compression ratio

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71Swingerman

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Just a question, the 78 30 I think had something like 72 cc heads. By using heads with 62 cc`s raise the ratio up from 8.4 to 9.4 or more? Possible a dumb question but I think that the only dumb question is the one you don`t ask. Thanks
 
Yes, it will drop your compression about 1.0-1.2 points.

However, I caution you on using the "book" numbers. Measure what you have and calculate from there.... I have found that the book numbers do not match up. I tested a set of 67 318 heads that are supposed to be 57 cc's and they came out to 63... Not as much improvement as I expected.

Here's a thread to help you calculate the compression on your parts:

http://www.forabodiesonly.com/mopar/showthread.php?t=278215
 
What he said. The 318 is really annoying when you want to up the comp. I have one with stock pistons @ .030 over. Used the Mr Gasket thin head gaskets, which if I recall are about half the thickness of the Felpros. Used closed chamber 273 heads which are supposed to be something like 57cc, they are not even after milling. After all of this according to my measurements I'm right in the 8.5 comp neighborhood. Would have liked at least 9, but it will be alright.
 
krazykuda said:
Yes, it will drop your compression about 1.0-1.2 points.

However, I caution you on using the "book" numbers. Measure what you have and calculate from there.... I have found that the book numbers do not match up. I tested a set of 67 318 heads that are supposed to be 57 cc's and they came out to 63... Not as much improvement as I expected.

Here's a thread to help you calculate the compression on your parts:

http://www.forabodiesonly.com/mopar/showthread.php?t=278215
Click to expand...

Those numbers are NHRA minimum numbers. In my experience it takes .040 milling the block side to achieve those numbers. 273 and J heads.
 
CultClassik said:
What he said. The 318 is really annoying when you want to up the comp. I have one with stock pistons @ .030 over. Used the Mr Gasket thin head gaskets, which if I recall are about half the thickness of the Felpros. Used closed chamber 273 heads which are supposed to be something like 57cc, they are not even after milling. After all of this according to my measurements I'm right in the 8.5 comp neighborhood. Would have liked at least 9, but it will be alright.
Click to expand...


It's easy to get a 318 into the 9's for compression.

Use 10.5 pistons and a 340/360 head will get you to 9.2 compression. That's what I did on my daily driver...
 
Ok, Thanks for the info. wasn't sure about how to get the numbers without ripping the heads off,
 
66fs said:
Those numbers are NHRA minimum numbers. In my experience it takes .040 milling the block side to achieve those numbers. 273 and J heads.
Click to expand...


Yes on my head lines we ran the block-to-head face "fat" in case we needed to do a clean up cut for any scratches or nicks that they may have gotten while handling them from tear down engines.

It also depends on how they maintain the head molds also to make the proper combustion chamber volumes...

My point was you have to measure the parts that you have to get accurate numbers. The book numbers do not "jive" with the spec's if you try to do the compression calculations from the specs in the service manuals....
 
I have formed the habit of adding 2-3 cc's over the 'stock' numbers that you find, based on some actual measurements and a lot of reading on what others have found. You also need to learn about pistons and the amount of chamber volume contained in the valve reliefs or dishes. For new pistons, you can get that off of most manufacturer's websites. And then you need to know the piston to block deck height; the stock 318's are 'down in the hole' by around .080" as an estimate.

With those factors, along with head gasket compressed thickness and standard bore/stroke info that you can find anywhere, you can use a calculator to get a good idea of what you will have. (It may take some time to make sure you are putting the numbers in right; some take + or minus for the same factor.) I like this one:

http://cochise.uia.net/pkelley2/DynamicCR.html
 
I mic'd a '79-360 and crunches the numbers to 7.8-1.
Same goes for my '78-400.
 
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