Horse Power and its Effectiveness, cylinder filling

[Aaron Willis]

At 10:36 AM 12/14/99 -0500, you wrote:
>
>
>
>>For a given displacement, long stroke means small bore means less force.
>>This exactly balances out the longer moment arm.  Long stroke motors have a
>>harder time at high revs (due to both inertia and limited breathing) but
>>they offer no advantage in low speed torque.
>
>Small bore doesn't mean less force.  You are still releasing the same
amount of
>energy in combustion.  You have just concentrated it over a smaller piston
area.
>How cou;d a stroker suffer from limited brreathing, if it is a "given
>displacement"?  Wouldn't the other motor suffer as well??
>
>
	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...



	Aaron Willis
	Garage TE51 International  http://surf.to/garage-te51
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	AOL IM: hemiyota