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Old 02-03-2011, 07:21 AM
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Dynamic Compression Ratio help

I have a 350 block .040 over with stock crank and rods with vortec 64cc 906 heads. the pistons i am looking at are the keith black 12cc d cup dish or the 7 cc flat top. my question is that with the cam im running which is the lunati voodoo 60102 my dynamic compression would be 8.26 with the 7cc and 7.99 or so with the 12cc. i am confused if i can go with the 7cc or not. the intake gasket is .016 thick and the pistons are .024 down the hole. the claculator i have been using is the one on project pontiac. just google the words PORJECT PONTIAC COMPRESSION RATIO and it is the first link. one more thing is that the ring to wall clearence on the keith black pistons is .0016 not the .0028 which is stock on the clalulator. I am trying for adivce as this is my engine build for a 69 chevy pickup that will be street driven with a th350 trannsmission with 3:73 gears with a posi out back.tires are 30 X 9.5 BFG A/T. thnk you for your answers.
P.S. maybe someone could ask Techinspector1 about this because he seems to have a good idea about this stuff. and i live in north dakota so it never gets over 100.

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Old 02-03-2011, 09:00 AM
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Simply change the 0.028" to 0.016" by manually erasing and entering. But the static CR remains 10.2 either way.

I do not get the same dynamic figures you are getting.
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Old 02-03-2011, 09:24 AM
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That combo has been ran on pump premium gas many times, a lot of people have used it and are happy with it, your tune better be really good though because that combo is borderline for pump gas.

If you want to run it on 87 octane instead of 93 I would use the 12cc dish. You're honestly not giving up a huge amount of power and it'll be more forgivable to bad gas or a slightly off tune.
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Old 02-03-2011, 10:33 AM
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Quote:
Originally Posted by 69chevyLWB
I have a 350 block .040 over with stock crank and rods with vortec 64cc 906 heads. the pistons i am looking at are the keith black 12cc d cup dish or the 7 cc flat top. my question is that with the cam im running which is the lunati voodoo 60102 my dynamic compression would be 8.26 with the 7cc and 7.99 or so with the 12cc. i am confused if i can go with the 7cc or not. the intake gasket is .016 thick and the pistons are .024 down the hole. the claculator i have been using is the one on project pontiac. just google the words PORJECT PONTIAC COMPRESSION RATIO and it is the first link. one more thing is that the ring to wall clearence on the keith black pistons is .0016 not the .0028 which is stock on the clalulator. I am trying for adivce as this is my engine build for a 69 chevy pickup that will be street driven with a th350 trannsmission with 3:73 gears with a posi out back.tires are 30 X 9.5 BFG A/T. thnk you for your answers.
P.S. maybe someone could ask Techinspector1 about this because he seems to have a good idea about this stuff. and i live in north dakota so it never gets over 100.
Rod length would be nice data to have if you're using something other than the GM 5.7 length.

Regardless of rear ratio, the big tires and automatic will tend to load the engine which makes it more ping prone. The 12 cc would let you get away with regular to mid grade fuel. The 7ccs would be a mid grade to premium. The exact fuel would depend on engine tune, operating temp and intake air temp. You can always play with these to dial it in. My preference would be the 7cc cup because I prefer to push the compression up and tune for it, especially with Vortec heads which burn quickly, you can shove the DCR up a bit and drop the ignition lead back if it's too pingy since these heads don't need so much spark lead anyway. For older heads you can't play in that zone without giving up a lot of power so the Vortecs give you some space to play with.

Bogie
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Old 02-03-2011, 10:50 AM
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The 7cc piston volume is the 4 valve reliefs in this case. The 12cc piston is an actual D-shaped dish.

Using the Lunati 60102 cam, the only way to get 8.26:1 DCR is to use 262 as the cam intake duration. BUT if you enter the intake valve closing angle (37.5) instead, you'll see the DCR jump up to 9.51:1!

Sometimes you'll hear guys say, "well, the stock L31 engine had such-and-such CR and it didn't ping..." Sure, and it also has EFI and knock sensors.

Either way, I would use the 12cc dished pistons, period.
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Old 02-03-2011, 02:07 PM
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Quote:
Originally Posted by cobalt327
Either way, I would use the 12cc dished pistons, period.
This assumes a street-bound engine w/no need to be "on the edge" tune-wise.

It will allow for some leeway in fuel quality, unforeseen issues w/engine temperatures- either by ambient conditions or possible cooling system issues, etc.

The dip in SCR (down from 10.2 w/7cc reliefs to 9.6 w/12cc dish) and the resulting marginal loss in output (~8.5 to ~10.5 HP on a 350 HP baseline) is minor to my way of thinking. Especially in the face of detonation-caused parts wear/breakage that might occur w/the 10.2:1 SCR.
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Old 02-03-2011, 10:13 PM
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thaNks for the replies guys I think I'll go with the 12cc just to be on the safe side and not have to always find 93 octane.
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