Optimum Cruise RPM

[JHumphries]

I am in the planning stage of a project truck, and am trying to figure out tire sizes and axle ratios, and came upon a question

What rpm should I shoot for when I'm cruising down the highway.

I am looking at Arvin Mertior's MX-120 axles, for some ridiculous load capacity. (I know, the frame can't handle near what the axles can, but I want to run 10 lug rims, without an adapter, and the ratio choices are better spaced. )

I am planning on running any where between 33 and 37 inch tires (I am leaning towards about 35)



To boil it down to a simple question what is a good cruise rpm? (with load of course. )

 

[JLEONARD]

Mine always sounded and felt good at about 1900/2000. Had no boost lag at that rpm so instant power was there, engine temp was stable, yada, yada.

 

[HBarlow]

I've never owned a 12 valve so don't know anything about them but all Cummins ISB engines produce maximum torque at 1600 rpm with the recent exception of the pickup version of the ISB6. 7 which provides max torque at 1500 rpm.

I would guess that your engine would perform fine at anything above 1600 rpm for highway cruise. Lower rpm usually produces higher fuel economy.

 

[RSchwarzli]

Yup. I agree with J. Always feels good round there.

 

[cerberusiam]

Best BSFC on an early 12V is going to be between 1600-1700 rpms. You have to gear it right to get it there without lugging it though. At 1900-2000 you are farther past the TQ peak and it will roll just about anything with little effort and still make good mileage.

 

[GHarman]

Best BSFC on an early 12V is going to be between 1600-1700 rpms. You have to gear it right to get it there without lugging it though. At 1900-2000 you are farther past the TQ peak and it will roll just about anything with little effort and still make good mileage.

I have a cummins Engine book for the '91. 5. The book states; "any engine operation, under 1700 RPMs, for more that one minute, is considered by Cummins, to be "engine abuse"".

I now have a '93, given the mods preformed on it, the stock cam is still in place. The best fuel mileage is right at 1950 RPMs, according to the tachometer. The 91. 5 did not have a tach, but the MPH for best mileage was 65-68 MPH, in 5th gear, 3. 54 axles and 32" diameter 235/85R-16 tires. This engine had no mods, other than the timing turned up. Knowing the 5th gear ratio, one could back calculate the engine RPMs. GregH

 

[HBarlow]

Greg,

Are you sure it doesn't say at "full throttle for more than one minutes below 1600 rpm ? The torque peak is and has always been at 1600 rpm. Why would it be harmful to operate the engine at max torque rpm?

I have a copy of the Cummins handbook, "Operation and Maintenance Manual ISB Engine" published in 1999 for the 24 valve engine right here on my desk and have posted quotes from it several times over the years. I even quoted it to Cummins, Inc. in an e-mailed question several years ago because the advice I was being given by TDR members was in direct conflict with the Cummins engine manual. Cummins confirmed the advice given in the booklet in their e-mail reply and I saved a print copy of the text.

The book I have says it is completely acceptable to operate the engine at full throttle down to but not below peak torque of 1600 rpm before downshifting to a lower gear.

I'll copy the exact page and paragraph quotes if anyone wants me to.

 

[JLEONARD]

In a marine application which is under heavy load all the time, "the book" said not to run under 1600 for extended periods. I would run often at 1400 mainly because the operating temp stayed at 170F which is fine.

The "gurus" of marine applications said max longevity could be had if you cruise for extended periods at whatever rpm gives you about 10-14 psi. Use that as a max. Anything over that would shorten life. (Of course that was assuming you could hit rated rpm meaning EGT would be in line with the spec which was slightly over 800F PRE Turbo).

That was 2000/2100 for my particular boat and setup.

So it seems that the 1600-2000 rpm number seems appropriate.

I know at 1600rpm cruise both my truck and boat felt like the engine was approaching a fast walk.

 

[GHarman]

Greg,



Are you sure it doesn't say at "full throttle for more than one minutes below 1600 rpm ? The torque peak is and has always been at 1600 rpm. Why would it be harmful to operate the engine at max torque rpm?



I have a copy of the Cummins handbook, "Operation and Maintenance Manual ISB Engine" published in 1999 for the 24 valve engine right here on my desk and have posted quotes from it several times over the years. I even quoted it to Cummins, Inc. in an e-mailed question several years ago because the advice I was being given by TDR members was in direct conflict with the Cummins engine manual. Cummins confirmed the advice given in the booklet in their e-mail reply and I saved a print copy of the text.



The book I have says it is completely acceptable to operate the engine at full throttle down to but not below peak torque of 1600 rpm before downshifting to a lower gear.



I'll copy the exact page and paragraph quotes if anyone wants me to.

Harvey, As soon as I can get over to the shop and access "Old Uglys" carcass, I'll get that book and check it out. My memory could be faulty. I'll get the book and confirm or correct my statement. Thanks for checking me. GregH

 

[cerberusiam]

I have a cummins Engine book for the '91. 5. The book states; "any engine operation, under 1700 RPMs, for more that one minute, is considered by Cummins, to be "engine abuse"".

I have no idea what that is but it goes against all knowledge of how to run these engines, and, what rpm they run at. At 55 mph mine is turning a modest 1400 rpm's and just loafing pulling the truck, great mileage at that speed also.



Would I tow at those rpm's? Definitely not but just the truck is not gonna hurt with rpm's down that low. Publishing a BSFC spec and gearing trucks so they run at those rpm's for extended periods just doesn't jive with that statement.

 

[HBarlow]

Yeah, I agree. My current truck and the previous one, both with automatic transmissions, will/would happily loaf along at light throttle under light loads at 1200 or 1400 rpm without a programmed downshift. I'm sure that if the engineers thought that would hurt one they would have programmed the ECM/PCM to force a downshift.

Like Cerberusiam said above, I would never tow or use a heavy throttle setting at such low rpms either but see no harm in operating at low rpm unloaded.

I ran my old '01 many a mile in sixth gear at 1600 rpm and pulled many steep grades at full throttle at 1600 rpm. I would never allow the tach to drop below peak torque rpm but if it would hold rpm and speed at 1600 I always let it pull. The old truck has 350k on it today and still runs good with no engine repairs. Apparently I didn't hurt it much.

 

[GHarman]

Green Cummins Engine Book, B Series c.1991

Here are the quotes from page 1-8;

[ "Caution: Cummins engines are designed to operate successfully at full throttle under transient conditions down to peak torque engine speed (RPM). This is consistent with recommended driving practices for good fuel economy. Excessive full throttle operation below peak torque RPM (peak torque RPM varies from 1100RPM to 1600 RPM, depending upon rated engine speed) will shorten engine life to overhaul, can cause serious engine damage, and is considered engine abuse". ]

["Caution: Operation of the engine below peak torque RPM can occur during gear shifting due to the difference of ratios between transmission gears, but engine operation must not be sustained more than 1 minute at full throttle below peak torque RPM". ]



This does not provide the specific RPM I mentioned in my original post, nor does define a specific engine peak torque RPM for the truck engine. I know I read that in some literature, but this is what I could come up with. You just have to know your peak torque RPM. 'Course, if you are a Diesel mechanic and love to tinker and tweek, who knows what changes can be made! GregH

 

[JLEONARD]

Excessive full throttle operation below peak torque RPM

I think HBarlow suspected this is the key phrase. I 'd expect that if any one did this you would see EGT skyrocket.

 

[Ncostello]

IE: Just don't lug it.



Use some super single 22. 5" Alcoas and super single tires! That would be amazing looking!!

 

[HBarlow]

Here are the quotes from page 1-8;
[ "Caution: Cummins engines are designed to operate successfully at full throttle under transient conditions down to peak torque engine speed (RPM). This is consistent with recommended driving practices for good fuel economy. Excessive full throttle operation below peak torque RPM (peak torque RPM varies from 1100RPM to 1600 RPM, depending upon rated engine speed) will shorten engine life to overhaul, can cause serious engine damage, and is considered engine abuse". ]
["Caution: Operation of the engine below peak torque RPM can occur during gear shifting due to the difference of ratios between transmission gears, but engine operation must not be sustained more than 1 minute at full throttle below peak torque RPM". ]

This does not provide the specific RPM I mentioned in my original post, nor does define a specific engine peak torque RPM for the truck engine. I know I read that in some literature, but this is what I could come up with. You just have to know your peak torque RPM. 'Course, if you are a Diesel mechanic and love to tinker and tweek, who knows what changes can be made! GregH

Yep, that's the exact same language contained in the ISB manual I have and that I was referring to.

Peak torque rpm has always been 1600 rpm in ISB engines except the ISB6. 7 in automatic transmission tune which is now 1500 rpm.

 

[glhs]

Depends on your turbo size.

An hx-35 would like 1700 -2000rpm; a s300 about 2000-2300.

 

[Ncostello]

JHumphries... you didn't mention your desired speed for highway crusing. For me that could range from 55 on many of our local highways... to 80 on I-70 all day.



I'm just saying that because you might get ideal cruise speed RPM's from someone who's thinking 75MPH and your thinking 55MPH.

 

[JHumphries]

I'm looking for good mileage at speed limits. If I'm towing ridiculous amounts of weight, I don't want to be too far above the speed limit. I also don't want to be riding the redline, if I want to drive a little faster unloaded.

The transmission I'm looking at is a 13 speed (it's a 9 speed with a splitter for the top four gears. ) That gives me two overdrives. If I set the truck up to cruise unloaded in 8th high, then I can haul load in 8th low, and be less likely to lug the engine under load. It's about a 300 rpm drop per gear in the top 4 gears with the splitter.