dynamic compression question
Thread Starter
Racer
Joined: Jan 2004
Posts: 398
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From: Eastern Shore Maryland
dynamic compression question
When looking at open/closing degrees, GM cams are listed @.006 and Comp are listed @.015. What is correct way to compare and get accurate numbers for dynamic compression?
Racer
Joined: Oct 2012
Posts: 281
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Race Director
Joined: Oct 2000
Posts: 15,017
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From: central FL, near the beach.
Seat to seat. Or lash point.
Engine won't even run if the valves are open at all, try it sometime!
Can NOT build compression if valve is open! even .001"
Very hard to get these seat to seat #'s from a cam co. Easy to do in engine. But then, u can just crank it over and read psi on a gauge.
Dynamic cr best used to solve a problem; say u have an xe256 comp cam and it pings, then u can compare any dcr # u plug in, to larger cams IN THE SAME SERIES, like xe262, xe268, etc, and see if you like the CHANGE. BUT it is much easier to find guys with successful combos here on cf, then you can duplicate the entire combo with confidence.
.
They call it "advertised duration" because it is an ad, a pitch to sell a product. GM uses lash, comp .006 H, .015" solid, Crane .004,
most also use .050" . To compare different BRANDS is nearly impossible, except in an engine or test fixture like Cam Dr.
Engine won't even run if the valves are open at all, try it sometime!
Can NOT build compression if valve is open! even .001"
Very hard to get these seat to seat #'s from a cam co. Easy to do in engine. But then, u can just crank it over and read psi on a gauge.
Dynamic cr best used to solve a problem; say u have an xe256 comp cam and it pings, then u can compare any dcr # u plug in, to larger cams IN THE SAME SERIES, like xe262, xe268, etc, and see if you like the CHANGE. BUT it is much easier to find guys with successful combos here on cf, then you can duplicate the entire combo with confidence.
.
They call it "advertised duration" because it is an ad, a pitch to sell a product. GM uses lash, comp .006 H, .015" solid, Crane .004,
most also use .050" . To compare different BRANDS is nearly impossible, except in an engine or test fixture like Cam Dr.
Last edited by Matt Gruber; Jan 29, 2013 at 09:53 AM.
Le Mans Master
Joined: Apr 2005
Posts: 6,350
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From: Seattle Area WA
Thread Starter
Racer
Joined: Jan 2004
Posts: 398
Likes: 0
From: Eastern Shore Maryland
what is throwing me is Comps 30-30 gm version shows a 103 degree intake closing, gm's numbers give 79 degree and using Joe's formula is 77 on gm's very close. Is comps version really that late and have an advertised duration of 346 degrees when gm gives 294 adv duration? I called comp and he reads off of the screen so no help at all.
