how do i select valve springs for my 1/4 scale engine? .125 total lift no more than .350 diameter. what do i need and where do i buy them. thank a bunch fellas.
You need to have some idea of the closing force you need on your valves. This will drive the wire diameter and number of coils to achieve that force with your installed length. The longer the spring (more coils) the closer the load on the cam (or rocker) is when comparing valve closed to full lift.
Obviously you have packaging constraints so the spring has to have limited length.
Take the spring free length and subtract the installed length. This difference is multiplied by the spring rate to calculate the valve closed force. For valve open force, multiply the difference between the spring's free length and length at full lift by the spring rate.
Look at the "solid length" and ensure you don't try to operate too close to that or you may break parts in your engine.
I have a OS model engine laying on my desk. The valve head diameter is about .25" and the force to fully open the valves feels like it is between 5 and 10 lb. You will need to calculate or guess the valve closing force you require to prevent valve float at max engine speed.
This calculation considers the inertia of the valves and rockers (and a portion of the spring). These are factored in with the engine speed and cam profile. I estimate that the valve on my OS weighs a gram and that means the spring load at full lift is over 2000 times the weight. The valve force fully closed is less, but probably 75 % or more of the load fully open.
If your springs are too strong the risk of "going solid" and binding the coils goes up because the wire diamter will be larger for a given OD. The greater load will tend to wear and fatigue the valve train prematurely and the added friction will steal power.
You could try a series of springs including ones that are probably on the wimpy side. Run the engine and see if it makes decent power as the revs are increased. If the engine will not rev beyond a certain speed with a softer spring than it did with a harder one, then you have valve float and need a stronger spring.
Google is your friend: Find places that make custom and stock springs and hopefully a stock spring will work for you.
http://www.asraymond.com/spec/catalog.asp and click on "compression springs", then on "more info". Jump to page 36 for the size range.
Last edited by dynosor; 10-15-2008 at 04:15 PM.
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