how to figure Compression ratio
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Tourer
Joined: May 2015
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From: Minnesota
how to figure Compression ratio
I am trying to figure out the compression ratio based on components selected.
The plan is a 107" with S&S cylinders, S&S CP pistons, S&S 89cc heads. I don't know which gasket yet because I need to find the compression ration for both .030 and .045
Is there a site that I can input this info and find where I would be at?
Thanks.
The plan is a 107" with S&S cylinders, S&S CP pistons, S&S 89cc heads. I don't know which gasket yet because I need to find the compression ration for both .030 and .045
Is there a site that I can input this info and find where I would be at?
Thanks.
Road Master
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From: Hill Country, Tx
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Tourer
Joined: May 2015
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From: Minnesota
I understand that many shops can flow a set of heads better than the S&S ones, but if I have to have them decked I plan to have these worked also.
Thanks
HDF Community Team
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From: san antonio
Cylinders and pistons purchased off the shelf are manufactured to +/- tolerances; they are not a matched set. If one happens to purchase a set of cylnders that is near the + side of the tolerance, i.e. max bore diameter and a set of pistons that is near the - side of the tolerance, i.e. minimum bore diameter, piston to wall clearance may be out of spec. Not to mention checking the bore for trueness. Always better to have cylinders, used or new, bored/honed to fit pistons which may require ordering .010" or .020" O/S pistons. Only way to guarantee precise piston to cylinder fitment and a true cylinder bore.
Thread Starter
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Tourer
Joined: May 2015
Posts: 474
Likes: 34
From: Minnesota
Cylinders and pistons purchased off the shelf are manufactured to +/- tolerances; they are not a matched set. If one happens to purchase a set of cylnders that is near the + side of the tolerance, i.e. max bore diameter and a set of pistons that is near the - side of the tolerance, i.e. minimum bore diameter, piston to wall clearance may be out of spec. Not to mention checking the bore for trueness. Always better to have cylinders, used or new, bored/honed to fit pistons which may require ordering .010" or .020" O/S pistons. Only way to guarantee precise piston to cylinder fitment and a true cylinder bore.
Last edited by pdevore; Nov 27, 2016 at 06:28 PM.
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I am trying to figure out the compression ratio based on components selected.
The plan is a 107" with S&S cylinders, S&S CP pistons, S&S 89cc heads. I don't know which gasket yet because I need to find the compression ration for both .030 and .045
Is there a site that I can input this info and find where I would be at?
Thanks.
The plan is a 107" with S&S cylinders, S&S CP pistons, S&S 89cc heads. I don't know which gasket yet because I need to find the compression ration for both .030 and .045
Is there a site that I can input this info and find where I would be at?
Thanks.
Scott
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From: deland,florida
agree with scott,checking staic compression is easy,but cams must be brought into the equation for corrected compression,this is the figure your looking for (& don't go by advertised compression of you pistons,always cc the heads and find out the true dome cc of pistons being used)
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Joined: May 2009
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From: Northern VA
You'll also find if you go piston shopping you can get the exact compression ratio you want for the cams you select. Those 89cc heads are pretty darn big for a 107 you'll probably find you want something down in the 83-85cc range once you start playing around with the calculator. Most pistons will put you 0.005-0.008" in the hole but you won't know for sure unless you measure.
As someone who went down the S&S off the shelf path and is now having issues if I were to do it again I'd use my stock cylinders and buy the pistons I want. It's more work finding shops you trust but less money.
Good luck with your decisions if this is your first time doing this it can be overwhelming but there's a lot of info out there. As was mentioned above you really need to find a cam you like first, or a range of cams that are similar, and what compression they like to be run at and go from there. The cam dictates what compression you should be running and consequently which pistons, which head gasket, and what kind of headwork.
Last edited by mattVA; Dec 1, 2016 at 08:04 AM.
HDF Community Team
Joined: Jun 2006
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From: san antonio
That is not clear in your OP. Any off the shelf cylinder should be checked for a true bore. If using off the shelf new cylinders, I make it a practice to order .010" or .020" OS pistons so even if the new cylinder bore is not true, it will be after boring. Additionally, I can set piston to cylinder wall clearance where I want rather than leave it to chance. Of course, if "seasoned" cylinders can be bored to the required spec to fit new pistons, why spend the $$ for new cylinders? Not so much an issue of one being better than the other to me but more an issue of cost.