compression test question
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
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
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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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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
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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).
