Re: John Stricker's engine post

["Bill Thebert" <bthebert@domain.elided>]

> From: Daniel Nees <cookiedan@domain.elided>
>    The BB article was about the engine that the Cohmers built for Terry
> Kuhl. They used the 304 because of a SHORTER stroke, thus giving more
> RPM's because of the shorter turn around of the pistons. So, wouldn't a
> shorter stroke make for more RPM's?

The key to understanding stroke vs. "red-line" is, as John Stricker said,
"piston velocity".  Let's use really round numbers to keep the math simple:

Our hypothetical engine has a 4" stroke.  The piston goes up & down (8" of
total travel) once per revolution.  At 5,000 rpms, that piston has to
travel 40,000 "inches per minute" (on the average) in order to accomplish
all that movement in the given time.  

An engine with a 3" stroke would only need to travel 6" x 5000 revolutions
or 30,000 "inches per minute" (again, on average) to accomplish the same
thing.

The "on average" part is important -- since the piston velocity is actually
ZERO twice per revolution (at TDC and BDC).  So the piston has to
accelerate like crazy from zero to its MAX velocity, and then slow back
down to zero -- all within a 3" or 4" travel range -- twice per revolution.
 

High-revving engines therefore strive to keep the MASS (less precisely,
WEIGHT) of the reciprocating assembly (pistons, rods, rings, etc.) as low
as possible, in order to minimize the INERTIA that must be overcome during
all this accelerating and decelerating.

If the requisite piston velocity (required to achieve a specific RPM) is
too high, or if the reciprocating mass is too great, then the engine will
either: a) come apart, or b) never reach the target RPM.

The Comer-built engine described in the article was designed to spin up
RAPIDLY from low to high RPMs -- for there is lots of accelerating &
decelerating involved in stadium racing.  

Bill Thebert
The Binder Bulletin