My build should show 203 psi. What I'm actually getting is 187 on the front and 195 on the rear.
Is the a +/- for psi or is there something going on here?

ACRs were disconnected, both plugs removed and fuel line disconnected. I ran tests with the tb open and closed with not much difference in results.

 

Assume Big Boys calculator or something similar was used to calculate CCP? Test done cold or hot? Are you sure you have accurate actual and measured inputs, i.e., chamber volume, piston dome volume, deck height, etc.? It is not unusual for calculated CCP and actual to vary and the same applies to each cylinder. If chambers and deck height vary between cylinders, so will the CCP.

If the performance is satisfactory, leave it alone.

 

I used BB calc based on the info I got back from the guy who ported my heads and bored my cylinders. It was checked cold.

When my bike was on the dyno a week ago it started knocking (not audible) at 4200 rpms and drastically pulling timing as rpms went up. Once I got back home I checked cold compression compared to what it "should" be.

It goes back on the dyno Tuesday. I'll bring a couple different sets of plugs to try out as it may just be a ghost knock.

 

Assume you have corrected for elevation? I would be more concerned about the variance between the two cylinders than the less than expected CCP. Suggest a compression test but leave the plug in the cylinder not being tested, make sure the battery is fully charged or connect to a battery charger that has the START option where it is putting out 50A, warm the motor up before testing close to operating temp, throttle wide open and re-test. A battery that is not fully charged or even a fully charge battery that is old and fades under load can produce lower than actual compression; very important to have a hot and fully charged battery when checking compression. Retest and post results. A leak down test may be the next step.

I don't think colder plugs will make the knock go away. Don't know your build configuration but pulling timing out as you describe would indicate high compression which has not been confirmed by your testing. Don't know what gauge you are using but a cheap or faulty gauge is also often the problem in this situation.

 

You know I always thought it was "cold" cranking pressure ...as in do it when the engine is cold.

I just checked my manual and it says to check it when it is warmed up as well ...thanks DJL

 

Also have your dyno guy do a ccp and see what he gets.... Also it's good to know all the specifics of the build so that we can truly come up with a better educated solution....

 

Lots of different view on the subject of checking CCP. I was taught that CCP stands for Cylinder Cranking Pressure; cold be hot or cold. I check a engine cold then hot and if I see more than 5-10psi difference (depends on cast vs forged pistons) a leak down test would be the next step. So there is value in checking both hot and cold. A cold check is pretty much the standard for comparison but cold lifters are bled off and operating tolerances have not been reached which will affect the reading.

 

Leak down shows leaking past the rings on both cylinders.
I pulled the pushrods for the leak down test. I could hear air coming from the lifter box covers. If I placed my hand over it the sound got quieter.

Stock heads ported, polished and valve job, CP pistons and stock cylinders bored to 107. Tman 577 cams with new SE pushrods and S&S standard lifters. Compression should be at 10.8 or 202 psi (cold) taking into account 100' above sea level.

 

What was the leak down percentage?

 

25 rear and 30 front.
Absolutely no signs of knocking on pistons or heads.
Excessive oil though in the exhaust port.

So getting oil past the rings isn't an issue?
Go troll elsewhere please.