Sport Pilot wrote:


Steve,
As you said power is torque * RPM, so for the same torque more speed
is power.

Or for the same SPEED, more TORQUE is more power. That's the part of the
equation that you've never acknowledged.



Your example is a poor one most diesels of equivelant size will deliver
more torque at less RPM and have less total horsepower.

Not always true (obviously), and when looking at modern diesels its not
even GENRALLY true anymore. That's EXACTLY why I picked the example that
I did! If you don't like the Dodge comparison, go look at a
Ford/Navistar 6-liter diesel and compare it to the closest-sized Ford
(5.4L) gasoline engine tuned for a truck application. 10 years ago, the
diesel would have won on low RPM torque, but been lacking in peak HP.
That is no longer the case most of the time, because diesels are now
*always* turbocharged, and most of the time have modern electronic
injection control over both injection rate AND duration.

I don't know
where you found that pitiful Dodge 5.9 liter engine.

There's nothing pitiful about it at all, it was an EXCELLENT truck
engine used from 1972 until 2003. When built for power rather than
torque, it can easily put out well over 400 Hp without turning 7000 RPM,
but as delivered in factory trucks, it was tuned for a torque band
that's flat as Kansas over about a 3500 RPM span, and that results in a
rather modest 230 HP.

I have a 4.7 V8
in my Grand Cherokee and it puts out 260+ HP.

The 4.7 is the replacement for the 5.2, and while a fine engine in its
own right, its a little bit low in the torque department. It doesn't
move the fullsize Ram pickup or even the Durango with much authority-
the old lower-HP rated 5.9 actually "feels" a lot peppier in around-town
driving because it has more torque below 3000 RPM than the 4.7L does,
despite a lower peak HP rating. That's a symptom mostly of its small
size- when you excessively constrain the displacement of an engine, you
start HAVING to spin it faster and faster to get the same power and you
sacrifice torque at the lower RPM levels- Which is a key part of the
point I've been driving home. Modern gasoline-powered cars and trucks
tend to have very high peak horsepower ratings- and yet many of them
feel weak compared to older cars, simply because all that high-RPM
horsepower comes at the expense of useful low-RPM torque, and they need
5- and 6-speed transmissions just to match the performance of older
torquier engines.

Gas engines have been pushed smaller and lighter by fuel economy and
emissions considerations, resulting in peaky torque curves, poorer
low-RPM torque, and higher peak HP to compensate.