Bob, Derek, Tigers,
Some more studying and experience recently gained with a higher quality
engine simulator program has led me to believe that Derek may be the
most correct in this matter. I now believe that the primary difference
between the big engine and the small engine in the high RPM extremes is
primarily due to engine friction. There appears to be some secondary
affects related to pumping efficiency like Bob anticipated but friction
seems to be the main reason why there is a point where the small stroke
motor makes more brake HP than the long stroke motor.
It had seemed to me that if cam timing, induction tract and heads are
identical then the big engine should always be the better pump, at least
when the motor configurations are not too dissimilar. The big motor
will max out the intake tract sooner and start seeing degradations due
to this before the small motor but I did not believe these effects
explained the dramatic drop off in power I was seeing with my initial
simulations. Friction would have to be the culprit I thought but even
though that program did model friction it predicted it was only partly
responsible for the discrepancy, it indeed showed the little motor to be
making more cylinder pressure and by inference moving more air than the
big motor at equal RPMs up in the high range. Wow, brain teaser!
The more sophisticated simulator I have been playing with the last
couple of days shows good correlation between engine total CFM and
indicated HP as can be expected and Bob predicted. It also supports
what I stated in the previous paragraph, the big motor is the better
pump (higher CFM) even when the little motor is making more brake HP.
This is in contradiction with the first program I used which predicted
that the little motor actually beat the big motor with respect to
indicated HP and therefore was somehow the better pump. This claim by
that program was a concept I had great difficulty with, in the end I was
willing to accept for the time being that it had to do with pumping
efficiency/tuning although I accepted it reluctantly. If that low end
program was correct what choice did I have? I now seriously doubt it is
correct, that program may in fact be a piece of crap in some respects.
So in the end Derek seems to get the prize, my world view is salvaged
and I can move on to more pertinent tuning issues. On the other hand, I
am just jaded enough to know that the issue is far from settled and
there is more here than meets the eye even still. Maybe the more
sophisticated program is the piece of crap, eh? I've got several engine
theory books on the way and perhaps they will settle this issue (at
least in my mind) or perhaps my world will once again lie in shambles on
the virtual dyno room floor.
Frank
P.S, the sophisticated simulator is from Performance Trends. It has a
lot more inputs and allows you to better characterize the intake and
exhaust tracts than the Mr. Gasket software does. The Mr. Gasket
software gives extremely optimistic power output numbers compared to the
Performance Trends software. The Performance trends software seems to
come pretty close to matching dyno curves I have for engine builds
documented in magazines and on the internet. I'm still evaluating both
but when I am done I may post a brief review of my findings.
>
> Consider this; why does any engine of a given displacement
> reach a point where it begins making less horsepower at
> higher rpm? The same reason, say a 289 would make less
> horsepower at 6,500 rpm than at 5,000 rpm is the very same
> reason a 376 ci engine makes less horsepower at the same rpm
> (5,000) as the 289. The specific reasons include, for
> example, the manifold vacuum is going to hell, which is
> indicative of volumetric efficiency, the result of turbulent
> flow which increases friction, inertia in stopping and
> starting the air mass, etc., etc. Even without valve float or
> any other extraneous reason, as you increase an engine's rpm
> past a certain optimum, horsepower starts to fall off. It's
> basically just a dynamic air flow problem; the same
> induction, heads, cam, exhaust, etc. will peak at the same
> cfm regardless of displacement or rpm.
>
> Bob Palmer
> Still, it might just be a crap program.
>
> cheers, derek
>
> Derek White
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