I know this has been discussed before but i would like a more definitive answer. If the Cummins engine produces 610ft/lbs of torque at 1600 RPM why are the 6-speed manuals geared so low. I thought torque is key to pulling heavy loads. I mean you are traveling 65 MPH and the RPM is pushing 2100, it seems if the peak torque is at 1600 RPM towing would be better at or around that RPM. Would someone please educate me on this subject.

 

Peak horsepower is reached at a much higher RPM (2600?). The "sweet spot" is usually always somewhere between peak torque and peak HP. So roughly 2000rpm is where you'd want to be for most of the time. Plus, the engine is much more responsive to throttle increases at higher RPMS, as opposed to 1600rpm where you're very close to lugging the engine.

Having said all that, I think most people would still agree that the six speed could really use one more gear, especially for when driving without a load. The automatic isn't perfect though, either.

 

I think you want a mix of Horsepower and torque to make the most out of the engines power. Meaning, if you are cruising at the engines peak tq and not in the hp band, you will not maintain speed with a load, but if you are making 3/4's of the hp and all the tq at say 2K rpms you have some margin for power making it over the mountain with a load. I do agree the trucks with a G56 and 3.73's run a little over rpm-ed, but man, with a load you cannot slow them down. I would like to find a tire combo that got me into the 2000 RPM;s at 70MPH and not have a huge mileage penalty. I guess there is give and take everywhere. Scotty

 

Thanks for the input guys makes better since now.

 

My OD unit gives me 1900 RPM's at 70 mph.

 

It doesn't is why. TQ starts at 1600 but peaks much later at about 2400 rpm's, which is right where the manual trucks run at 75 mph. Best engine efficiency is at or slightly above peak TQ. Under a load you have a very broad band to roll thru and maintain power which won't happen at lower rpm's. Doesn't do much for empty mpg but thats not what the truck is designed for.

 

That seems fishy to me. Not trying to be an a$$ - but from what Ive read about this truck, the peak torque (roughly 610lbs) is reached at 1600rpm, period. Maybe you are confused, because once you reach 1600, I think the torque curve is pretty much flat up until about 2600rpm - so you have your max torque at a good range of rpms. But I'm pretty confident that it is FALSE to say that peak torque isn't reached till 2400rpm. You've reached peak torque way before that.

Hopefully someone else can chime in to settle this one way or the other. Anyone have a link to a dyno graph/chart?

 

Don't believe everything you hear/read.

 

Can't argue with that, thanks for posting it. I stand corrected!

I forgot to take into account all the torque management software in the truck's ECM that controls the engine's output. I suspect that if the engine itself were to be dyno'd (independently from the truck), the result would be similar to what I had been saying (peak @ 1600) - otherwise Cummins & Dodge would be guilty of false advertising?

But yeah it's interesting to know that the truck wont allow the engine to reach max torque till around 2400 rpms.

 

Exactly. Everyone quotes the published numbers that are used as "industry" leading and never stops to realize that is NOT real world view. As far as I know neither DC or Cummins has said the peak TQ was at 1600. Here is the quote from a web site talking about the new 610 series:

"The Cummins 610 Turbo diesel delivers a class-leading 610 ft.-lbs of torque, starting at only 1600 rpm, along with an unbeatable 325 horsepower."


You right,the fuel management will never let the full TQ come on under about 2200 rpm's due to the drive train being too light.

The Dmax's are advertised the same way and in real life they don't pull as well as the Cummins. A friendly dyno operator always helps sell more trucks.