Question about cranking compression, hot and cold etc

[ViennaHog]

Gents,
I have been chasing a problem on my personal 2008 FLHTCU and decided to go back to the basics and check the fundamentals of the engine. (original post is here: http://harleytechtalk.com/htt/index.php/topic,25479.msg260742.html#msg260742)

I checked the cold cranking compression and both cylinders read around 190 psi which is close to the calculated value of 200 psi. Then I did a hot CP with a thoroughly warmed up engine and I saw 175 psi on both cylinders. Conventional wisdom would tell you otherwise.

Anyway, did another CCP this time with an oil injection into the chamber to check on the condition of the cylinders. To my surprise the front jug read 225 psi and the rear 200 psi. Repeated it and verified the initial readings.

Couple of things:

• Is a jump of 10 psi between oiled and dry CCP normal?
  Why would the front go up to 225 (+35 psi)  which is way beyond the design CCP of 200?
  Why would the cold cranking compression be higher that the hot cranking comp?

I will do a leak-down tomorrow and find out about the valve conditions as well.

[axtell]

A intake lifter that has bled down shuts the valve early and that will raise the compression....If you shut down your engine and it stops with a intake fully open it can bleed down a lot....this makes your starter try to exit the chassis :-) it can also explain why the two cylinders would have a different test result....

[ViennaHog]

I understand the impact of lifter bleed down on CCP and it is definitely an option.
But why would two tests w/o oil done immediately before show about the same ccp in both cylinders and only after adding oil such a difference?
Why would the coll cranking comp be higher than a hot test?

Thanks

[Vosselman]

How about setting your hydr. lifters just a hair above bottom-out.
Almost like a solid. You will not ride the bike like this, it's only for a compression test.
This way you want have any bled down. And therefor same 'setup' between the tests.

[harleytoprock]

I've never been a fan of the oil test. Test is not accurate depending on how much oil is put in. Throw in an ounce of oil and the compression will go threw the roof. And the better the ring seal, the better the oil won't leak down, the higher compression. The shear volume of the oil raises the compression on any motor.

[rbabos]

I agree with the oil volume as well as improving ring seal causes an increase. It will on any engine. Hot test should be higher normal or oiled. I'd suspect a valve sealing problem when hot , but the best is a leakdown test will show what's going on. Aircraft engines are leak tested hot. I would do the same here,  considering your findings so far with the opposite readings you are getting.
Ron