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 Post subject: Compression ratio help
PostPosted: Wed Sep 05, 2018 5:38 pm 
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I have a short block with undesirable 425NP dish pistons. I have measured and the pistons are .045 in the hole. The compression height of these pistons are 1.630. It seems the deck has not been milled. I would like to mill .020 off the deck and run a .020-.025 cosmetic head gasket. This would allow .045-.050 quench. Using a 66cc D0AE closed chamber head what would the static compression? I am shooting for 9.1:1-9.5:1 this extremely pump gas for friendly. Thanks in advance for your help.


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PostPosted: Thu Sep 06, 2018 5:09 am 
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I have never used that piston so I don't have any volume data on it....nor could I really find any. I did a rough calculation of the dish volume and got about 8cc, but that doesn't count the valve reliefs. So, if you're looking at a 4.030" bore, 3.500" stroke, 4.100"x.025" gasket, with a 62cc chamber and the pistons .025" in the hole, you're looking at around 9.7-9.9:1.


For the amount of trouble of whacking the deck that much and buying Cometic gaskets, why don't you just buy another set of cast or hypereutectic pistons that will give you the results you want? The heads will have to have the correct surface finish as well to run the Cometics and you're looking at around $160-200 for gaskets alone.

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PostPosted: Thu Sep 06, 2018 1:50 pm 
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Thanks Brent. I will take your advice. I have a set of 555P flat tops. I can sell the closed chamber D0AE’s. I will build a set of open chamber 4V heads and shoot for 9.0:1-9.5:1 static compression. That should do good with the mild Comp hydraulic roller, FC 33148/33158 HR10.


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PostPosted: Thu Sep 06, 2018 3:05 pm 
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Why not run a set of Keith Black 148 pistons? 1.670" compression height, with a 13cc dish. Puts them at zero deck and with a 62cc chamber, you're at 9.7:1.

$320...

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PostPosted: Fri Sep 07, 2018 3:53 pm 
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Have you measured - cc'd - the head to be sure about your chamber size? Because they vary much.

Agree with Brent - you'd be better served to get pistons with a pin height that took care of most of the issue with the deck, then spec a dish that puts your compression where you want it. I used some Cometic last year on the dragster motor. Inspite of all the care and deck surfacing specifically for the Cometic, they did no seal well. This year went back with Felpro 1013 and no issues.

I'd look to trim as little as possible from the deck and heads, only what's necessary to provide a good fresh seal. I'd cut more on the hed if required to hit a compression number - within reason. Most heads I have cut in the .012~.016 range to land in the 64~65 cc range, no more than maybe .010 off the deck is worst cases. .005 if it cleans up.

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