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Old 02-12-2013, 01:10 AM   #43
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I dont know about you big guys but I only have a super C... I thought my 6.6 Duramax would be under-powered but coupled with the Allison trans, my wife constantly has to tap or hit my arm to slow down (have bruises sometimes to prove it)...lol.
I am not deaf in the right ear I have just acquired the means to tune her out so well.
With all the suspension improvements learned and installed from this site, she operates more like smaller sized vehicle ... sometimes I fool myself into thinking it handles better than my pickup truck
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Old 02-12-2013, 07:59 AM   #44
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Most of you probably already know this but for those who dont it might be helpful to know where your peak power is via some calculations. I am an automotive engineer and used to run engine dyno's. But its been a while so hopefully I remember things well enough to explain in a way that makes sense.

Everyone talks about HP and seems to be familiar with it. Seems odd that a corvette with 556 hp has more than a semi pulling 20 times the weight of the vette. To think that your RV may only have 300 HP seems a bit undersized right?
Thats where torque come in. HP is really only a calculation of torque and RPM. So RPM has a huge impact on calculating HP. Which is why a little 600cc motorcycle can achieve 125 hp because it cranks the rpm up around 12,000rpm. Diesels typically run at a much lower RPM and so they typically have really high torque to achieve that modest HP number and yet is a much stronger engine when it comes to pulling.

To calculate your HP you take the torque in ft/lbs x the RPM and divide it by 5252
So to find out the RPM range that your engine pulls the most you reverse the calculation and take your known HP x 33,000 and divide that by the torque x 6.283.

So for my 330 engine and I know it puts out about 950ftlbs of torque I take 330hpx33,000= 10,890 and divide that by the 950 T x 6.283= 5968
so 10,890/5968=1824rpm So my engine pulls the best at 1824rpm

For those that are interested HP was developed by James Watt for a way to calculate the power of steam engines (not sure on the date would have to google it) He found out that an avg horse could pull 330lbs for 100 feet in 1 minute which is where the 33,000 number comes from. 33,000 ft lbs/min. Which is also equal to 746watts.

This would be easier to explain if I could have entered it more like a formula than typing it out but hopefully it makes sense.
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Old 02-12-2013, 04:18 PM   #45
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[QUOTE=So for my 330 engine and I know it puts out about 950ftlbs of torque I take 330hpx33,000= 10,890 and divide that by the 950 T x 6.283= 5968
so 10,890/5968=1824rpm So my engine pulls the best at 1824rpm[/QUOTE]

So if I understand you right, if you are pulling a grade you would try to gear down to keep it in that 1824 RPM sweet spot?
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Old 02-12-2013, 04:22 PM   #46
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It's a trick question. There is no such thing as enough power!
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Old 02-12-2013, 04:44 PM   #47
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greatlakes, so if I am doing the math right, my 380 Cummins with 1050 ft lbs would like to be geared down pulling a significant grade and held right around 1900 RPM.

Does gearing change this number?

Same question worded differently, sorry bout that.
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Old 02-12-2013, 04:54 PM   #48
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greatlakes, so if I am doing the math right, my 380 Cummins with 1050 ft lbs would like to be geared down pulling a significant grade and held right around 1900 RPM.

Does gearing change this number?

Same question worded differently, sorry bout that.
The gearing will not change the engines peak power just at what speed that equates to.
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Old 02-12-2013, 06:08 PM   #49
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But of course, I should have figured that out on my own. Gearing that is.
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Old 02-13-2013, 12:00 PM   #50
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Quote:
Originally Posted by greatlakes View Post
Most of you probably already know this but for those who dont it might be helpful to know where your peak power is via some calculations. I am an automotive engineer and used to run engine dyno's. But its been a while so hopefully I remember things well enough to explain in a way that makes sense.

Everyone talks about HP and seems to be familiar with it. Seems odd that a corvette with 556 hp has more than a semi pulling 20 times the weight of the vette. To think that your RV may only have 300 HP seems a bit undersized right?
Thats where torque come in. HP is really only a calculation of torque and RPM. So RPM has a huge impact on calculating HP. Which is why a little 600cc motorcycle can achieve 125 hp because it cranks the rpm up around 12,000rpm. Diesels typically run at a much lower RPM and so they typically have really high torque to achieve that modest HP number and yet is a much stronger engine when it comes to pulling.

To calculate your HP you take the torque in ft/lbs x the RPM and divide it by 5252
So to find out the RPM range that your engine pulls the most you reverse the calculation and take your known HP x 33,000 and divide that by the torque x 6.283.

So for my 330 engine and I know it puts out about 950ftlbs of torque I take 330hpx33,000= 10,890 and divide that by the 950 T x 6.283= 5968
so 10,890/5968=1824rpm So my engine pulls the best at 1824rpm

For those that are interested HP was developed by James Watt for a way to calculate the power of steam engines (not sure on the date would have to google it) He found out that an avg horse could pull 330lbs for 100 feet in 1 minute which is where the 33,000 number comes from. 33,000 ft lbs/min. Which is also equal to 746watts.

This would be easier to explain if I could have entered it more like a formula than typing it out but hopefully it makes sense.

I tired this a couple of times before I realized 330X33000 is 10,890,000 and that works out. My 330X33000=10890000 and divide that by 860 Torque X 6.283 (5403) and the rpm would be 2015. Does that sound right? I usually pull long grades about 2000 rpm because that "seems good" but cruising is closer to 1800 rpm.
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Old 02-13-2013, 12:29 PM   #51
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Good information, I did the math and it is saying 2,188. Just so I understand when going up a grade I should downshift to keep my RPMs around 2,188?

Cumming 275 ISB 275hp/660tq

275x33,000 = 9,075,000
660x6.238 = 4,146
9,075,000/4,146 = 2,188
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Old 02-13-2013, 02:06 PM   #52
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I tired this a couple of times before I realized 330X33000 is 10,890,000 and that works out. My 330X33000=10890000 and divide that by 860 Torque X 6.283 (5403) and the rpm would be 2015. Does that sound right? I usually pull long grades about 2000 rpm because that "seems good" but cruising is closer to 1800 rpm.
Yes that seems about right. Just because your peak TQ is at 2015 they would not necessarily be where they would gear the engine for crusing, (because of fuel economy reasons).
Makes even more sense with a passenger car. The peak TQ would typically be more along the 4500 RPM range but you wouldnt want to drive down the HWY so they gear it down close to 2000 RPMs or less (hence OD). The nice thing about these big Diesel engines are the torque curves are pretty flat. So although your peak power may be at 2015 your probably within 10% going 500 RPMs on either side of that 2015.
Ideally though yes pulling up a steep hill you would want the engine in that range so downshifting to achieve that RPM would be your best option.
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Old 02-20-2013, 03:29 PM   #53
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It's a trick question. There is no such thing as enough power!
X2!!
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Old 02-20-2013, 11:48 PM   #54
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My last coach was a 37 footer with a Cummins 275 HP engine...it had plenty of power to pull even the toughest hills. It just wouldn't do it as quickly as the larger engines would. But never had a problem towing my 2002 Jeep Grand Cherokee. It got a lot better milage than the CAT 350 HP that I've got now.
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Old 02-21-2013, 12:19 AM   #55
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On our ISC with Banks it works out to 1,900 rpm
435*33,000 = 14,355
1200*6.283 = 7,539
14,355/7539 = 1,900 approx.
The published charts from Banks show approx 435 HP @ 2000 rpm.
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Old 02-21-2013, 02:03 AM   #56
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This is some good information. I tried to work out a formula that makes it easier to remember the calculation. I also did poorly in algebra so if it is wrong or not written right then please educate me.

Here goes: (HP x 33,000) / (T x 6.283) = Peak Power in RPM's
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