Quote:
Originally Posted by ponyakr
There are lots of smart guys here who can probably give you all the math involved in figuring the ACTUAL CR of an engine.
I'm not that smart. So, I use the Wallace CR calculator.
http://www.wallaceracing.com/cr_test2.php
It has been posted here many times that many of the factory published head chamber sizes & CR's of some engines were not correct.
For example: Several have posted that when they measured the chambers sizes of some virgin heads they were quite a bit larger than advertised, like maybe a "72cc" head actually had chambers that were 75-78cc, or even 80cc.
So, the ONLY way you can compute the actual CR is to know the actual chamber size & all the other measurements that are required for an accurate calculation.
Here's a calculation of a std bore 400 with 72cc heads. I don't know all the other exact factory numbers, so I used 6.6 valve reliefs, .045 x 4.3 head gaskets, & .020 deck height. Calculator says 9.75 CR. I think some of the 72cc head engines were advertised to have 10:1 CR, some 10.25:1, some 10.5:1, & some 10.75:1. Just guessing that most of these adv numbers were wrong, on the high side. Also assuming some hp ratings were not correct, some on the low side & some on the high side, for different reasons. |
Ok so mine are 1970 heads with 87cc chambers can I take 7 cc off these heads?
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