Sweet build! What tranny are you using? How many rpm's are you running at 70 mph? What's your differential gearing? What size tires are you running? How does it do off-road when wheeling? I assume your not running high rpm's on the trails? Are you getting the 20-25 mpg's folks claim?

 

T176 tranny, no tach so have to just do the math on RPMs. 3.73 gears. 33" tires. So far so good on the trail for being at 9k feet MSL and above. Not sure of MPG, but not that high. I'm revved up all the time to get over these mountains. Overall, the engine runs like a dream, but it sure could use a lower 1st gear. The low end torque is not great. Lots of RPM and clutch feathering to get it off the line at a stop light. Maybe even lower axle gears. Cheap conversion, but it's no Cummins.

 

Although both run on diesel, the OM617 and the 4bt are completely different engines. The OM617 is designed to rev high and the cummins is a lower reving torque monster. Till you get used to the OM617, you will always think its revving too high. To tune this beast in you definately need a tach. for example, the 300d/sd your OM617 came from spins around 3400 rpms at 70 mph before 1985.

While all 33" tires are not exactly the same height, the rpm calculator I used has you at 2625 rpms at 70 mph which is too low. Some 4.10's would get you closer but still be on the tall side. If you could get access to some 235/70R15's or something just slightly taller to put on for a test drive, those would get your rpms just north of 3000 at 70 mph which is where I think you want to be with a Jeep's weight and aerodynamics. If you liked the drive train's performance with the smaller tires, maybe axle regearing with the larger tires would be in order to complete the project?

Good luck!

 

I agree. I've done Cummins swaps, and they are much different. You have to gear them pretty intensely to keep them in a very small RPM range (1700-2200). I also agree that this engine likes to rev. I think the axle gearing will definitely help at highway speeds, but not sure it will do much for the off the line issue here in the mountains. Turbo is not spooling till engine is revving, so not much power available off the line. Considering a T18, but hard to find the one with the 6:1 1st gear. Then have to mate it with the D300, etc etc, more money, etc etc. I think I will put some 5.35's in there as I am really never above 60 -65 MPH. 3300 rpm would yield about 61 MPH. 3800 RPMs to go 70. I don't know,,,What do you think? Are you an OM617 owner/Guru? As I have read, the engine makes it's torque at about 2600, which is where I am now, but it makes its HP up in the 4000's. Most diesels that I've owned like to be in their torque curve for best econ and throttle response.

Thoughts? 3300 RPMs on the highway good for economy?..Not wasting fuel just to feel more power?

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Looks like 4.88 is as Low as I can go after shopping around a bit

 

I'm not a Guru. There's been a couple of OM617's in my family and I have one right now on an engine stand awaiting a repower. I commute in an MB 190D right now with the OM601 so I'm pretty familiar with their characteristics. My 190D is a 4-speed auto and revs at 3400 rpm's at 70 mph.

To me, a 5-speed makes these motors a more complete drive train but it doesn't appear you are really wanting your scrambler to hit the interstates so the overdrive would be less of a need for you.

Mercedes made a G-wagon 300 GD with the OM617 which is close in size and function to your scrambler . It would be interesting to see their gearing selection. The OM617 also came in the Unimog and some other delivery type trucks which may be even closer to what your wanting to do with your scrambler? You might want to post your issue on Unimog - Benzworld.org - Mercedes-Benz Discussion Forum to see what those folks suggest for gearing.

If you are rarely above 65 mph, then I would be comfortable with gearing that produces 3500 rpms at that speed. I don't think its the best gearing for highest fuel economy, but thats where all those 300D's driving around are running. Folks who have re-geared those for economy drop the rpm's down in the 2800-3000 range for crusing speed and report a 5% - 10% decrease in fuel use. Its not going to hurt the motor to cruise at 4000 rpms if you need to go 70+. If your doing alot of off road crawling, then a t/c with some low gears would be useful.

I still think you ought to find / borrow some smaller tires to simulate gear changes before investing in new R&P's.

As for off the line, if you are geared right I wouldn't think the OM617 would be gutless if its running right.

 

I ended up getting a NP435 tranny out of a Ford Bronco for 60 bucks on Craigs List. They bolt right up to the bellhousing, and they have a 6.68:1 first gear and 8.26:1 reverse. Had to get an adapter from Novak for the Dana 300. Total cost: about 700 bucks. Had to take it apart to install new mainshaft, but it was nice to see inside for inspection anyway. Will install tomorrow and post results. That should certainly solve the off the line issue, as the T176 has a 3.52:1 first gear. As for the highway, I'm not displeased with highway performance. If I need to, I can run into the high 3000's in 3rd gear and still run highway speeds. Even here at high altitude, I'm in 4th gear most of the time with no trouble. When I'm climbing the passes, I just have to drop a gear, no sweat.

BTW, I also just removed the wastegate line from the turbo and blocked it. Folks report 30psi from that mod, which is useless in this engine at sea level, but up here, above 10k feet, I'll get some use out of some more boost. Atmospheric pressure up here is only about 10.1 psi as apposed to 14.7 at sea level. 17-18 psi would bring the engine back to sea level pressure. As long as heat does not become an issue, I'll run the boost wide open. If it becomes an issue, I'll regulate it. Might clear up some black smoke at wide open throttle.

 

NP 435 installed....Problem solved..This low first gear is the absolute ticket. I now have 67:1 crawl ratio, and easy off the line performance. Turbo is running wide open, and there is almost no black smoke at wide open throttle. This is a new machine!

 

George "Hannibal Hayes" Peppard voice on* "I love it when a plan comes together." *GPv off*

 

HI DieselGreg, Looks good. What sort of water temps are you seeing. I was concerned about running a e-fan only, but then I saw yours, I think you have a bigger fan than I put on the XJ. My gauge is showing a little over 200. my point and shoot thermometer, is showing about 190 it think.
If you still need a tach, I got a "TinyTach" for my XJ, cost $95, AND made in the USA.. I am very happy with it.
I used to live in CO, and I know what you are going through with the altitude. I wanted to put a turbo on my 302 Bronco with a manual wastegate from a Cessna. I got by on the Bronco with a re-jetted carb, but you really lose a lot of power at altitude.

 

I'm running the stock Jeep gauges, so temp is bottom of the green arc. Very important to use a properly fitted shroud. Do no put e fan directly on the radiator. Those little "zip ties" will eventually wear through and water will leak out. I used the stock Jeep shroud, Put an aluminum plate in the fan hole and cut out the exact size of the e fan in the aluminum. I'm not sure of the size 16"? I have it wired through a thermostat, so it only runs when it needs to. Don't need a tach, but thanks anyway.