07 ultra presently at 96" with SE 4" slip on's and stage 1 intake. Spend all my riding time in the 2500 - 3000 RPM range. Can't remember ever going above 3500 RPM's. I have a DIY stage II 103 ready to start. Kit comes with 255 CAM's. Also have SE comp install pending.

Question is about compression ratio's. My understanding is SE255 CAMs work best with a 10.0:1 compression ratio.

I Sent a second set of low mile take offs heads out for MCR and got them back. I noticed that the MCR holes are pretty deep when the MCR's are in the engine run position and wonder if the CC's of volume added as a result of the MCR's will lower my compression. With out MCR's my stock heads with 103 flat tops are theoretically supposed to produce 10.0:1 compression. However , whenever I read about compression there's always a multitude of reasons why theoretically compression never happens , especially with folks that have ACR's or MCR's put in and it's always some number less, and folks wind up using thinner gaskets, machining, or both.

So if I assume that the theoretically compression of 10.0:1 will not happen with my combo of parts, and I don't mill heads and don't use thinner gaskets.....what about a different piston. I also have a set of SE 103 forged pistons with a theoretically compression of 10.5:1. If we assume, that the theoretically compression of 10.5:1 won't happen either , but keep the goal of trying to achieve 10.0:1 I'm thinking considering the added volume of the MCR's, and wonder if a slightly thicker head gasket (opposite direction from the folks who are trying to add 0.25 more compression) I can probably wind up very close to the targeted 10.0:1 goal. AND what if I miss it, and wind up at 10.25:1, whats the consequence...will it hurt my low RPM range (doubtful) or is it a limitation for max power at higher RPM's (likely)?

I know what to do once I assemble and compression test and figure out my actual compression ratio...but I want to try to get close as possible before I assemble. I guess I am trying to understand what the tolerance should be 10.0:1 plus or minus what .....0.25, 0.125, etc. If I get 10.25:1 I would probally run with it.

I had posted this question in the touring forum where I spend most of my time...lots of great folks there...but I wound up getting referrals on who does good engine work......testimonials on 3rd party parts combo's which folks liked that didn't involve my SE parts ......but no real answer to my question. So lets see what the engine related section has to say.

Thanks for any input.

 

Too many of you guys get hung up on static CR which is good to know but should not control chamber volme or cam selection; corrected CR and cranking CR control. Your static CR right now, Stage I 96" is about 9.2; just increasing bore from 3.750" to 3.875" will increase static CR to about 9.7 with flat top pistons and stock head gasket. Use a .030" head gasket and static increases to about 10:1.

"Safe" targets for corrected and cranking CR for well mannered, tuneable street build are 9.3 and 190psi, IMHO. Some will say 9.6 and 200psi and I might do that for one of my own bikes but not recommend that to a friend.

The SE255 intake valve closes at 25* ABDC; the intake valve timing is what determines corrected and cranking CR. You bolt in a set of SE255s, right now, without doing anything else, and your corrected and cranking CRs will be about 8.9 and 183 and, with a good tune, the bike will run much better, particularly in the rpm range you like. Change cams and intake valve close timing and your static CR won't change but corrected and cranking CR will.

Increase bore size to 3.875" fpr 103", install a .030" head gasket, flat top pistons, stock head chamaber and the SE255s and your static CR will be about 10:1 as pointed out above but, guess what, corrected and cranking CRs increase to 9.7 and 204psi; IMHO, OTT for a street build and asking for trouble.

As a matter of fact, if you do upgrade to 103" and run SE255s, you will have to increase chamber volume to get the corrected and cranking CRS within manageable limits. Or select a cam with a later intake close.

BTW, I don't know anyone that allows for lost chamber volume to the tiny hole for the compression releases.

 

your points well taking......also thinking along the same lines as para 5.

I measured the volume of the internal chamber of the MCR when open and got 3 cc's. Some MCR's have 4 cc's internal volume.the 3 cc increase changed compression from 10.14:1 to 9.85:1 according to the bi boz calculator.
SO in light of the 3 cc increase to the chamber volume and your comments in para 5 about increaseing chamber volume to get corrected and cranking CRS, am I good, or do I need to go with .060 head gasket?