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Old 06-24-2012, 04:24 PM
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350-flat top pistons..vortec heads?

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
james

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Old 06-24-2012, 06:05 PM
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A few factors enter the picture. First off, at top dead center, how far down are the pistons in the hole? What thickness head gasket do you intend to use? Add the two figures together and you can calculate the overall height and add it up on a compression ratio calculator or one of us here can do it for you. There is a way you can run those pistons with vortec heads, however it requires careful tuning. If you use Vortec heads and if the cam has over .450 lift you will need to have the valve guides clearanced for the additional lift.
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Old 06-24-2012, 10:14 PM
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Quote:
Originally Posted by DoubleVision
A few factors enter the picture. First off, at top dead center, how far down are the pistons in the hole? What thickness head gasket do you intend to use? Add the two figures together and you can calculate the overall height and add it up on a compression ratio calculator or one of us here can do it for you. There is a way you can run those pistons with vortec heads, however it requires careful tuning. If you use Vortec heads and if the cam has over .450 lift you will need to have the valve guides clearanced for the additional lift.
when we tore the engine down,we checked piston height,they are even with the deck,i havent purchased any gaskets yet.i will be picking the block up from the shop monday and will have the cam specs,also i have a edelbrock carb stamped 2702 on the front,trying to figure out specs-cfm etc.do you think going for vortec heads will be worth the effort,or should i stick to gm 76cc heads?
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Old 06-25-2012, 06:59 AM
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Quote:
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
james
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
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Old 06-25-2012, 09:02 AM
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Quote:
Originally Posted by cobalt327
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
thanks for the info! as this car is also going to be a daily driver,it will be too finicky with this set up,sounds like my best bet will be to change the pistons first, i think summit has a kit,as i still need to pick up bearings,gaskets etc,is there a better type of piston to use with this set up? we do want this car to be a little lumpy but don't want a stall converter,some economy as a daily driver(summer cruiser) with some attitude and punch when needed.only 11000 of these cars were produced,so we want to do it right.
thanks for your help
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Old 06-25-2012, 09:16 AM
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Quote:
Originally Posted by jameshemi
thanks for the info! as this car is also going to be a daily driver,it will be too finicky with this set up,sounds like my best bet will be to change the pistons first, i think summit has a kit,as i still need to pick up bearings,gaskets etc,is there a better type of piston to use with this set up? we do want this car to be a little lumpy but don't want a stall converter,some economy as a daily driver(summer cruiser) with some attitude and punch when needed.only 11000 of these cars were produced,so we want to do it right.
thanks for your help
Hard to get a lumpy idle w/o any increase in stall speed, but I recall someone recommending a Summit(?) cam that should fit the bill.

You don't need much of a volume increase to put you into the safe zone, considering the CR is 10.35 w/your specs (you're positive about the zero deck?) and a 0.041" head gasket. D-shaped "inverted dome" pistons (these maximize the quench/squish effect) are available, a 12cc piston dish volume puts you at 9.5-9.6:1.
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Old 06-25-2012, 12:25 PM
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These pistons will fill the bill:

http://www.summitracing.com/parts/SLP-WH670CP30/
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