68BLUCUTTY

Hello,

I have a 1968 Holiday Coupe that started out new as 350 CI 250 HP 2 Barrel with a Jetaway transmission. The engine is still the original but now has an Edelbrock 0L4B 4 barrel intake with a 650 holley, dual exhaust with a 350 trans swapped in. I'm going to have the engine rebuilt and I am wondering what is the difference regarding the compression ratio between my engine and the higher horsepower engines? I am showing my engine stock is a 9.0 compression ratio and all the other 350's are 10.5. My plan so far is to rebuild with Edelbrock 650 cfm carb, use same 0L4B intake, edelbrock 226/236 cam, lifters and valve springs along with shorty headers and dual exhaust with H-pipe.

My questions are
1. What is the difference in the pistons on my engine and the higher horsepower 350's?? Are they dome?
2. Are the cylinder heads the same for my engine and the higher horsepower 350's??

Thanks

brownbomber77

coppercutlass

Chances are your compression is under 9 to 1 the low compression 350's had a 14cc piston and factory tolerances are not exact . The w31's 325 hp 350's had a flat top piston and I wanna say where 10.25 to 1 there was a 7cc dished piston as well which I wanna say was 10 to 1.

What you plan to do with your car will dictate what parts you should be using. Depening on the mileage of your engine you might get away with more of a freshen up vs a true rebuild with new pistons. Piston to wall clearances and many other factors will truly tell you if you can keep your stock pistons.

The difference between the "hp" heads vs regular is pretty much just big valves. Those bigger valves do help . Good bang for the buck upgrade if the budget allows along with a little bowl work under the valves. If you are going to rebuild it you can also just have the heads milled to accept the thicker head gaskets .

in my experience for a cruiser your 650 should work fine you may leave a little hp on the table but for street use it should be fine . I ran a 600 cfm on a 350 that ran low 13's maybe a little undercarbed but it never missed a beat. My last engine I ran a 750 holley double pumper but it was worlds away from stock.

olds 307 and 403

As said you probably have 14cc pistons which equal high 8 to 1 compression. Don't go with that lazy Edelbrock RPM cam but upgrade to the RPM intake, much better than your current intake. Cutlassefi has brought modern Mahle 10cc Olds 350 pistons to the market with a 1mm ring pack, lighter than the other aftermarket pistons and give perfect low to mid 9 to 1 compression. He also does custom cams for your build as well.

Fun71

One thing to keep in mind about pistons and compression ratios is that almost all of the aftermarket cast pistons are shorter than the factory pistons, so even though the pistons have the same size dish the compression ratio will be a lot lower.
This is not an issue if you use forged pistons, or if you do the machine work to the heads and/or block deck to compensate for the shorter piston.

Bottom line is you need to determine what compression ratio you will need for your build, then ensure the piston dish, block piston-to-deck height, and head combustion chamber volume all work together to get you there.

As said, talk to Cutlassefi (Mark) or another knowledgeable Olds builder and learn what needs to be done to achieve your build requirements. You can also ask questions of the masses and get feedback as there are lots of folks who have been there and are willing to help.

68BLUCUTTY

Thanks for everyone's response. I will reach out to Cutlassefi on some recommendations for my build.