jododo

I cross posted this in the performance section as it pertained to both - please indulge the culture clash.
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Alright I admit it I'm empty headed when it comes to cars let alone trucks. The ditty is - I bought a 07 Dodge 2500 4x4 (manual) to carry a 2000lb truck camper. So I need advice on the best gear range under load and without.

The guy who owned this truck put a bullydog system on but did not include the programmer. Long story not worth the feed, service techs flashed it and now the bullydog acts as a monitor which has been interestingly informative. What I've noticed is that the pyro drops; which I assume is a good thing when I drop down a gear. Why is this?

The last 3k miles I have been running the truck between 1200-1750, usually 6th gear I shift over 50 MPH.

Younger days I used to drive a little miata and I would run this at much higher RPM's as I enjoyed winding it out. But I'm concerned doing this with the diesel will drop MPG and performance and wear.

Laughing aside, at what speeds or RPMs should I shift? And would/how/ why would this change underload?

KISS please.
TIA

BTW: I love everything about this truck, never thought I would as the power, sound, smell are in an odd way a turn on.

Dartmouth 12V

When you drop a gear, your engine is turning more rpms so it is moving more air. The easiest way to think about it is that if you go from 1000-2000rpm, your cylinders fill twice as much. The other thing is that you need less power from each combustion cycle because there are more of them so you are burning less fuel each time your cylinder fires. Basically your egt's drop because you have a higher ratio of air to fuel.

As far as what rpms to drive around at, it is dependant on the load of the vehicle. Since you seem to be concerned with fuel economy, you want to avoid running high rpms when it isn't necessary. However, it is just as bad to lug the engine. Lugging is when the rpms are too low and you are having to inject too much fuel during each injection event to get the desired power. By increasing the rpms, you have more combustion events so each one needs less fuel.

Most engines are the most fuel efficient at wide open throttle close to their torque peak. I don't remember the exact rpm where the engine is most efficient but I think I remember it being around 1900rpm at WOT. If there is no load, they are most efficient at idle. All other loading conditions fall somewhere in between. If you are pulling a big hill, you don't want to be down at 1400 rpms, it would be better to drop a gear and be at 2000rpms and if you are still at WOT in that gear, dropping another would probably be required. Since you have a pyro, that will be a good indicator of lugging. If it starts climbing quickly, you probably need to drop a gear.

If you want to improve your fuel economy, the easiest thing to do is usually to slow down a little on the highway. The economy really drops off once you get over 70mph or so.

DBLR

IIRC the 5.9 hits its rated TQ at 1600 RPM's in high gear after that your using HP. Its also best to keep the RPM's under 2200 or 2000 or lower to get the best fuel mileage.

Jason Foster

Diesels are all about torque. Keeping the RPM low, but not too low, is a good thing. Keep your RPM at as close to the highest TQ rating as possible and, like DBLR wrote, below 2000. Don't let it lug down. You'll know by feel and sound where the sweet spot is to shift.

Number47

i had a miata, i put a 302 in it, it was fun, but diesels are better. when you are loaded you wanna bring rpms up alittle higher than unloaded to reduce work load

Philabuster

My girlfriend is jealous of my truck (after installing the Smarty) because it accelerates faster than her '99 Miata.

She hasn't seen nothing yet.

jododo

Sorry - what is TQ and HP? Yes I know laughable learning here.

I'm thinking of buying the programmer for the bullydog as I'm starting to really realize significant mileage decreasing this last week. I'm shifting around the 1700 lately, so this might be the hit. The engine sounds better when I'm running it higher rpm; 1700-2100, just wasn't sure if I was doing more harm than good.

Dartmouth 12V

Torque is a measure of the force being exerted by the engine to turn the flywheel. It is equal to the force exerted multiplied by the radius at which the force is exerted. For example, if you have a 1 foot wrench and you apply a 10 lb force on the end of it, 10lb*1ft=10ftlbs of torque.

Horsepower includes a time component. Power is the work done divided by the time. Basically it is how quickly you can get something done.

I don't know why your mileage is dropping but if it is dropping, I would try to find the cause before you put a programmer on it. Then, once you get back to the mileage you were getting before, put a programmer on it. That way, the programmer won't mask the symptoms.

HOHN

J-do:

NO laughing here. None of us was born knowing this stuff, so you have to start somewhere.

The proper RPM to shift gears will vary with the amount of load on the engine-- including which gear you are already in.

A good general rule of thumb is to shift at 2000rpm when running empty. In lower gears (like first starting out) you don't need to run it up that far, but it doesn't hurt anything.

If you are pulling a substantial load, run the engine up to the RPM of rated power-- in your case it should be 2900rpm, then shift gears.

Some will say that these rpm points are higher then necessary, and many times they will be. But until you develop a "feel" for the minimum rpm in each gear, it's a good safe rule of thumb.

Don't be afraid to "wind it up" because even at higher rpm, the engine is still not running very fast, and it is built in such a way that it can handle maximum rpm at full load all day, day in and day out. So don't worry about hurting the engine from running at rated rpm (2900). You also aren't going to wear out your Cummins. Abusing one may break it, but wearing out a Cummins is a thing confined to only the rarest of circumstances.

Higher RPM is safer under load. First, it means you're in a lower gear, which means more torque multiplication getting to the load, and hence less load on the engine. It's much easier for the engine to pull the same load in 4th gear than in 6th-- though you'll be driving slower to do so. Second, the given load is easier on the engine because each RPM has to do a smaller portion of the job. Just like carrying a large tree is tough to do with 20 guys, but not so hard with 2000 men. Same job-- less load in each member.


As Dartmouth mentioned, tq is a measure of twisting force. You can think of it as leverage. For example, if I have pry bar 2 feet long and I apply 10 pounds of force to the end of it, I have a "prying force" of 20ft-lb. This would be the same if I had a 10ft pry bar and only 2lb of force. They multiply out the same-- force times distance.

So the tq of your engine is a measure of twisting force. (ft-lbs). But the units of ft-lbs also apply to "work". For example, if I lift 10# off the ground 3 feet, I have done 30ft-lbs of work. Again, the same as if I lifted 3lbs 10ft off the gorund (I'm not that tall, though).

HP is simply how fast you can get work done. For example, it might take 400lb-ft of torque to pull a load up a hill. But if you can only apply that torque slowly, you have very little power. You'd have to go up that hill at 10mph or so. Doubling the power would double the speed at which you can pull the load up the hill. I could pull a large load with a low HP engine, but I would have to have so much gear reduction that the travel speed would be barely above a crawl.

TQ and HP become relevant by gearing and the way that effects time. Remember, tq is just a force, but power is force times speed (or time). So if an engine can make the same tq at an RPM that's twice as high, it will have double the HP.

Gearing can multiply tq, but NOT HP. The reason for this is that for every doubling of tq by the gears, the also cut in HALF the rate of power application, so the two counteract each other by equal amounts.

Let's say I have a drive shaft spinning at 5252 rpm with ONE HP of power. The formula (HP=tq*rpm/5252) says I have one lb-ft of tq on the driveshaft. If this shaft feed a gearset that provides 4:1 gear reduction, what happens?

Well, my axles will turn at 1/4 of the speed, but they will have four times the TQ applied to them. So instead of 1 ft-lb at 5252 rpm, I will have 4 ft-lb at 1313rpm. Notice that the tq has been mutliplied by four, but the POWER IS THE SAME.

So, the conventional wisdom that "tq is low rpm and HP is high rpm" is clearly wrong. There is low-rpm power and high-rpm power. POWER is KING. TQ is simply a derivative of power and gearing. You can always turn HP into tq with gearing, but you cannot turn tq into HP.

jh

Dartmouth 12V

HOHN, that was a good job of explaining both shift points and the horsepower torque relationship. I am always amazed at how many of the people in my engineering classes still don't understand that relationship.