allow a builder to cut the block decks to zero while removing a minumum of material for better structural integrity of the block. I know of no piston that fits this bill and is available for a BBC, but there should be.

I will edit this post to give you the math for figuring static compression ratio in a little bit......

.

Well, as fate would have it. My truck decided to start hem-ridging oil from somewhere. I had already planned to pull the motor and start a tare down for pistons, but I'll do some research on having the block deck evened out. Upon that not being feasible(I live in a smaller town and the closest place that will even Port/Polish the heads I can find is 2 hours away). I'll sadly have to go with the "time bomb" combo that's being used. Thank you very much for taking the time to help and go into the detail as you have. Wasn't expecting to get a response quickly, yet alone a very detail oriented reply addressing the issue. I suppose time to buy a motor stand and get busy watching how to videos to get pistons out .