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Old 06-14-2004, 05:14 PM   #1
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Compression questions.

Just a few questions about compression. Stock compression is 9.36:1. If you mill the heads .030 and bore the block .030-.040, and use flat topped pistons, what compression would you be likely to end up with?

Would the compression need higher octane gas than 89?

Would I run into problems with valve clearance?

I don't think this matters but this would be with a 4.2 crank. I'd Love some input on this. I was trying to map out my plans for the next bit of modding, I've decided to go the N/A route, and see what I kind of horsepower I can get out of it that way.

Thanks for the input guys.
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Old 06-15-2004, 12:35 PM   #2
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Hmm, either I've asked the wrong question, or I've stumped everybody... damn. Ok, second question. If someone could help me out, maybe I can figure it out myself. I know the bore and stroke. The only thing I don't know is the volume of the chamber in the head. The headgasket I'm going to guess is a little under an 1/8 of an inch, probably around .110 in. So does anyone have the volume or dimensions of the head? Or the volume that is lost if the heads are milled .030? As I remember the bowl isn't the same diameter as the cylinder, it's smaller. Thanks for the help, who ever can find the info, or happens to know it.
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Old 06-15-2004, 12:59 PM   #3
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I believe most flat top pistons run in the 10:1 or slightly higher. But the cylinder heads resurfacing and deck height has a lot to do with it. But on a stock virgin block and virgin heads with flat tops 10:1 or 10.5:1 would seem close. But like I said it all depends on what else is done to the block and heads.

RGR or Mikael at RPM could give you specifics on pistons they sell.

As far as octane requirements using flat tops might require a higher octane. I would probably say low octane 87 would be out if bumping up to 10.5 compression.

I hope this helps you out some.
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Old 06-15-2004, 01:30 PM   #4
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Brother Corey comes through again. Thanks man.
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Old 06-15-2004, 03:25 PM   #5
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If the heads are milled .030 and the pistons are changed to flat top pistons with a 4.2 crank, does anyone know if there would be valve clearance issues? Or would valve reliefs need to be cut into the piston?
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Old 06-15-2004, 06:01 PM   #6
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Quote:
Originally Posted by Horse
If the heads are milled .030 and the pistons are changed to flat top pistons with a 4.2 crank, does anyone know if there would be valve clearance issues? Or would valve reliefs need to be cut into the piston?
As long as you use the 4.2 rods there should not be as long as the pistons are also made to go into a 4.2 motor. Piston height varies ( measured from the center of the piston pin to the top of piston deck. As long as the measurements are the same clearance in not a great issue. There have been people that have had to shave a bit of the piston deck off to prevent the piston deck from being to tall.
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