Detroit 6.5 or Cummins 12v

[usar17]

So I just spent the fall doing a 2nd Dodge restore with my buddy. That Cummins is a beast lol he asked me if I'd ever do a diesel swap into my 85 K20. So I poked around and it seems like a 12v is a massive undertaking in wiring, mounting, suspension, and driveline work. So I went looking at early GM diesels. I see the 6.2 was more an eco daily driver than anything. But the specs I'm reading on a 6.5 look closer to a 12v.

What are everyone's thoughts on the 2 motors and which would be simpler swaps in from my TBI350/4L60e/NP208 current setup?

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[1987 GMC Jimmy]

I think it depends on the year of Cummins 12V you go for. Earlier years are simpler but have more static components in terms of tunability (e.g. P Pump vs. VE Pump). They can still be turned into monsters, though. You’d have to do the 47RH for an auto or an NP4500HD for a manual. I think they came with a Getrag something or another manuals, too. There are certain blocks to watch out for and the KDP issue is another thing.

My brother has an ‘02 OBS Dodge 2500 with a 5.9 Cummins. What a piece of crap, wiring nightmare, jacked up truck. I used to want to undertake a Cummins swap but not anymore after being around this. It’s left a horrible taste in my mouth towards them. Maybe if I was around an older, simpler setup that wasn’t all hacked up wiring, hick exhaust, and crappy shifting, I’d like it better, but that truck’s been an awful, unreliable form of transportation in my opinion. I still like the 7.3 PowerStroke a lot, and I would do the conversion if that trail had been blazed enough. I don’t think it has, though.

It seems like you like the Cummins a lot, and there are a lot of good points to them. If I had reasonable exhaust, good sound insulation, an early motor with simple components, and a transmission that had fluid gear changes rather than throwing you around/jerking around, I would consider it. I’ll play devil’s advocate here and stick my neck out for the Detroits. Late model 6.2s would be a better starting point, and some people combine parts between the two for their motor builds. I value the simplicity of the GM diesels. At its core, the 6.2 is a very rudimentary NA, V8 diesel. Super easy to work on, super reliable, and the most plug and play thing you can go for. The 6.5 follows the same idea. I don’t think it could ever hold a candle to the Cummins in terms of performance, but it’s definitely possible to build it outside of its traditionally anemic state into one where its competent and can hold its own. It also depends on what you want to do with it.

 

[usar17]

The Dodge I just worked on was a 95, so still all mechanical. Only wiring was for the 47rh. I know the earlier 1st Gen Dodge 12v was even more simplistic. It's a tank. His setup runs like a dream since we worked on it. So I was looking at diesel setups with similar performance but a manual trans.

Regarding the detroits, I keep reading they are no better than a 305 or 350 v8. So what am I really gaining by doing that switch?

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[shiftpro]

The Detroits are cheap and reliable and don't weigh 1000 lbs.

 

[usar17]

What is the overheat issue that everybody complains about?

And going from a TBI 350 to a Detroit what am I really gaining? Trust me I'd love to throw a 6.2 with a turbo add-on and 5spd just for the simplicity alone. but if I'm not changing anything except higher fuel price what is the point?

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[Frankenchevy]

For the money, I’m not sure if anything can match the torque potential, reliability and economy of a good 94-97 intercooled 12v with the kdp fix. There’s a reason why people don’t swap 6.2s and 6.5s into their first gen dodges and fords.

My cousin’s 97 can get mid 20s mpg turned down and beat a lot of “sport” cars off the line turned up. Problem is, from the mid 90s on, dodge trucks have been ugly as sin.

This is the swap I have planned for the future. I went simple now because it will most certainly be a hearty undertaking. There was an episode of Finnegan’s Garage where they swapped a 12v into the c30 ramp truck. The company he worked with has adapters galore. All depends on your budget and time constraints.

Also, there’s a company in Texas with a great reputation on rebuilding 12v Cummins. Iirc, they’re pretty reasonable. ~$5k for a zero mile unit.

Unless you’re super accurate with planning, take your budget and timeline, then double or triple it, lol

 

[usar17]

Ya the timeline and budget of a 12v is what turns me off most haha. So much to beef up and change for the motor. And yes the 2nd gen Dodge is fugly lol.

So the base question is, are the 6.2 Detroit's with a turbo worth the swap from a 350 SBC?

oh ya I have 4.10 axle gears.

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[Frankenchevy]

Unless you have big tires, I think you’ll be regearing at the very least. The engine speed is much lower on a diesel. You’ll have engine parts flying out at 65mph.

You’ll be changing quite a bit to go from gas to diesel. Hydroboost, glow plug circuit, dual batteries, gearing, front springs, etc. your trans is fine for the 6.2 but not a whole lot more unless it’s really built, etc etc

In other words, probably be better to build a gasser over swapping in a 6.2 or 6.5. That or buy a 6.2 square to begin with.

 

[Blue Ox]

Having made the mistake of doing a 350 to 6.2 conversion the first thing I would suggest is don't. Frankenchevy is right about the accessories i.e. the hydroboost. There's a lot of things to change, but the advantage of the 6.2 is that there are factory parts so it's not necessary to fabricate EVERYTHING.

These trucks usually came with 4.10 rears but they do tend to scream a bit at highway speeds if they don't have an overdrive. Automatics use a lower stall speed converter in Diesel applications and I don't know how the controller for a 4L60E is going to respond to the lack of engine inputs. There really is a lot of stuff to change if you want to get the most benefit out of a diesel.

 

[usar17]

I have a 4" lift from more front leafs. Currently have hydroboost already. I'd put in a 5spd manual with the detroit. 31' tires currently. I'd never go over 33'

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[usar17]

Currently my 350 runs nicely. Im at around 2100 rpms at 65mph. No clue my fuel milage.

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[CSFJ]

The Detroits are cheap and reliable and don't weigh 1000 lbs.

Umm, actually the 6.2 is damn close to 1000lbs.

 

[shiftpro]

Umm, actually the 6.2 is damn close to 1000lbs.

About 650lbs... like a BBC.
The Cummings is something like 950.

 

[Ilgy84]

I pulled the 6.2 out of my k30 and put in a 12v cummins and it was the best thing that ever happened to that truck. That 6.2 was incredibly under powered. I've got a 6.2 and the 6.5 turbo intake manifold you can have.

 

[Turbolover]

I would do a cummins any day of the week over an anemic 6.2 or 6.5.
A 12 valve only takes 1 wire to get it started and you can shut it off with a pull cable.
You can run a 98.5 - 02 24 valve with 5 wires.
Even in plain jane stock form a Cummins will outperform a 6.5. Do a few tweaks and free tuning and you will leave a Detroit in the dust all day long.
They have adaptors for just about any trans to a Cummins and stand alone controllers for most anything from an Allison to a 4L60 or 80.
If you use a Dodge manual you can even adapt and bolt in the clutch linkage to the chevy body.

A cummins motor is very easy and simple to work on. Chrysler parts are ridiculously expensive but if you go to a big truck cummins dealer with a part number you can buy the same thing for about a third of the price.

They don't have glow plugs, it is a glow plate and rarely does it burn out. They frickin start in extremely cold weather when other trucks dont even want to try.
A good used one can be had for $2500 - $3400. (A complete donor truck will save a bunch in rounding up parts and wiring harneses)
300,000 miles is no concern to worried about. Pull the oil cap off and if it doesn't float around while running you have a good motor.
The infamous 53 block would develop a crack in the lower right side towards the rear of the block. If it has that many miles on it, it probably isn't going to develop and become a problem.