REFLECTOR: Engine Selection

Al Gietzen ALVentures at cox.net
Fri Jun 18 11:42:29 CDT 2004


Subject: Re: REFLECTOR: Engine Selection

 

In a message dated 6/17/2004 10:25:59 AM Mountain Standard Time,
ALVentures at cox.net writes:

You can have high torque a lower rpm but if it gives you the same HP you've
gained nothing

Well Al, I have to disagree on that aspect. If you have higher torque at a
given RPM, Then you can turn more pitch at that RPM. If you have an
adjustable prop, that should relate to a change. Either lower RPM setting
for the same cruise speed (more pitch) or a higher cruise speed for the same
RPM. Unless I am totally hosed (sometimes I wonder). 

 

Kurt;

We don't disagree, just look at it a little differently.  Since HP and RPM
are directly proportional, a higher torque at a given RPM also means higher
HP.  Peak HP generally (always?) occurs at a higher RPM than peak torque,
probably at an RPM above where you want to run.  So you are thinking more in
terms of the torque curve, and getting the torque peak into the cruise range
is a good thing.  Somewhere around peak torque is usually where best economy
is.

 

It would be interesting to see the torque and HP curves for your engine;
that's what really tells the story.  My torque curve is very flat, and has
abroad peak over the 2300 - 2750 prop RPM range.  HP ranges from about 215
to 255 over that range, and continues nearly linearly upward to 285 at 3225
RPM (prop) - but, of course I can't go there.  I'd like to see a bit more
peaking of the torque around 2400.  Still, the calcs suggest I can get about
15 mpg at 225 mph at 8000 ft.  Won't it be fun to see what I really get?

 

Aircraft engines all run at RPM considerably below the peak HP.  A 200HP Lyc
(rated at 2800, I believe) is typically running around 150 HP at cruise.

 

Only time will tell. I have either convinced myself into buying a Packard in
a dragster suit... 

Or built a decent performing machine.  Wish me luck.

 

I'm sure you'll do fine.  I wish you all the luck possible.

 

Al

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