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Another Compression Question!!!!

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Old May 1, 2007 | 07:05 PM
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From: Iron Station N.C.
Default Another Compression Question!!!!

I have tried using the compression programs recommended on this forum, but it seems that I get different results or the results seems like it cannot be right. Last time I tried I got a DR of over 11.0:1 and a static of around 7.1. Can this be right?

Bore: 4.030
Stroke: 3.75
Pist Vol -5
Pist Compression Height: 1.425
Cam:
Comp Cams 270 Magnum Hyd. Flat Tappet
Int. 224@ .050 Exh. 224@.050
Heads:
World S/R Torquer: 67cc Cast Iron
Head Gasket: .041 with 4.166 Bore
Advance: Hopefully 32-34 Degrees

Any help greatly appreciated. I just want to make sure I can run with premium fuel.
As always, Thanks in advance.
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Old May 1, 2007 | 07:14 PM
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I feel your pain brother I just went throught the same thing. The CR programs available all disagree with each other. I got anywhere from 10.2 to 11.8 SCR on my build....It is NOT 12.8 or 10.2. My best guess right now is about 10.7SCR.

BTW are you sure your DCR and SCR aren't backwards??


A DCR of 7.0 will run on pump gas.....If your DCR is really 11.0:1 then you better fill up on the 115 leaded


BTW DCR will always be lower than SCR


Good Luck
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Old May 1, 2007 | 07:17 PM
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After looking at your specs again there is no way that you are 11.0:1 SCR.

Even at zero deck with 67cc heads and an .041 head gasket you should be more like 10.2 (with -5cc flat tops)
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Old May 1, 2007 | 07:46 PM
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From: Iron Station N.C.
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Yes I did get the static and dynamic backward. Even so it still would seem that a dynamic of around 7.1:1 and static of over 11.1:1 would not be right with a short cam like this one.
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Old May 1, 2007 | 07:52 PM
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I almost forgot this spec. The deck is stock, which should be 9.025, right? I could not find a machine shop to deck the block without taking of the numbers. It was checked and found to be OK.
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Old May 1, 2007 | 09:26 PM
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Originally Posted by blueonblue
Even so it still would seem that a dynamic of around 7.1:1 and static of over 11.1:1 would not be right with a short cam like this one.
Cam or not this does not sound right...I really think that your SCR will be under 10.5.....Your DCR under 8.3
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Old May 1, 2007 | 09:59 PM
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From: Iron Station N.C.
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I agree. The piston mft. states 10:1:1 for a 68cc head. I was just concerned about using pump fuel, and was trying to figure dynamic compression. Guess I was just putting the wrong figures somewhere.

Thanks anyway
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Old May 1, 2007 | 10:00 PM
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Originally Posted by oregonsharkman
After looking at your specs again there is no way that you are 11.0:1 SCR.

Even at zero deck with 67cc heads and an .041 head gasket you should be more like 10.2 (with -5cc flat tops)
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Old May 2, 2007 | 12:06 AM
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My calculations show a SCR of 10.1:1 and a DCR of 8.34:1.

I think you are running a little high for iron heads and no quench effect.
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Old May 2, 2007 | 12:23 AM
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Originally Posted by blueonblue
Yes I did get the static and dynamic backward. Even so it still would seem that a dynamic of around 7.1:1 and static of over 11.1:1 would not be right with a short cam like this one.
You use 5cc NOT -5cc in the calculator. It is piston volume. That is why your static compression is coming out so hi. -5cc volume is a dome piston. Double negative is a positive. Math 101.
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Old May 2, 2007 | 12:27 AM
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Originally Posted by dgruenke
My calculations show a SCR of 10.1:1 and a DCR of 8.34:1.

I think you are running a little high for iron heads and no quench effect.
Stay under 8.0 MAX for iron heads 7.8 is safer. You need a bigger cam or lower static compression for sure.
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