Quote:
Originally Posted by Barry_R
I suspect that the primary driver for the non-roller tip NASCAR rockers is weight reduction at the valve end. I tend to agree that the roll couple at the tip is quite small, but do not see how directional reversal inertia could be much of an issue due to that short travel - perhaps .080". The benefit of the roller tip lies in the pivot function - reducing side load imparted to the valve tip. Skidding or scrubbing is most likely the result of uncontrolled valve bounce taking the roller out of contact with the valve. Reducing the weight would help with high RPM valve control - something of a "Devil's agreement"...
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Barry - Sounds like you're exaggerating the production problem rate of the blocks a bit! Not that I'm naive to the likely anomalies, but unless you've been there when it was being done back in the day, it's also difficult to know what repairs weren't done after the fact (just like people bad mouth original Cobra production quality, when the vast majority of issues were done by people working on the cars subsequent to delivery). I've been into FE powered Ford cars for about 30 years myself, so seen a few virgin FE's in my day too.
You should come up with a test rig and verify rocker roller tip skidding for yourself, it would just need to be fairly sophisticated to see the small relative motions. I've been told by multiple leading edge engine developers that lab testing has shown the roller tip to skid progressively more as engine speed goes above 2,000 rpm, depending on individual engine attributes. As skidding occurs, the side load on the valve is the same or worse than a pad rocker at the same point in the lift curve (usually worse, because a good design pad tends to rock across the tip). I know the engineer responsible for the new Vette Z06/ZR1 valve train design and he said they went through all the vacillations/testing of evaluating whether to go with a roller tip and went with the pad - in spite of the fact that the rest of the engine has all the trick high buck things like Titanium con rods and intake valves, lightweight pistons, roller lifters, etc.
By the way, while people think the FE is so antiquated, it may be interesting to note that the Z06 rocker is virtually the same ratio and offset, weight, inertia and, of course, pad design as the 427 Ford. In fact, you can go through the whole design of the Z06/ZR1 engine and it seems almost certain the GM engineers had an FE there during development: Valve head diameters within about .010" of 427's, hollow stem sodium filled steel exhaust valves just like the 427, bore and stroke almost identical to the 428 Ford, Titanium retainers vs. aluminum retainers used by Shelby American on team car engines, valve inclinations/layout similar to the FE with spark plug angled toward the exhaust valve just like the FE, roller lifters similar to roller lifters that Shelby American used in their top of the line race engines back in the day, lifter diameter increased to I believe FE diameter from the smaller size used in earlier Chevy small blocks (smaller lifter is a problem in achieving higher cam ramp rates), heart styled combustion chambers similar to the aluminum comp Cobra HR head, <individual> shaft mounted rockers similar to the FE (in line, GM just left out the extra material in between), dry sump utilizing the stock internal pressure
oil pump just like the original Cobra setup, crank main thrust bearing move to the middle of the engine like an FE from the rear that earlier Chevy's had, distributor moved from the back to the front like an FE, intake and exhaust ports spread like Ford engines, cross bolted main caps with skirted block like the 427, cylinder head bolt pattern so close to the small block Ford that I've been told all you need to do is slightly elongate the holes in the head to swap heads between engines, tuned IR runners with FI which is the evolution of the tuned IR 58mm weber setup of the Cobra team comp car engines, aluminum block and heads just like the Cobra big block team cars had been originally planned, overall weight of the Z06 engine within about 10lb of the comp Cobra aluminum team car engine with 58mm webers and dry sump, 7100rpm redline vs the "7,000 rpm kit" hollow valve comp/MR/HR engines Cobras had back in the day, and probably a host of other attributes. It sure is interesting how both engines ended up so similar when you consider the fact that the Corvette engineers employed all the modern, sophisticated tools like FEM and CFD.
Most of the differences in the newer engines focus on subtle refinement details, like valve coatings, harder guides, better sealing rings, etc.
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