Articles on Completed Swaps

Uncle Douglas

Well-known member
Interesting. He kept his LT77 transmission.
And tiny clutch assembly, with small friction surface area, not ideal behind these engines. Wonder if this is the fellow that bolted a piece of plate steel to the cummins fly wheel to move the friction surface back and allow him to incorporate the rover pressure plate bolt pattern.
The second radiator behind the front skid plate due to insufficient cooling from the primary generic Summit sourced one means you can't off road the
truck as it will fill with mud/silt/dust and stop exchanging heat.

The radiator we had made drops engine temp 2 degree's per second that the fan ran. The ecu asks for cooling when it needs it via a switched ground for an electric fan. You can watch the coolant temp drop on the Murphy gauge.
In my opinion, if you have spent 9k on a brand new Cummins, your cooling system should be a topshelf solution, not a kludge.
 
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LR Max

Well-known member
Did a quick mpg conversion, which came out to 16~18 mpg (US miles and US Gallons). Either that seems low for a diesel or they were in the throttle the entire time. But it is a heavy truck with a giant brick on the roof (to extra brick the already brick front end) so there is that.
 

rocky

Well-known member
I'd be hoping for 23-26 mpg from the conversion. Is their result typical for this conversion?
 

Uncle Douglas

Well-known member
In a Jeep running 70-75 mph to trade shows the Cummins owned display truck consistently got high 20's.
Guessing this fellow did not switch to a 1.2 tcase since he kept the lt77 so he's turning 15% higher rpm than ideal.
Also keep in mind that the intercooler piping and exhaust system on a diesel plays a large role in how the engine breathes and that drives economy.

Since this guy did his own work, and judging by the rest of what we can see, I'm guessing he kludge'd it.
 
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MountainD

Well-known member
The cummins Disco was getting in the high teens, correct? It’d be interesting to hear real world mileage— I’m guessing 20 or below based on what I’ve heard so far, but I am interested as the 2.8 is on the table for my next build...but not at those levels. I’m hoping it’s considerably more...
 

LR Max

Well-known member
The Cummins test jeep was a late 90s TJ. So it has an AX-15 with a 0.79 over drive and a 1.1 transfer case. It could've had 3.07s or 4.10 gear ratios (depending if it was a 4 or 6 cyl initially).

But otherwise it was stock. Stock tire size and nothing else bolted onto it. Using the R2.8 gear calculator, with 3.07s, at 70 mph, it puts it near the top end on the "efficiency" range of the R2.8. So 3.07s make sense.

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Looking at that rig they built, here is what it looks like. Assuming retained pre-TDI LT77 and 1.4 Transfer case:

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So yeah, pulling an extra 1100 rpm to do the same as that TJ, and I'm sure its significantly heavier. Bump it up to 85 (pretty much the average speed needed to cross the country on any interstate without getting run over) then you are really pushing that engine at 3700 rpm. Of course it wouldn't be an old rover unless it was screaming along at 3,000 rpm.
 
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