Compression Ratio
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Intermediate
Joined: Jul 2006
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Compression Ratio
Can someone please give me an approximate CR on my 95 Build?I'm trying to match the cams for my build. My choices at this pointare Andrews 31, 37 and the HQ 0034. I have the SE Heads(Not ported or polish), HSR 42, 95 BB cylinder w/SE Flat pistons.I have no intention bumping the CR further to avoid any pinging. Replies appreciated.Thanks.
Road Warrior
Joined: Dec 2006
Posts: 1,530
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From: Diamond Bar, California
RE: Compression Ratio
So is this thing put together already? you could to a compresion test and that would give you an aproximate compresion.
or you can go to big boys cylces and use there compresion calculator, you will need a head gasket thickness and cylinder head CCs to get it acurate.
it could be as low as 9.4 or as high as 10 to 1! If your carburated I would get a HQ ignition with one of Dougs timing curves and you could up to 10.5 with out problems on 91 oct. you could also run many other cam grinds too.
or you can go to big boys cylces and use there compresion calculator, you will need a head gasket thickness and cylinder head CCs to get it acurate.
it could be as low as 9.4 or as high as 10 to 1! If your carburated I would get a HQ ignition with one of Dougs timing curves and you could up to 10.5 with out problems on 91 oct. you could also run many other cam grinds too.
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Intermediate
Joined: Jul 2006
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RE: Compression Ratio
I have all the parts now with the exception of the cams and gaskets. I have been doing research and I am not sure whether my build will retain the CR of 10.1 as indicated on the SE catalog using flat top pistons with a stock SE performance heads. Some have indicated that it will be lower than that. I was hoping that someone with the same build can share what their CR ended uponce it is installed so that I can match the cams. Andrew 31 requires a CR of 9.8 to 10.2 while the 37 will be recomendable at CR lower than 9.8. I have the SE adjustable ignition installed. Thanks
Tourer
Joined: May 2005
Posts: 473
Likes: 8
From: Chelsea, AL
RE: Compression Ratio
As it was stated before, go to http://www.bigboyzcycle.com. Click on "Calculators" then "Compression Calculator" and just enter the pertinent info. It will show you the compression ratio, and you can then compare cams to give you a corrected compression ratio for each cam setup. Most of the info you need is already there for you to choose. The head gasket thickness would be a great help to know and any special work done to the heads or pistons. You'll be able to get pretty much dead on though with the info they supply.
Tourer
Joined: May 2005
Posts: 473
Likes: 8
From: Chelsea, AL
RE: Compression Ratio
Well using the limited info that you gave, I went to BigBoyz and plugged in this info into their compression calculator. Using the .30 head gasket thickness, I got a 9.36 static compressio cold at 194 lb. of compression. The problem here is that this figure is for a 0 degree cam closing on intake. The actual compression will be determined by the cam you choose and where it closes the intake valve. The lower the number, the higher the actual running compression (corrected compression) will be. The higher the lift and the longer the duration of the cam will serve only to aid top end performance.
Go to BigBoyz site and use the compression calculator for yourself and see how this is shown. Plug in the cam your thinking about using and see how it affects the compression ratio. With the stock cams it's around 8.6 CCC with 168 lb. of compression.
If your still not sure about your choices, call the cam manufacturer and ask them if they're talking about static or corrected compression in the description they give in their advertising. I'm sure it will be the corrected compression. I had to think long and hard on my choice of cams, because I have high compression pistons I was concerned about raising my compression and the affect it would have on drivability. I settled on the Andrews TW55's and they seem to be a very good choice.
I hope this helps you! But be prepared, cause if your not careful you'll go crazy trying to figure out the right choice.
Good Luck!
Go to BigBoyz site and use the compression calculator for yourself and see how this is shown. Plug in the cam your thinking about using and see how it affects the compression ratio. With the stock cams it's around 8.6 CCC with 168 lb. of compression.
If your still not sure about your choices, call the cam manufacturer and ask them if they're talking about static or corrected compression in the description they give in their advertising. I'm sure it will be the corrected compression. I had to think long and hard on my choice of cams, because I have high compression pistons I was concerned about raising my compression and the affect it would have on drivability. I settled on the Andrews TW55's and they seem to be a very good choice.
I hope this helps you! But be prepared, cause if your not careful you'll go crazy trying to figure out the right choice.
Good Luck!
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Intermediate
Joined: Jul 2006
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RE: Compression Ratio
Thanks , that calculator is very helpful. Still playing with the numbers. The correct site is http://www.bigboyzheadporting.com/TwinCamComp.htm
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Novice
Joined: Jun 2007
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From:
RE: Compression Ratio
Have you figured out your cams yet? What are the part number for your heads because the new S/E heads will give you 10.5 to 10.8-1 static compression with a combustion chamber size of 76cc.
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