Actually it comes down to mechanics. You need to visualise a
single cylinder engine in you mind. OK. Now put it at top dead center,
and give it a very long rod. Now slowly rotate the crank pulling the
piston down. When the rod is exactly at 90 degrees to the crank's rod
journal, this is the point of maximum mechanical advantage. In other
words, this is the point where the piston can twist the crank the
hardest. With long rods this will be around 75 degrees (rough estimate
here) of crank rotation from top dead center. With short rods, this
point same point of mechanical advatage will ocure sooner like around 65
degrees of rotation. This is because the short rod will have a greater
maximum angle than the long rod will.
The force of the explosion drops rapidly when the exhaust valve
starts to open. Depending on the cam chosen, this can happen anywhere
from 36 degrees before bottom dead center (BBDC) on the power stroke, to
60 degrees BBDC with the real lumpy cams. (specs taken from the
edelbrock cam web page). With short rods, the maximum thrust will
happen earlier AND with greater force (because it will happen before the
exhaust valve opens). This also raises Piston aceleration which is bad.
It will have a higher piston speed because of the increased angle at 45
degrees of crank rotation which is like snapping the piston up and down
in the cylinder. and also slams it from side to side, again because of
the angle.
A long rod will be a smoother engine and more reliable, but will
have less torque and will be better with mild cams, and won't be able to
take advantage of radical cams.
Rich
> ----------
> From: RPalmerBob@aol.com[SMTP:RPalmerBob@aol.com]
> Sent: Thursday, September 11, 1997 8:31 AM
> To: Richard Atherton (Entex)
> Cc: tigers@autox.team.net
> Subject: Re: Rod Length
>
> Rich,
>
> Thanks for the response. Let's see, currently I think we have one
> vote for
> more torque with shorter rods and one vote for more torque with longer
> rods.
> Any more votes out there? Whichever answer is right, if either is,
> my
> question is "Where does the power go in the case with less torque?"
> Friction? If both cases have the same displacement, volumetric
> efficiency,
> etc., etc., burn the same amount of air/fuel mixture, etc., etc, then
> what
> happened to the difference in power output? Kinda want to hold on to
> that
> old consevation of energy concept for awhile.
>
> Something to ponder for awhile.
>
> Bob
>
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