Compression ratio, DCR, cam duration, cast pistons, etc
Discussion
Sorry, I phrased that question badly. When I said "can you substantiate this?" I meant can you prove there is actually no dynamic trapping of gases even at cranking speed or indeed any other factor that might affect the simple assumption that DCR can lead directly to a cranking pressure calculation?
I submit that either there is some dynamic trapping taking place or that perhaps the effective valve closing lift point actually changes as the cam duration increases and therefore the piston speed increases due to it being further up the bore.
The piston speed at IVC on a race cam closing at 80 ABDC is about 50% higher than with a road cam closing at 50 ABDC. One might speculate this has some effect on how reversion flow or dynamic trapping takes place.
As the piston speed increases it is trying to displace air back through the rapidly closing valve faster and faster. This might indicate the effective choke point in flow comes at a higher valve lift than it would at lower piston speed. So the notional closing point for a race cam is actually closer to BDC than the stated duration would indicate. For a road cam this effect is less pronounced as the closing point is closer to BDC, the piston is traveling slower and the actual choke point is closer to the notional closed lift.
So we conclude that cranking pressure with a long duration cam at high CR should be higher than with a short duration cam at low CR but the same DCR based purely on a stated duration at a given valve lash point.
I submit that either there is some dynamic trapping taking place or that perhaps the effective valve closing lift point actually changes as the cam duration increases and therefore the piston speed increases due to it being further up the bore.
The piston speed at IVC on a race cam closing at 80 ABDC is about 50% higher than with a road cam closing at 50 ABDC. One might speculate this has some effect on how reversion flow or dynamic trapping takes place.
As the piston speed increases it is trying to displace air back through the rapidly closing valve faster and faster. This might indicate the effective choke point in flow comes at a higher valve lift than it would at lower piston speed. So the notional closing point for a race cam is actually closer to BDC than the stated duration would indicate. For a road cam this effect is less pronounced as the closing point is closer to BDC, the piston is traveling slower and the actual choke point is closer to the notional closed lift.
So we conclude that cranking pressure with a long duration cam at high CR should be higher than with a short duration cam at low CR but the same DCR based purely on a stated duration at a given valve lash point.
Edited by Pumaracing on Sunday 8th February 05:55
Dave,
You point out the difference in position and speed of the piston, but did not answer my question about intake closing ramp rate / speed which to me is the other half of that picture. Is there a difference in intake valve seat angles between your two engines which can effect low lift flow?
Stan
PS - Do you have any idea about what the starter motor RPM's are for your engines?
PPS (Edit) - Let me add these graphs which shows the change in cylinder volume
You point out the difference in position and speed of the piston, but did not answer my question about intake closing ramp rate / speed which to me is the other half of that picture. Is there a difference in intake valve seat angles between your two engines which can effect low lift flow?
Stan
PS - Do you have any idea about what the starter motor RPM's are for your engines?
PPS (Edit) - Let me add these graphs which shows the change in cylinder volume
Edited by Stan Weiss on Sunday 8th February 15:52
Edited by Stan Weiss on Sunday 8th February 15:53
Stan. The stock CVH engines have a hydraulic lifter cam. The Peugeot one a solid lifter race cam. I have no idea how the ramp rates compare or the starter rpm. The valve seats are both 45 degrees though.
But the premise is more general. I've compression tested many engines and never seen a stock road engine get anywhere near 200 psi on 9:1 CR. You need 10:1 or a tad over to hit that regardless of cam duration. Race engines behave very differently from how the DCR might indicate. Always a higher cranking pressure for a given DCR on a high CR and long cam duration.
But the premise is more general. I've compression tested many engines and never seen a stock road engine get anywhere near 200 psi on 9:1 CR. You need 10:1 or a tad over to hit that regardless of cam duration. Race engines behave very differently from how the DCR might indicate. Always a higher cranking pressure for a given DCR on a high CR and long cam duration.
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