Unimogadventures - Our build and travel thread

[Iain_U1250]

I put the truck on a dyno last weekend,



Our truck makes 123HP / 440Nm at the wheels, and has a nice flat torque curve between 1800-2200rpm.



The air/fuel ratio is where it gets interesting, as it shows that the truck is running quite lean for a diesel, especially when the boost climbs. It is quite visible in the video where the EGTs start to drop once we get over 1800rpm.




I put these two graphs into ChatGPT and asked for an interpretation as in description for the video.


Unimog U1250 Dyno Run – 123HP / 443Nm at the Wheels

This dyno run was carried out on our 1984 Mercedes-Benz Unimog U1250, fitted with a GT2860RS ball-bearing turbocharger (replacing the factory unit), a water-to-air intercooler, 4-inch exhaust, and UG100 gearbox on 365/80 tyres.

Results:

• Max Power: 123 HP at the wheels (~91 kW)
• Max Torque: 443 Nm at the wheels
• Peak Torque: ~1,800 rpm
• AFR: ~28–30:1 across the rev range

The torque curve is broad and flat, with over 400 Nm available from 1,500–2,200 rpm before tapering gradually. The GT2860RS spools quickly and maintains airflow consistently across the working range, giving a much stronger spread of torque than the stock turbo.

The AFR trace explains why power is capped where it is:

• At idle through ~1,100 rpm, AFR is lean (≈30:1).
• Between 1,100–1,400 rpm, AFR dips slightly as fuelling rises with boost onset.
• By ~1,800 rpm, torque peaks at 443 Nm, yet AFR is still lean at ~29:1 — showing fuelling, not airflow, is the limiting factor.
• From 1,900–2,400 rpm, AFR stabilises around 29–30:1 while torque holds strong, again pointing to untapped headroom in fuelling.
• Even as torque falls after 2,400 rpm, AFR stays lean, meaning the turbo still has capacity.

Takeaway: The GT2860RS and intercooler setup have plenty more to give. At ~30:1 AFR, the engine is running conservatively lean. With more fuelling (via pump adjustments or injector upgrades), and proper EGT management, the same hardware could deliver well over 500 Nm and a noticeable increase in horsepower — while still being reliable for heavy-duty use.

This video shows the dyno pull from start to finish, with power, torque, and AFR traces logged directly.

I think this is pretty true, still have a fair bit of range to play with fueling wise on the little turbo.


This is a dyno run from a stock U1700 ( 170hp OM352A)





CHat GPT compared the two results.

Power​

• The U1700 makes ~119 HP, while your U1250 makes ~123 HP.
• On paper that’s similar peak power, but remember:
  • The U1700 is running near factory boost/fuelling.
  • Your U1250 is on a modern GT2860RS turbo, but fuelling is holding it back (AFR ~30:1 is too lean for best power).

Torque​

• U1700: 363 Nm
• U1250: 443 Nm
• Your truck makes ~80 Nm more at the wheels — a big deal in low-speed off-road pulling.

AFR (Air-Fuel Ratio)​

• U1700 runs in the 23–25:1 range at load → closer to ideal diesel performance tuning.
• U1250 runs very lean (28–30:1) → safe, but leaves significant headroom for fuelling adjustments.
• This explains why the U1250 makes more torque (thanks to modern turbo/intercooler airflow) but doesn’t make much more HP: fuel is the bottleneck.

Boost​

• U1700 graph shows ~8–9 PSI, fairly typical of a stock OM352A turbo.
• Your U1250’s GT2860RS can comfortably support 15–20 PSI with good response. At current fuelling it’s under-utilised.

Torque​

• U1700: Peaks at ~363 Nm
• U1250: Peaks at ~443 Nm → +80 Nm gain
• Your U1250’s torque comes on earlier and stronger, then holds steady across the working range — much more usable for off-road and towing.

Horsepower​

• U1700: 118.7 HP
• U1250: 123 HP → only +4 HP difference at peak
• Why so close? Because your AFR is very lean (~30:1), limiting fuel delivery. The GT2860RS turbo clearly has airflow overhead, but fuelling is capping top-end HP.

Power Delivery​

• The U1700 has a smoother “stock” curve, but runs out of breath above ~2,400 rpm.
• The U1250 feels stronger in the mid-range thanks to modern turbo efficiency, but top-end HP hasn’t increased much due to conservative fuelling.

 

[DzlToy]

Scheid has your pump, as long as you swap in a 12 valve ISB, which can be had with an SAE#2 bell housing. I know of at least one 424 or 427 Series truck that has an ISB 5.9L Cummins in it, here in the States.

INJECTION PUMP, P7100 850CC AG GOVERNOR, 13MM, SCHEID

INJECTION PUMP, P7100 850CC WITH AG GOVERNOR, 13MMCOMPETITION USE ONLYP7100 PUMP WITH AG GOVERNORUP TO 850CC FUELQUICK RATE CAMUP TO 7000 RPM'SBILLET GOVERNOR WEIGHTSEXTERNAL FUEL SCREW4…

haisleymachine.com

There is a massive aftermarket for these engines and you can easily get 200 - 500 HP out of them and make them live quite a long happy life.

Note: The pump above is massive overkill, but Scheid does make nice stuff.

Cheers

 

[Iain_U1250]

Scheid has your pump, as long as you swap in a 12 valve ISB, which can be had with an SAE#2 bell housing. I know of at least one 424 or 427 Series truck that has an ISB 5.9L Cummins in it, here in the States.

INJECTION PUMP, P7100 850CC AG GOVERNOR, 13MM, SCHEID

INJECTION PUMP, P7100 850CC WITH AG GOVERNOR, 13MMCOMPETITION USE ONLYP7100 PUMP WITH AG GOVERNORUP TO 850CC FUELQUICK RATE CAMUP TO 7000 RPM'SBILLET GOVERNOR WEIGHTSEXTERNAL FUEL SCREW4…

haisleymachine.com

There is a massive aftermarket for these engines and you can easily get 200 - 500 HP out of them and make them live quite a long happy life.

Note: The pump above is massive overkill, but Scheid does make nice stuff.

Cheers

I have heard of people putting in the Cummins motors, but they need quite a lot of mods to the cab to fit - not as easy upgrading the OM352 to similar power, you can even get a factory OM366 with 300hp/800Nm but I would not like to see the fuel consumption if you use that power.

 

[BSR]

I have heard of people putting in the Cummins motors, but they need quite a lot of mods to the cab to fit - not as easy upgrading the OM352 to similar power, you can even get a factory OM366 with 300hp/800Nm but I would not like to see the fuel consumption if you use that power.

Now, here's a question...
Everyone knows the reputation of the Merc engine in the Unimog, and I would suggest that most people are familiar with the reputation of the venerable 5.9 Cummins, much loved in the US pickups and over here in quite a few DAF trucks.
I'm guessing the only reason you'd swap is a quest for power. Let's try and ignore all the cries of "heresy" from the Merc purists!
My take, purely an armchair critique, is that the Merc has durability, but the Cummins has more stomp.

Thoughts?

P.S. Truck is still looking amazing.
I remember back in the days of the original build, the joy of seeing there was another upload and seeing it progress. I think it's still one of my two favourites ever.

 

[Iain_U1250]

If the Cummins was easy to fit, I would fit one It is a great motor, and with the modern fuel injection etc, should be more ecconomical, which is what we want - increases the range and costs less to drive.


I enjoy making modification to my true, it a fun hobby. One of the things that has made a big difference was fit a modern ZF steering box. The LS3 steering is no longer made, and whilst the one I hadwas new when I put it in, it was never light, always felt like a truck. Now I can steer with one finger, and it is so much more precise.

 

[gator70]

Curious, what is the annual maintenance expense on this rig? (all monies that is spent)

 

[Iain_U1250]

Maintenance wise, not much, just filters and oil normally, I run long life synthetic oil costing $250 for 20litres on the long trips, so I only have to change the filters every 5000km, and since I buy in bulk, they only cost $15 each. ( All these are AUD) An air filter is around $120, and they last around 30,000km. Fuel filters are $30 each and I change them every 30,000km as I have a finer pre-filter which I change every 10,000km and the two of them are $10 each. So normal maintenance would be less than $1000 per year.

Tyre are quite expensive, with the two Michelin XZLs costing $1900 each which we had to get after destroying one on a remote beach in Western Australia. The first set of XZLs were $1100 10 years ago, the second set were $1300 4 years ago. I have now fitted the Advance tyres on the rear, as they were only $800, so will see how they go. I get around 30-40,000km out of the tyres as I replace them when they are about 70% worn, and don't re-groove them. The places we go tend to chew up tyres due to the sharp rocks etc, and it is just not worth the risk of getting stranded due to old tyres.

As I do all the things myself, it is just parts cost. For example overhauling the diff has cost me around $700 in bearings, seals etc but a lot of time and effort The portal bearings lasted 10 years, (apart from the one that broke when the truck fell off the jack) and they are around $1500 per corner. I've just replaced them all now, and fitted two new gears which were $1500 each.

I tend to hunt out better prices for parts if I have time, for example, MB portal seals are $120 each from the aftermarket suppliers, about $200 from MB, but they are all just Corteco seals, and I got them for $50 each from a seal place, similar with the inner portal bearings, I got SKF bearings for about 1/4 of the price of the exactly the same bearing but without the MB box. Unfortunately, the are a lot of OEM parts that are not available elsewhere, so I go with the aftermarket suppliers. For example, the special steering shaft for the steering box upgrade is an MB only part being a bit longer than the standard one, and that cost $1000.

If I had to pay a workshop to do these things, it would be really unaffordably expensive to do some of the things I have done.

I do a lot of extra things on the truck which are not really necessary, but I enjoy doing it, things like the new steering, new transmission, new wheels etc, were not necessary, but I do it to improve the truck, and mainly to keep me sane when working my day job and we can't get away as much. I am running out of thing to upgrade though I would not say that it is cheap to own a Unimog, but ours gets used for what we built it for.