VZM

I am just north of Toronto. The machine shop recommended the flat top silvolite piston with the 50 thou head gasket.

BillK

Most of those flat top pistons with 4 valve reliefs are about a 5-6 cc "dish" for the valve reliefs. Unless I am doing my math totally wrong with a 1.560 compression height the piston will be about .020 down in the cylinder. That combined with a .050" gasket and the 64cc Vortec heads gives you a compression ratio of just about 9.5 to 1 ? I guess as long as you are ok with 93 octane fuel then it should work.

VZM

Machine shop said that would run 87... Stock mercruiser 350 compression is 9.4, but it has 50 quench to help. With this flat top option the quench is out the door.

kornegle

Many years ago, I had "260 Mercruiser (350 non Vortec GM) in my 23 ft Wellcraft. I removed the engine around 250 hours and installed brand new GM Vortec heads, different valve springs, a mild flat tappet Comp Cam around 212 / 218 @ 0.050. New Edelbrock intake, roller rockers, timing chain, HiPo Mercruiser exhaust manifolds, and kept the stock flat top pistons with 4 valve reliefs and never even removed them. I calculated around 9.3 compression and was concerned as well, since I think I used the stock Mercruiser head gasket. I started at 32* total timing and 89 mid grade. It gained 9 mph and I switched from a 21 Mirage to a 23 Mirage and it turned that prop to 5100 rpm. Later, I put the timing back to 30* total and ran a few tanks of 87 octane and could not detect any detonation or pinging. I went back to 89 for safety and ran the boat like that 8 years, no issues, sold it to a friend and he has had it with no issues for 5 years.