I decided to do a compression test to try and get a better idea of what my compression ratio might be. When I bought the bike used with 650 miles on it I was told the compression was 10.1. I have poor off idle response and a guy told me that my cams want at least 10.5 to 1 comp. The motor was professionally built etc etc and runs very strong. I just hate the off idle throttle lag.

My tester said 182 front and rear cylinder. I cranked it 3 times with throttle wide open.

I talked to an engine builder from this forum and he said I should be 10.5 to 1 with 182 on the compression tester.

Any thoughts?

 

Don't know how accurate this is, but it sure is fun to crunch the numbers.

http://www.bigboyzcycles.com/TwinCamComp.htm

 

I wouldn't be concerned about the static number, I'd be figuring how to get at least a 15-20 pound increase in cranking pressure. A 10.5 piston with all else remaining the same will probably get you 10 pounds. I am assuming a sound motor.

 

qtrracer; I did call the shop you advised me to call and the guy there said with my cams 180 cranking compression was at least 10.5 to 1 compression. I would think the motor is sound, no signs of anything outside of normal. I'm sure there's a lot of factors that go in to this but I'd thought I'd ask anyway.

My bike run's hard, no complaints other than off idle throttle response.

I sent my Map over to (Zippers) Tmax and they worked it over for me. It's much better on the low end so maybe some more tuning will make me happy.

I will say (Zippers) Tmax has been great to work with.

 

If tuning helped thats great. I can only speak to my experience and that has been, 175-180ccp kinda doggy on the bottom and once you hit 3k, or a bit higher depending on displacement, she's off to the races. You get it up to 195 or above, tuned correctly, things get fun earlier. There are caveats to this ccp though as you probably know. To be clear I have no idea how that motor was built and with what components. For example, what pistons are in it? You were told 10-1 but what head cc was used in the calculation to get that cr? What do the heads used on your motor cc out at? What is the piston to cylinder deck height? Head gasket thickness? If you are happy enough with it you can wait until you have a real reason to tear it apart and when you do all these things can and should be checked out so before you do put it back together you can decide what ccp you desire and choose the right parts and get the head cc's where you want and you will know within a couple of pounds what it will crank at before hand. This is where getting with a good shop is invaluable.

 

Make sure you also factor in altitude.

 

I forgot to say that all my comments are assuming you would not be changing cams.

 

My motor has stock heads (Ported) Stock pistons; The heads were decked and I have no idea what head gasket was used?? Thats kinda why I'm going off cranking compression.

Tuning did help my throttle response off idle but it could be better, and you're right after 3000 rpm my motor pull's like a freight train in all gears.

Sorry I can't give much more info on the motor but I didn't build it and I've talked to the Harley shop that did the build but only talked to the service manager not the guy who did all the work. This mechanic used to own his own race shop, this is his formula that he likes for the 110's.