compression test question
05-24-2006, 09:21 PM
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compression test question Ok, question of the day. If you do a compression test on a motor, does the overlap of the camshaft effect the results? I have heard both ways and I am doing a test right now on a car and I have a few issues that are racking my freaking brain. Any explanations?
Rick
Rick
05-24-2006, 09:42 PM
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Originally Posted by MadBill
The only cam spec that affects cranking compression is the actual (not 0.050") intake valve closing point.
That I know, so if I have more overlap on the seat timing then I will have less cranking correct?
Or does only IVC effect cranking?
Rick
05-24-2006, 10:32 PM
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Overlap is at top dead center. You are reading the compression from the intake valve closing to top dead center. The intake valve closing is after bottom dead center.
05-25-2006, 06:43 AM
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The only way overlap affects cranking compression is if with a specific intake duration, say 240°, you grind a new cam with the intake centerline reduced from say 113° to 108°. This would increase the overlap by 5°, but the only reason cranking compression would change (rise) is because the intake valve closes sooner after bottom dead center, thus trapping more mixture to compress.
Example: We tested a 13:1 CR 358" Chev with a mechanical roller 300/311° cam (~270/280 @ 0.050") with a 114° LCA cam 'straight up' (no advance or retard). The cranking compression was only 130 psi. We then tried a 300/306° cam with 107° LCA 2° advanced (~12° more overlap, but the relevant feature being a 9° earlier IVC) and the compression went up to 180 psi.
(BTW, the first cam made 735 HP @ 7,900 RPM, with ~505 -510 lb-ft. of torque from 6,000 to 7,000. The second was down 30 HP but made 12 more lb-ft over a narrower band and had almost the same average power from 6,000 to 8,000.)
Example: We tested a 13:1 CR 358" Chev with a mechanical roller 300/311° cam (~270/280 @ 0.050") with a 114° LCA cam 'straight up' (no advance or retard). The cranking compression was only 130 psi. We then tried a 300/306° cam with 107° LCA 2° advanced (~12° more overlap, but the relevant feature being a 9° earlier IVC) and the compression went up to 180 psi.
(BTW, the first cam made 735 HP @ 7,900 RPM, with ~505 -510 lb-ft. of torque from 6,000 to 7,000. The second was down 30 HP but made 12 more lb-ft over a narrower band and had almost the same average power from 6,000 to 8,000.)
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05-25-2006, 08:57 AM
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Originally Posted by MadBill
The only way overlap affects cranking compression is if with a specific intake duration, say 240°, you grind a new cam with the intake centerline reduced from say 113° to 108°. This would increase the overlap by 5°, but the only reason cranking compression would change (rise) is because the intake valve closes sooner after bottom dead center, thus trapping more mixture to compress.
Example: We tested a 13:1 CR 358" Chev with a mechanical roller 300/311° cam (~270/280 @ 0.050") with a 114° LCA cam 'straight up' (no advance or retard). The cranking compression was only 130 psi. We then tried a 300/306° cam with 107° LCA 2° advanced (~12° more overlap, but the relevant feature being a 9° earlier IVC) and the compression went up to 180 psi.
(BTW, the first cam made 735 HP @ 7,900 RPM, with ~505 -510 lb-ft. of torque from 6,000 to 7,000. The second was down 30 HP but made 12 more lb-ft over a narrower band and had almost the same average power from 6,000 to 8,000.)
Example: We tested a 13:1 CR 358" Chev with a mechanical roller 300/311° cam (~270/280 @ 0.050") with a 114° LCA cam 'straight up' (no advance or retard). The cranking compression was only 130 psi. We then tried a 300/306° cam with 107° LCA 2° advanced (~12° more overlap, but the relevant feature being a 9° earlier IVC) and the compression went up to 180 psi.
(BTW, the first cam made 735 HP @ 7,900 RPM, with ~505 -510 lb-ft. of torque from 6,000 to 7,000. The second was down 30 HP but made 12 more lb-ft over a narrower band and had almost the same average power from 6,000 to 8,000.)
Rick
05-25-2006, 09:36 AM
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On a cranking PSI test yes IVC will effect your results. On a leak down test no, the cam will not effect the results (unless you try to run the test with either one of the valve open which you shouldn't do).
