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have a motor im rebuilding and just had the heads done with new valves and the comp cam springs. this is a basic application in a f550 so im not going for more horsepower just want to do it correctly. is it commonprocedure to shim a brand new spring on a brand new valve?have yet to measure according to spec am wondering what others have done in same scenarios. thankx!
If you're gonna be running high boost, an exhaust brake, or high rpms, then I would shim for a higher seat pressure. Otherwise, shim them to spec' installed height.
I did my homework on this one. The 910 is a shorter spring than stock, but it's stronger. There's a lot of data here, but suffice to say the installed height of the 910 spring with the .030 shim is 1.803 inches. Stock spring pressure is 79 pounds, where the 910/shim is 117 pounds - that's a 40% bump in holding strength. Doing the math on the square inches of the valve face, it worked out that it takes about 35 PSI of backpressure with no boost to bleed a valve. My new limit will be over 50 PSI backpressure while braking.
I would definitely shim the springs - in fact... I did. I did the big power mod, but that's not my primary reason for shimming - it's engine braking. When feeding the throttle red meat, the exhaust backpressure equalizes against the boost, so the valves aren't inclined to lift on either intake or exhaust. Now... take engine braking with no boost on stock spring pressure, and there's plenty of opportunity for exhaust valve and piston facetime - no iPhone needed.
The springs and engine braking provide me with a whole new problem: My 38R turbo "blew" past the MAP pressure limit, so I had to install a boost fooler to protect the MAP sensor from damage. The stock EBP sensor is good to about 40 PSI - and I'm gunna "blow" past that. Yuck. I don't know of any EBP foolers, and I doubt I'll easily find anything that takes the punishing heat of exhaust. Not even our own sensor can take that heat - that's why it's on the end of a tube. This is gunna get weird.
If you are 100% certain your 550 is never going to utilize engine braking, then no shims needed. The stock height on a 910 with no shim is still close to 100 pounds - about 20% stronger than stock.
11-24-2014, 08:49 PM
