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I gave you my 2 cents' worth on the subject already. The valvetrain weight and the valve acceleration rate (determined by both the cam profile and the rockerarm ratio) are significant factors, there's no simple answer that's good for all combinations. Some combinations will rev to RPM using single springs, some will float long before that. Valvesprings have harmonic frequencies at which they lose a substantial portion of their strength; if a harmonic is within the operating RPM range, there will be trouble. By shimming up the springs to the point where they still do the job even at their harmonic frequency, you put more strain on things at all speeds. With duals, the inner and outer have different harmonic points so one spring can help to "cover" for the other when it's weak; there's also the benefit of the damping effect caused by the springs contacting each other. Duals can almost always be set up with lighter seat pressure than singles for the same RPM capability, so IMO they're the better choice if you're uncertain...but for your current project, which you've stated will be a with 40x35.5 valves and a planned upper RPM of ~, a decent set of HD singles should suffice for any cam/valvetrain components you'd be expected to use. DO fit steel shims under the springs - they come in various thicknesses to allow adjusting the spring's installed height and seat pressure, but with fresh heads they should all be relatively close, the shims are just to keep the springs from chewing up the heads so thin (~.015") ones should be all you need. http://www.shoptalkforums.com/viewtopic ... 8&t=
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