Ask Away! with Jeff Smith: Optimizing Compression in a SBC Corvette Boat Engine

I’m rebuilding a Corvette small block Chevy engine for my cruiser boat in Australia. It’s a mid-range torque engine. I’m using Scat forged, floating pin rods, and a mild Isky cam which is right for my purpose. Is Mahle a better choice than JE SRP pistons, and is a dished piston a better choice than a flat top?

My boat is a 22-foot cruiser, old but honest like me. I did about 35 mph with the old motor that had been neglected. I’m building this engine as strong and efficient as I can without shoving a wind mill down its neck. It will have forged pistons and I am paying a lot of attention to heads with quench, deck crank alignment, and type of piston design.

The numbers on the heads are 462624.

I’m running a standard 350 stroke and 5.7 rods. If I go with a Mahle piston I can get 4.040-inch pistons. The compression height of the old pistons is 1.562. The nearest I can get to that in a Mahle is 1.550. I’m just about to measure the deck height. I can get a JE here at 4.030-inches. The desired quench gap is 0.050-inch. So how do I achieve that with a dished piston? — M. V. K

…

Jeff Smith: As for the difference between Mahle and JE — they are both great pistons. I would suggest the 4032 alloy pistons for your application. Both JE SRP and Mahle are made in this alloy. The 4032 alloy allows a tighter piston-to-wall clearance which should eliminate piston slap noise when the engine is cold. Since this is a boat, it likely will run at a cooler engine temperature.

The casting number you offered appears to be from an early ’80s L82 Corvette engine. My accurate casting number research material stops at 1975 so that means I must rely on the internet.

Calculating compression ratio the old manual way is quite involved and would get quite lengthy to explain. It’s better just to use one of the free online compression ratio calculators. Summit Racing offers a very simple calculator that works very well.

The heads appear to be a set of 76-cc castings. With a 4.00-inch bore, 3.48-inch stroke, 76-cc chambers, flat-top pistons with small 6-cc valve reliefs, a 0.038-inch thick gasket, and a piston 0.005-inch below the deck will produce 8.9:1 compression.

This combination would put your quench (piston-to-head) at 0.043. The tighter the quench distance, the better the combustion efficiency. This creates squish that more effectively mixes the air and fuel in the chamber.

Quench is very desirable and can improve efficiency almost as much as raising compression — the combination of the two is ideal. It’s best to shoot for a tighter quench if the deck is truly square to the crank centerline. The better machine shops use a tool from a company called BHJ which establishes a mandrel that represents crankshaft centerline to measure the distance to the deck at any point on the block.

For the mild engine like you are building, 0.040-inch or less is perfectly acceptable. That 0.050-inch figure is rather wide and will only hurt performance and likely require more ignition timing.

Those old iron head chambers offer poor combustion efficiency, but they will be okay in your situation.

This combination will need a flat-top piston to maintain the compression.

If the piston is too far down in the hole and you don’t want to mill the block, another trick is to use a Fel-Pro steel shim gasket with a rubber coating. It has a thickness of only 0.015-inch — more than half that of the 0.038 gaskets. Because it’s a steel shim gasket, the block and deck surfaces have to be flat and parallel to create a good seal.

The Fel-Pro PN is 1094 — you will need two. All of this assumes your chamber size is 76 cc. A 74-cc chamber helps a little to 9.2:1 so all these little things add up.

A small-block Chevy’s standard deck height is 9.025 inches. We can figure out a rough piston position by adding rod length + half the stroke + piston compression height. You offered a compression height of 1.56 and we will put half the stroke at 1.74.

If we add 5.7 + 1.74 + 1.56 = 9.0 inches.

If we assume the deck has not been milled, this would produce a clearance of 0.020-inch between the top of the piston and the deck. Aftermarket pistons sometimes make the compression height shorter which will produce even more piston-to-head clearance. The only way to know for sure is to mock a piston up in each of the four corner cylinders and measure the piston-to-deck. Then the block can be machined if necessary to create the desired compression height.