Quote:DBM with a DR cam I would expect to see a compression test with lower pressure, with valve overlap the exhaust valve is also open.
I'm not sure what you are getting at Batman. Valve overlap occurs at the end of the exhaust stroke. It should have no effect on cranking pressure. Intake closing point determines how many degrees of crank rotation are left to compress the fresh charge of air. Close the intake at BDC and you have 180 degrees of crank rotation to compress the charge. Close the intake at 20 degrees ABDC and you have 160 degrees of rotation to compress the charge. Close the intake at 48 degrees ABDC (stock cam) and you have 132 degrees of crankshaft rotation to compress the charge. Close the intake valve at 42 degrees ABDC (DR cam) and you have 138 degrees of crankshaft rotation to compress the charge (6 degrees > than stock). How does the valve overlap event that occurs at the end of the exhaust stroke affect the compression stroke?
Regarding low end torque, my stock engine dyno run showed 30.5 max HP, Suzuki says it makes 31 HP so I'm pretty comfy with the results of the dyno pull. I think it was accurate. The stock engine made 30 ft-lbs at 2000 rpm. The guy in Finland with the DR cam in a 692cc engine and 10.?:1 compression makes 25 ft-lbs at 2000 rpm. That's 5 ft-lbs less low end torque. I attribute that to the valve overlap. The intake & exhaust are swappin holes at low speed. Fresh charge is running out the exhaust. Also, he runs a straight pipe so there is most likely quite a bit of reversion during overlap (sonic wave running back up the pipe, through the open valves, and back out the carb).
I put that DR cam in my engine over the weekend and have been fiddling around with it. Of course I checked compression before and after. It was interesting. To my surprise, it didn't change a bit (155 psi before, 155 psi after). Since I had the head cover off and the cam out, I checked the stock cam timing. It's a bit different than what I recorded when I checked it through the valve covers with everything together. That's understandable since you have to fight the valve springs and alternator, and access doesn't permit optimum placement of the measuring instruments. I will be posting an update to the valve timing data I previously posted. For this discussion, we only need to know the intake closing point, which is 48 degrees ABDC for the stock cam, and 42 degrees ABDC for the DR cam. The 6 degrees of additional rotation did not increase compression pressure.
Keep in mind that as the piston moves further up away from BDC, the movement accelerates (the piston moves further for each degree of rotation). At some point, the intake valve closing is going to start having a significant effect on compression pressure. That point is obviously somewhere past 48 degrees ABDC.
I can tell you this much for sure, the DR cam does not produce low end torque like the stock cam. The stock cam pulls harder at slow speed on surface streets. The DR cam comes into its own on the freeway. I'll post the results shortly.