...Asking for power at low rpms means you're stressing the engine more than if it was turning more moderate rpms...

...Asking for power at low rpms means you're stressing the engine more than if it was turning more moderate rpms...

Mark,

Truly, truly, not wanting to start a flame war, just a quiet discussion, because what you state is a very common opinion, but I haven't been able to back it up, at least for normally aspirated engines.

Detonation and preignition problems aside (which are a problem at whatever rpm they occur), horsepower (how hard we're working the engine) is determined by rpm and torque. When the revs are low, the available horsepower is low (assuming WOT), so the engine demands are low. Also, low rpm have a direct on fuel economy (duh, Mike, when did you figure that one out?) :D. It also reduces engine wear (rings & cylinders, mainly). Charles Lindberg demonstrated to the Army Air Force in WWII that running their aircraft engines (both air and water cooled) at high manifold pressures and low rpm (which gives the same horsepower as lower manifold pressures and higher rpms) resulted in longer range with no adverse effect on the engines.

I've heard that "Well, the bearings take a beating." The only time I've seen bearings significantly worn was in a turbo'ed engine that had been overboosted. Other than that one, all I've seen is just ordinary wear.

Now, acceleration performance is going to be pitiful in 6th, but that's what lower gears are for! :D

So, what'cha think?

Have a good one,
Mike

Mark,

Truly, truly, not wanting to start a flame war, just a quiet discussion, because what you state is a very common opinion, but I haven't been able to back it up, at least for normally aspirated engines.

Detonation and preignition problems aside (which are a problem at whatever rpm they occur), horsepower (how hard we're working the engine) is determined by rpm and torque. When the revs are low, the available horsepower is low (assuming WOT), so the engine demands are low. Also, low rpm have a direct on fuel economy (duh, Mike, when did you figure that one out?) :D. It also reduces engine wear (rings & cylinders, mainly). Charles Lindberg demonstrated to the Army Air Force in WWII that running their aircraft engines (both air and water cooled) at high manifold pressures and low rpm (which gives the same horsepower as lower manifold pressures and higher rpms) resulted in longer range with no adverse effect on the engines.

I've heard that "Well, the bearings take a beating." The only time I've seen bearings significantly worn was in a turbo'ed engine that had been overboosted. Other than that one, all I've seen is just ordinary wear.

Now, acceleration performance is going to be pitiful in 6th, but that's what lower gears are for! :D

So, what'cha think?

Have a good one,
Mike

My rationale is that if I am calling for 100 hp from my engine, its going to require less torque at 2000 rpm than it will require at 1000 rpm by a factor of 2. In my mind, less torque means less stress on the engine. The reason that I've picked 100 hp is that you're not likely to be at WOT in 6th gear down at 1000 rpms, but you may need that amount of power for climbing a hill.

I understand the arguement that turning less rpms reduces cylinder wear and so forth. But I also think you need to balance torsional stresses with wear.

I just feel better with keeping my rpms in the 1500 to 2200 range when cruising.

- Mark

I think my manual says not to use 6th gear under 900RPM in that gear. If I have to do under 40MPH, I stay out of 6th. Anything 40 and above, it runs just fine.

True, torsional loads, like friction loads, do place wear on an engine. But, what we're mainly considering here is highway cruising with slight rises and dips. Sure, when you're going to be climbing a significant incline (or wanting to pass that old geezer in the pretty blue '00 hardtop ;)), you'll want a lower gear. It's just that you want that gear from a performance standpoint rather than a reliability standpoint.

And now consider this (which I'm not claiming as a fact but it seems logical). The harmonic dampener is mounted on our engines to maintain control over the torsional loads that you mention. But, those loads are peaking in the upper rpm ranges, not the lower ranges.

Good discussion for a Monday morning.

Have a good one, :seeya
Mike

I do some towing with my truck, and I know that often times the truck runs cooler and with better fuel economy at higher rpms than lower rpms at the same speed...

One indicator that I use for gear selection is how much throttle input I need when driving. If I find I'm needing to use lots of throttle input to compensate for hills or whatever, I'll select a lower gear that tends to not need so much throttle input.

I'm not exactly sure what your point is regarding the harmonic balancer. My understanding of the harmonic balancer is that it counters some of the transient spikes in torque that occur during fractional rotations of the crankshaft. I really have no idea how it impacts reliability or wear.