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Old 12-07-2007, 05:53 PM   #1 (permalink)
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Mill heads or high compression pistons

Is it better to mill the heads for more compression or use high compression pistons. I don't want to get into throttle body fitment issues, and I would like to reuse the stock pushrods if possible. Which is the best approach to getting the compression, that most aftermarket cams need. I'm starting a 95" build and need some advise.
Thanks
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Old 12-07-2007, 06:18 PM   #2 (permalink)
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Quote:
Originally Posted by ride4daze
Is it better to mill the heads for more compression or use high compression pistons. I don't want to get into throttle body fitment issues, and I would like to reuse the stock pushrods if possible. Which is the best approach to getting the compression, that most aftermarket cams need. I'm starting a 95" build and need some advise.
Thanks

If it were me I'd do the heads.....
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Old 12-07-2007, 06:39 PM   #3 (permalink)
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Quote:
Originally Posted by ride4daze
Is it better to mill the heads for more compression or use high compression pistons. I don't want to get into throttle body fitment issues, and I would like to reuse the stock pushrods if possible. Which is the best approach to getting the compression, that most aftermarket cams need. I'm starting a 95" build and need some advise.
Thanks
What kind of compression do you want? Which type of cams you going to run. It make a difference. Going to 95 with an .03 gasket will bump the compresion a bit as well.
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Old 12-07-2007, 07:07 PM   #4 (permalink)
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I was thinking about using the Andrews 37 , so I'm guessing 9.5 to 9.8 to 1. The SE cast HC pistons are 10:25:1 so with a stock gasket they might work and keep things close to the same height as stock. What are the advantages to one method over the other are. Also if the piston makes to much compression, why not just mill the piston for the right amount? I'm just looking for some ideas to talk to the machine shop about.
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Old 12-07-2007, 07:21 PM   #5 (permalink)
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Quote:
Originally Posted by ride4daze
I was thinking about using the Andrews 37 , so I'm guessing 9.5 to 9.8 to 1. The SE cast HC pistons are 10:25:1 so with a stock gasket they might work and keep things close to the same height as stock. What are the advantages to one method over the other are. Also if the piston makes to much compression, why not just mill the piston for the right amount? I'm just looking for some ideas to talk to the machine shop about.
Milling .060 off the head and a .030 cometic head gasket will get you what your looking for in compression...and you can still use your stock push rods. A flat top piston is better than a dome piston because it is the floor of your combustion chamber....the low floor doesn't interfere with air flow during valve overlap to fill the cylinder better. The dome piston will divert some of that early air flow.

What you have to be careful of when milling heads is the size of the valves...the larger the valves the closer they get to the head gasket surface and this can and will interfere with shrouding the valve or the valve can hit the eyebrow of the piston.
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Old 12-07-2007, 07:28 PM   #6 (permalink)
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I'm assuming you are going to bore your cylinders to 95". If that's the case then you're going to buy pistons anyway. Chose a set that will achieve your target compression, as mentioned they can be milled if needed.
Like you I'm a fan of stock pushrods. They're paid for, don't go out of adjustment and they're stiff.
Good luck
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Old 12-07-2007, 07:50 PM   #7 (permalink)
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IMO, the combustion chambers should be measured before making those decisions. The combustion chamber sizes typically fall between 86-88cc but they could be anywhere from 84 to 91cc in stock form. Until you know exactly what you have to start with it's all guess work.
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Old 12-07-2007, 07:52 PM   #8 (permalink)
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Thanks for the advise everybody, I shipped my heads and cyls. to Big Boyz this week, I was just trying to get some ideas for questions to ask when I call to discuss the build next week.
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Old 12-07-2007, 08:58 PM   #9 (permalink)
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Thanks for the advise everybody, I shipped my heads and cyls. to Big Boyz this week, I was just trying to get some ideas for questions to ask when I call to discuss the build next week.
good decision!!!!
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Old 12-07-2007, 10:34 PM   #10 (permalink)
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I got a better idea.. Increase the displacement until you get the compression ratio you want with flattops... Bruce
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Old 12-07-2007, 10:53 PM   #11 (permalink)
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I got a better idea.. Increase the displacement until you get the compression ratio you want with flattops... Bruce
hehe, another great idea!
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Old 12-08-2007, 08:11 AM   #12 (permalink)
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I like to avoid cutting the head to a large amount. As there are two sides to the coin. Yes you can mill the heads and go that route. But if you spend the money on the porting and later you want to go bigger say 103 then you are at a much higher compression than if you would have used a very small dome. Example the piston that we spec'd out to JE has a 3 cc dome now this is a very very small dome and does not cause the problems that a larger dome can, as DOc pointed out. The upside is that you have to remove very little from the heads this way. You hit your target compression with a thinner head gasket, and very little millig on the head. You are not shrouding the valves. Your piston to valve clearance is much larger, and the cam selection is more open for larger cams in the future ( more lift etc) and to go bigger in engine size.

And until you know what size chamber you have you are playing a guessing game. And this was pointed out by Springer. We see most of the heads in the 86-87 range.

The pushrod thing I like adjustables, as I am not a fan of running the plunger that low in the lifter body. that is my opinion . You can get a adjustable that is stronger and stiffer than the stock unit. Money wise yes it is great that you can reuse parts. You are back to checking what size chambers you have , maybe you only need to remove 20, or 30 from the deck . Or you have to remove 80??

You are on the right track and the other guys have all given you great advice. Good luck with the new build.
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Old 12-08-2007, 10:27 AM   #13 (permalink)
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Quote:
Originally Posted by HDMD88
Milling .060 off the head and a .030 cometic head gasket will get you what your looking for in compression...and you can still use your stock push rods. A flat top piston is better than a dome piston because it is the floor of your combustion chamber....the low floor doesn't interfere with air flow during valve overlap to fill the cylinder better. The dome piston will divert some of that early air flow.

What you have to be careful of when milling heads is the size of the valves...the larger the valves the closer they get to the head gasket surface and this can and will interfere with shrouding the valve or the valve can hit the eyebrow of the piston.
That much difference shoundn't be used with stock pushrods, that is a big difference (about .077). I would use adjustable push rods here.
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Old 12-08-2007, 10:39 AM   #14 (permalink)
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That much difference shoundn't be used with stock pushrods, that is a big difference (about .077). I would use adjustable push rods here.
By all means please do....if the rods spin the valves are closed, if the rod doesn't spin valve is still open you need adjustables. Personally if the rod doesn't spin I install a -.030 S/E perfect fit rod. What ever floats you boat.
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Old 12-08-2007, 11:15 AM   #15 (permalink)
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I usually go with the domes on the street. The tight ports and quench work very well and I have seen many drag heads (and some street) leak under the exhaust ports so like to see as much material thickness there as I can get.
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