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post #1 of 9 (permalink) Old 12-09-2017, 12:18 PM Thread Starter
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calculating compression?

Is there a way to tell what your compression actually is? I mean, 10:1, 9:1. etc. If I had a cylinder compression tester, which only shows pounds, would there be a way to convert that into a ratio?
Not sure if I'm wording this correctly. I read so much about it. I wanted to figure what mine was.
I have a 08 rg 103, w/ 225 cams se ac, d&d 2-1. Any help would be appreciated.
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post #2 of 9 (permalink) Old 12-09-2017, 12:25 PM
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great write-up about static vs. dynamic pressure...

Static Vs Dynamic Compression Ratio | Piston Ratio

long story short you have to cc the heads if any volume changes from stock

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post #3 of 9 (permalink) Old 12-09-2017, 02:20 PM
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There are a couple calculators out there that will assist you. Big Boys being one of them, but without knowing your deck heights and actually cc of your heads and pistons it would only get you in the ballpark
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post #4 of 9 (permalink) Old 12-13-2017, 10:46 PM
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http://www.bigboyzcycles.com/TwinCamComp.htm

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post #5 of 9 (permalink) Old 12-15-2017, 09:34 PM
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So say I have a 2016 Road Glide Ultra wet head bike. Stock compression is 10-1. I add 10.5 -1 compression pistons to it with a se 585 cam. Leave heads stock. Would the compression be higher than 10.5 since it is already 10-1 stock ?


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post #6 of 9 (permalink) Old 12-15-2017, 11:58 PM
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Quote:
Originally Posted by freedom1974 View Post
So say I have a 2016 Road Glide Ultra wet head bike. Stock compression is 10-1. I add 10.5 -1 compression pistons to it with a se 585 cam. Leave heads stock. Would the compression be higher than 10.5 since it is already 10-1 stock ?
one of the benefits of changing the cam(s) is that it can lower the dynamic compression.
intake and exhaust valve timing determine the dynamic compression of an engine.
is there a reason you're concerned about compression?

you need to have a sit-down with whomever is selling/installing the cam/piston/ and discuss the characteristics of each.
they should be able to provide you with dyno charts that show when the power comes on.
plan everything according to your everyday riding style and what you want to accomplish before you start your build.
i.e. a longer stroke will typically give you more low-end torque, which is more useful for cruising.
a cam with a lot of overlap will sound great but will move your torque higher in the rev range.

throwing parts into an engine willy-nilly is an expensive experiment.

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Last edited by Little Wing; 12-16-2017 at 12:04 AM. Reason: forgot dyno charts
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post #7 of 9 (permalink) Old 12-16-2017, 12:05 AM
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So adding a cam to a stock bike will lower compression in some cases and make the torque suffer on the bottom end ? I tour a lot and am not wanting a bike with so much compression that it pining in Florida in August when it’s hotter than hell. That is what I am trying to figure out. The tech told me it will be fine. Just looking for others opinions.


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post #8 of 9 (permalink) Old 12-16-2017, 12:14 AM
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Quote:
Originally Posted by freedom1974 View Post
So adding a cam to a stock bike will lower compression in some cases and make the torque suffer on the bottom end ?
that's not what i said. please read the post again.

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post #9 of 9 (permalink) Old 12-16-2017, 01:19 PM
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Cam timing can either add or subtract from your compression, depending on the design of the cam. If you will enter the values from your bike into the calculator that Desert posted above then play with the different cams that are programed in the calculator you can see what cam timing can do to your compression
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