Valves, Cams, and Ports

I recently answered an email from a potential customer who was interested in having some performance work performed on his 95" Twin Cam, and had been shopping around, asking other shops for their input. One of the shops he contacted suggested keeping the stock valve size because a high lift cam and larger valve would lose bottom end torque. He had been led to believe that velocity would be greater with a smaller valve. He was also concerned with picking a different cam grind to replace his Screamin' Eagle 203 cams that would keep the low end punch he was used to.

By the time I saw the length of my answer I realized I should just turn it into a post here on my blog. The following is most of my answer to him.

There are more factors than lift and duration that effect how a cam performs. Higher lift by itself does nothing except allows more fuel/air in provided the heads flow more at the higher lift than they do at stock lift. Stock heads do not flow any more at .570" than the do at .500". Thus, a higher lift cam with the same timing specs would not give a performance benefit with stock heads. Duration does have effects on where the motor makes its power, but when the cam opens and closes the valve (timing events) can make a big difference. In other word two cams can have the same duration, but the timing of the opening and closing can be different giving different results.





Ported Twin Cam Intake









Many performance cams are designed such that they add top end power and take away some low end. They absolutely need more static compression ratio to make up for cylinder pressure lost to cam timing. Most cams in the .550" to .590 lift area fit into that category. They often loose a little low end, but make decent mid range power if the compression ratio is high enough. Other cams (such as those from Wood Performance) make huge torque numbers from combining a high compression ratio with cam timing that gives really high cylinder pressure. They tend to be hard starting and touchy about pinging because of it. The Kuryakyn cams are the only ones on the market that offer a third option. They make power throughout the RPM range, are easy starting and not prone to pinging.

I know I am starting to sound like a Kuryakyn commercial, but trust me I have no stake in selling their product except that they work the best and so make my headwork look even better than it would with another cam.

As for the larger intake valve loosing torque, That is only an issue if the flow does not increase enough to maintain or increase the velocity compared to the smaller valve. Velocity is a function of CFM and area. There are two places to consider the velocity. One is commonly called valve velocity and the other is port velocity. Valve velocity is calculated using the CFM and the diameter of the "choke" just beneath the valve seat. Port velocity is calculated using CFM and port volume (which gives an average port cross sectional area). The computer that is hooked up to my flowbench calculates both of these for each valve lift that I test. A good porting job will increase both of these velocities as well as the CFM for any valve size (within reason). Of course there does come a point where too large a valve will not increase flow and then the velocities will go down though the CFM may stay the same.

In a Harley head with its extremely short intake tract, velocity is probably not as critical as in an engine with a longer one. It is the inertia of the intake charge that increases the volumetric efficiency by "overfilling" the cylinder. Inertia is a function of mass and velocity. With the mass of the charge limited by the short length of the intake tract, velocity becomes much less the critical factor it is with a longer one. I have to thank Mike Roland (the designer of Kuryakyn cams) for pointing that out to me. Sometimes the obvious just totally escapes me until someone points it out to me. That being said, my porting work invariably increases velocity.







Ported Twin Cam Exhaust






On the exhaust side, the smallest valve and port that will adequately evacuate the cylinder will work the best. At moderate RPM all of the spent gases will have been cleared before the cam gets into the higher lifts. Larger exhaust valves and more high lift flow are needed for high RPM operation when there is not as much time available to clear the cylinder. For street use I like to keep the stock size exhaust valve.