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Neil Harvey Neil Harvey is offline
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Join Date: May 2004
Posts: 210
One of the many "ball park" procedures when assembling these engines that has carried over since round wheels were first put on automobiles.

Years ago it was never thought that springs pressures had such a big influence on the engines running. Even today, RPM limits are thought to be the only consideration when installing springs.

Installed height, (valve closed) is what you are measuring here. What changes, would be the seat height , lock groove position differences and any change to the backside of the valve. Plus, add in manufacturing high tolerances. You are also hoping that the springs used are close in their rates.

The correct way to do this is to measure the retainer heights, test each spring for its rate, closed and open pressures with known installed heights and open valve lift, then set each spring installed height based on the closed seat pressure, not the installed height.

I can tell you the difference when done this way is huge at RPM's above 2500 RPM. Run the engine to 6500+RPM and the vibrations set up within the valve train are huge and any differences in seat pressures affects the valve motion off and back onto the seat.

Doesn't bother most as they don't understand what is going on, but if I told you I couldn't hit you directly in the face with a 20lb sledge hammer has hard as the cam hits the valve at 6500 RPM.

Differences in seat pressures at these speeds affects how the spring is accelerated off the seat and how it closes and sits back down on the seat. A lot of cam profiles sold today are very bad in controlling the closing motions. Have differences in seat pressures and the valve is doing a dance back onto the seat.

It falls under that common saying, "never had a problem before".

So how do you measure the seat pressures. You need special equipment that most have no access to. So installed heights is the "ball park" way to do this. At least get that somewhere close and accurate. Otherwise measure the retainer heights, and have someone with this equipment measure the springs and give you the proper shimming required. Believe me it does make a difference.
Old 05-22-2018, 12:12 PM
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