Thursday, January 17, 2008

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.

2 comments:

Professor Howdy said...





There is a time for everything,
a season for every activity
under heaven. A time to be
born and a time to die. A
time to plant and a time to
harvest. A time to kill and
a time to heal. A time to
tear down and a time to
rebuild. A time to cry and
a time to laugh. A time to
grieve and a time to dance.
A time to scatter stones
and a time to gather stones.
A time to embrace and a
time to turn away. A time to
search and a time to lose.
A time to keep and a time to
throw away. A time to tear
and a time to mend. A time
to be quiet and a time to
speak up. A time to love
and a time to hate. A time
for war and a time for peace.

Best wishes for continued ascendancy,
Dr. Howdy

P.S. Here's some blogs & videos
that I found of interest as
I negotiated my way through
cyberspace:


Every Student
My Blog Video
Religion Comparison
Avoid This Place
Danish Cartoons
Arabic Cartoons
Muhammad or Jesus???
Answering Islam
Is Jesus God?
A Short Look At Six World Religions
God's Word in different languages...
How to become a Christian
Who Is Jesus?
See The Word
Watch The Jesus Movie
Spanish Cartoons
German Cartoons
Chinese Cartoons
Italian Cartoons
Greek Cartoons
Japanese Cartoons
Portuguese Cartoons
Around the Well
French Cartoons
Hindi Cartoons
Russian Cartoons
Little Girl
Get Saved?
You & Sin City
Mysterious Disappearance
Evolution Video
The Departure
You Need To See This
'Thought & Humor'


Tell me sometime what your
thoughts are about all this:O)

gate valves said...

what a great blog. good thing i dropped by