Diesel economy based on RPMs, right? - PeachParts Mercedes-Benz Forum

Hit Man X · #1 10-09-2004, 08:33 PM

From what I've read about light duty truck diesel motors, their economy is considerably better at lower cruise RPMs.

If this holds true for car diesel motors, the max I run on the freeway is 75mph on a road trip, around town just 70mph. Keeps the law off my back and I get there quick.

So seeing as how the Mercedes Autos (and Manuals) 4th gear is 1:1, would it be wise to try to get a taller (numerically lower) gear like a Wagon diff as they have 3.07:1 and 2.88:1 ratios.

Sure the car's going to be slower, but will the economy go up enough to warrant this swap?

~3500@70mph, 3.69:1 or ~2900@70mph, 3.07:1 or ~2700@70mph, 2.88:1 (25" tire, 1:1)

Seems like 600rpm lower would be considerable. I'd still have an overall 1st ratio of 11.3:1 so that's not too bad... considering my F150 I have built with a 2.40:1 1st in the trans with a 4.10:1 rear axle has a 9.8:1 only.

What do ya guys think?

Gurkha · #2 10-09-2004, 09:29 PM

Here is my take on this.

I have two vehicles, the Gurkha which has the OM 616 turbo 91bhp with nice shifting 5 speed and a 69 Nissan Patrol with 4009cc, four cylinder HINO truck engine. Both vehicles weigh the same roundabout, the Patrol has a ancient, mismatched 3 speed tranny and the HINO is a typical low revving engine with max revs of 4000. The beauty is that it develops max torque at 1200 rpm and max bhp 110 at 3200 rpm.

Needless the say even with all this handicaps the Patrol outpaces my turbo OM 616 Gurkha and still manages to give me around 17 km per liter of diesel whereas the OM 616 turbo manages best figure of 13 km per liter.

Obviously the OM 616 is being kept at near abouts max rpms to gain any appreciable performance and this contributes to its low economy whereas for the HINO engine, it is almost like idling.

sixto · #3 10-09-2004, 09:54 PM

70mph in town? Define town.

Should be worth it. The downside I see is if you can't live with the slower acceleration and mash the pedal every time off the line. Might offset the cruising gains. Don't forget to recalibrate the speedo.

Sixto
95 S420
87 300SDL

Hit Man X · #4 10-09-2004, 10:18 PM

Quote:

Originally Posted by sixto

70mph in town? Define town.

Should be worth it. The downside I see is if you can't live with the slower acceleration and mash the pedal every time off the line. Might offset the cruising gains. Don't forget to recalibrate the speedo.

Sixto
95 S420
87 300SDL

I mean in town freeways... lol not in town like city streets.

It's an economy car, I have a few stop lights between my pad and work. Everyone that lives in my neighborhood is early 50s+, so we all drive slow. hahahaha It's simply not made for going fast I realize that. If I could up the fuel economy (or drop it's consumption, which term you prefer

) for $300 or so installed by me and make the vehicle 1 sec slower 0-60mph so be it.

boneheaddoctor · #5 10-09-2004, 10:28 PM

Optimum economy is going to be achived by keeping the engine in its sweet spot, not above or below. That will be different on different engines because its a factor of engineering. That is usually at the point it makes peak torque. Past that point economy decreases, below it it decreases if the engine is being lugged. But what that point is??????????????? You need a dyno sheet to tell you.

BobK · #6 10-09-2004, 10:36 PM

Diesels actually are most sensitive to work performed. Much more linear than gas engine I'd say. Low revs help but if you have low revs and your foot mashed you will still drink the fuel. Thing with revs is the work it takes to make thoses revs. Considerable work involved in compressing air at over 20:1 ratios. Add in the considerable friction losses which go WAY up with revs and you can see the losses in the engine. Thus best bet is low revs and working with the torque of the engine. At or slightly above the torque peak is usually the best all around engine speed for the amount of work done vs. fuel consumed.

Hit Man X · #7 10-09-2004, 11:08 PM

Quote:

Originally Posted by boneheaddoctor

Optimum economy is going to be achived by keeping the engine in its sweet spot, not above or below. That will be different on different engines because its a factor of engineering. That is usually at the point it makes peak torque. Past that point economy decreases, below it it decreases if the engine is being lugged. But what that point is? You need a dyno sheet to tell you.

Well I doubt 3500rpm is optimum by anymeans for any diesel motor, heck most of the truck motors rev limiters are right around that area.

A diesel motor is known for making torque at low RPMs, so that being said... anyone know the Benzo specs for TQ peaks of the ol' diesel motors? The 2.4 NA and the 3.0 TD?

boneheaddoctor · #8 10-09-2004, 11:15 PM

Quote:

Originally Posted by Hit Man X

Well I doubt 3500rpm is optimum by anymeans for any diesel motor, heck most of the truck motors rev limiters are right around that area.

A diesel motor is known for making torque at low RPMs, so that being said... anyone know the Benzo specs for TQ peaks of the ol' diesel motors? The 2.4 NA and the 3.0 TD?

the 617 and may mercedes diesels for that matter are high reving motors. Unlike many over the road truck engines. Like I said max economy is at or very near to peak tourque.

Much lower and you are lugging engine (relative term) and too high and you do get a real drop in economy.

Brian Carlton · #9 10-09-2004, 11:19 PM

I've read your post several times and cannot determine which vehicle we are speaking about.

Proper advice is not possible until we know if we are dealing with a n/a 240 or a turbocharged 300D.

If you have the 240, the available horsepower will require you to keep the current gearing. Trying to put taller gears in it will result in no better fuel economy, but, will reduce the top speed, and will drastically increase the acceleration times.

However, the 300DT, having twice the horsepower of the 240, would certainly allow you to use a 2.88 axle. This would provide some fuel economy benefits, but, you would have to carefully monitor the consumption to document it. You certainly will not get anywhere near 15% out of it.

Hit Man X · #10 10-09-2004, 11:19 PM

Quote:

Originally Posted by boneheaddoctor

the 617 and may mercedes diesels for that matter are high reving motors. Unlike many over the road truck engines. Like I said max economy is at or very near to peak tourque.

Much lower and you are lugging engine (relative term) and too high and you do get a real drop in economy.

Hrm, well why the vast differences in axle ratios...

230,240D RATIO 3.69:1
280E RATIO 3.58:1
300D RATIO 3.46:1
300DT 85 RATIO 2.88:1
300DT TO 84 RATIO 3.07:1

Do the smaller motors really need that extra higher RPM cruise to keep moving? I understand about lugging, my '01 Z71 runs at 1800rpm@70mph with 3.73:1 gears, 33" tires, and a 0.70:1 OD

boneheaddoctor · #11 10-09-2004, 11:23 PM

Quote:

Originally Posted by Hit Man X

Hrm, well why the vast differences in axle ratios...

230,240D RATIO 3.69:1
280E RATIO 3.58:1
300D RATIO 3.46:1
300DT 85 RATIO 2.88:1
300DT TO 84 RATIO 3.07:1

Do the smaller motors really need that extra higher RPM cruise to keep moving? I understand about lugging, my '01 Z71 runs at 1800rpm@70mph with 3.73:1 gears, 33" tires, and a 0.70:1 OD

You are comparing apples to oranges. except for the 300 series the others different. and the 280 is a gasolene engine.

And smaller engines generally do because they have less torque, and make up for it by creating max torque at a higher RPM.

Lugging actually creates a heavy load on the engine because you run have it working harder to maintain speed than you would with more favorible gears and a higher RPM

Hit Man X · #12 10-09-2004, 11:24 PM

Quote:

Originally Posted by Brian Carlton

I've read your post several times and cannot determine which vehicle we are speaking about.

Proper advice is not possible until we know if we are dealing with a n/a 240 or a turbocharged 300D.

If you have the 240, the available horsepower will require you to keep the current gearing. Trying to put taller gears in it will result in no better fuel economy, but, will reduce the top speed, and will drastically increase the acceleration times.

However, the 300DT, having twice the horsepower of the 240, would certainly allow you to use a 2.88 axle. This would provide some fuel economy benefits, but, you would have to carefully monitor the consumption to document it. You certainly will not get anywhere near 15% out of it.

Well my initial intentions are concering either an NA 2.4 (I found a nice car that needs a motor, so I'd keep an NA 2.4 in there or step to an NA 3.0 swap I read about via the search here) or a 3.0TD as I found another decent one locally for $1200 cash. I should have tried to be more clear but I guess it could apply to either motor, but that seemed to be wrong.

Brian Carlton · #13 10-09-2004, 11:27 PM

The smaller engines put out considerably less horsepower.

The 240 puts out something like 67hp at 4400 rpm. If you try and run the engine at 2400 rpm, you will get something like 42 hp. This is not sufficient to attempt to run the vehicle at 70 mph. There is nothing left for upgrades and nothing left for acceleration.

The 300DT puts out 120 hp at 4400 rpm. If you try and run this engine at 2400 rpm, you will get about 75 hp. This is probably the bare minimum to operate, comfortably, at 70 mph. The 2.88 axle would allow the 300DT to turn at about 2900 at 70 mph. This is perfectly OK.

So, the axle ratio depends on the available horsepower and the tolerance of the driver.

Hit Man X · #14 10-09-2004, 11:28 PM

Quote:

Originally Posted by boneheaddoctor

You are comaring apples to oranges. except for the 300 series the others different. and the 280 is a gasolene engine.

And smaller engines generally do because they have less torque, and make up for it by creating may torque at a higher RPM.

Oops, didn't catch the 280E axle gearing in there.

Okay, let's stick with the 3.0 motors to keep things simple. Why do the Wagons have taller gearing as opposed to the Sedans?