Horse Power and its Effectiveness, cylinder filling

[David Cooley]

At 11:58 AM 12/14/1999 -0800, you wrote:
>         One important factor is that for two engines of identical 
> displacement,
>one oversquare and one undersquare, the oversquare one will have more room
>in its wider bore for big valves and thus big ports.  Imagine a typical
>stroked engine - the bore size, valves, and ports remain unchanged while
>the stroke increases.  Hence the engine can burn more mixture at any given
>RPM (thus producing more torque, HP, joules per farthing, whatever) but
>only up to the point where the head limits gas
>flow.  This same would be true if the induction or exhaust were the
>limiting factors, but of course they are not dictated by bore diameter.
>         I dare not say that the stroker will absolutely not make more 
> ultimate
>horsepower than the stocker, just that its HP will probably not rise in
>direct proportion with its torque.
>         Something I have wondered about is cylinder filling relative to 
> bore size
>vs. valve size - i.e., for a given head, how would cylinder filling vary
>between two cylinders of identical displacement, one undersquare and one
>oversquare?  Would the small-bore, long stroke design prevail over the
>big-bore, short short-stroke?  I tend to think not, cannot imagine any
>reason why, but I see a lot of current hi-po engines going undersquare
>(Honda and VW spring to mind).  perhaps has to do more with overall engine
>size considerations...

Oversquare has a longer dwell time at BDC and TDC, due to the shorter 
stroke.  This is a good thing for High RPM, as the piston velocity is less 
than a long stroke motor, and the extra dwell time allows the cylinder to 
fill more efficiently at higher RPM's.  A small bore/Long stroke motor is 
more suited to low end torque (IE truck) because the piston velocity will 
be higher at a given RPM, adding stress at the high end of the RPM band, 
and attributing to less efficient cylinder filling as well... smaller 
valves/ports and less dwell time for TDC and BDC.
Seems a small bore/long stroke engine would benefit more from 
exhaust/intake tuning than a large bore/short stroke engine would.
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David Cooley N5XMT Internet: N5XMT at bellsouth.net
Packet: N5XMT at KQ4LO.#INT.NC.USA.NA T.A.P.R. Member #7068
We are Borg... Prepare to be assimilated!
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