From: CTDreher@aol.com
Date: Thu, 30 Nov 1995 10:17:24 -0500
Subject: Long/short stroke
Tom Gentry asked, in regards to my long/short stroke article..
>>The major advantage slow-reving long-stroke motors have over short-stroke
>>motors is their superior low-rpm velocity of the incoming charge
>I don't get this part. Can you explain why the long stroke engine would
>produce higher intake velocity? Since both engines would have the same
>displacement it seems they would "want" to pump the same amount of air. Are
>you assuming that the smaller bore equates to smaller valves and/or ports?
Tom-
Yes! In the bad old days of two-valve heads (and seat-of-the-pants
engineering) a large-bore engine had large "high performance" valves, which
meant the intake charge moved much slower at low rpm when there wasn't much
air/charge momentum built up in the intake runners. Conversely, a small bore
had necessarily small valves, which generate a higher intake velocity and
better combustion chamber filling.
You might find it fun to read about the junkyard-built "Old Yellar" racing
car of the 50's, which had a Buick "nail head" engine... so called because
the valves were so small they looked like nails! It continually gave
European pure-bred sports cars a thrashing on the track because this engine
had terrific low-end torque AND (more importantly!) a driver who knew how to
exploit it.
All of this bore/valve discussion goes OTW (out the window) when you get four
valve heads and non-symmetric cam designs and variable intake runners and....
As I tried to point out earlier, engine design is not a single-variable
problem. Like any engineering discipline, a hundred trade-offs are made to
determine engine characteristics. (Make that a thousand given pollution
controls now-a-days.)
- Doc Dreher
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