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2019 Heritage
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Discussion Starter #1
As I continue getting ready for my M8 114 to 131 upgrade I have realized that I might have too much of a compression ratio. The 131 Kit I want to use contains SE-517 cam which I want to replace with S&S 465 cam. SE Heads with the SE-517 cam have compression ratio of 11:1. SE-517 closes the intake valve at 40 deg After Bottom Dead Center. The S&S 465 closes the intake valve at 15 deg ABDC. That will, according to my calculation, increase the compression ratio to 12:1. Am I right or wrong here? Would 12:1 be too much? I am trying to build as much of a low end torque as possible. To bring the compression ratio back to 11:1 I would need a Head gasket which is 0.041" thicker than the original gasket. Any other remedies, suggestions?
 

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Your compression ratio doesn't change when you change the camshaft. Compression ratio is the ratio of the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel. Closing the intake valve sooner or later changes the cranking compression (cylinder pressure). There is a "corrected compression ratio" that is used by some that helps explain the effect of changing the intake closing and I assume that is what you mean.

Using the numbers you provide, I'd say that the cranking compression would increase to a dangerous level (probably in the 230 to 235 psi range) when changing the intake closing point from 40 to 15 degrees. To be honest, I haven't worked on a M8 engine. I don't know what the piston design/combustion chamber looks like. In engines that I have worked with, using thicker head gaskets to reduce compression ratio is a double edge sword. You drop the compression ratio but you reduce quench. Not an effective move.

I think tall terry was responding to your post in another thread when he said,

just a fyi,,, large somewhat high compression engines,, like the 131, have a response to small short overlap cams,,, they build so much cranking compression the smoke batteries and starters religiously,, the longer over lap is needed to bleed of some of the compression..
also, are prone to detonation/preignition once running,,, check the numbers before commiting to a problem

best of luck to you
 

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You need more cam w that displacement. At minimum the S&S 475 or one just above that. That big engine will tame down a camshaft and have a butt load of torque. The higher duration will help reduce cranking compression and keep the starter happy. Still need compression releases however...

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Discussion Starter #4
Thank you guys for your excellent replies. Yes, I did mixed up the engine compression ratio (which does not change) with the "corrected" CR. However, I will have to do more homework on the cams. The 131 kit comes with SE-517 cam which closes the exhaust valve 7 deg past TDC and opens the intake valve 7 deg before TDC, hence the total overlap is only 14 deg. And Harley, in its "head installation instruction", has a procedure which requires to check valve to piston clearance at TDC. SS 475 or even SS 540 have much larger overlap so what worries me is that the valves could hit the piston at TDC.
Maybe I should get custom cam. Is there anyone out there who grinds custom cams? Or maybe I am over thinking this whole thing. Any input is appreciated.
 

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Leave it alone, it is not broken. If you want it to run use a better pipe and an open tune. What are you shooting for to shunt the cam timing, certainly not torque. That 131 will perform well and be a lot of fun.
 

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i believe you are about to make a mistake ... i expect to see your ccp get in the 250psi range with your combination... post it when it has been fired up and ridden a couple hundred miles,, im just guessing,, i didnt bother to do the math
 
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