I have swapped an OM617.951 into my 89 Jeep Wrangler and I am trying to figure out what the best gearing would be for the differentials. Both vehicles weight about the same and have the same 1.00 final drive. I want to have good acceleration in 1st gear, but I also want to maintain proper speed at optimum RPM in the final gear.


The 85 300 SD was as follows:

Auto trans:
1st gear: 3.54
Final Drive: 1.00

Rear Differential: 2.88

Tires: 25.3" Diameter

Weight: 3500lb ish

The Jeep is as follows:

Auto trans:
1st gear: 2.74
Final Drive: 1.00

Rear Differential: 3.73 (currently)

Tires: 32.07" Diameter

Weight: 3200 lb

Currently I see it like this (if this is even a correct way to look at it...):

300SD (1st gear): 25.3/(3.54*2.88)=2.48
Jeep (1st gear): 32.07/(2.74*3.73)=3.14

So.... 3.14/2.48 = 1.27
I think this means I am asking 27% more out of the engine in 1st gear than the 300SD did.

Further

300SD (Final Drive): 25.3/(1.00*2.88)=8.78
Jeep (Final Drive): 32.07/(1.00*3.73)=8.60

8.6/8.78=0.98, so this is almost exactly where it should be.

If I go to 4.10 gears, the 1st gear calc goes to 2.85/2.54=1.12 but the Final Drive calc goes to 7.82/8.78=0.89. So while in 1st gear, acceleration would improve, at highway cruising speeds I would be pulling higher RPM.


If anyone has read through all that, would the higher RPMs in final drive be ok for engine performance, or do you think it would be better to live with the slower acceleration and keep the highway performance right around where the 300sd was?

I didn't read your entire post, but I did catch the first and last paragraphs. Here's a couple of suggestions:

1) You might want to search this forum for various results when folks changed the differential on their w123 300D with the same OM617 engine. The results/opinions are mixed.

2) You might want to venture over to 4BTswaps dot com and look at some of the OM617 swaps. There may be a few into Jeeps. If you do venture over the and you are a 4BT fan, check out my build thread (swap) on my 2000 Dodge Dakota.

Of course, it's really going to boil down to what your major objective is. Are you looking for higher MPG or is low end torque more important? If you are going to be doing 40K+ miles of interstate driving a year and want to maximize your fuel economy, you can probably live with the lower engine RPMs at top speed. On the other hand, if you want to haul, pull, or do ANY extreme offroading, then you are going to want a different configuration. And if you just want something in the middle......then, there, you go.

I hope those questions and this post are helpful to you.

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

Assuming your transfer case has a 4lo option, I would go with the lower engine revs on the highway. If you're offroad and want more torque, then toss it into 4lo.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

YOu really have no idea where you are until you tell us the diameter of the tires in both cases. the jeep will have taller tires, most likely so to equal the 2.88 gears of the sd you'll need 323 or perhaps 343 gears in the jeep.

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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.

He did in the first post unless it was added after you asked your question;

The 85 300 SD was as follows:
Tires: 25.3" Diameter

The Jeep is as follows:
Tires: 32.07" Diameter

With the poor aerodynamics of the Jeep, I would set my gearing to spin the OM617 at 3000-3200 at 70 mph in top gear. there are several good rpm calculators on the web that would help you determine the diff gears to achieve this.

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You can't always get what you want, but if you try sometime you just might find you get what you need.

I believe that would be as follows:

(3000 rev/min)*(32.07in*pi/rev)*(mile/(63,360in))*(60min/hr) = 286.08 mph before applying the reduction from the rear diff.

so 286.08mph/70mph=4.08 differential to achieve that.

With my 3.73 gears, I would be doing 76.7mph @ 3000 RPM
or, @70mph it would be pulling 2730 RPM

Doesnt the OM617 achieve maximum torque at 2800 RPM??? So wouldnt that be the optimal range?

With an EGT gauge and some DIY tweaking inside the pump you can move that torque curve around FYI.

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

EGT gauge and probe are sitting in the garage right now, just need to tap the exhaust manifold and install it. I have already removed the ALDA and adjusted the valves. The cam timing is absolutely dead on.

Adjusting the power output was going to be my next thought. Do you have any details on what adjustments I should look into? I know there are many different ways people propose to increase output on these engines, but there is a very wide range of opinions on what works and what doesnt...

Looks like you're pretty close to ideal with your 373 gears. I'd start with them since they are in there and see how you like it.

__________________
[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.

Make sure your injectors are set to pop properly, and are spraying a nice pattern.

Advance your timing to 26*-28*, if it knocks at all retard it.

Fuel return spring stretched back to spec

While you're installing the EGT probe port the exhaust manifold. Mine had some pretty big casting irregularities. Use your judgement when you work on it.

Without an ALDA the guestimate boost level should be around 12psi.

Pulling prechambers and boring their holes has been discussed.

With the pump tweaked (check out superturbodiesel forums for the doc) you might need to run some more boost (don't know the # offhand)

A VNT will reduce spool time. Methanol/water spraying is good for 25hp-ish. propane can blow out your prechambers you're dancing with the devil if you go that route.

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

3 speeds , lowish power and limited redline don't work well together.

Did the Wranger ever come with the 4 speed over drive automatic Torqueflight? If so, the conversion _will_ be worlds better to drive with 4 speeds and a lock up converter.
( Some Jeeps came with a Toyota automatic that I know little about )

OD and lock up on a Torqueflight are triggered by a solenoid for each. The OD bypass switch is usually on the dash so the shifter will look the same as a 3 speed.

Yout trans with 2.74:1 first, 1.54 second and 1.00 third has the wide ratio gear set where older stuff had 2.45 1.45 1.00 . OD is somewhere around 0.69 and I'm not sure if the wide ratio 1 and 2 were used with OD.

So, you will be able to gear the axles for good acceleration then use overdrive to keep engine RPM down on the highway. What also matters is the motor is running at / near torque peak for good fuel use with the ability to drop down a gear so RPM rises into better HP numbers for acceleration.

Mercedes seems to run counter to most USA cars by running numerically large trans ratios and small axle ratios.

"""""
Currently I see it like this (if this is even a correct way to look at it...):

300SD (1st gear): 25.3/(3.54*2.88)=2.48
Jeep (1st gear): 32.07/(2.74*3.73)=3.14 """"

The numbers need to be equalized to RPM Vs road speed,

Somewhere there is a conversion formula ( tall tires + high number ratio = short tires + low number ratio )

If I had 3.73 gears currently, I'd try them out first and see how I like the performance. That 2730 would be sweet if the OM617 can handle it. My thinking is the larger tires and poor aerodynamics of your Jeep may require the OM617 to rev a little higher for steady 70 mph cruising. If the gears are too tall, then the motor will struggle to maintain constant rpms on inclines. MB set up the OM617 in trucks in europe to cruise at rpms higher than 2800 for some reason but definately try what you have first.

__________________
You can't always get what you want, but if you try sometime you just might find you get what you need.

Due to the cost of adapting the trans to the engine and the fact that I just did it, I cant really afford to do it again... So I am stuck with this trans for now. In hindsight, I would have swapped to a 4 speed AW4 auto and bought the trans adapter for that trans, but its too late now... Maybe in the future...

After driving it yesterday, I think 3.73 is the right combo for the 3 speed, and I just need to do a little adjusting to get a little more power off the line. I would like to install a VNT turbo, but I can not find any useful info on one that would work...

The equation I had above does what you say... With dimmensions it is:

300SD
(25.3in*pi/1 Axle_RPM)*(1 Trans_RPM/3.54 Engine_RPM)*(1 Axle_RPM/2.88 Trans_RPM)= 2.48*pi in/Engine_RPM

Jeep
(32.07in*pi/1 Axle_RPM)*(1 Trans_RPM/2.74 Engine_RPM)*(1 Axle_RPM/3.73 Trans_RPM)= 3.14*pi in/Engine_RPM

If you further convert inches to miles, you get mph/Engine_RPM

Since each side is in equivalent units and pi is on both side, no further conversion is needed and pi can be dropped with no effect on the comparison of the two results. If you set each equation equal to each other and replace the 3.73 with variable (x), solving for x would be as follows:

1/[(25.3/32.07)*(2.74/3.54)*(1/2.88)]=x and x = 4.72

So 4.72 would give a straight equivalency without load differential in first gear, but when applying this to final drive, you get:

1/[(25.3/32.07)*(1.00/1.00)*(1/2.88)]=x and x = 3.65

If I had the torque curve for the engine, and the road load vs speed for each vehicle, I could theoretically figure out what the actual best equivalent would be, but I can not find the road load vs speed for the Jeep... So I do not know what is required to push my brick through the air at each speed...

The turbodiesel is notorious for being sluggish off the line, but once rolling it has good throttle response, but always better the higher the rpm, up to a point. With the engine over 3000 rpm the turbo is really humming and you get instant acceleration on kickdown of the tranny....(in a Mercedes). If your three speed has wider gear spacing you may not be able to kick down and immediately hit the sweet spot like the Mercedes with the gear ratios tailored to the engine though. But really you probably installed the diesel for its low down torque and better economy so talk of perfect acceleration is probably not that important.

__________________
[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.

The 4 speed OD torque flight is a 3 speed with a OD unit hung on the back. The back of the case is different so you can't just hang a OD on.

So, it is highly likely a 4 sp exists with your current bell housing. Drive shaft length and trans cross member location may be the only changes needed.

Given the current trans works, drive it around for a while and decide if you need more acceleration ( lower axle and OD to get back to the same cruise RPM ) or lower RPM on the highway ( OD alone )

Another bit, to help off the line acceleration, a high stall converter helps. Then use lockup to reduce slippage.