Best speed, rpm and Peak Performance

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Best speed, rpm and Peak Performance

May 29, 2013, 11:28:45 am

There was a post by Bret Wolfe related to this in April 5, 2010, "best RPM to dirve up a mountain road" and Tim Fiedler gave good educational links in February 16 of this year, "gear ratio and transmission in 1995 U300 and U280 GV". You may want to read both. A friend with a new to him cat 3126 asked me what rpm he should use? So I began to research, only for the ISM. (I still need numbers for the 3126)

Question: What is the best rpm range to drive an ISM in a motorhome? What is peak performance in that context? This is what I have found to this point, perhaps way more than most want to know but some gear heads and mechanical engineers on this Forum can probably help me out. Thanks in advance.

Per Cummins, the ISM has peak torque at 1200 rpm and brake horsepower maximum at 1800 rpm. The best performance and recommended cruise rpm is 1700 -2000 rpm. The best economy is 1200 - 1800 rpm. Their FMCA presentation though says to go with the lowest rpm (for economy), "allow engine to lug-back while climbing grades...". They say that their engines are most efficient at lower rpms.

So, what does "peak performance" need to mean to me? If I were get into that peak peformance range and the tail end of the best economy range, I would be at 1700 rpm and going way too fast unless I dropped into fifth gear, I guess. Do we just ignore peak performance, is it more for very heavy loads?

I tend to stay in the 1300 - 1400 range at about 63 mph and around 1350 rpm. Seems to give a good speed, emphasizes economy but am out of that Cummins cited best performance range.

I would like to think I could distill into something like, "keep the rpm at 1356 once you are up to highway speed". That hits pretty much what Bret reported, "If your coolant temperature does't rise, that is the way I do it (150 - 200 rpm over peak torque)" and as usual he has the technical background to support what he says.

I have begun to think that our motorhomes are set up intended that we do not operate at peak performance, as you might want in an airplane that adjusts peak torque and horsepower (power) to more coincide. See this interesting article, Power and Torque: Understanding the Relationship Between the Two, by EPI Inc.

The site that Tim gave us....http://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

Your corrections and further explanation are appreciated,

Thanks....Mike

Re: Best speed, rpm and Peak Performance

Reply #1 – May 29, 2013, 01:05:30 pm

Mike

I've been reading those links. My head hurts.

Glen

Re: Best speed, rpm and Peak Performance

Reply #2 – May 29, 2013, 01:18:43 pm

This has always been a topic of interest to me as the engineer in me refuses to retire...

That said, I decided some time ago that I was going to drive like an old man...wait...I am an old man...

Driving at the C8.3 sweet spot of about 1500 to 1600 RPM gets me to about 60 MPH. That's been good to me for many years and yields the optimal fuel economy on the Cummins. Of couse, the average mileage will vary as other factors enter into the equation.

Nonetheless, driving at the lowest RPM when maximum torque is reached for your engine is the best for the engine performance, economy and longevity. IMHO. (as you all know, tire pressure, wind, terrain, throttle usage habits, etc. have a lot to do with engine economy).

Then again, some folks like to drive and are comfortable at faster speeds, and that's fine with me. I'm not doing their driving for them.

Enjoy!

(Oh, oh...I'm begining to sound like Dave M.)

Re: Best speed, rpm and Peak Performance

Reply #3 – May 29, 2013, 01:45:26 pm

Quote from: Peter & Beth – May 29, 2013, 01:18:43 pm

(Oh, oh...I'm begining to sound like Dave M.)

Peter, You need to be careful, I would not suggest even knowing that idiot.

Re: Best speed, rpm and Peak Performance

Reply #4 – May 29, 2013, 02:08:15 pm

Quote from: Dave M – May 29, 2013, 01:45:26 pm

Peter, You need to be careful, I would not suggest even knowing that idiot.

You're way too modest...

Re: Best speed, rpm and Peak Performance

Reply #5 – May 29, 2013, 04:11:05 pm

I was on US 30 in PA yesterday. Peak RPMs for me was about 1800 and I was going up the hill in third gear at 35 mph. Came down in second at 20 mph speed limit. On the flat lands I will run the speed 65 usually in 6th gear and get around 7.8-8mpg. However at 55 I will be in5th gear but get 8.5 mpg. I find that going less than the legal limit by 10 or 15 or so less than the avg speed makes for a very dangerous trip. Out west you are ok but east coast I 95 will be an issue of cars zooming up pulling in front and stopping or trying to get around you to get to their exit. More close calls that way then when driving at speed in the middle lane.

Oh and RPMs are anywhere from 1300 to 1500.

Re: Best speed, rpm and Peak Performance

Reply #6 – May 29, 2013, 04:59:53 pm

Generally speaking best fuel economy/performance will be at peak torque. Best giddy-up will be at max horsepower.

On the subject of fuel economy. I had a friend back home who owned a 1908 Napier racing car. It was driven originally by S.F. Edge at Brooklands but was banned from racing in France because of it's detachable wire wheels gave it an unfair advantage. Lots of punctures back then.

Anyway, when asked by a driver of an D-type Jaguar at a historic race meet why he got such amazing fuel economy for such a monster Ronnie relied "My cylinders are larger than yours but they don't fill as often".

Explanation: Although the D-Type had an engine 1/3 the size of the Napier it revved so much higher as opposed to the Napier's 1200 rpm max.

Keith

Re: Best speed, rpm and Peak Performance

Reply #7 – May 30, 2013, 12:58:28 am

I find when I run at full torque (at torque peak) the exhaust gas temp goes way up (1100-1200 after the turbo, and I get nervous. Anyone else with an EGT gauge see that behavior? Just had it in to Cummins to see if any concerns, and the tech did pretty extensive testing and said all was OK, but I still wonder...

Re: Best speed, rpm and Peak Performance

Reply #8 – May 30, 2013, 04:17:35 am

What is the intake temps? my intake runs about 10f above ambient, meaning the CAC is doing a great job, no I do not have the egt readout, but I am not hot rodding my engine either. With that temp range, are you reading the before or after turbo temps ? before that would be fine, after you are in the trouble zone, per Cat, Cummins etc. Just maybe your egt is not real accurite ?
The more you know, the more you realize you dont.

Re: Best speed, rpm and Peak Performance

Reply #9 – May 30, 2013, 08:42:42 am

Dave K I have intake temps but not an EGT temp on my coach. I know I run cooler since the resonator on botht the tranny and the engine. I think it is because the engine is running cooler the radiator is not working as hard and the transmission is cooler as a result. I have not had any issues with high EGT temps as would be noticed by failures. I am at `143K miles on this unit and not not baby it up hills but rather hammer at them and hit them at speed if I can and hold the speed if it is a short hill. Out in Denver, I did not put my foot into it and let it go up a bit less than 100 percent engine load do to the high temps outside (90s) and the grade and the length. I did not have any issues but did get up to 220 degrees. I have hit that one other time and that was going west out of Rapid City and the outside temps were 110 degrees. It was hot and there is not a lot of air higher up. Now that would have been interesting to see the EGT but I do not have that gauge in there.

Re: Best speed, rpm and Peak Performance

Reply #10 – May 30, 2013, 08:45:30 am

Not to divert you two Dave's from the temp and cooling issue related exactly to this discussion, and noted by Bret too in selecting rpm, let me move the non-hot ridding gear head to what I guess I am hearing.

Dave, am I to interpret that Cummins adjusted their torque and power and rpm relationships so we run near peak torque to maximize fuel economy whereas if we were heavy loads they would make the torque and power somehow overlap more so they get less economy but better pulling power? That would mean they made the ISM in two versions, Motorhome and truck?

(Not to imply Dave M in the hot rod comment that you exceed the speed limits but you have the ability if you wanted to!)

And too, for my friend with the 330 hp Cat, I gather he may have different rpm for his running, an idea what that might be? I somehow think it higher rpm than the ISM?

Re: Best speed, rpm and Peak Performance

Reply #11 – May 30, 2013, 09:14:01 am

Yes, the Caterpillar 3126 makes max torque (860 lb-ft) at 1,440 RPM. With all engine makes, the smaller the displacement (that Caterpillar is 7.2 liters) engine speeds are higher.

Re: Best speed, rpm and Peak Performance

Reply #12 – May 30, 2013, 09:18:18 am

Quote from: Dave Katsuki – May 30, 2013, 12:58:28 am

I find when I run at full torque (at torque peak) the exhaust gas temp goes way up (1100-1200 after the turbo, and I get nervous. Anyone else with an EGT gauge see that behavior?

I haven't noticed correlation between RPM (considering torque curve) and EGT. To me, the EGT appears to be related to how hard the engine is working. According to documents I found in the coach, 1400F is the redline. I put a piece of red tape on the EGT gauge at 1350F. Marilyn and I watch when we are climbing mountains. If it appears that we are approaching the red line, we back off a bit from our charge up the hill. I don't think we've ever reached 1350F.

Boost and EGT gauges were installed by PO along with Banks Stinger. Engine is Cummins C8.3-325.

Re: Best speed, rpm and Peak Performance

Reply #13 – May 30, 2013, 09:42:18 am

Thanks Bret, will pass that on to the family you helped buy the coach with the Cat. They having lots lots to learn, not knowing what they need yet but will get there in time.

Thanks

Re: Best speed, rpm and Peak Performance

Reply #14 – May 30, 2013, 10:05:16 am

Mike,

Lots of information here on diesel motorhomes in general and Caterpillar in detail: Diesel RV Club, an FMCA Chapter - Index

And on that site, for an excellent document on performance/mpg that applies to all diesel powered motorhomes, go to "Driving and Operation" and click on "Understanding Coach/RV Performance".

Brett

Re: Best speed, rpm and Peak Performance

Reply #15 – May 30, 2013, 10:18:35 am

does a diesel engine reach stoichiometric ?
Old round engines were leaned to the lean side of stoichiometric and showed an increase in torque for less fuel burned, the cylinder head temp's actually dropped up to 30 degree centigrade

Re: Best speed, rpm and Peak Performance

Reply #16 – May 30, 2013, 10:31:33 am

Bill,

I am not aware that diesel engines have a stoichiometric ratio.

Unlike gasoline engines that must have very close to the "proper" fuel/air ratio (13- 14.7/1 air/fuel), a diesel can happily run very lean (or rich for more power and smoke!).

Re: Best speed, rpm and Peak Performance

Reply #17 – May 30, 2013, 11:30:58 am

It would be neat to map the voltage to that gage and ID the 1300F point. Somebody like Neal or Jim F could design a simple circuit to give you a yellow, red light or an alarm. I'm just a simple electrician. and computer geek. If i were retired I would break out the books and 'figger it'...

That would have made me far happier when I had the Banks kit in my 93!

Re: Best speed, rpm and Peak Performance

Reply #18 – May 30, 2013, 12:33:04 pm

I love firing up the experts, that said I need a short lecture on how to run a diesel lean ?

Yeah for having enuff power to get-r-done.

Re: Best speed, rpm and Peak Performance

Reply #19 – May 30, 2013, 12:39:41 pm

Bill C,
I never heard that term, I just leaned to +50f (rich) of peak. I ran 75% as much as possible, never had an issue with cylinders on the Lycoming IO360 A1A, always was a great engine at 1800 hrs, 1200 + I put on it.
Cheers

Re: Best speed, rpm and Peak Performance

Reply #20 – May 30, 2013, 01:04:04 pm

Back to Dave K ,
Jus had pow-wow with a Cummins brain, says Max 1200f after turbo will be ok if your careful not going over ever. Your playing with $$$.

Re: Best speed, rpm and Peak Performance

Reply #21 – May 30, 2013, 01:38:00 pm

When we talk about peak torque RPM (peak= high?) are we talking the RPM the engine starts to develop rated torque or the RPM the engine will produce that torque to?

Re: Best speed, rpm and Peak Performance

Reply #22 – May 30, 2013, 02:14:39 pm

Peak torque RPM is the lowest RPM at which that torque is produced. Yes, many engines have a fairly flat torque curve that stays reasonably close to that torque figure throughout its operational RPM range.

Re: Best speed, rpm and Peak Performance

Reply #23 – May 30, 2013, 03:25:15 pm

Depending on the amount of fuel injected, diesels can run an air-fuel ratio of 16-25 to 1 at full throttle and over 145 to 1 at idle. That's why Euro taxis could idle all night using less than a pint of fuel.

Remember, there is no throttle plate on all but a few very old diesels. The intake passage is unrestricted all the way from the air cleaner to the intake valves/ports and the power is controlled by the amount of fuel the injectors inject.

Too much fuel can run the EGT to a point of melting the pistons. Bad turbo seal can do the same thing. Measuring EGT and managing the amount of fuel/temperature depends on where the EGT probe is mounted. Naturally, the closer to the turbo, the higher and more accurate the reading will be. Some engines have been damaged by a customer locating the probe a couple of feet away and adjusting the fuel accordingly.

Pierce

Re: Best speed, rpm and Peak Performance

Reply #24 – May 30, 2013, 03:32:40 pm

John, if you would like I can send you the torque and horsepower curves for my ISM in the 2001. Really interesting to me, two other threads plus a friend with a new coach led me to study this some versus just driving it. i knew the folks that enjoy this technical topics would give me help and open up related topics i had not thought about but are important. i now wonder if i need a way to monitor that temperature?

But for your need, the Cummins charts that came in the Motorhome operating manual may be in your manual but if not I can scan and send to you. Let me know if need

Mike

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