OK I'm starting to look into a new tow rig. I'm thinking diesel and am looking at medium duty trucks.

But I am getting confused as they boast of 250 hp Cat 7.4L. Is hp different on a diesel? Seems to me 250 hp isn't much, at least not a ton in the gas tow rig world. :confused:

Horsepower is a function of torque and rpm. Diesel engines make a lot of torque at lower rpm, which does not translate to a lot of hp, but still makes a good tow rig. Still, more hp is better. You really need to look at the torque curve (torque vs. rpm) to characterize the performance of an engine. The peak hp or torque numbers do not tell the whole story.

How much torque does it make?? That's where your power is needed w/ a tow rig who give a shit about HP....

Figures, they don't list torque.
GMC Medium Duty (http://www.billfarlow.com/gmctruckspec.htm)

Yep, I've got a lot of homework ahead of me.

diesel is a different world all together, look at the big trucks on the freeway, big power is 600hp and 2000ft lbs of torque!

my powerstroke is pumped to 355hp, but also 765ft lbs tq

stock PSD's come with 250hp (auto) 275 (stick) and 525 ft lbs tq, my old V10 was 310hp and 425tq,

the gas motor made killer power and towed awesome, but it would hang in the 4k rpm range on good long steep pulls, the PSD now that it's running right (bad PCM) doens't shift out of overdrive on the same pull going the same speeds!

Dude go see my dad at Capital ford and have him and the fleet manager find you something.

DSI is right, the diesel will always put out a sh!t load of torque compared to hp. If you want to see an example, let me know and we can load your truck on the trailer and use my buddy Tims Powerstroke to pull up hwy 50. I was amazed at how that thing pulls and it's only 250 hp stock.

Stay away from those POS Cat 7.4Ls. All kinds of problems.
The Cummins 8.3L is better, but still has a lot of problems with the CAPS fuel pump taking a shit.
Go Detroit if you can. Series 50, 300HP, real Jake Brakes:D. Good luck getting the OEM to spec it though.

you want the torque curve to be essentially flat and early. I am partiall to Ford, worked there two years. Their only weak point is the glowplugs in the cold. Under 35degrees and your pluggin in. The manual trannies are ZF's with titanium synchros. They rarely die. Here in the rockies, Fords sell like hotcakes, especially the autos. On the western slope or in the super cold areas dodges sell real well due to the lack of complexity of the glow plug system and injectors.

The my father-n-law sells duramax diesels, but I don't see a ton of them towing up the passes here. 12-13k feet.

Get a stroker.

Think of HP as a measure of how fast and torque a measure of how much. My diesel can move a lot of things slowly, vs a gasser that can move less, but maybe more quickly.

Bryan

Originally posted by Oxjockey
Think of HP as a measure of how fast and torque a measure of how much. My diesel can move a lot of things slowly, vs a gasser that can move less, but maybe more quickly.

Bryan

Ummm, OK. That makes sense, I think:confused: J/K, Bryan.

Torque is the ability to do work. The mesure of horsepower is how fast you can do the work (a function of torque and engine speed). Torque and HP always cross at 5252 RPM, so if you have a 350 gas engine that makes 300 HP at 5252RPM it is also making 300 foot lbs at the same time. Before 5252 RPM the torque curve is on top and the HP curve is on bottom, and above 5252 RPM the HP is higher than the torque. Most big Diesels redline before 5252 RPM so there HP numbers never catch up, but there ability to do work is huge.

On the flip side small gas engines can rev way higher than 5252 RPM, which is why you see small four popper rice burners that make an honest 200-250 HP at 12,000 RPM. It is not that they have a huge amount of power, but they have the ability to put it down so much faster.

Hope that is easy enough to understand.


JYS

As already mentioned, HP is the product of torque and RPM.

HP = torque*RPM/5252
torque = HP/RPM*5252

If you notice on any dyno chart, the RPM and torque crosses at 5252 rpm (that's if the engine revs that high). Below, 5252 rpm, HP will always be lower than torque; above 5252 rpm, HP will always be greater than torque (speaking about the numerical values, not what the actual values represent).

WRT 250 hp, if it peaks at 2500 rpm, torque at that RPM is 525 ft.lbs; and that may not even be the peak torque produced. If that same engine produced 600 ft.lbs of torque at 1500 rpm, your HP at the rpm would be 171 horses. Not a lot of HP with respect to a higher-revving gas engine, but torque is where the diesel excels.

I dont understand why the RPM numbers are the same, but I found the same numbers on several sites.

Edit: I think these are for different versions of the engine, i.e one set of numbers HP/TQ per engine.

Cat 3126B

Fuel
Diesel w/ HEUI Displacement/Type
7.2L Il6

Hp@RPM
175@2200
207@2300
210@2200
210@2200
230@2200
250@2200
250@2200
275@2200
275@2200

Torque lb-ft@RPM
420@1440
520@1440
520@1440
605@1440
660@1440
660@1440
800@1440
800@1440
860@1440

Torque Rise
25 %
40 %
23 %
45 %
37 %
26 %
52 %
39 %
49 %

Originally posted by Shrock
I dont understand why the RPM numbers are the same, but I found the same numbers on several sites.

Edit: I think these are for different versions of the engine, i.e one set of numbers HP/TQ per engine.

Cat 3126B

Fuel
Diesel w/ HEUI Displacement/Type
7.2L Il6

Hp@RPM
175@2200
207@2300
210@2200
210@2200
230@2200
250@2200
250@2200
275@2200
275@2200

Torque lb-ft@RPM
420@1440
520@1440
520@1440
605@1440
660@1440
660@1440
800@1440
800@1440
860@1440

Torque Rise
25 %
40 %
23 %
45 %
37 %
26 %
52 %
39 %
49 % Probably the same engine just different E.C.M calibrations( flash files)
FIGURES THEY DONT LIST TORQUE / GMC TRUCKS Read the spec tag on the valve cover of the cat engines/ cummins spec tag is on the front gear housing

The Cat 3406B in my '88 Pete only makes 425 hp, but it makes 1450 ft.lbs. of torque at 1250 rpm. It also has a 2100 max rpm limit in stock trim. :D

You'll get more miles out of diesel and much better fuel economy. Some of the diesels in the big semi's can see a million miles and beyond!

If you can get over all the noise, it's hard to beat a diesel.

Originally posted by redrangie
I am partiall to Ford, worked there two years. Their only weak point is the glowplugs in the cold. Under 35degrees and your pluggin in.

IIRC, PSDs haven't used glowplugs since the late '99s.


Here in the rockies, Fords sell like hotcakes, especially the autos. On the western slope or in the super cold areas dodges sell real well due to the lack of complexity of the glow plug system and injectors.
...
Get a stroker.

The Dodges have never used glow plugs.

I've never had mine not start, I plugged it in ~3 times last year, and that was only so I wouldn't freeze my arse on the way to work (although the truck is happier when you plug it in when its real cold).

Pete

so michael ya wanna make me a real good deal on that chebbie of yours :flipoff2:.

Dallas

Originally posted by pcorssmit


IIRC, PSDs haven't used glowplugs since the late '99s.



The Dodges have never used glow plugs.

I've never had mine not start, I plugged it in ~3 times last year, and that was only so I wouldn't freeze my arse on the way to work (although the truck is happier when you plug it in when its real cold).

Pete

Ok you got me. I left ford in the 99 model year. Right before it actually. On the dodge thing, I figured that saying "glow plug system" would give the non-diesel people the idea. Sorry for sounding dumb. But I just can't help it sometimes:rolleyes: