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#1
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Questions on Compression Test
I did a cold compression test on my 455 some months back and was satisfied that all the numbers were within a few lbs of each other, right at 140. I was chasing a smoke issue. There was a bit of curiosity involved too since the history of the shortblock wasn't completely clear.
All is well, the engine runs great, uses no oil and does not smoke....replacing the stem seals and getting the PCV system in correct functional order did the trick. Now I want to do a hot compression test....just because. Warm it up completely, pull all the plugs, disable the fuel and ignition systems, lock the carb wide open and proceed. My question is how does the cam affect the numbers? Not just my cam, but in general, how does a cam change the numbers on a compression test? I have a Comp hyd roller (230/236) on a 110. It has approx .510-.520 lift with 1.50 rockers and I run Crower solid roller lifters set at .006 cold. This is a completely factory 73 model short block. Just as it left mother Pontiac. 6X-8 heads, 97cc.
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. Last edited by David Jones; 02-28-2014 at 11:49 AM. |
#2
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Quote:
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Karl |
#3
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Thank you Karl, but I'm still a bit lost on this.
Think pre-school with your explanation....with examples.
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#4
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Quote:
"Event 1 - Intake valves opening The camshaft opens the intake valve, and the piston moves down the cylinder. As the pressure drops in the cylinder, the air starts moving past the intake valve to fill the cylinder. This period of the engine cycle is known as the intake stroke. Event 2 - Intake valves closing At some point, usually after the piston reaches the bottom of the intake stroke, the intake valve closes. The piston moves up the cylinder, beginning the compression stroke and compressing the fuel/air mixture within. At some point, usually before the piston reaches the top of the compression stroke, the spark plug ignites the mixture, causing it to burn and expand rapidly. The crankshaft has rotated once at this point. Event 3 - Exhaust valves opening After the piston reaches the top of the compression stroke, pressure from the burning, expanding mixture pushes the piston back down the cylinder. The exhaust valve starts to open, usually before the piston is all the way down, allowing some of the burnt gasses to exit the cylinder. This is commonly referred to as the blowdown phase. The piston begins to move back up, forcing the rest of the hot gas out of the cylinder. Event 4 - Exhaust valves closing As the piston moves back up the cylinder, the exhaust valve remains open, usually until slightly after the piston reaches the top of the cylinder. We refer to this as the exhaust stroke. As the piston reaches the top again, the intake valve begins to open again, often before the exhaust valve is fully closed, and the whole cycle begins anew. The period when both valves are open simultaneously is referred to as “overlap.” The crankshaft has now gone around twice. In a nutshell, here’s how it all happens. Overlap Intake stroke Compression stroke Blowdown Exhaust stroke Repeat" http://hiperfautotech.blogspot.com/2...le-engine.html
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Karl |
#5
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LSA along with actual compression ratings can really effect a compression test.
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#6
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I've got that part Karl. The basics.
Transferring those events to specific cams.....maybe specific examples of a early closing and late closing, that's what I'm unsure of. What is considered early/late?
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#7
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Quote:
My engine has approx 9.25-1 and my cam is your Butcher Special.
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#8
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Generally, short cam; higher cranking compression. Long duration cam, reduced cranking compression. High compression; higher cranking compression. Low compression, lower cranking compression. LSA - tight, higher cranking compression. Wide - lower cranking compression.
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#9
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This graph show a computer simulation of cranking compression for the same engine with 4 different compression ratios. You can see where each crosses the 180 PSI line there is line that runs down to the bottom where it show intake valve close in degrees ABDC (needed).
Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#10
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Ummmm, not sure how this relates. I see the trend lower with various compression ratios but those numbers seem somewhat higher than what I've seen or read about. I thought a really good number was lower than 200.
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#11
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Lots of variables. While I can't tell you what yours should pull, my 10.7:1 462 pulled the following numbers after about 1000 miles with old faithful installed at 108
2 - 175/180 4 - 175/180 6 - 170/175 8 - 180/185 1 - 180/185 3 - 180/185 5 - 180/185 7 - 180/185 According to cliff, at a true static of 10.7 to 1, and the cam installed around 108-109ICL, it should be around 175-185psi You could loosen up the #1 intake and exhaust rocker and spin it a few times and see how your compression changes. That would be a neat experiment.
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Skinned knuckles and empty wallet! Could I be any happier? 66 GTO Convertible. LS3-525 HP. Legend LGT700 5-speed, Wilwood 4-wheel disc brakes, Ridetech coil over front susp, PMT rear susp, Hotchkis bars, Billet Specialties 18" Dagger's (18X9 rear, 18X8 front). 2002 Ram Air WS.6 convertible Trans Am. Wife's car. |
#12
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Quote:
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#13
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David, you can go to Wallace racing and get rough ideas of what it would be using one of their calculators.
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1969 GTO 4spd. Antique Gold/black, gold int. 1969 GTO RAIII 4spd. Verdoro Green/black, black int. 1969 GTO 4spd. Crystal Turquoise, black int. 1970 GTO 4spd VOE Pepper Green, green int. 1967 LeMans 428 Auto. Blue, black int. |
#14
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Or ask Karl, he has the daddy engine analyzer
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1969 GTO 4spd. Antique Gold/black, gold int. 1969 GTO RAIII 4spd. Verdoro Green/black, black int. 1969 GTO 4spd. Crystal Turquoise, black int. 1970 GTO 4spd VOE Pepper Green, green int. 1967 LeMans 428 Auto. Blue, black int. |
#15
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GW, I looked on Wallace but didn't really see one that fit. Which one?
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#16
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http://www.wallaceracing.com/dynamic-cr.php
Try this one. You need a more advanced program like Karl has to put in perameters like lsa.
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1969 GTO 4spd. Antique Gold/black, gold int. 1969 GTO RAIII 4spd. Verdoro Green/black, black int. 1969 GTO 4spd. Crystal Turquoise, black int. 1970 GTO 4spd VOE Pepper Green, green int. 1967 LeMans 428 Auto. Blue, black int. Last edited by GOAT WHORE; 02-28-2014 at 10:46 PM. |
#17
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Quote:
I ran a 455 @ 9.25:1SCR with the Butcher Special on a 110ICL and 110LSA using solid rollers with a .006" lash and the Engine Analyzer thinks you should have a theoretical Cranking Compression of 175PSI.
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Karl |
#18
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Thank you Karl.
I ran these numbers on Wallace Racings dynamic compression calculator and it said 165.44 There is not a camshaft detail provision on the calculator. 9.25-1 compression 6.625 rod 4.15 bore 4.21 stroke Intake valve closes at 45 abdc 0 boost 600ft elevation
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
#19
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Engine Analyzer is using both the Advertised and .050" numbers to make this calculation. Is your cam installed on the 110°ICL?
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Karl |
#20
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I think part of my struggle has been terminology. Dynamic. Static. Cranking. And just plain old compression ratio. I obviously need to dive into what does what.
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frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. |
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