Jump to content

Engine RPMs


Jimalberta

Recommended Posts

I discovered that torque on my MH does not improve with rpms. We were making that long climb out of Laughlin and started gaining on a Dutch Star half way up. I had it in manual and was doing about 2200 rpm and was making very little gains on this other MH and didnt want to tie up the left lane but wasnt happy having to ease up and follow him as he was slowly slowing down. Finally I geared up one gear and dropped to 1600 rpm and the MH took off and passed the Dutch Star with ease. Lesson learned .....keep the rpms in the torque band which for me is 1200 to 1600 rpms.

 

What do you guys find as your best power band?

Link to comment
Share on other sites

Usually torque peak is around !,400 rpm. Peak HP is around 2,100rpm. As you found, peak torque works the best for pulling. The reason to run at higher rpms is for increased coolant circulation, and increased fan rpm, both to prevent or reduce overheating the engine.

Link to comment
Share on other sites

Torque peak and HP peak can vary on engines. Get the SNO off YOUR engine & call the engine mfr, most of them can give it to you. Or, a (engine brand) dealer should be able to access your particular config. And, can tell you if it can be bumped up, etc

Link to comment
Share on other sites

My engine is a Cummins 8.9 ISL. 370 HP , 1200 ft lbs of torque at 1300rpm. Exhaust brake...wish it was a jake.....Allison is set to drop to 4 th on engine brake activation.

 

I like to travel at about 58 to 60 mph. Usually keep the economy mode on...less up and down shifting on hills.

Link to comment
Share on other sites

If you look at the HP and torque curves for these diesels you will notice that they are rather different than those for gasoline engines. The first thing you notice is that rpm cutoff is much lower than you are used to; the comment about 2100 being a governor stop is probably quite accurate. The next thing you notice is that the torque curve is rather flat over much of the rpm range; in fact some diesels will exhibit their highest torque just before stall speed. This is what gives big diesels their incredible pulling power; the slower they turn the more torque they have!

 

With my CAT C12 the torque peak is down around ~1100 rpm and peak HP is in the 1700-1800 range. With my gearing I can run 65 in 4th and I find this a delightful gear to be in when either climbing or descending hills. For climbs this puts the engine at ~1600-1700 rpm just below the HP peak. Locking it in 4th also keeps the Allison from hunting from one gear to another.

 

On long, steep descents by being in 4th before the Jake engages eliminates the downshift "jolt" and the Jake simply goes on and off as I touch the accelerator.

 

Each MH will be different depending on the gearing, weight and engine size. For example, my friends with the larger Beaver Marquis in the same period as mine are geared higher so they can't go above ~57 in 4th so this strategy isn't as useful for them. As you get used to your MH you will find what works best for you.

Link to comment
Share on other sites

Interesting Ray....know what you mean on the coolant circulation but with a side radiator my fan runs independant of the engine rpm. I have never seen my heat guage move up from normal.

I know, I have a Spartan MM GT chassis. My heat gauge will climb to ~220 on a hard pull when turning about 13/1400 revs, but if I manually downshift to bring rpms up to ~1800, the increased coolant flow will lower the temperature back to just under 200, which is where it normally stays when on level ground during summer. 220 is still not enough to trigger the fan solenoid.

 

Link to comment
Share on other sites

Do you find at 1800 rpms that your pulling power is still good?

 

We bought diesel today and after the drive from Las Vegas up the Virgin River Gorge and continued climbing it seems all the way to Idaho....I got pretty dismal mileage...about 6....ouch. And that was keeping my speed at around 60 mph but it was a good pull for the engine.

Link to comment
Share on other sites

On a largely uphill climb like you just finished I don't think you can expect your mileage to be any better. Las Vegas is at ~2100 ft and, no matter what the grade of the road is, you effectively have to "lift" your MH and toad from there to whatever elevation you ended up at, not even considering all the ups and downs along the way. Physics is physics and work is work; there's no way around it, so your mileage is always going to be poor on a day like that.

Link to comment
Share on other sites

Our ISL370 (now flashed to ISL400) was built by Cummins in July of 03, and in an 04 coach. Suspect our engines are very similar. (The flash to ISL400 was reported by many to be not worth the $$. Very minor changes in both the HP and Torque curve, with the exception of top end is now 400HP at the upper end of the HP curve. Plus the early side of the HP curve is a bit better too, so less need to go up in RPM to obtain HP.)

 

I would suggest you try the same hill two different ways, and see what works best for you.

1) Climb it with the RPM being kept between 1550-1650 RPM, which should work well of you on the Torque side of things.

 

2) Climb it with the RPM being kept between 1850-2000 RPM, which should work well for the HP side of things.

 

You should find that you climb the hill easier, and a bit faster, with the 1850-2000 RPM being maintained. It will also keep your coach running a bit cooler too.

 

As mentioned, many differences between coaches with the same engine. (Coach weights; Gearing; Wind Resistance - are the big three.)

 

MPG on long haul climbing at 6 MPG could be a bit on the low side. Our 40', with Tag added weight, runs about 7-7.5 MPG on average. (Some runs at close to 9.5 MPG, and some runs in the 6 - 6.5 MPG - so MPG does vary.) We run in California at 57 MPH. And other states very seldom get above 65 MPG. I like 62-63 MPG for comfortable cruising. But again, our coaches have variances between them. And also, learning to drive for MPG is different in a DP vs a Gasser. Slow but steady acceleration from stop. Anticipating the hill ahead, and traffic, and down shifting in time to keep RPM's in the sweet spot for hill climbing - all make a difference on MPG.

 

Best to you on your travels. And great advice on calling Cummins with you engine Serial and getting your specific HP and Torque curve values!

Smitty

Link to comment
Share on other sites

Slightly off subject, but if you want to monitor your diesel mpg in real time, etc., I can recommend getting a VMSpc unit and using a laptop or other device to monitor the engine - http://www.silverleafelectronics.com/node/6. http://www.rvupgradestore.com/ seems to have the best prices on them, $306.25 right now. If your engine is new enough, you can get the one that works with Bluetooth or Wifi. Otherwise you need the USB version and you have to find out if you have a 6 or 9-pin diagnostics port, and a Windows laptop or tablet. It is best to get a 32-bit Windows machine, since the installation of the drivers is much, much easier.

Link to comment
Share on other sites

If you look at the hp and torque curve for your engine you will find that the peak is produced at a certain rpm. Climb above that rpm and they start dropping off. More rpm does not equal more hp or torque, it equals less. If you can find a graph that overlaps them and see where they cross try holding that rpm. My peak torque is at 1200 rpm and peak hp is closer to 1800. I hold 1550 rpm and get my best mpg and can climb most hills without downshifting and cruise at 63 mph

Link to comment
Share on other sites

So much varies between manufacturers, that what is ideal for one coach, may not be the same for another. Key variances are of course gearing, tire size, and 'specific year and build within year' of an engine.

 

For example, the CAPS era ISL's that the OP and I have, will vary in both HP and Torque ranges from a newer VGT equipped, or VGT and DEF equipped engine. They may all be ISL and 8.9, but they vary. Toss in other variable such as side vs rear radiator, and the real world differences of actual rig's weight when rolling down the road - and the ideal RPM and speed for MPG can vary.

 

As FYI to the OP, as we have very similar engines, while still different coaches and mentioned above variables - my rolling Silverleaf reported MPG is best at between 63-65MPH. I shift into 6th at between 61-62MPH, and find 6th to 5th to 6th hunting takes place if I drop below 63MPH.

 

Still recommend getting the actual Serial of the engine, and calling Cummins for your specific engine's Torque/HP curves, and then experiment when driving around those two key peak areas.

 

Best to all,

Smitty

Link to comment
Share on other sites

Do you find at 1800 rpms that your pulling power is still good?

 

We bought diesel today and after the drive from Las Vegas up the Virgin River Gorge and continued climbing it seems all the way to Idaho....I got pretty dismal mileage...about 6....ouch. And that was keeping my speed at around 60 mph but it was a good pull for the engine.

Sorry I haven't responded sooner; I've had computer ills.

I get about 6 in heavy climbing too. Yes, 1,800 revs seems to work well for my MH climbing steep grades. I know ISC peak torque is 1,300 rpm, but I don't like to hammer the bottom end like that, As others have recommended, call Cummins with your engine S# and request HP/ TQ curves in order to determine your engines "sweet spot".

This downloadable pdf on Cummins fuel mileage is interesting, most things are simple, but effective.

Link to comment
Share on other sites

  • 2 weeks later...

In following this thread I have come across the following info. It may been seen before but it really speaks to this discussion.

 

www.weisstravels.com/3.html

 

Go to documents and Cat technical paper.

 

I do not know how to make this a clickable link.

Thanks, Bob

Link to comment
Share on other sites

In following this thread I have come across the following info. It may been seen before but it really speaks to this discussion.

 

www.weisstravels.com/3.html

 

Go to documents and Cat technical paper.

 

I do not know how to make this a clickable link.

Thanks, Bob

 

Here's a clickable version of the link. I'm glad you find it useful: http://www.weisstravels.com/resources/CAT+fuel+economy+paper+reduced.pdf

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...