1988 r20 Suburban Cummins swap

[dwntwn]

This has been an ongoing project. I picked this Suburban up Nov of 2016, its a 2wd TBI 7.4 liter, TH400, 4.10 gears. Over 200k on the odometer, but overall a good running truck. Within 60 days of owning it I'm trying to figure out the best path for overdrive (4l80, gear vendors, etc)

I accidentally found this 91.5 Dodge D250 July 2017, it drove great, was beat up but ran outstanding. 200+ on the broken odometer, no blowby, original—nothing done to the VE pump, etc. Just a good truck. I'd always wanted to do a Cummins conversion, and figured I could at least start on figuring how to get an intercooler into the rectangular Chevy core support.

I started with the idea of using as much as possible off of the Dodge—radiator, intercooler, evaporator, trans cooler. I had a tape measure and thought I had enough figured out to start on an extra core support. (core support images are from August of 2017)

[dwntwn]

I wanted to use what I had as much as possible—I was even going to use the first gen engine mounts if I could. I followed the builds on this site and there was one on Facebook that was informative. There was a lot of photos of second gen intercoolers.

These photos show the stock First Gen setup... I liked how everything fit together. Brutally simplistic.

The engine and driveshaft angles are from my D250 sitting on level ground before anything was taken out. I was going to keep the angles as close to this as possible when the Cummins goes into the Suburban. (Note: Driveshaft photo is upside down...don't know how to fix that)

[dwntwn]

FFWD to April 2020, I started taking the sheetmetal off the Dodge. Next month I needed new tires for the Suburban and snagged the second gen wheels that were on the Dodge. They actually look alright, and I think thats the first part from the Dodge put on the Chevy.

We had one of the worst summers as far as heat but when I could I was kept busy doing a lot of work on a 1969 c20 (replaced the motor, PDB conversion, etc) and sold that Oct. 2020.

As soon as the c20 was out I pulled the Dodge back in and worked on removing the drivetrain.

The last photo shows the Cummins and a518 out of the Dodge and on a cart. Besides pushing it around the shop, it remained like that for about 11 months. Its what I have, and for what I'm doing with the Suburban it'll be fine. If I had a later one like a 47RE I'd have made that work instead.

[dwntwn]

I forgot to mention while I was getting that c20 ready to sell I started cleaning and painting a c10 frame. Its an 84 short 2wd, I've got a clean AC cab (no rust) I'm gonna put on it. The original cab was from back east, non ac and rusty. This truck has a rebuilt 454 on the stand all ready to drop in.

Another thing I left out was my yard was full with a lot of accumulated junk—I scrapped an 86 Suburban, made some room for a 40 foot Conex box and spent 3 weeks basically organizing everything i've been dragging home the past decade. It was a lot, but needed to happen. I'd spend way too much time searching for stuff I knew I had here somewhere.

Next photo shows the KDP getting peened. A lot of people stress over that, this one was fine, not even close to sticking out. Easy piece of insurance while the motor was out either way. This is around February 2021.

I still have the Dodge on a trailer... I'm glad because I keep thinking about little parts here and there I might want. Plus i've got a lot of interior parts rat-holed in the cab, so for now its ok. Plus it prevents me from dragging another non-running project home. heh heh

This has nothing really to do with the build, but it fits the timeline...we went to Rocky Point Mexico May 2021 (its a 4 hour drive south of Tucson) its an amazing spot—but check out this rusted-out roof on this poor Chevy!

Next photo shows that c10 in July... got the frame painted, suspension back on it.

[dwntwn]

Up to this point, I haven't really mentioned goals. The main thing with the Suburban is its primarily a third vehicle that we use a couple times a week. I'm not anxious to have it down for too long, so I haven't just started pulling out the drivetrain. Another thing is I will need to sort out local emissions. Its due to get tested June 2022, so I'm probably going to leave the 454 in until then, as soon as it gets smogged I'll have a year to decide next steps.

I'm trying to sort out as much as I can so the conversion itself takes as little time as possible. This is my first Cummins conversion and the threads here and elsewhere have given me a lot of information to choose which path makes the most sense.

AC is a must, and I want to use as many parts as I can that I already have.

Something I've been thinking about is whether the floor on the Suburban will clear the a518. I'm not sure why I'm so against a body lift, but its just something I don't want to deal with if I can avoid it.

I decided to do a test-fit with the Cummins/a518 on the c10 frame since it had no sheetmetal in the way, and I could sort things out. I went with a set of Summit engine mounts and a piece of angle iron to support the transmission until I could figure that out.

I had both a Chevy and Dodge transmission crossmember, but neither quite did what I was looking for—Dodge put the transmission too high, Chevy would work, but had exhaust clearance on the driver side, Cummins os on the passenger.

In the end I ended up springing for a Dirty Dingo tubular crossmember—fits perfect, gives the driveline angle I want, and the exhaust clears. They are located in Phoenix and offer a ton of LS conversion stuff. I was pleased with the crossmember. It was like $165 shipped unpainted. Something cool was I was able to use a TH400 bushing—the holes almost perfectly line up with the a518, I had to do a tiny bit of filing to enlarge the holes by 1/16" inboard, but that was it.

Now I'm trying to figure out the core support, and up to this point, this has been the most challenging. I want to use the fist gen intercooler, but the Chevy radiator is too wide. I could use the Dodge radiator, but its too tall. I can either get a custom radiator made or just go second gen intercooler.

As it stands right now, second gen intercooler is most likely what I'm going to end up doing.

[dwntwn]

Since the cab I'm putting on that c10 was a 4 speed manual, it had the high hump/removable cover. This was a good opportunity for me to see how the 518 might clear the Suburban floor (low hump) It fits easily, but I'm most likely going to cut the Suburban floor so I can use the c30 cab cover (I needed an excuse to buy a new carpet kit for the Suburban anyway)

The fan clutch I'm using is 1st gen, on the Dodge it was 1.5" from the face of the radiator. I've got the engine "frame towers" pushed back as far as possible, there is enough room for the stock exhaust to clear the firewall, I can easily remove the last Cummins valve cover. If someone wants specific measurements hit me up.

This is about as far as it sits now. The next thing I need to sort is the core support. I think I've landed on keeping everything GM (radiator, evaporator, trans cooler) and making the Second Gen intercooler fit.

Questions I have are making the First Gen intercooler piping match with the second gen. (I think they're about 1.25" smaller first gen—I'm away from the shop right now to confirm)

[Dieselwrencher]

Nice project! I think the only thing I did on a square body low hump floor was roll the lip of the firewall so a 4" down pipe would clear and so the dipstick would pass by. That was a C20 also and the frame was a little taller but it all worked out. That was the only 2wd square body I have done. I used a champion radiator and a 1st gen dodge intercooler that I had one side tank widened to fit around the chevy champion aluminum radiator. I did a 4x4 89 gmc burb the same way and they worked well. The 1st gen intercooler piping is smaller but nothing some chop cut rebuild won't fix.

That 518 is also a little closer to the engine than a 47re or rh so keep that in mind if you ever plan to swap to one. The difference is in the engine adapter plate. The 518s are non lock up so the converter is shallower. The engine plate is only 1" thick where the 47rh and re engine plates are almost 1.5" thick/deep. This puts the trans farther back and can get right into the firewall. Just something to think about. 518s are great transmissions. The non lock up thing just gets old. But if you never have it you don't know what you're missing.

[dwntwn]

Originally Posted by Dieselwrencher

Nice project! I think the only thing I did on a square body low hump floor was roll the lip of the firewall so a 4" down pipe would clear and so the dipstick would pass by. That was a C20 also and the frame was a little taller but it all worked out. That was the only 2wd square body I have done. I used a champion radiator and a 1st gen dodge intercooler that I had one side tank widened to fit around the chevy champion aluminum radiator. I did a 4x4 89 gmc burb the same way and they worked well. The 1st gen intercooler piping is smaller but nothing some chop cut rebuild won't fix.

That 518 is also a little closer to the engine than a 47re or rh so keep that in mind if you ever plan to swap to one. The difference is in the engine adapter plate. The 518s are non lock up so the converter is shallower. The engine plate is only 1" thick where the 47rh and re engine plates are almost 1.5" thick/deep. This puts the trans farther back and can get right into the firewall. Just something to think about. 518s are great transmissions. The non lock up thing just gets old. But if you never have it you don't know what you're missing.

This is great info, thanks Ryan!
I'm still undecided on a custom radiator or second gen intercooler...gonna reach out tomorrow to Champion, see what it would run