Valve Train Basics
Last month we indicated that an effort would be made to talk about valve compression spring settings. Please refer specifically to the four considerations stated last month because we won't take space to repeat them. Well, anyhow, here goes.
These are my personal thoughts, right or wrong: The powers that be have improved performance of the stock class by permitting camshafts to be ground, causing very rapid valve opening and closing. In addition, the .233" lift ends up being .250" to .270", depending on the set up. One visual confirmation is to remove the head and actually see where the valve head has come in contact with the head surface. This sounds like a great deal, but at what expense? Lifter and lifter guides began to break often resulting in block breakage, cams bent, etc. This stuff can get downright expensive. To avoid potential failure, many have resorted to welding up the bore area and installing aftermarket alloy steel lifters. With 20/20 hindsight, this was probably not in the best interest for karters running stock engine classes. Since it's in place, the only thing to do is make the best of a bad situation.
A few terms need to be addressed. These may not be exactly what all shops call it, but the basic idea should be the same.
Coil bind - when the valve spring is completely compressed. Most kart shops measure this by installing the valve, retainer and spring in the block. Next, a dial indicator is placed over the head of the valve and zeroed. Finally, some device (big screwdriver) is used to push up on the bottom of the valve stem. Once fully compressed, a reading is taken from the indicator, for example .340", 360", 400", etc. This information will become important during final set-up.
Free length is the length of the valve spring when it is setting on the bench and not attached to anything. WKA has a length, wire diameter, and diameter tech. It may be wise to make a tech check before installation.
Compressed spring length is how long the spring is when the valve is at its maximum opening, installed in the block with cam at maximum lift. A special tool is available for doing this through Am. Power Sorts. Ballpark length is around .845".
Compressed spring tension is how much tension (in pounds) the spring exerts at the valve’s maximum opening. This requires compressing the spring and retainer to the length indicated by the tool used above. In addition, some type of valve spring compression tool must be used to come up with an accurate pound value. This value will vary because all springs are not created equal. Ballpark for this runs 26 to 30 pounds.
Installed free length is how long the valve spring and retainer are when installed in the engine with the valve completely closed. You can measure this with tools described above or add the running length to the compressed spring length. Example .845 + .228 = l.073"
Installed free length tension is how much tension (in pounds) the spring exerts when the valve is completely closed. This is often referred to as seat pressure. This value must be measured with some type of valve compressor testing tool. This value can vary anywhere from 12 to 18 pounds.
Ok, so what does one do with all this stuff? What good is it? What is the best approach? If you are limited as to resources and tools available, the only option is to set up according to the coil bind dimension supplied by the cam manufacturer. It’s not the final answer, but it should get you in the general area where things will work up to 80% most of the time. For those wanting the ultimate, other factors listed will come into play. Coil bind settings do not guarantee reliable compressed spring and free length settings. Once you determine exactly what has worked well, exact duplication is in order.
Getting that magic seat pressure may not get the compressed tension you want. What is the solution? Research and development on the bench. Assemble, check, disassemble, assemble, and re-check. This requires accurate record keeping and part identity. Springs, retainers, upper retainers, retainer groove relocation, and depth of back facing become a juggling act in order to get that exact set-up. Don’t forget to record your final settings on your Rix build sheet, which you already secured from www.Rixkartengines.com. (I assume you did your homework.)
Generally, a low rpm engine (rookie) will not require as much spring tension as a high rpm engine. Too little tension equals uncontrolled valve float which causes misfire at high rpm. Too much tension won’t allow controlled valve float to get that rapid lift and valve opening combination for ultimate performance. A number of publications are available that include valve spring tension specifications for most common classes run.
Thanks for the response from last month’s article. Let us know if we can make this section more worthwhile. We welcome your comments and recommendations, good as well as bad. Thanks for listening, Yours in karting, Rick at Rix.