Originally Posted by jameshemi
i am doing up a new (rebuilt) 350 for our mercedes sec conversion.
it has a 350 with edelbrock manifold and 600cfm carb.motor seems stock otherwise and has a knock in the front main,so we are building another motor.
what i have is a 72 chev 350 4 bolt,278 cam, flat top pistons. 30 over i was going to use the stock gm heads (supposed to have been reworked with triple ground valves and z28 springs)but i doubt it.so i am leaning to the vortec heads.i know i will have to change the intake manifold.but the question is..can i use the existing pistons? or is the compression too high? what other mods can be or should be done.i am looking for a respectable 300+hp.
i have found a number of articles on this build ,but they showed engines with the ashtray pistons.
thanks for your input
It will be hard to NOT make 300-plus HP w/Vortec heads on a 350. I suppose if you used the lowest lift factory cam, you might pull it off- but even then it would still make good torque. With goals as modest as that, you need no go overboard on ANY specs, be it the cam, compression, carb cfm, etc.
Compression is going to be about 10.35 w/a zero deck, FT pistons w/a 5cc valve relief volume, and a head gasket thickness of 0.041" (do not go thinner than this if the deck height is really zero).
You can use the Vortec heads, but I would want you to pull some compression out of the equation. This can be done several ways but the easiest and cheapest is to stray from the 'holy grail' quench distance of 0.040" to a wider- but still acceptable- 0.050" to 0.055", which puts the static CR at about an even 10:1 to 10.1:1.
Reworking the combustion chambers to deshroud the valves is another way, although the chambers are pretty darned good as-is, and an inexperienced or overzealous hand could do more harm than good, and to have a pro do it will co$t.
Now, the engine can be cammed to run w/~10.4 CR, providing premium fuel is used and close attention to the tune- especially timing- is being paid. And there are guys running Vortecs and FT pistons w/a 0.041" quench and are getting away w/it. But two things come to mind:
1. A M-B shouldn't have a "lumpy" cam w/a rough idle, at least in most cases
2. If anything less than maximum power timing is used, the engine power and economy will suffer. In some cases, a LOT.
If you were going to run aluminum heads, I'd say run it w/the 10.4 CR (0.041" quench) and hi-test. Also let it be known that I tend to be on the conservative side when it comes to compression. The tiny gains (relatively speaking) to be had from an increase in CR do not come close to offsetting the possible losses due to having to run less timing than the engine needs to make the most power. Or the loss of the entire engine due to detonation.
L31 5.7L Vortec combustion chamber