Calculating Comp. ratio??
 

I am currently dying to dust off my truck and take it out of storage. I have been planning doing a couple more preformance mods.
Other than adding a MSD ignition box with boost control, I am looking to increase my boost pressure, from a 6 lb pulley to at least an 8 lb. I have been running the calculations several times on many different web sites. I keep getting ultra low static compression ratio of only 7.0:1 or so.

Here are some numbers to crunch

431 cubic inch

4.28 bore
3.75 stroke
.040 head gasket
36.1 cc dome pistons
124cc combustion chamber

Trw 226 pistons
Open chamber oval port from 72' passenger
My guess was about 8.0:1 but after running the numbers looks like I have lots of room for more BOOST.
70rs/ss some feed back would be great.
thanks

Re: Calculating Comp. ratio??
 

What is your deck height? With the info you've given, I come up with anywhere between 9.5:1 and 10:1 depending on what your actual deck height is.......

Re: Calculating Comp. ratio??
 

http://www.mopar1.us/compratio.html
I ran my numbers agian in this calculator and still ended up with a 7.69:1 ratio.
I ran the deck heigth at 0.
I believe the deck height can only very a couple thousands of an inch.
If my compression ratio really was 9.5 or 10.:1 I would already had detenation while running 6 lbs pulley.
Compression ratio would have easily exceeded 12:1..

Re: Calculating Comp. ratio??
 

My CR calculator says 10.07 with zero deck, it assumes the head gasket is the same bore as the bore so it is a hair high, but it would still be like 9.9 or so.

Re: Calculating Comp. ratio??
 

I think I see what you're doing. You stated above that your pistons have a 36cc dome. In the calculator you listed, you need to use -36 for a piston volume instead of 36 (which would be a 36cc dish).

Your deck height is a crucial measurement in figuring compression ratio. Stock deck height is ~.025" below deck. The difference between stock and zero deck is over a half point of compression.

Re: Calculating Comp. ratio??
 

I see where you are getting the 10:1.
Your using the volume of -36.1 for piston dome
I was using a .226 for dome shape
http://cgi.ebay.com/ebaymotors/Genui...QQcmdZViewItem

But even if you look at the example in the link above the seller uses an example of the same pistons with 124cc closed chamber heads and he claims only 9.25:1
If you use
these numbers

10.0:1 for compression
6psi boost
0 altitude sea level
In the link found bellow I would currently have a dynamic compression ratio of 14:1.
The numbers just don't match or balance with how the engine runs.

http://rbracing-rsr.com/compression.htm

Re: Calculating Comp. ratio??
 

Those claimed compression ratios in the ebay auction are just flat wrong. Compression ratio with those pistons, .029" gasket and .008" deck height (figures they give in auction) is 10.17:1 no matter how you figure it.

I'm not well versed in forced induction (I'm a nitrous junkie) so I can't accurately comment on what your dynamic compression really is under boost or how much you can get away with on pump gas.........but if the specs you're giving us are accurate, your static compression is absolutely landing between 9.5-10:1 (deck height unknown).