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tom1394racing tom1394racing is offline
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Join Date: Jan 2004
Location: CT
Posts: 9,891

I found the CR measurement to be difficult and it took me several tries to get repeatable measurements. Check this post that details some of my efforts and those who helped me through it.

What's Your Head Volume?

Reviewing your numbers and comparing with my final numbers, a couple of observations & questions:

1. CR is too high even for twin plug on street gas

2. Head volume is consistent with my twin plug heads that were machined to give 10.5:1 CR

3. What base gasket and deck ht do you have? Adding another base gasket of 0.25mm will drop CR by about 0.3 pts. You need to make sure that adding a base gasket does not take you deck ht up too high

4. I don't recognize the swept volume and dome volume in your numbers. I used the PCNA (Bruce Anderson) method. I used a small carpenters level in place of jo-blocks and the depth gage on my digital calipers to measure the "theoretical volume". The technique is demonstrated pretty clearly in the photo tour of Jerry Woods shop that Wayne has posted on this site. I can send you a spread sheet to make the calculation using this method if you like.
Tom Butler
1973 RSR Clone
1970 911E Restoration in Progress
Old 10-23-2005, 04:49 AM
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