Engine HP Output

[DrPhil]

I have a 2017 DSDP 4369 with the stock 450 HP Cummins ISL. I also have a ScanGauge mounted on the dash in close sightline. I'm sure that I've run through the RPM power band thousands of times (including the stated 2,100 rpm where one should see max HP from the engine), and I've NEVER seen anything more than 350 HP registered on the ScanGauge. This bothers me, but maybe this is normal based on the meter respond time or something. One would think something close to 450 HP would be registered sometime over a long hill climb with the throttle pinned, but admittedly it likes to run in these situations more at 1500-1700 rpms where the torque is higher, and looking at the engine curves, this would be more consistent with ~350 HP. Is this indeed normal? Maybe there is an engineer out there that could explain this situation so we could all understand the torque/HP tradeoff unfettered by the 450 HP marketing tagline.

[twinboat]

HP is a calculation from torque and RPM.

Torque is pulling you up the hill. It drops off as HP climbs

https://www.capitalone.com/cars/lear...ters-more/1428

[shootist]

Dud scan gauge or dud motor?

[jarwiebe]

I don't have the ISL450 but for comparison I have the ISL370. I use VMSpc to monitor my engine parameters and have it set to log the data every 6 seconds of run time and store the data in a CSV file.

I just looked at some of my files (save each trip in a separate file) and filtered the data to show the HP hitting 370 which it hits at 950-1000rpm at 1850-2000ftlbs.

I also filtered on the highest ftlbs which was 1200 and it hits this at 1400rpms and 320hp. The spec plate on my ISL370 shows HP rated at 370hp at 2000rpms.

[jharrell]

Originally Posted by DrPhil

Is this indeed normal? Maybe there is an engineer out there that could explain this situation so we could all understand the torque/HP tradeoff unfettered by the 450 HP marketing tagline.

Its is not a trade off between power and torque, torque is one component of power, the other is rpm. Gearing can increase torque to the wheel, power can only be increased by the engine.

At full throttle your ISL should create 450 hp at 2100 rpm, if it is not then something is wrong. The motorhome will accelerate fastest and go up a grade the fastest at 2100 rpm creating its maximum power. At its maximum torque it will only be creating 350 hp which will accelerate and go up grade slower.

Around maximum torque it may have its highest BSFC (best efficiency) which would mean the least fuel use per hp created which is useful for maximizing fuel economy at the expense of speed, which is perhaps your tradeoff.

My guess is the the gauge is not calculating power correctly, but there could be an engine issue.

[jarwiebe]

See new notations in red

Originally Posted by jarwiebe

I don't have the ISL450 but for comparison I have the ISL370. I use VMSpc to monitor my engine parameters and have it set to log the data every 6 seconds of run time and store the data in a CSV file.

I just looked at some of my files (save each trip in a separate file) and filtered the data to show the HP hitting 370 which it hits at 950-1000ftlbs at 1850-2000rpms.

I also filtered on the highest ftlbs which was 1200 and it hits this at 1400rpms and 320hp. The spec plate on my ISL370 shows HP rated at 370hp at 2000rpms.

[Dutch Star Don]

In terms of what the scan gauge sees, it's an estimate when it shows your HP and Torque.

I have the VMSpc program and have been running it for years. I often see HP into the 435 range but on rare occasion, I've seen it briefly hit 449 and once 450. Again, it's an estimate of the flywheel HP and Torque.

Hers is a photo of my VMSpc screen showing the torque hitting 1249. I've never seen 1250, but a few times its hit in the 1240 + range.

You can get into HP/Torque discussions until the cows come home and typically does on here. The reality, these coaches don't have enough gears to get or keep the coach in a range where you can optimize the HP/Torque. Other factors include the steepness of the grade you're climbing, which can often change within the climb, leveling off and then getting steep again.

Unless you're running a 600/605 HP ISX, I sometimes make a little bit of a run before the grade and downshift to 5th at the base. After that, I let the Allison do the rest.

In the photo, it shows 1249 torque and I'm at 100% load. However, I'm not at max turbo boost in this particular instance, about 3 pounds shy.

[n2zon]

Originally Posted by jharrell

Attachment 388728

Its is not a trade off between power and torque, torque is one component of power, the other is rpm. Gearing can increase torque to the wheel, power can only be increased by the engine.

At full throttle your ISL should create 450 hp at 2100 rpm, if it is not then something is wrong. The motorhome will accelerate fastest and go up a grade the fastest at 2100 rpm creating its maximum power. At its maximum torque it will only be creating 350 hp which will accelerate and go up grade slower.

Around maximum torque it may have its highest BSFC (best efficiency) which would mean the least fuel use per hp created which is useful for maximizing fuel economy at the expense of speed, which is perhaps your tradeoff.

My guess is the the gauge is not calculating power correctly, but there could be an engine issue.

A couple of minor clarifications, if I may.

Because F=MA, maximum acceleration comes at maximum torque at the drive wheels, not at the flywheel. The transmission gear selection and differential ratio play here. Likewise, it's HP at the rear wheels that determines steady-state climbing speed on a given grade.

"Assuming" the transmission ECM is doing its job well, the engine might never develop peak HP at the flywheel on a given grade. Maintaining speed up the grade is prioritized, which means maximizing HP at the drive wheels. Engine speed constraints and toque multiplication in the transmission and differential could place engine RPM in a range of possible speeds on the HP curve. The HP at the drive wheels will always be less than at the flywheel for a number of reasons (measurement standard, accessory load, driveline losses, etc.).

Back to the original question: To figure out whether the reading is right, the OP needs to select a gear that lets the engine reach the RPM for max HP at full throttle and read the Scanguage when the tach crosses that RPM. If it's not pretty close to the engine's rating, one of them has a problem.

[DrPhil]

Thanks for all the info. These replies give me somethings I can try. I've been meaning to set up something like VMSpc and have a bluefire to do so. Just lazy. So therefore the suggestion to gear my tranny to hit the right RPM range and check makes sense. I've often thought that the ScanGauge probably does some weird averaging to get its numbers. Bottom line is I have no sense that the engine is doing something wrong. I have no issue hill climbing or acceleration.

[Dennis C.]

It would be nice if BlueFire allowed you to see HP and torque values, even if they were calculated estimates. It allows me to see engine load and engine torque, both shown as a percentage rather than the values.

[jharrell]

Originally Posted by Dennis C.

It would be nice if BlueFire allowed you to see HP and torque values, even if they were calculated estimates. It allows me to see engine load and engine torque, both shown as a percentage rather than the values.

Would be nice but its would just be calculated value that would require knowledge of the specific engine as the ECU doesn't report that directly as far as I know, this seems to be confirmed by the VMSpc manual and probably why scangauge showing odd values to the OP:

[Dutch Star Don]

I've been running the VMSpc since 2007 on three different DP's. Since the VMSpc connects to the J-port of the coach, it sees exactly what the Cummins ECM sees.

However, the ECM does not give HP and Torque numbers, that's an estimate done by the program. The Cummins ECM doesn't look at exhaust temps either. If you have an exhaust temp gauge, that is added by the manufacturer as an option. Instead, the Cummins ECM reads exhaust manifold temps, so you'll see on my screen above, I watch the manifold temps.

I looked at the Bluefire system when it came out a few years ago but didn't care for the available gauge configurations. Either program though reads some things exactly, which I like. The oil pressure, coolant temp, voltage, speedometer and trans temp are all the same as the ECM sees.

The engine load, HP and Torque are all estimates, but fun to watch and track. The Rolling MPG I find useful. Often you can't see it, but you may be on a slight grade or a slight downhill or being pushed or slowed by the wind. I can see that in the Rolling MPG and will often adjust my speed.

Lastly, I use three Trip Odometers. The first one is for each fill-up, the second one is for the entire trip and the third one is for lifetime. The VMSpc allows you to "dial in" your actual mpg by adjusting percentages in the program. It only takes a few seconds to adjust the numbers so the fuel used matches the mileage. Once you make this adjustment a few times, it's fairly accurate, to about 5%. The only thing that throws it off to about 10% error are long hill climbs.