1983 300SD

Recently I have been toying with the idea of not putting shifter to D untill my car reaches 45-50 MPH. This is mostly slow driving situations, around town, or when going up hills at a speeds less than 50MPH which I do allot due to where I live.

Where I live I have to go up allot of long fairly steep hills and the car definitely goes allot faster in S compared to D when I climb these hills with the fuel pedal in the exact same position.

It amazed me when I first saw the car do this time and time again.

The reason I THINK this happens is because the engine if running in a better power band for effeciently.

I was sort of surprised of this but it happens. Try it.

As you would expect the turbo speeds up as the engine starts to burn the fuel more effeciently.

I was wondering if anybody else has noticed this or am I over looking something or misinterpretting this.

This cars shifting bushings have been rebuilt since I purchased the car so I'm thinking the previous owner my have been doing it also.

These engines don't seem to run very effeciently when the engine speed is below 2500 RPM, give or take.

Do any of you turbo owners do this on a regular basis?

Thanks.

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1982 300SD

No, if you have an automatic gearbox, learn to use it.

If you want to drive it like a manual, buy a manual, or a modern tiptronic.

You are just forcing the car to drive "on cam", which is bad for fuel economy and longevity.

Average speed over the ground isn't a function of engine RPM or being "on cam"

What do U mean by On-Cam?

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1982 300SD

I do this occasionally, of course mileage will suffer, you're at higher revs.

You are not harming the engine as long as you keep it below 4000. Max horsepower is attained at 4350 rpm, not that is should stay there, I'm thinking.

Yes, you are in the turbo zone, read power band above 2500.

These cars don't have the best of fuel mileage, at least in some eyes, so MB did what it could to help. Some start in second gear and lope around town to get higher mph.

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83 SD

84 CD

I only do that if I happen to be driving at a speed that causes it to shift frequently (i.e., it downshifts on every small hill). Most of the time it works just fine in drive.

I hardly ever go as high as 4,000 rpm, usually when I'm doing this hill climbing it is 2,500 to 3,000 in S.

This seems to be the sweet spot on these cars, where it runs the most effeciently.

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1982 300SD

I do as Craig does to force it not to shift up and down as you climb hills and make turns. My incline is about 8 miles with allot of turns. Anybody understant what the term On-Cam means?

When I do it my car speed goes up about 10MPH with the same fuel postion on the rack, ie Go pedal. So for me this is a big difference.

But we all do it to hear the turbo scream, right?

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1982 300SD

If it upshifts too soon on a regular basis, consider adjusting the cable (a little tighter) to keep it in the lower gear longer. The term "on-cam" just means running the engine at a higher rpm to get more power.

OK Craig. Little tighter on the cable, gotcha. TY for On-Cam explanation. The hill climb here is such that I'd prefer just to go with the S postion but I know now which way to go with the cable should I want to go this way in future. Thanks!

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1982 300SD

see this, the red line is the torque curve


You can see this engine is "on cam" at around 2000 RPM, basically it is the revs at which the engine starts to "pull"

the more tuned the engine, the more pronounced this is.

staying off cam improved economy and improves longevity.

staying on cam maximises performance, fuel consumption, and wear.

do not equate rack position in a diesel with fuel consumption, you have to multiply by rpm... eg "x" rack position at 1500 rpm = 3/4 the fuel consumption of the same rack position at 2000 rpm.

also if you double rpm you quadruple reciprocating loads and therefore wear, so a 33% increase in rpm from 1500 to 2000 represents a 78% increase in recprocating loads and therefore wear, also loss of volumentric efficiency, etc etc

if you hold the throttle steady, the auto box will select the right gear to maximise economy and minimise wear.

The term "on cam" has very little to do with a turbo diesel engine and is mostly just confusing to anyone who wasn't working on engines 30 years ago. What you mean to say is "peak torque."

With my trucks and my cars, I have found that greatest efficiency is had when at torque peak. That said, I almost never manually select a gear position.

I do, on occasion however, hold a lower gear in the benz on a cold day to warm it up while starting out on my journey. Out of my neighborhood 10-15mph in 1st-2nd gear, then hold it in 3rd on the main street for a mile, then back into Drive to let it do its thing.

At no point am I concerned about wear and tear or efficiency. I only want the oil to warm up, while not sitting in my driveway going no where.

For the OP: Bottom line, its your car, do what you want. Your findings are reasonable. Don't let fanatical wisdom change your mind.

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.

I never shift over 3000rpm unless I want maximum power. The only time the 240 sees more than 3000rpm is when I go over 75mph.

Nope, if I had wanted to say peak torque, or meant to say peak torque, I would have said peak torque.

On cam also applies to every engine with a camshaft, turbo or not, diesel or not.

In the graph above 4000 rpm is well below peak torque, but it is still very much on cam, and will stay that way until the valves bounce or it can't suck enough charge air in any more.

see how adjusting the cable works for you, but MB put the lower gear positions on the shifter for a reason. climbing hilly areas at moderate speeds seems to be one of those reasons.
that said, if you can do it with the pedal, you can always floor it, to drop down a gear on difficult hills.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!