dave.williams
rec.autos.tech  06-11-94

-> Besides which, is the rotational inertia of the camshaft itself a
-> limitation on how fast/high an engine will rev?  Seems to me the
-> reciprocating weight (valves, pushrods, lifters, springs) and lobe
-> profile are much more important than making the cam 50 percent (or
-> whatever) lighter.

 Ford's trying to make out like the rotational inertia of the cams is a
big deal, but as far as I'm concerned it's all in the imaginations of
their marketing people.

 A normal cam is cast iron, with the lobes cast in place.  It's
center-drilled, the bearing journals are ground, any gear teeth are cut,
and then it's put in a special cam grinding machine, where an eccentric
master lobe moves it in and out as it rotates against the grinding
wheel.  As the wheel wears the actual lobe profile generated on the cam
changes slightly, and sometimes you'll get chatter marks if there's an
imperfection in the wheel.  Mostly, though, the process is slow with
normal machines - only one lobe at a time is ground; the cam is indexed
to the next lobe and the grinding wheel moves, and then the next, etc.

 A built-up cam uses powder-metal lobes which are sintered into the
proper shape to start with, no grinding necessary.  The lobes are simply
pressed onto the shaft - probably completely automated - and that's
pretty much that.

 As a SWAG, I'd say the rock-bottom price anyone could make a
conventional cam for would be around $35, and you could probably make a
built-up cam for $10.  When you're talking about four cams per motor,
that's $100, and a Detroit automaker would run barefoot through fire to
save that much money.

 I still don't like built-up cams, and I wouldn't trust one in any
performance application unless the lobes had been brazed to the shaft,
not just pressed or swaged.  Even then I'd be continually suspicious.

 A powder-metal cam has one clear advantage over a casting or forging,
though.  The composition of alloys in the powder can be much more
precisely controlled, and anti-wear elements like zinc, lead, or
graphite can be included in useful form without causing alloying
problems.