Diesel engine comparison for dummies

[Craig R]

HELP! Somebody please treat me to a lesson from Diesel engines 101. Please remind me why I can't repower my MC5c with a GM Duramax 6600 diesel that puts out 650lb-ft of torque and 360 HP. I know I can't, but the why escapes me. This [physically] small pickup truck engine appears to turn out half again as many horses (and equal torque) as my 6V71 -- which you couldn't haul home from the rebuilder in the bed of most pickups. What gives?

[Jerry32]

I had a motorhome with one of those PU engines in it( B 5.9 Cummins) and it was underpowered so I jacked the power up to 350 Hp which was great on the open road with the engine winding . All the power came from turbo boost pressure ( 38 lbs max  ) and when the rig was in traffic it took a half a mile to get enough boost to really get rolling if the RPM droppedtoo low. I constantly had to manually shift the auto transmission to get the rpms up to get power. Once up to 60 mph it would walk over the mountains like they weren't there but like most with boosted power in hot weather it would overheat. The difference between a 5.9 L engine and a 12 L engine is certainly worth it. Jerry

[skipn]

IMO and I will keep it somewhat general.

In a way you are right but let's look at the number game a bit.
     PU engines run a 2:1 ratio between torque and hp
     bus/med weight engines run close to a 3:1 between torque and hp.
  Even when you increase HP you don't really change this ratio to much.
  So when you bump up the PU your max torque band width become quit narrow and exhaust temps go through the roof.
   That is one of the reasons why the bullydogs have a pyror max temp programmed. My son-law hits that all the time pulling
   a 13k# 5th wheel.
  Where as the med. duty engines will run that way until bumped up and they run into the same issues as the PU being bumped up.
   It would probably be a lot cheaper to turbo your current engine than to spend the money on radically revamping.

  The biggest plus to the PU engines is that they can be worked on in almost every town in the country.

  FWIW
    Skip

[kyle4501]

I think it all goes back to duty cycle. How long will it make rated power? Or put another way, how much power can you get out of it for an extended period before bad things happen?

Those duramax & powerstroke engines are in light duty trucks whose gross weight is usually well under 26,000 lbs & are not constantly making the advertised HP. An 80,000 lb truck will require full power for extended periods.

Your 6-71 can put out full rated HP 24-7 for ~ 400,000+ miles. But you pay for that in extra metal in the engine.

My brother's '06 duramax is rated at 650 lb-ft torque, but I seriously doubt it will last the mileage the heavier engines will under similar conditions.

I'll bet that if a 6-71 was tuned to have the same service life as the pick up engine, it would make twice the power. After all, they do it in boats, don't they?

[Kristinsgrandpa]

I think a differential gearing change would be necessary also.

I don't know about the Duramax but my Powerstroke redlined somewhere in the mid 3000rpm range.
3300 to 3500 I think.

Ed

[Melbo]

I asked a friend of mine a similar question -- he operates trucks for highway work -- he called them pop truck motors. He said they work fine if you are just bumping around but if you really use the motor it won't hold up.

Just another opinion for what it's worth.

Melbo

[TomC]

The Duramax at that setting is at about the peak power that engine can put out.  And, if you got close to 200,000 miles out of it, I'd be surprised.  On the other hand, the 6V-71N at 220hp can last well over a million miles with a couple of rebuilds (the Duramax would most likely be worn out at 200,000 miles).  Detroit made a 6-71TTA marine engine that was rated at 465hp at 2500rpm, that deminstrates how unstressed the 220hp version is.  If you do want to go to a smaller power plant, the only ones I'd consider is the 8.3 or 9 liter Cummins, DTA466 International, Mercedes-Benz 900 series 6.4 or 7.2 liter, or 7.2 or 9 liter Caterpillar (3126/C7 or C9).  The International/Ford Powerstroke, the Cummins 5.9/6.7, the GM 6.6 Duramax are light duty pickup truck engines that can put out big horsepowers, but not for long durations-like climbing a long grade in 100 degree weather.  Good Luck, TomC