Originally Posted By: Scdevon
Aren't there lots of other areas in any engine that see high psi loads besides flat tappets? What about pushrods against rocker arms? (Way less contact area and very high PSI). What about a rocker arm against a valve stem? What about a wrist pin bushing? What about a rocker arm fulcrum? Don't these areas need ZDDP too? Seems like if low ZDDP was an issue, there would be lots of other engine parts failing and not just flat tappet cams.
Toyota to this day still makes bucket and shim OHC engines, don't they? Buckets and shims are still "flat tappets" when it comes to cam lobe loading. There isn't a more trouble free setup than an OHC Toyota with buckets / shims. They seem to do just fine on the cheapest store brand oil people can find.
Same with Nissan and my Saab B235 has cam on bucket lifters, no rollers in sight
Just a couple of clarifications:
-American Metallurgy for cams has traditionally/historically been poorer than Japan or Europe (just admit it, you know it's true)
-Most Japanese Engines have oil holes in the cam lobes to directly oil the cam and lifter interface surface (it's just a better system.)
-Due to the design of a DOHC cam on bucket design, the lifter is not required or expected to be rotated by the cam, this is only a function of push-rod designs. Thus like a V8-roller-push-rod, the cam / lifter interface is at a perfect 90° = less wear.
-Modest displacement Japanese and European engines with 4V-cylinder heads typically run much lower lift numbers 8-9mm in many cases. 2V-pushrod large displacement American lumps are almost always +10mm, if not +12.5mm.
-The mass of the pushrod assembly + positive rocker ratio's requires significantly stiffer valve springs to ensure stability at high RPM. This then puts a much higher stress on the lobe / lifter interface vs DOHC designs. It's also why nascar specify a maximum crank to cam spacing for their race engines. as the shorter you can make your pushrods, the less mass you have reciprocating. take a look at the amazing Ilmor pushrod indy V8 from the early 90's to see this taken to the extreme.)
-Thus because of the extra valve area (2 vs 1 intake) = lower lift required to flow + lower mass: DOHC can run softer springs and still perform easily past 7000RPM.
My CA18DET race engine (circuit, not drag so it's built to survive) makes peak power at 8,500RPM swallowing 25PSi of boost; the cam is a 270° 8.8mm. We use a stiffer than stock, but still single valve spring. No issues. Run Helix Ultra 5W40.
2 modest sized valves opening a modest amount is always preferable to 1 large valve having to open a large amount
regards
Jordan