Frank,
Tim is talking about reciprocating weight, not rotating weight. It takes a
lot of force (and power) to continuously accelerate, decelerate, accelerate
the rods and pistons. Since the acceleration and thus the force is related
to the stroke x rpm squared, as the rpms go up, the forces go up
exponentially and there will be a point where the extra power used to move
the reciprocating masses exceeds the power gain from burning air/fuel (which
is only increasing linearly and is dropping off anyway due to breathing
limitations.)
here's a useful set of formulae
http://www.engineersedge.com/engine_formula_automotive.htm
cheers, derek
Derek White
37/2 Buller's Lane
Colombo 7
Sri Lanka
Tel: 94 1 581175
Cell: 94 777 475955
-----Original Message-----
From: owner-tigers@autox.team.net [mailto:owner-tigers@autox.team.net]On
Behalf Of Frank Marrone
Sent: Monday, February 10, 2003 4:58 AM
To: 'Ronak, TP (Timothy)'
Cc: 'Tiger News Group List (E-mail)'
Subject: RE: Desk top Dyno fun
Tim,
I'm not sure it's all that simple. There is an effect of friction with
the longer stroke configuration but that only accounts for part of the
advantage the smaller motor. I'm not sure extra reciprocating weight is
accounted for by the program and I'm also not sure it would matter.
Wouldn't a configuration with more reciprocating weight just take longer
to get up to RPM assuming all else equal?
The premise Bob set forth about scaling RPM based on the size difference
does seem to resolve things once you account for the extra friction in
the long stroke motor. I'm having some problems with Bobs idea though,
it doesn't really answer why the smaller motor is such a better pump at
the higher RPMs given that all other engine parameters, except the
stroke, are the same. I was going to get around to posing this question
to Bob and I guess now is as good a time as any. I think there must be
something that has to do with the bore to stroke ratio that makes the
smaller stroke motor breath better at higher RPMs. Then again maybe Bob
is correct especially since it resolves things. Got to think on that
one.
Frank
> -----Original Message-----
> From: owner-tigers@autox.team.net
> [mailto:owner-tigers@autox.team.net] On Behalf Of Ronak, TP (Timothy)
> Sent: Sunday, February 09, 2003 2:11 PM
> Subject: Desk top Dyno fun
> Listers,
> My guess as to why the Power falls off at high RPM for larger
> displacement is pretty simple. The bigger the slugs the
> faster it eats up horsepower at RPM. That is why the larger
> inch motors are best for low rpm duty. You can make em rev
> but it may result in diminishing returns due to the extra
> reciprocating weight.
> Tim Ronak
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