HP vs Torque

[Sky_Boss]

OK...I know this has been discussed and cussed a lot. Let me ask this in a way were we can compare apples to apples.

The following is data on the Cummins ISL series

Cummins Engines - On-Highway - Motorhome - EPA 07 Engines - EPA 07 ISL

Engine Model Advertised hp Peak Torque lb-ft @ rpm Governed Speed
ISL 425 425 1200 @ 1300 2200
ISL 400 400 1200 @ 1300 2200
ISL 370 370 1200 @ 1300 2200

3 different HPs with the same Peak Torque at the same RPM. So...from a seat of the pants perspective, what does this mean to us?

As a follow up...

Compare this to the above ISLs, especially the ISL 370

Engine Model Advertised hp Peak Torque lb-ft @ rpm Governed Speed
ISB6.7 XT 360 800 @ 1800 2600

Thanks in advance.

[Route 66]

The ISL wins hands down. 33% more torque at a lower RPM and max HP at a lower RPM.

It's the same as hot rodding.......there's no substitute for cubic inches.

A smaller engine pushed to it's limits is never as good as a larger engine not working as hard.

Compare this to the above ISLs, especially the ISL 370

Engine Model Advertised hp Peak Torque lb-ft @ rpm Governed Speed
ISB6.7 XT 360 800 @ 1800 2600

[spike45]

Originally Posted by Sky_Boss

OK...I know this has been discussed and cussed a lot. Let me ask this in a way were we can compare apples to apples.

The following is data on the Cummins ISL series

Cummins Engines - On-Highway - Motorhome - EPA 07 Engines - EPA 07 ISL

Engine Model Advertised hp Peak Torque lb-ft @ rpm Governed Speed
ISL 425 425 1200 @ 1300 2200
ISL 400 400 1200 @ 1300 2200
ISL 370 370 1200 @ 1300 2200

3 different HPs with the same Peak Torque at the same RPM. So...from a seat of the pants perspective, what does this mean to us?

As a follow up...

Compare this to the above ISLs, especially the ISL 370

Engine Model Advertised hp Peak Torque lb-ft @ rpm Governed Speed
ISB6.7 XT 360 800 @ 1800 2600

Thanks in advance.

Torque on the ISL for all HP ratings as above does peak at 1300 RPM. HP is actually (torque X RPM) / 5252. The ISL rated HP (load) occurs at 2200 for these applications.

(T X 2200) / 5252 = 370HP
This will calculate to 883 pounds torque at 2200 RPM
Each higher HP rating will yield higher rated load speed torque.

What that means to you is more pulling power with fewer downshifts. At peak torque the cylinders are being filled with more combustion air for more complete combustion and greater cylinder firing pressure....downward push on the piston crown creating torque peak. As engine RPM rises above peak Torque RPM, the volumetric efficiency of filling the cylinders with air is decreasing. Between torque peak and HP Peak the governor in the ECM is in control. As RPM under peak torque continues to fall below Torque peak, the governor is no longer in control. Further reducing of RPM due to excessive load leads to lugging and damage to the upper half of rod bearings. Since you have an automatic trans, you will not experience lugging.

The high torque rise of this engine is what keeps the trans from having to downshift as much. The down side to this is that while the ECM is dialing in more fuel delivery to increase torque, the RPM is falling and along with that is lower water pump speed and reduced fan speed (if driven by the crankshaft). What this all means is the engine is burning more fuel at a time when the cooling system is not at its best.....reduced WP speed means coolant stays in the engine longer acquiring more heat. Even with the thermostat fully open, the flow is not high enough. Fan speed is reduced so the really hot coolant is not being cooled as much as it needs at this particular time. Translated: many of you complain of the engine temperature rising while hill climbing....now you know what is happening. The easy fix is to be certain the radiator is always clean especially if your engine does NOT have a closed crankcase vent system. Second thing, down shift to get engine RPM up, decrease fuel delivery rate, increasing water pump speed thus the engine cools better.

For those of you who try to race that big Peterbilt next to you where he is running an ISX 600 at 2050 pounds torque.....give it up! He has got torque to burn and has a higher torque rise than you do.

[Mr_D]

Great analysis Gary! Most people don't realize the HP is calculated from torque and rpm. It also favors higher rpm engines.

For comparison here's what an ISC with Banks has (the black is an ISL 400 plotted from Cummins literature) I guess I should plot the 425 and 450 ISL on it too.



But, the ISC is being stressed more so it may last only 200,000 miles rather than 300,000 miles! Not sure I'll ever see that point.

I also find that I use the economy mode far more than when the engine was stock due to the wider torque band.

[Mr_D]

Originally Posted by Route 66

A smaller engine pushed to it's limits is never as good as a larger engine not working as hard.

That can depend on how it's built and what it's used for!

[Route 66]

OK. Let's say motorhomes and trucks.

Originally Posted by Mr_D

That can depend on how it's built and what it's used for!

[JohnBoyToo]

Agree - for that application Higher torque is always our winner

BUT, you want to not just look at PEAK, also look at the duration...

i.e. it's important to know how much is "under" that dyno curve or
how 'wide' that rpm range is ...