Cam Theory Phase II
Camshaft performance comparison has been discussed and written about many times over. Common factors include manufacturer and model of camshaft, spring pressure and coil bind settings, RPM and power range, header selection and carburetor fuel draw. By following manufacturer recommendations performance will usually be good. To get the ultimate, it requires extra effort based on accurate data accumulated for analysis. Some folks are looking for a simple foolproof set up that will work exceptional for their particular situation. With over 30 years karting experience we have yet to find a quick answer set up formula, it just does not exist. The first step is to accept the fact that an ultimate performance set-up requires some type of trial and error dyno or track testing program. To be of value, testing must be based on some type of a tangible and accurate analysis record keeping system. This month a cam valve spring and set up analysis record keeping process developed and successfully used by us will be discussed.
Those new to engine set-up must learn the basics. The proper use of a degree wheel, dial indicator set, and how to set-up a cam are minimum requirements. A number of publications are available from the APS catalog. Terms related to coil bind, spring lengths, and pressure settings can be found in an article we wrote a few months back in Hooked-Up. This month’s article will include only terms and factors necessary to understand the proper use of this analysis system. Camshaft terms and events used as part of this system include: overlap, overlap duration, overlap centerline, and crossover. It’s necessary to have some understanding of these events and terms in order to analyze data collected. Another major factor is to have tools that make set-up changes quickly and accurately. The one move-around gauge method will make the work more difficult. We manufacture a number of specialized valve set up and tech tools. Check out our current specials that happen to be running the next few weeks. For additional information check out these items on our web site at www.rixkartengines.com Our products are designed to ensure accuracy and make work progress at a much faster pace. A "Cam and Valve Spring Set-Up Analysis" form will also be available within the next week. We hope it may prove to be of use for your record-keeping program. It’s free with just the click of a button.
So, what does "overlap" have to with cam performance? Big whoop! Overlap is when the intake and exhaust valves are off their seats at the same time. It occurs near the end of the exhaust stroke and the beginning of the intake stoke. This can be observed by removing the head and running the piston up to TDC. Every other time the piston reaches TDC, both valves will be open just off their seats at the same time, allowing both ports to be open simultaneously. Duration of overlap can be calculated by adding degree specs for the intake valve opening to the exhaust valve closing. (Example, if the intake valve opening is 16 deg BT and the exhaust valve closing is 18 deg AT, the overlap duration would be 34 degrees.) In operation, some of the intake mixture can be drawn out with the last bit of burnt gasses in the exhaust flow. Effects can vary depending on engine RPM, but this draw-through effect always increases at higher engine speeds. Specialty headers can aid in producing a ram kick back effect that forces un-burnt fuel back into the cylinder at the instant the exhaust valve closes.
Ok, what about this crossover stuff? "Crossover" is probably one of the most important cam events, but it is the least talked about. It’s the time during overlap when the intake and exhaust valves are off their seats at exactly the same amount of opening. (Example might be .039" for each valve lift at 3 degrees BTC.) This 3 degree value is known as "cross over centerline". Knowing cross over centerline can be useful when making performance comparisons at specified RPM values. As a rule, crossover usually occurs plus or minus 5 degrees near TDC. Cam manufacturers for stock engines automatically have this value calculated if cam card suggested settings are followed. Modified cams with adjustable gears can be readily altered, but it’s a different story for stock classes. It requires juggling valve settings, cam twisting, or lobe altering, while keeping within WKA tech profile specs.
Our cam and valve spring set-up analysis chart provides a format for recording a number of cam set-up variations or cam-to-cam comparisons. Use the chart to record set-up values, then compare them to track or dyno testing results. This sounds like a bunch of work. You’re right, it is! Those wanting the ultimate in performance realize that it requires a lot of hard work. Remember, there are no quick answer set-up formulas. They just do not exist. Thanks for listening, Rick at Rix