Head Flow Effect On Compression

[WhipLash96]

If one head flows a different number than the other, will you see that in a compression test?

[hrdtail78]

I doubt it.  I don't believe the air has enough velocity at 500 rpms w/ a wot to take advantage of it or to indicate it with what we have.  A 2-3 psi difference on a compression check is mostly ignored.  You would have to eliminate some variables such as batt voltage, cam to engine timing, friction....

[aswracing]

hrdtail's answer is correct.

I would also add that head flow and cylinder fill are two different things. They may go in opposite directions, particularly in a low rpm situation like cranking.

[WhipLash96]

Ok. I'm just asking for process of elimination reasons. I will be doing a leak down test to see what's going on soon. The heads were hand ported and are not equal to each other. Rear cylinder is 20 psi lower than front. Not what I want to see.

[kouack]

20 psi difference on a fresh rebuilt? Is this a twincam by any chance? one cam could be one teeth off?

[WhipLash96]

20 psi difference on a fresh rebuilt? Is this a twincam by any chance? one cam could be one teeth off?

About 4800 on the engine build. Yes it's a Twin Cam

[kouack]

Did you check the compression during the buildup? Prior the first start?

[FXDBI]

Did you check the compression during the buildup? Prior the first start?

Took the words right out of my mouth. Second question did you try a wet test after?  What were the results? 
Bob

[C-Cat]

Are you using a battery charger when doing the compression test?

[WhipLash96]

Are you using a battery charger when doing the compression test?

When the compression difference was discovered, the bike was at the shop getting converted to EFI so I didn't personally do it. I can only assume that it was on the charger. I can and will do one myself to verify.

[WhipLash96]

Did you check the compression during the buildup? Prior the first start?

IIRC, I don't think so. Dummy me.

[98fxstc]

I just did a top end refresh and thought about doing a compression test prior to startup.
I decided No, the rings aren't seated, it won't really mean anything, but I see now that it may have been worthwhile to give an an early indicator if there was a problem.

[Hossamania]

Maybe not at first turnover on a new build, but shortly after break in has occured.

[kouack]

I just did a top end refresh and thought about doing a compression test prior to startup.
I decided No, the rings aren't seated, it won't really mean anything, but I see now that it may have been worthwhile to give an an early indicator if there was a problem.

You are 100% right the rings are no seated and will not give you a perfect picture of the engine. For me, when I built my 117 from slip cases to rocker box covers, this was more a quick confirmation everything was put together properly. Also to give me a good feeling in my pants before hitting the start button for real. If I would have found a 20 psi difference between my two cylinders, a CCP +/- 15 psi from expected or a high rate leak down, I would have probably called a stop drop right there and recheck my stuff.

[98fxstc]

A preliminary test could be carried out before complete re-assembly ?
Without fitting of the exhaust, the intake and the fuel tank and instruments, a mechanical compression test could be carried out with a remote starter button on the starter motor and a reasonable measure of ccp obtained ?

[FXDBI]

A preliminary test could be carried out before complete re-assembly.
Without fitting of the exhaust, the intake and the fuel tank and instruments, a mechanical compression test could be carried out with a remote starter button on the starter motor and a reasonable measure of ccp obtained?

Yes that would let you know before all the other pieces are on. I think I will try that next time actually. I normally do one before initial startup after I have turned it over without plugs and fuel pump disconnected and got the oil light to go out. Then again after the first oil dump in 50 miles. Every oil change I do a check after that. Its the old millwright in me PM checking. Cheaper to fix when you find things early.  Bob

[Hossamania]

You do a compression test every oil change, even after the first couple changes, which I can understand, but even years down the line? I don't want to judge, but that's a little OCD .....
 

[FXDBI]

You do a compression test every oil change, even after the first couple changes, which I can understand, but even years down the line? I don't want to judge, but that's a little OCD .....
 

Well in 50 years of fixing machinery being I have learned it easier to keep it up than fix it up. It dont cost me to check has I do all my own work. I push it to its limits every time I ride it being ocd keeps me warm and fuzzy hitting the redline.  Bob

[Hossamania]

You do a compression test every oil change, even after the first couple changes, which I can understand, but even years down the line? I don't want to judge, but that's a little OCD .....
 

Well in 50 years of fixing machinery being I have learned it easier to keep it up than fix it up. It dont cost me to check has I do all my own work. I push it to its limits every time I ride it being ocd keeps me warm and fuzzy hitting the redline.  Bob

Works for me!

[Dan89flstc]

If an engine is performing well, and not consuming oil, there is no reason to perform routine compression tests (unless it is an airplane engine ). 

[FXDBI]

If an engine is performing well, and not consuming oil, there is no reason to perform routine compression tests (unless it is an airplane engine ). 

Because I want to and can is enough reason for me. I see no downside to it and it cost nothing.  Bob