Posted By: Mopar493
Compression test - 08/04/22 09:55 AM
Hi, i did a comp test on my pretty new built 499cui 10.95:1 and 7,49:1 dynamic. See added camcard. I get 210 psi on the 4 pulse, it seems high for my set up with pretty large camshaft, hydraulic roller. Can it be that the bleed down from hydraulic lifters give a false to high reading? I can not hear any pinging or see any sign on spark plug or piston top with 23 initial 33 full at 3500 rpm and 6 degrees vacuum advance. Pro flo 4 controlled ignition.
Posted By: jwb123
Re: Compression test - 08/04/22 12:47 PM
bleed down on lifters will reduce compression test. I use performance trend engine analyzer software, it will calculate cranking compression, generate an advance curve, tell you if detonation is an issue with different octane fuels, idle vacuum, besides HP and torque. I never build an engine without running it through the program. But like any program you have to take the time to put good numbers in it. Things like really knowing what your static compression is, intake and header lengths, flow of your heads, etc.
The higher the cranking compression the more power an engine makes, unless detonation becomes an issue.
Posted By: Mopar493
Re: Compression test - 08/04/22 12:53 PM
bleed down on lifters will reduce compression test. I use performance trend engine analyzer software, it will calculate cranking compression, generate an advance curve, tell you if detonation is an issue with different octane fuels, idle vacuum, besides HP and torque. I never build an engine without running it through the program. But like any program you have to take the time to put good numbers in it. Things like really knowing what your static compression is, intake and header lengths, flow of your heads, etc.
The higher the cranking compression the more power an engine makes, unless detonation becomes an issue.
Thanks for answer! I do know the values and the cam is a custome grind that i let a company with known reputation Ppm racing here in Sweden count for me to my combination. Static compression is 10.95:1and dynamic is 7,49:1 with this cam. Thats why i be surprised by the high cranking value i get.
Posted By: HardcoreB
Re: Compression test - 08/04/22 12:56 PM
If there's no problem knocking then it is what it is. I use compression MAINLY to gage overall ring and valveseal etc and only get concerned if there's a major disparity from cylinder to cylinder. Or if it's much different than the day it was built.
With the bleed rates that most aftermarket HR lifters have, combined with the elevated spring loads most of the associated cams require....... I’d expect the lifters are substantially bled down, which shortens the cam timing during the cranking pressure test, which gives a higher number than what the true dynamic CR would be with the motor running and the lifters pumped up.
It’s like an extreme example of the principle behind Rhodes Lifters.
If one was curious what the difference might be, they could bottom the lifter plungers out on one cylinder, and see how that impacted the cranking pressure results.
Posted By: Mopar493
Re: Compression test - 08/04/22 07:55 PM
With the bleed rates that most aftermarket HR lifters have, combined with the elevated spring loads most of the associated cams require....... I’d expect the lifters are substantially bled down, which shortens the cam timing during the cranking pressure test, which gives a higher number than what the true dynamic CR would be with the motor running and the lifters pumped up.
It’s like an extreme example of the principle behind Rhodes Lifters.
If one was curious what the difference might be, they could bottom the lifter plungers out on one cylinder, and see how that impacted the cranking pressure results.
Thanks for answer, i just be surprised over the pretty high value i get and have zero issues with pinging or such things.
