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Old 01-10-2016, 10:41 PM   #1
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Speed going up grades: max HP vs. max torque

I was looking through the Workhorse chassis guide and after a recent trip, got to wondering about going up grades properly. Maximum horsepower is at 4,200 RPM, with maximum torque at 3,200 RPM.

When going up a long grade, what is the best reference of what RPM I can sustain? In the past, I would run at about 3,200 RPM so as not to 'beat' on the motor by running at a higher RPM. Recently, I decided to run at 4,200 RPM over the same grade, which gave me a little more speed but not that it really mattered. What is the best RPM reference that I should be using in terms of engine longevity? Speed up the hill or fuel economy is not of primary concern.

Thank you. -RT
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Old 01-10-2016, 10:55 PM   #2
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Old 01-10-2016, 10:58 PM   #3
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I only have opinion. Someone may come along and give you a HP vs Torque run down and both responses probably have validity. My 03 Dolphin Workhorse had a "sweet spot" for hill climbing at 3700 RPM. That was the point where the engine sounded good and preformed well; meaning had ample power to pull 6 or 7% grades easily. 4200 RPM simply sounded too stressful on the engine. I'm suspect there is a fuel consumption issue as well between the two. I personally found little reason to run at 4200 RPM for any extended time such as pulling a long grade.

Generally on a 6 or 7% grade I could run approx 50 MPH, maybe a little less (I've since sold my MH). Patience, planning, relax you will get there was my attitude so the speed was plenty satisfying. Overtime I have read a number of comments about the "sweet spot" from others, I think it has merit. You certainly are not lugging at the RPM which is the worst you can do.

I suppose the Vortec 8.1 engine is designed to run at 4200 RPM but I was not, it just made me wince sensing that I was putting too much stress on the engine.
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Old 01-10-2016, 11:38 PM   #4
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Think you answered your own question

Your ran at 3200 RPM (max torque) and then you ran at 4200 RPM (max HP) and only gained a little more speed.

1000 RPM more on engine, more fuel spent, more heat generated and for what?
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Old 01-10-2016, 11:49 PM   #5
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I climb mountains at 3,600 rpm. Whatever gear I have to go to that keeps me moving at that rpm is where I keep it. Every now and then I'm down to second gear. Fine by me, all the better to enjoy the scenery.

I never exceed 4,000 rpm.
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Old 01-11-2016, 06:52 AM   #6
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I have run at higher RPM a few times but prefer not to. I would rather keep the RPM in the 3,700 vicinity. I kind of wish we had the push button gear selector I think it would aid in keeping the RPM where I want it. Don't care about speed, most times I am struggling up a hill I have plenty of company with the trucks.

For me, downhill is the bigger issue. Hit some hills in North Carolina where I was manually in 1st gear with climbing RPM. No help at that point except the brakes. Coach actually did really well though.
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Old 01-11-2016, 09:05 AM   #7
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Quote:
Originally Posted by Arch Hoagland View Post
I climb mountains at 3,600 rpm. Whatever gear I have to go to that keeps me moving at that rpm is where I keep it. Every now and then I'm down to second gear. Fine by me, all the better to enjoy the scenery.

I never exceed 4,000 rpm.

That's what I do too.

Uphill-
Very little gain above 3600 except for noise and gas consumption..

Down hill-

I gear down and brake hard at about 4200 until down to 3500 and repeat. Always works for me!
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Old 01-11-2016, 09:23 AM   #8
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Quote:
Originally Posted by Old-Biscuit View Post
1000 RPM more on engine, more fuel spent, more heat generated and for what?
That's what I'm trying to figure out. It's not that I'm in a hurry, it's that I'm trying to figure out, from an engineering standpoint, what's the best for the engine. There must be a reason that vehicle manufacturers give us the specifics of torque and horsepower ratings. At 4,200 RPM, it's not like the engine sounds like it's going to blow-up and at 3,200 RPM, it's not like the engine is lugging.

I can understand that some here don't want to go over, say, 3,700 or 4,000 RPM, but it sounds like it's more of a personal choice. Nothing stated WHY running at maximum horsepower is a bad or unwise thing. And again, it's not that I need to, just trying to understand the differences, since 4,200 RPM is not redline.

Some mention fuel economy but I can tell you that pulling a long grade at 3,200 or 4,200 RPM has no practical real-world difference. At either RPM, depending where I am on the grade, the fuel mileage can be 2.8 to 3.2 miles per gallon. I can get a lot more than a .4 MPG difference on flat-roads just between slight elevation changes and head-winds or even coming up behind a semi-truck.

I'm hoping that someone can explain the differences from an engineering standpoint. Naturally, running at 3,200 RPM is 'easier' on the engine (as long as it's not lugging) than 4,200 RPM, but then I could say that running at 2,600 RPM might give the engine more longevity than running at 3,200 RPM.

And as I have mentioned, I'm no speed demon, I'm in no hurry, (from a safety standpoint, some people on the highway might argue the speed differential as they come up behind me in my lane IS pretty important) just trying to understand it from a mechanical / engineering standpoint.

All opinions to my question are welcome... -RT
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Old 01-11-2016, 09:41 AM   #9
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Torque is the instantaneous twisting power of the engine, and hp is the cumulative torque over a period of time. So while 3200 is the peak torque rpm, you have more overall power available at the hp peak due to the additional rpms providing more total power. Is the engine safe at the hp peak rpm: yes. Will it be a lot of extra heat, noise and fuel used: yes. Just remember that power comes from fuel burned, so at higher rpm you are burning more total fuel, assuming full throttle and same basic conditions.

My suggestion is use something around 3500-3700 rpm range when you really need it on hills. It will be a bit more pleasant in the driver's seat than 4200 rpm. Watch trans temp, that is where you really have more concern than the engine.
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Old 01-11-2016, 09:58 AM   #10
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I focus more on engine and transmission temperatures and find the RPM and speed that keep the temperatures manageable. The higher RPM has the fan, water pump, oil pump and transmission pump spinning at a fast rate pulling heat out and circulating coolant/oil. Will there be a noticeable gain with a 500 RPM increase, hard to say.

I try for a high RPM with moderate throttle and I'll down shift as necessary so I can back off the throttle while minimizing speed lose if possible. Not too worried about RPM since the governor will protect the engine. I know I'll get to the top eventually even if I'm in with the truckers doing 25MPH in second gear but don't want to overheat doing it.
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Old 01-11-2016, 10:11 AM   #11
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When you operate above the peak torque rpm, you can pull a grade more easily. This is the result of the increasing torque as the grade/load tends to slow the speed/rpm towards the peak torque point. Once below the peak torque rpm, a downshift will be needed to advance above the peak torque point again.
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Old 01-13-2016, 12:13 PM   #12
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Being the torque curve is so flat on the newer engines and especially the 8.1.
Older engines did have a pointed torque peak which was a good RPM for cruising and possibly climbing. But it is horsepower, which needs rpm to get you to the top at a reasonable speed.

If you are anywhere between 2000 RPM and 3500 RPM the torque is much the same on the 8.1, and it does not drop that much even at 4000 rpm. You need the rpm to make the horsepower.

I would rather be at 1/2 to 3/4 throttle climbing at 3200 to 4400 RPM, than at 2400 to 3200 rpm at 3/4 to Full Throttle. As mentioned in the above Post, all the fluids are moving better at the higher RPM's. I perfer to monitor the road and even manually select a gear that will get me to the top with minimal shifting (no gear hunting) be it 3200 to 4200 RPM at half to 3/4 throttle.

Yes 3000 to 4400 rpm does not sound or feel good at full throttle and losing speed but sometimes that is only for a half to a full minute or two, to get over the top.

"Quote"

Because about 25% of GM's larger engines are typically sold for marine use, much of the testing on the Vortec 8100 was done using the marine market's more severe-duty parameters. In one particular test, called the "Marine Dock," an engine is run at virtually wide-open throttle for 300 hours straight (60-minute cycle—55 minutes wide-open-throttle/5 minutes idle). According to GM, this testing is more severe than standard durability testing used for truck applications.

Source http://www.strokerengine.com/BBCspec.html

"Quote"
So you’ve decided you want an 8.1L engine in your muscle car or your tow rig, and who can blame you? They make more torque at 800 rpm than a factory LS1 produced at any point in its rpm range. Because they were also intended for marine and medium-duty truck use, they were subjected to grueling development tests, including the “Marine Dock” test that involves running the engine at full throttle for 55 minutes and then idling if for five minutes, 300 times. In other words, they’re incredibly durable.

Source
How to Find an 8.1L Big-Block
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Old 01-13-2016, 12:43 PM   #13
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Until somebody straightens me out... my understanding is that the peak torque point is where the engine is really running efficiently so you want to get there. Once that is achieved the second issue is how well you can hold that force. The HP peak is more or less an intersection of more speed and dropping torque. If one can get into that band and stay there then they get the best results. That is where gear spacing comes into play. Stay above peak torque and below peak HP.

When I say efficiency I am talking about the ability to turn fuel into force for the motor in question. That is probably not the most fuel efficient place to be in terms of driving down the road.
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Old 01-13-2016, 12:56 PM   #14
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Torque is a measure of Work in foot - lbs...
HP is how FAST you can do that work...

so I like to be in the peak torque curve most of the time... and therefore like a WIDE torque band in an engine tune...

As a reference, look at the difference in the 'drama' between the large turbo diesel pickups I had or the larger cummins in the DP, and the N/A gas engine in the Jeep toad...

It is more of 'what I'm used to and like' then a safety thing, but the low rpm PUSH of a diesel seems so effortless to me and
so much less drama than the 6500 screaming rpm I need in the lil sewing machine of the jeep to really get anything done...

So I am more of the quiet and efficient runner than the pedal to the floor type.
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