Thread: Domed pistons and quench?

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Posted by jimmyjeep:
"you referred to the cam being long, you mean in duration?"

Yes.

"Could you go into a little more detail on how the lift and duration parts of the cam work in correlation with high and low compressions?"

Concerning lift: If you are looking to optimize your build and make the most horsepower per cubic inch that you possibly can, then use the highest lift cam you can bolt in the motor. There is a price attached though. You must be willing to very closely check every component of the valvetrain to insure that everything is going to work together. You'll want to measure the exact valve to piston clearance through both intake and exhaust cycles with lightweight checking springs, clay on the pistons and solid lifters and rolling the crank through a complete 720 degree cycle. You'll also want to check for spring coil bind at max lift, rocker to retainer interference and binding of the rocker on the stud. The other downside to doing this with a street motor is the maintenance required on the springs. When using very high lift, the springs are going to take a beating and will fatigue over time. Now, there have been some advances in spring technology and they are getting better, but you still have to check them every so often. Your cam grinder will have exact information on this. If you are looking to put the motor together and not remove the valve covers again except for the occasional valve adjustment, then back off on the lift. Again, consult with the tech at your favorite cam grinder.

Concerning duration: It takes time to fill a cylinder with mixture. As engine speeds increase, the available time the valves are open becomes less and less. How do we extend this time at high rpm's? By keeping the valves open longer with more duration.

The most important event on the cam lobe is the intake closing point. Compression in the cylinder cannot begin until the intake valve has closed at some point after bottom dead center with the piston ascending up the bore on the compression stroke. With a low scr, you'll need to close the intake fairly early to trap enough mixture to make a good bang when the plug fires. If you use a long cam with a low scr (exactly what most newbies do), most of the mixture is blown out the open intake valve and back into the manifold and out the carburetor bores before the valve closes and you have a motor that won't pull your hat off your head due to low cylinder pressure. This is called reversion or standoff and you can see it as a white, foggy cloud that stands off just above the carburetor with the air cleaner removed. Reversion upsets the signal to the carb so that it doesn't know how to meter the fuel/air properly and things go to **** in a handbasket. It won't make any power, but man she sure sounds good at idle. Conversely, with a high scr, you must delay the intake closing point so that some of the mixture is lost back into the intake tract or you'll make too much cylinder pressure for the available fuel. It is a very delicate balancing act and is the reason you should have ALL, and I mean ALL the information about your motor before you talk to a cam tech at the grinder for a recommendation. I, as well as others on this and other boards will give advice based on available information, but it is only a recommendation that is meant to get close so that you understand what a cam in that neighborhood will do for you and why. Under no circumstances should you take anyone's advice but the cam tech's for your final decision.

There is a new wave of thinking in choosing a cam being talked about as the result of extensive testing by David Vizard. He suggests that the cam should be chosen for the correct overlap first, based on the cylinder displacement divided by the diameter of the intake valve, then duration will be determined as a result of that figure. It's taking me some time to get wrapped around this thinking and I'm not there yet. As soon as I do, I'll try to explain it to you guys. If you get it first, please help me to understand it.