Thread: compression/HP change calculation

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Just a thought: Most compression calculators can figure out dished and domed pistons. If you have flat tops with only valve reliefs or even dishes you have to type "-6cc" in the box. If you forget the minus the calculator thinks you have a 6cc dome on your pot. Maybe that's where you went wrong calculating you DCR. A 12cc difference can easily make a change of one point on DCR. be careful you get the CR right before you go shopping for a cam, otherwise

MadMax,yeah,thats what I did,I forgot the -(minus) sign,
Thanks.

Can anyone tell me why pluggung in -6cc for piston head volume gives a increase in CR? Isn't this a decrease in area and should make less compression? I don't understand.

It depends on how you look at it. The calculator sees it as a decrease in the volume of the total combustion chamber, thereby making the squeeze tighter. Don't use the - sign when entering piston dishes/eyebrows.

Thanks Richard. Using the- sign gives me almost 1 1/2 point more compression.Someone else said use it??? Oh well,,,9.537dcr sounded to good to be true anyway Guess I have some learning to do about the importance of dcr and I guess some people build their engine around that factor,i.e.,head & cam selection,etc. But I still can't understand why cr is so low with 58cc heads.Evidently those hipo 283's came with domed pistons.

I don't think you'd get by with 9.537 DCR, I suspect the motor would spit the head gaskets out.

OK,,,crank is turned,have bearings.On a check with pistons 1,7,2,8, installed,minus rings.I checked each of these at the 3 o'clock & 9 o'clock positions and have .030 deck height.Plugging in,,,58cc combustion chamber;6cc piston head volume;gasket thickness .016;gasket bore 4.100;cyl.bore 3.905(.030 0ver 283);.030 deck clearance;stroke 3.00;rod length 5.7;intake closing point 41 deg. abdc; have 9.031 static compression;8.276 dynamic compression.
Seems that this wouldn't be bad,but I'd like for this motor to get a S10 into the high 14's and this CR has me worried. Maybe I'm wrong?
Please,,,while I have your attention,,,I want to run a cam with no less than 480 lift,300 duration,with early opening on the intake valve.Trying to build around the 3.00 stroke.However,,,I am using,305HO heads.1.84" intake 1.50"exhausts,58cc;These are 416 heads that a certain magazine(moderator will abort if I say the name)says have a intake runner same as a 350 head).
NOW,,,,LET ME HAVE IT WITH BOTH BARRELS,,,POS OR NEG DON'T CARE.I need advice because I trust this site more than any.Thanks,Jerry.