Hi All,

I just bought my first diesel ever, a 1979 300SD.

I'm interested in determining the torque peak of this engine by putting it through a dyno run.

I've noticed several other members have posted dyno runs, but the torque peak varies quite a bit -- some as low as 2200 rpm while others are up over 3000 rpm. I'm assuming this is due to different turbo settings?

So I thought that perhaps the way to determine what the torque peak is with regard to the valve timing alone, I would need to take the turbo out of the equation. So I'm thinking I need to do a dyno run, but force the wastegate to be open the entire time, so that the turbo is (somewhat) disabled.

Why would you want to know the torque from a turbodiesel without a functioning turbo? 40% of the torque comes from the additional air and fuel that is enabled by the turbo.

Torque peak rpm is most likely determined by cam and injection timing.

I'm going to be pulling the OM617 out of the 300SD and put it into a 1967 Chevy half ton pickup.

I'd like to set it up to run at peak efficiency at 70mph. Between the rear-end and tires, I have some flexibility with changing the highway rpm, but first I need to know the torque peak.

However, after looking on this forum, the torque peaks appear to not be consistent, and I presumed that people's individual turbo settings had a lot to do with that.

So my thought was to:

1) figure out the engine's "native" torque peak (ie, independent of influence from the turbo)
2) change the rear-end to accomodate that
3) use a boost controller to give desired boost in that rpm range.

Its not that I plan on running the thing with no turbo, its that I want data which isn't influenced by the turbo before I make any more drivetrain decisions.

does that make sense?

The turbo is a fixed ratio and boost is based on engine load. The wastegate and boost controllers only control the max boost pressure.

1) The engine doesn't have a "native" torque peak. It has a torque peak with it's accessories..........which includes the turbo in this case. Data without the turbo is meaningless for a turbo engine.

2) Agreed. However, if you're looking for fuel efficiency, you run into a wall with regard to insufficient horsepower. The typical gear ratio for the sedan........3.07........allows the sedan to run at approx. 3100 rpm at 70 mph. The horsepower peak is at 4400 and the vehicle usually has sufficient power to climb most hills with that gearing.

Now, as you raise the gearing..........to 2.88 for example.........the sedan runs at 2900 rpm at 70 mph. Better fuel ecomomy..........but, further from the horsepower peak. Hills become more difficult for the sedan at 70 mph.

Now, raise it again to 2.47 in the back...........the sedan runs at 2500 rpm at 70 mph..........but, it's very far from it's horsepower peak and hills become impossible at 70 mph without a downshift. Once you start downshifting, your fuel economy disappears in a big hurry.

I can tell you, without any dyno runs, that a pickup truck is going to need the 2.88 as a minimum if you expect to drive it at a consistent 70 mph. Any consideration to go to a taller ratio would be suicide in the truck and it will not result in any better fuel economy.

Note that all of the aforementioned info is with sedan tires........215-70-14. If you use larger truck tires.........you'll need to shorten the gearing even further.

There is no magic with the 617...........you've got to have rpm's to compensate for the lack horsepower and torque...........it's only got 123 hp at 4400 rpm.

oh, ouch. that could be a problem.

The ring and pinion in my truck are 3.08, and I have (I think) 29.5 inch tires. According to this rpm calculator, that puts me at 71 mph at 2500 rpm.

http://www.angelfire.com/fl/procrastination/rear.html

so that means I'm looking at downshifting, or changing out the rear end.

If the truck is a manual, you're going to have trouble with that ratio, unless you don't mind downshifting. The problem is that a 4-3 downshift will raise the engine speed to 3800 rom at 71 mph and the engine will be screaming..........very undesirable from a driveability standpoint.

The auto will allow the rpms to climb to 2800 or so..........but, it's still weak for a truck.

Maybe a Detroit 4-53 would be a better option.
http://www.youtube.com/watch?v=k3qyR2z8xEs

wow, that's quite a rig

the other option I was considering was a cummins 4BT out of an old chevy step van (cummins did a diesel repower of a large number of step van fleets in the 80's, and many of those vans are being retired now).

however, those engines appear to be heavier and more costly (I snagged my 300SD for $1100!).

I'm still undecided if I want to stick with the mercedes automatic 4 speed, or find one of the manual mercedes 4 speeds from a 240D.