General Rules of Thumb on Tighter Lobe Separation
Pluses
Higher peak torque
Higher cranking compression
More area under the curve between torque peak & horsepower peak
Minuses
Lower idle vacuum
Power drops off faster after power peak
Torque drops off faster below torque peak
If intake port design is very efficient, and lobe separation is very tight,(106 or less) raw air/fuel mixture can be drawn through on overlap, which can cause peak torque and power to drop & bfsc to go up
If intake valve closes too early, peak power can drop due to insufficient cylinder filling at high rpm
Why it happens
Lets start out with the basics. The strokes of the 4 stroke internal combustion engine. Intake, compression, power, exhaust. Each stroke represents 180 degrees of crankshaft rotation, for a total of 720 degrees to go through all four cycles. Overlap occurs at the end of the exhaust cycle & the beginning of the intake cycle. Overlap begins when the intake valve opens prior to the piston reaching tdc at the end of the exhaust cycle, and ends with the closing of the exhaust valve after tdc on the intake cycle.
I will use the cam I recommended for wigsplitter as the example, as that is what started this discussion. For those that dont recall, it is a single pattern Lunati profile with the following specs:
[email protected], 290 advertised, 106 degree lobe separation, the cam is ground 4 degrees advanced, for an installed intake centerline of 102. For the purposes of this conversation, the lift is irrelevant, but it was .520 at the valve. If we use the .050 numbers off the cam card, overlap ends when the exhaust valve closes 10 degrees after tdc on the intake stroke, or 170 degrees before the compression stroke begins. At this point, anyone who has even a minimal understanding of how engines work can clearly see that overlap has NO effect on cranking compression.
At this point, we are 10 degrees into the intake stroke, the exhaust valve has closed, the intake valve is open, and the piston is traveling down the cylinder. As we reach bdc on the intake stroke, and the piston changes direction, the compression stroke is beginning. The intake valve is still open, and will not close until 42 degrees after bdc. This is with the 106 degree cam.
I picked 110 degrees for the wider lobe center version of the same cam. As this is a common number to be used on Comp Cams small-block street/strip cams, & they are pretty popular, I felt it would be a good example. If the lobe separation is widened to 110 degrees on this same Lunati cam, the intake valve now stays open till 46 degrees after bdc. You could actually run 8 degrees more duration on the 106 degree lobe separation cam before you would end up at the same intake closing point as the 110 lobe separation cam. Obviously, the cylinder cannot begin to build compression until the intake valve closes. The intake closing point is what is going to govern the cranking compression, not the overlap. The later you close the intake on the compression stroke, the less the cranking compression. As I said, simple first year high-school auto shop.
I also quickly changed the file on my 360 in my Performance Trends Engine Analyzer to the same cam specs and static compression ratio as this cam would have in wigsplitters combo. It lost 10 pounds of cranking compression. Based on the dyno test & my real-world experience (not that that is worth anything) this is probably on the conservative side, as the computer program assumes perfect ring seal ect.
Ultimately, the lesson here is, when you read something in a magazine, dont just blindly accept it, think it out for yourself. Work it through in your head, ask yourself if it really makes sense. I have been reading magazines for long enough that I have read NUMEROUS articles on cam design, half the articles contradict each other. The guys who write this stuff are, for the most part, journalists who HAPPEN to work on or have an interest in cars. They arent cam designers, or engine builders. They get things wrong. ALL the TIME.
