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The Cummins Bronco

104K views 236 replies 88 participants last post by  GodStang 
#1 ·
Hello, I spent 8 months building my '78 Bronc with a '91 6BT Cummins, and I figured I'd show you all the build process.

First off, back in April of 2009, I found a '78 Bronco Custom on craigslist for a mere $700. I'd wanted one for.. oh about 30 years, and one just never fell into my lap, until now. Finally getting my favorite truck of all time, I wanted to use my favorite light truck motor of all time, which is the early Cummins all-mechanical 5.9L 6BT. I wanted to go diesel for a few reasons.
1) Fuel mileage - I was building this truck to be my daily driver.
2) Power - I hate driving a slow vehicle.
3) Longevity - These motors generally last a very long time
4) Diesels are really cool.

So here we go: My 1978 Cummins Bronco Project. :rockon
 
#2 · (Edited by Moderator)

Just bringing it home on the trailer. The '78 Bronc had been sitting in a garage since 1992 (the inspection sticker on the windshield) The previous owner had purchased it solely for the engine and transmission, because the truck only had 84,000 miles on it, and it ran perfect, but the drivetrain had been pulled by the guy he bought it from, who was going to restore the truck.

Original '78 Tailgate.. flawless condition! :)


Typical Bronco rusty quarter spots. The entire underside of the cab, the chassis was all sandblasted and painted with epoxy by the original owner. He also installed a complete 6" Skyjacker suspension, poly bushings, new shocks and all. Although some of it was still in the boxes, I had to finish quite a bit myself, but believe me, I wasn't complaining a bit. I installed new tie rods, and extended the track bar mount so the bar and draglink were parallel.







As you can see, this truck is in amazing condition. Even after 32 years of Upstate New York weather.
 
#3 · (Edited by Moderator)




Yes, that's a brand new Ford core support, that the guy had already installed and painted.
 
#5 · (Edited by Moderator)

I happen to have an F800 laying around, so I grabbed the upper steering joint and welded it into the Bronco's shaft. I'd never before seen a steering joint so completely seized tight. I originally thought the steering box was bad, until I took the shaft off and checked it. It had become more like a crowbar.
Anyway, the F800 joint has zerk fittings, and it's pretty hefty.

I got a new booster and master cylinder at Napa for a '78 F350, and it bolted right in. I also installed F350 rear wheel cylinders.

I bought this set of 35x12.50R15s from a buddy for $300. It looks a whole lot better..

I installed the new shocks that came with the truck.

Drop pitman

This is the Cummins for the Bronco, still in the '91 Dodge parts truck. As ugly as it looks, it had only 159k miles and ran like new.
I paid $1300 for the parts truck, pulled the motor, and sold off the rest of the truck for $1500. So I got my Cummins for negative $200. I think that's reasonable! I got $650 for the rear axle, which was a D70 with the rare 3.07 gears.

As I was cleaning the motor, I discovered that somebody had repaired a crack in the timing case with silicone, and it had been seeping and covering the whole motor. Obviously a KDP(Killer Dowel Pin) incident at some point in the past, and after removing the case, I could see the damage to the case was pretty bad.


As you can see, it wasn't pretty. The steel alignment pin falls out on these early Cummins, and it's carried around by the timing gear, and when it hits the side of the case, either the case gets a hole and the pin flies out, or the pin keeps going and destroys the motor. That's why it's called Killer Dowel Pin, and it's the Achilles heel of the Cummins.
I found a used timing case from a guy in Kansas for $75 shipped, but the fun part is, the cam gear doesn't come off the cam, you need to pull the cam. So off to lowes I went to buy some 1/2" wooden dowels. I rounded them on the ends, and after removing the valve covers, rockers and pushrods, I tapped the dowels down into each of the lifters, then carefully pulled each one up and secured it with a wire tie. In this way, the lifters stay up and you can pull the camshaft out of the motor.


Ah, now that's better. The Cummins is the color it's supposed to be (Ford Blue) and the new case and cover is perfect.

The sheet metal cover is painted with a color called CAST COAT. I like it.
 
#6 · (Edited by Moderator)


Believe it or not, you can actually buy motor mounts for this swap. Destroked has them, and Fordcummins has them. I got the ones from Fordcummins. They position the motor properly and all, and it saved a lot of time compared to making my own.

I wanted to move the motor back 4 inches in the chassis. Mainly so the oil pan would clear the front crossmember, but also to get some of the 1000 lbs of Cummins off the front end of my 1/2 ton truck. I made these mount plates, they position the motor exactly where I want it, and they bolt to the 2 existing holes in the frame that already have integral nuts and all.

I welded the studs in place, and ground the other sides off flat. Then I countersunk the holes in the plates

This is how they hold the rubber mount supplied by fordcummins


You can see how they mount. I tried to take a shot of how close the front of the oil pan is to that crossmember, but the pic is all blurry. I can just barely fit my fingers between, so it's close. By mounting the motor in that position, I didn't need to touch the crossmember, the exhaust manifold cleared the firewall by just enough, and the motor is sitting low in the chassis. That means lower center of gravity, but also it means I can fit a huge intake pipe on the manifold without worrying about hood clearance. The dodges have about half the room that my Bronco does between the intake manifold and the hood.
 
#7 · (Edited by Moderator)

With the 3" body lift, you can see the motor clearly from the side. :D


35" tires, 6" suspension lift, 3" body lift, Cummins in place.

I threw the stock trans adapter on the Cummins just to check clearances. As you can see, there's a LOT of room in there. The motor fits better in the Ford than it did in the Dodge. Seriously. That big indentation in the firewall really helps.

This is the stock Dodge 3" downpipe, and guess what? The Ford C6 transmission adapter from destroked places the starter right there. So I knew I had to do something with that downpipe.

Enjoy that "dodge ram" on the intake pipe while it lasts, sorry dodge boys, but that's gotta go!

The belt sander made it look nice, don't you think? :)

The cnc machined C6 adapter for the Cummins, from Destroked. WOW the quality.. If I could have afforded a second one, I would have hung it on the wall, because it looks that awesome. Compare this piece to the factory part I posted earlier..

Billet cnc flexplate from Destroked. Again, wow. It's about 3/8" thick, too. These don't break.

Compared to the stock stamped sheet metal flexplate, up top..


I did, in fact, clear coat the adapter.
 
#8 · (Edited by Moderator)

The adapter requires a Ford 6.0L diesel starter.


In my opinion, nobody builds automatics like Dynamic Transmissions. They built this C6 for my Cummins, along with a billet 1200 rpm converter.

Blasted and painted NP205. As you know, these are tough. But good god, lifting one into place without a jack by yourself in the pouring rain SUCKS!!

I bought a big roll of that special green treated brake line, and I like it so far.

For the power steering, I bought a new '78 Bronco high pressure hose, then used a steel compression fitting to mate it to the Cummins pressure fitting.

Fits like a glove.



The extended length brake hoses are from Super Lift, lined with Teflon, braided stainless and covered with urethane. They look awesome.
Since the Bronc's was screwed up, I used the proportioning valve from the Dodge, and it was a perfect fit. Lines and mounting holes were exactly the same.

I used the battery that was in the Dodge parts truck. And ran all new heavy cables, soldering everything and used heat shrink to seal it up.


For the fuel return, drilled a hole in the tank and used this bulkhead fitting from Summit.

I found the grill shell at the scrap yard for $20, and the grill insert from a parts truck at a shop for $20.
 
#9 · (Edited by Moderator)

I used driveshafts from my '83 parts truck, and I cut them down to fit in the '78. The u-joints and double cardan are larger than the stock '78 pieces.


It's almost finished in these shots. The brand new Ford fenders came with the truck!

The cheesy little air filter is temporary.

You can see how far the motor is from the radiator now. Speaking of which, I had a buddy change the inlet and outlet of the stock radiator that came with the truck, and it works perfect with the Cummins. The radiator looked like it had been recently rebuilt, so I wanted to use it.


B&M shifter is a Z-Gate. I lined it up with the transfer case selector, I think it looks good.

I had to weld up the 4-speed hole and move the t-case selector back in the cover, since the whole drivetrain is moved back 4 inches.
 
#11 · (Edited by Moderator)

B&M thinks they're cute by only including car shifter brackets with their kits. I wasn't amused. I ended up making this one, which angles up. The problem was, the brackets for the cars come straight back, which means the cable would hit the transfer case. My bracket puts the cable over the t-case.

The Cummins Bronco is ALIVE. And hungry for imported vehicles.

As you can see, my old stock '83 is tiny. :D


Oh, this is the heat exchanger bracket for the transmission. I modified the stock part to work with the Destroked trans adapter.
This setup doesn't use the radiator's trans cooler, it has its own very large separate cooler, mounted to the side of the motor.


Thusly.

This is how I routed the 3" downpipe from the turbo. I cut up the stock stainless downpipe into several sections, and welded them all back together correctly. The exhaust goes outside the frame, and is hung up with rubber doughnuts. As a side benefit, there's no more exhaust heat transferred to the transmission. I also trimmed the inner fender plastic liner to clear the downpipe.

3" downpipe, into 4" exhaust, no muffler, with stainless band clamps, and painted in satin black. This is about the only place on the truck you can mount the exhaust that won't blow in through the rear window when it's down. Even when I'm full throttle and blowing black smoke, not a trace of it gets inside the truck. :)
 
#12 · (Edited by Moderator)

This tank came from the '83 parts truck. I used copper tubing for the overflow.



Rear custom driveshaft, painted and with stickers, of course. You can see the stickers clearly from the side when the shaft is in the truck.

A collage I made for a contest.

My yard ornament. :D

I got it on the road last November, in fact, right on my birthday. Which was an awesome birthday present, because I'd worked so long to build it. I ironed out some bugs over the winter, but overall, it's been a great truck. This is BY FAR my favorite truck ever.
The acceleration is insane, and I'm averaging 20-22 mpg. The noise, the smell, I love it.

It's a great fishing vehicle. The kids all fit, the poles and gear all fits. I can even put my row boat in the back when I need to.

Me and my son lounging on the beast at the park.
 
#14 · (Edited by Moderator)
Oh yeah, the tricky part.. Making the C6 vacuum-modulated transmission shift properly behind a motor that makes zero vacuum? Daunting?? Nah! The diesel has a vacuum pump on it, to power the brakes, but it's constant at about 21".
During the early days of Ford 6.9 and 7.3 diesels, they were still using the C6 behind them. So naturally, I figured I could use whatever Ford used to make them shift properly. And the answer was, a VRV valve. I ordered one from Ford, and then I had to make a bracket to mount it to the throttle assembly on my Cummins. To do this, I first made a small shaft that would weld to the throttle.


Like this. Having a brother with a lathe that I can use is AMAZING. :D I turned this thing down, and then welded the small end to the throttle pin. I then cut keyways into the big end, so that it would fit into the VRV valve.

The bracket is simply a bent piece of sheet steel, bolted to the throttle assembly. It holds the VRV valve in place. The VRV is a plastic piece that costs a friggin 100 dollars from Ford, but it works really well. The keyed shaft I welded to the throttle pin twists as I hit the throttle, and that twisting turns the inside of the VRV valve, which changes the vacuum output. The VRV bracket can be adjusted. So at idle, the VRV puts out 13" of vacuum, and at wide open throttle, the VRV puts out 6" of vacuum. It shifts like a computer controlled truck. It works better than a gas setup, because it's so much more adjustable. I love this thing.

Finished product! See the adjustment slots on it? As you gradually increase throttle, the vacuum gradually drops. I have it shifting into 3rd gear at 30 mph at half throttle, and with over 600 ft/lbs of torque, believe me, you don't need to be in a lower gear.
 
#15 · (Edited by Moderator)

The exhaust is tucked right up next to the frame, as you can see here, and is well protected from off roading hazards.
 
#16 · (Edited by Moderator)



I took apart the injector pump, installed a 3200 rpm governor spring. I installed a Denny T fuel pin, adjusted the pre-boost fueling, removed the safety collar from the full power screw, and then turned the screw in 5 turns. It's 1.5 turns from run-away. I also advanced the pump timing by 1/8 inch.
Whenever I get some money together, I plan on installed a much bigger S300 turbo with waste gate, 5" exhaust, 3" intake tubing and intercooler. It will be really powerful.
The Cummins Bronco will top out at 93 mph, if I had the nuts to go that fast with it. haha, I did have it cruising at 80 one time on the Interstate, and it felt great. But I think that's as fast as I'm going to push it.
 
#17 ·
#19 · (Edited by Moderator)
Thank you very much for the compliments, it was a lot of work and a lot of saving up for parts.

Ah yes, the paint.. hmmm. I'm a big battleship fan, especially the Iowa-class American BBs. The USS Missouri and USS Iowa wore an awesome gray and black camo scheme for a time during WWII, and once I get the body rust repaired on my Bronco, it's getting a coat of satin finish BB camo. :D
It's about as cheap as you could get, easily repaired if scratched, and it will be a tribute to our armed forces.

This is what the camo will look like.

Since the truck will have the battleship theme, I designed this decal for the rear window:


I don't like blending in with the crowd.
 
#22 ·
Damn! That is Sweet....
Don't like much dodge stuff, and ain't in any way a diesel fan outside of semi trucks,
but man, that is too freakin' cool... Great conversion.

What are the torque/HP ratings on that motor?

G.
 
#23 ·
I haven't taken it to the chassis dyno, but from reading up on the Cummins websites, with the mods I've done, it should be around 260hp and 650 ft/lbs of torque, give or take 10%.

I've been driving heavy trucks for a living on and off since '86, and in fact, just hit my millionth mile in the summer of '09, so I'm right at home with a diesel under me. I wouldn't trade diesel power for anything..

p.s. I'm not a big fan of dodges, either, although I certainly would take a Dodge with Cummins power over a GM with an Isuzu maxipad diesel any day, but Cummins is a separate entity, and they make pretty awesome motors. Especially the old all-mechanical models. In fact, the bigger Ford F series heavy trucks have Cummins in them, too.
 
#41 ·
but Cummins is a separate entity,
Did not know that.. Nor did I know that bigger Fords had them, Always seemed to me that it was just a dodge brand name for heir diesel engines. Nice to know! :thumbup

650 ft/lbs of torque, give or take 10%.
Yikes!

responsible for the torque peaking at 1600 rpm.
Whoa... it peaks that 650ft/lbs at 1600 rpm?
:rockon:rockon:rockon
I guess I gotta start looking a little more seriously at diesel engines!

G.
 
#25 ·
A big reason I like the Cummins is that inline configuration.. Lots of room to work in an engine bay makes me happy. Another reason I like the inline is because it has a 4.72" stroke, and is partly responsible for the torque peaking at 1600 rpm.
 
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