Hillclimbing - What's harder, revs or gas?

[Hokiefyd]

Originally Posted By: Velo_Fello
Speed for speed, you'll be creating the same power.

But if you have more revs, you'll have a greater flow of oil around the engine, and a lower load on your bearings.


That's a good way to look at it I think. As you say, to drive a given vehicle up a given grade, you need a given amount of power. Whether the engine's turning 2500 RPM or 3500 RPM, it's making the same amount of power. At 3500 RPM, you're likely generating more heat, but you also have more coolant and oil flow.

I agree with gearing down one gear. There's a similar grade on I-77 coming out of North Carolina into Virginia. My Dakota won't downshift out of 4th gear unless I REALLY push the gas. It's got such a low-revving V-8, it'll pull most any hill in overdrive. The van will stay in overdrive, but will cycle in and out of torque converter lock. So I pull it down into 3 and let it rev up the hill. It's about 2800-3000 RPM at speed in 3rd gear. I haven't driven the Camry up that grade yet, but I imagine shifting down into 5th would be about right for it.

 

[rslifkin]

Hokie - My Jeep is the same way as your Dakota on the hills. I've never found a hill that it wouldn't accelerate up at highway speed in OD with the TC locked. The engine is perfectly happy and pulls strongly down to about 1200 - 1300. At 1100 or so, the power starts to drop off.

 

[Kurtatron]

I've gathered the following responses:
A)Downshifting flows more oil.
B)Downshifting causes greater heat.
C)Downshifting shifts the engine revs closer to the power band.

The reason I held the impression "Lower revs" is better is because:
A)Oil life lights use total revs to gauge oil condition.
B)The stress on internal parts increases exponentially with revs. I don't have the source at hand, but I read in an old Turbo Handbook that reciprocating motion doesn't take to high revs too well. So I conjured up the idea minimized bearing wear = less revs. Now I'm wondering from you all: Is this factor just not significant at lower revs? Is the greater oil flow enough to negate this?

 

[expat]

"A)Oil life lights use total revs to gauge oil condition."

Sorry I don't understand??

"B)The stress on internal parts increases exponentially with revs. I don't have the source at hand, but I read in an old Turbo Handbook that reciprocating motion doesn't take to high revs too well.

Well we are not talking about 'Red Lining' it are we?
You will not be stressing anything to any significant degree.

 

[mechtech2]

'Lugging' MAY be harmful to the engine under some severe/special circumstances, but NOT to the oil.
Climbing a hill like this in the wrong gear will not hurt your engine.

So the answer depends on heat.
Which method is getting the oil hotter? It will take the same power, no matter what the gear.
I suspect that being in an inappropriately low gear[high RPM] would make it hotter - more shearing/drag through the oil.

But full throttle in top gear? So what? The engine is actually more efficient, as it does not have to produce intake vacuum constantly. This takes power.

 

[440Magnum]

Originally Posted By: Kurtatron
My commute requires a climb of 2000ft each way. This occurs within 15 miles. The steepest grade is about 7%. I.E. Lowest gear for a cyclist. I've been of the mindset "Less revs is better." But my miata has double digit horsepower and torque. I really have to give it gas to maintain speed in 5th. At about 2500rpm, my acceleration also stinks.
What would you think strains the oil and motor more? Remaining in 5th and reduced revs or increasing the revs but less gas pedal?


Lugging is much harder on mechanical parts than a few more revs. If you're lugging in 5th at 2500, you should still be just a little over 3000 in 4th (just guessing- don't know your exact ratios) and the engine will be much happier. If it were a question of putting it ANYWHERE near redline, the answer might be different- but you're still talking about being in the happy middle of the power band vs. lugging along at the bottom of the torque curve.

I have no idea where the general conclusion that "higher RPM generate more heat" came from- combustion chamber temperatures and exhaust gas temperature will actually be lower at a higher RPM and lower manifold pressure (throttle more closed) than lugging at low RPM and high manifold pressure. There is also less chance of detonation.

 

[Captain_Klink]

as long as you are in the power band, you are fine. if your choice for uphill is the lower part of power band vs upper part of power band, take the upper part of power band. the power band is nowhere near the redline.

 

[dparm]

Lugging is hard on lots of components of the car like the transmission and motor mounts. I doubt there's a significant difference in the lubrication in this scenario.

 

[JustinH]

Don't lug the motor, downshift it and let it rev a bit. Slow down and keep the car on the right lane, you aren't driving anything with a lot of power.

 

[Nyquist]

See if you can find the power curves for your engine on Mazda's web site. GM makes those readily available. What I generally do is try to stay within that since the engines in my vehicles have a nice flat torque curve. That will yield great results for driveability and fuel economy.

 

[White 03]

I believe lugging is defined as when you cannot accelerate. I don't have one now but that is what I have read in owners manuals for big trucks.

 

[Kurtatron]

Now I thought lugging occurred at very low revs and under load. Lugging causes the motor to vibrate violently. The low oil flow supposably is terrible for the bearings.

 

[Gary Allan]

You would be correct. What most are calling lugging here occurs on every engine dyno to determine what the peak power points are.