Yes, peak HP is something above that.......around 4400 rpm typically.......when the torque starts to drop like a rock..........

im happy with anything over 20 mpg in my 240D... she is 29 years old so i dont ask too much from her

Not a big difference but, I think max torque in the 617.952 is at more like 2850.
These graphs are from when I put Marlene the Wonder Wagon on the rollers.

Dyno chart - torq/revs



Dyno chart - hp/speed

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Never a dull moment at Berry Hill Farm.

The five speed is a few inches longer...basically, it's a 4-speed with a different tailpiece.

This is basically an academic discussion since 5-speed are rare as hen's teeth (mine cost me $1500 and had a 240D Euro bolted to it) and the costs and engineering needed to retrofit an aftermarket OD are beyond the capabilities of most of us. But, for discussion's sake, I'll hold forth by saying that I think you have to look at more than fuel economy when considering the ROI of a 5-speed.

These cars were imported when the national speedlimit was 55mph. As delivered, (at least with the 3.07 in the turbo), they're geared for highway cruising at that speed with the engine spinning right in the middle of the torque curve at 2800something RPM. Fast-forward to 2006 when the national speedlimit is 70-75 and now you're turning the engine well beyond the RPMs it was intended for cruising...3400+ IIRC.

Regearing a turbo with a 2.88 diffy is a good start. I can detect no decrease in performance after regearing my 300D with a 2.88 which now turns, interestingly enough, 2850@65mph. I cannot speak to economy because the car is mainly an in-town driver rarely, if ever, getting on the highway but, it does manage to extract 24-26 mpg in town.

So, you have to ask yourself: What is going to happen to these engines (other than consume more fuel) if we spin them a little bit too fast for too long?

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Never a dull moment at Berry Hill Farm.

They wear out. I am thinking about air compressors as an example. Industrial compressors have motors that always spin at 1725 RPM and last a long time. Cheap compressors have motors that spin at 3450 RPM and they wear out quicker.

Thanks Randy.

Actually, that makes much more sense. The butt dyno tends to agree with 2850-3000 rpm as peak torque.

But, didn't Forced Induction or someone else do a run with a peak torque at 2400 rpm? Could that have been a n/a engine?

The owners manual calls out peak torque at 2400 rpm, but, it never made any sense to me. The torque is clearly building all the way up to 2900 rpm or so.

IIRC, Lance's curves were pretty much the same as mine except that his car (also a turbo) had a few more HP out on the top end. I was doing my runs at 80-85°F and high humidity...not a good recipe for making HP.

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Never a dull moment at Berry Hill Farm.

So were the models that stayed in Germany re-geared for the Autobahn? Weren’t 4 speed manuals fairly common there?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Can you imagine trying to drive a 240D on the Autobahn? Hope they have a far,far, far right lane reserved for 240Ds and mopeds.

You would die! That is a right lane car, stick with the RV's and family sedans pulling trailers.


Venture into the left lane and you would probably have an S500 6in from your bumper with his brights on.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Good queston. Next time I'm at the undisclosed location where my Euro is stored, I'll crawl under it and take a look at the diffy to see what ratio it is.

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Never a dull moment at Berry Hill Farm.

Make sure Dick Chaney doesn't borrow it during orange alerts.

Watch out for camo covered 300D's following you 500ft back.

the 616 and 617?

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Though another gear may help a little, I think that adding a fifth and the reported positive results are wishful thinking.
A diesel is best as a steady state engine running constantly at it's torque peak or slightly lower.
Engine loading determines fuel usage and drivers of diesel powered cars must use the eggshell on the accelerator technique keeping the engine as close to it's peak under all conditions, regardless roadspeed in order to maximize economy.

I'm not sure why there is little discussion of the use of pyrometers in this forum. It measures exhaust gas temperatures which rise and fall rapidly along with load conditions. With such a thing it is possible to drive with an eye on the gauge in order to keep the temperatures even and low. Low temperature = low load = low fuel usage. Any truck driver knows this.

Also, keeping EGT's low will aid engine longevity. This should be obvious as fewer problems with head gaskets and broken pistons is an immediate benefit.

They can't be driven like the cars you want them to be........like sports cars. Buy that gasser for fuel economy if you have a heavy foot. 70 mph is a heavy foot.