Valve Theory and Actual Dyno Power
A number of activities have been experienced since the last opportunity to write a few lines for Hooked-up. Kartfest was the highlight by far. Most all who attended would probably agree. A number of photos related to this event have been posted on our website at www.rixkartengines.com, just click on the red "Kartfest" title.
It was good to finally meet Lorin, owner and publisher of Hooked-Up. He seems to have many good ideas in store in his continual efforts to make this publication a grass roots karting information source to meet the needs of the average karter. Keep up the good work, Lorin.
Our past articles submitted to Hooked-up have not contained much theory and formulas related to camshaft design and operation. There is a general idea that the more theory one understands and applies to engine building or set up, the result will automatically ensure better performance. In other words, the more cam theory one knows the faster you’re going to go. With this reasoning an automotive engineer will always be a faster competitor. In reality this is not always true. It can be counter productive, time wise, if an engine builder doesn’t come to realize there just is no substitute for an actual trial and error testing program. I’ll try to give an example.
Traditional camshaft symmetrically ground lobe theory is usually viewed by the following basic concepts. The more duration (amount of time the valves are off their seats) the higher the peak power band will be. Intake lobes that open 30 deg. BTC, when compared to a lobe that opens at 20 deg. BTC, would traditionally be viewed that the 30 deg. lobe would be preferred for higher RPM applications.
Another example would be exhaust valve closing. A camshaft that closes the exhaust valve at 20 deg. ATC, as compared to one closing at 16 deg. ATC, would traditionally lead one to believe that the cam with the 20 deg. shut lobe would provide more top RPM useable power. Although this may remain consistent for the exhaust timing theory, on the most part this has not proven to be fact with the camshaft lobe configurations currently used in Briggs WKA sanctioned racing.
Individual lobe centers (this is the point that the valve will be at its maximum opening in related to a given fixed degree value point in crankshaft and piston position), lobe separation angle, crossover, and overall cam center have been analyzed almost to death by many karters before final camshaft selections are made. After installation and set up actual performance experiences do not always match theory expectations, especially in the stock engine classes.
There is one formula that seems to have merit and makes some sense that causes others seem to be inaccurate under racing conditions. The formula relates to how fast a valve can open in relation to time and height, better known as lift rate ratio. I don’t remember the rate ratio, but indications are that parts would be damaged if camshafts were ground to exceed the said ratio value. The flat head engine has a very durable and efficient valve train, but do broken lifters and lifter bores ring a bell? Yes, we have gone beyond theory and are now operating in the Twilight Zone. The most logical question would be, why?
How quickly and how high a valve remains off its seat is more important than how long. The ultimate would be to have an intake valve open immediately to its maximum lift, somewhere around 20 deg. before TDC, remain open at its maximum lift, then slam close immediately from its maximum lift at around 20 deg. after BDC. In reality this can’t happen. Current slapper style camshafts are designed to imitate this action by having base circles that act something like a steep ramp. They launch the lifter up much like a springboard. At high RPM the lifter doesn’t even come in contact with the nose of the lobe, provided your spring pressure is set up correctly. Horsepower gains are substantial.
Current engine builders do not have an option, they must know how to set coil bind, nose and seat pressure. Billet lifters are no longer an option and exhaust guide bores must be welded up to insure durability. It doesn’t end there. There are variations among classes and track types. Springs must be checked and re-checked at regular intervals to keep tabs on tension changes. This will require investing in some type of accurate testing and set-up tools and equipment. Additional set-up and build time is also required with the bottom line being more expense to the karter.
What might be good or bad for racing is not the primary issue. The nature of the sport results in an ongoing constant search for that extra bit of power advantage. Technology is the culprit. Attempting to control advancing technology always results in more rules and limiting the karter to have less variables. Sometimes it’s tough to decide what is in the best interest for all concerned. That’s why we have kart organizations and the powers that be. For the majority we must learn to live with whatever comes down the pike in the form of our annual rulebook.
Back to the original thought concerning valve theory. A few days ago, in a phone conversation with Mr. Mike Bordeau, of Dyno Cams, he put it this way: "Forget theory and do whatever it takes to make power on the dyno". Take this advice for what it’s worth. If you don’t have access to a dyno, track testing would be another way. Whatever method you may use, an efficient, accurate record keeping system is a must. Best of luck in your efforts to get to the front of the pack and beyond.
Yours in karting, Rick at Rix.