We're kind of saying the same thing--it's the usual problem of language
imprecision.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Randall Young
Sent: Thursday, May 20, 2004 11:25 AM
To: fot
Subject: RE: cranks
> Long stroke engines have high torque for an obvious reason--the length
> of the lever arm (the distance of the rod centerline from the crank
> centerline).
I don't see it that way Bill ... to a first approximation, max torque is
determined by displacement, not stroke. IOW, to keep the displacement
constant when increasing the stroke, you have to reduce the bore, giving the
combustion gases less area to press against ... the two effects cancel.
> Overlap becomes necessary and
> the long stroke is mostly wasted because you're stroking against open
> valves.
Except that max torque rarely goes down for a high overlap cam, it just
comes at a higher rpm ...
Main problem with long stroke engines is they can't turn very fast because
of piston speeds (which are directly proportional to stroke) ... high piston
speeds both increase mechanical stresses exponentially and eventually the
piston outruns the expanding mixture. Plus for a given displacement, long
stroke means small bore which leaves less room for valves. There's also the
"TR6" problem ... long crank throws tend to get floppy and heavy.
Randall
|