Re: Compression test
[Re: jwb123]
#3065085
08/04/22 08:53 AM
08/04/22 08:53 AM
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Joined: Dec 2019
Posts: 138 Sweden
Mopar493
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Posts: 138
Sweden
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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.
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Re: Compression test
[Re: HardcoreB]
#3065163
08/04/22 12:49 PM
08/04/22 12:49 PM
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Joined: Jan 2003
Posts: 13,357 Marion, South Carolina [><]
an8sec70cuda
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I Live Here
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Posts: 13,357
Marion, South Carolina [><]
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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.
CHIP '70 hemicuda, 575" Hemi, 727, Dana 60 '69 road runner, 440-6, 18 spline 4 speed, Dana 60 '71 Demon, 340, low gear 904, 8.75 '73 Chrysler New Yorker, 440, 727, 8.75 '90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt '06 GMC 2500HD LBZ Duramax
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Re: Compression test
[Re: an8sec70cuda]
#3065168
08/04/22 01:19 PM
08/04/22 01:19 PM
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Joined: Jan 2003
Posts: 14,492 So. Burlington, Vt.
fast68plymouth
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I Live Here
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So. Burlington, Vt.
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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.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Compression test
[Re: fast68plymouth]
#3065237
08/04/22 03:55 PM
08/04/22 03:55 PM
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Joined: Dec 2019
Posts: 138 Sweden
Mopar493
OP
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OP
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Joined: Dec 2019
Posts: 138
Sweden
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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.
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