Swapping a 6.6l Duramax/Allison into a '94 Roadmaster Wagon - The Duramaxster

So, I'm a middle-aged dad with a wife and 3 kids.  For 10+ years now, I've had this car-guy dream project of buying a late 1930's touring sedan, like a big-body Olds or Cadillac, then putting a modern diesel truck drivetrain and suspension under it, a giant air conditioner, and using it as the family road trip vehicle for vacations like visiting national parks.  However, my oldest kids (twins) are 12 already, and I realized a few years ago that even once I started on that project, it would take me 5+ years to actually build it with my full-time+ job and other dadly responsibilities.  I woke up one day and realized that I just didn't have enough years with the kids still at home to actually use and enjoy something like that if it took me that long.


So I decided to take a smaller bite at the vision of the dream Family Truckster and shoot for an "easier" version.  I spent a fair amount of time thinking about it and narrowed it down to 3 primary criteria:

• It needed 3 rows of seating
  
• It needed to be able to tow 7000-ish pounds comfortably, and when I say comfortably I mean with a chassis and brakes that can handle taking that weight over the Rockies and also enough power to do 80+MPH up the side of a mountain with that trailer hooked up.  I've been spoiled by more than a dozen years with big-block 3/4 ton trucks, and 200-300HP small-blocks were not going to cut it for me
  
• With that power, it still had to get 25+mpg on the highway when not towing
  

After thinking about it for a while (this was in 2018 and early 2019), I came to the conclusion that humans had not yet produced a vehicle that would meet all of my criteria, so I would build it myself!  Yes, I know that GM's new SUVs with the 3.0l Duramax would pretty much meet the criteria (not really enough power IMO, but respectable), but they didn't exist then, and I don't have $60k+ to spend on one, so I'll slowly spend $60k building this myself instead



I settled on what I felt was my best option - Buying a GM B-body wagon and diesel-swapping to a Duramax 6.6l V8, which I'll almost certainly do some mild modifications to so that I can break drivetrain parts with 800-ish lb/ft of torque.  And after several months of searching, I found one in the glorious pre-COVID heyday of summer 2019.



Of course, it was 1300 miles away and had a blown headgasket, so getting it was going to be a challenge.  

I should also add that my buddy who helped me buy the donor truck for this project put a condition on his help - he required me to promise to put the project on YouTube:

Well, once I had the beast home, obviously the first priority was disassembly and assessment.  Here's some stuff that isn't in the video:

The seller told me that it had a blown headgasket.  He was a mobile mechanic and had a couple of other cars he was restoring, so he wasn't without knowledge.  He said it was pouring white smoke when he parked it.  I decided to take the heads off under the theory that I'd maybe fix it and just drive it for a while before doing the diesel swap.  I pulled the heads, and frankly they looked great.  No obvious issues with the gaskets, the surfaces looked good, even after resurfacing them by hand with a wetstone.  HOWEVER, the #8 runner on the intake and head was literally full to the brim with fuel.

So....I think it just had a stuck fuel injector.  I could have probably fixed this for $80 or so and had a perfectly good LT1 Roadmaster wagon, and even by the time I was confident in the diagnosis and that the bottom end was good, I could have probably put new headgaskets and injectors and the other "while you're in there" stuff for $300.

But...I didn't want to.  I wanted to get on with the project or I thought I would risk never getting it done.  So I tore it down.



Of course, my main problems were self- and space-inflicted, and it took several tries to get it into the right position and get the body separated, but here's my main reason for never seriously considering a scissor lift as an alternative to my MaxJax - you can't do this on a QuickJack:




I remain completely smitten by how rust-free this thing is.  Between this and the Escalade, I might never buy another used car that isn't from the desert.  I don't mind replacing the bushings and A/C parts that go out on the desert cars...not when you can take everything apart as if it was made yesterday.

Hope you enjoy my latest trials and tribulations - anybody want to start placing bets on how much I have to cut out to fit the diesel?  I left a preview for the sharp-eyed in this video at how bad it's going to get:

And then, finally, I started doing the real cutting on the wagon and trying to see how bad my predicament really was.

I also knew that the Allison trans was really large, but I really wanted to use it for a few reasons:

• I've been driving this one behind an 8.1 gas V8 for 6 years and maybe 20,000 miles.  The Allison is just a really good transmission for this era.  Shifts are smooth but firm, with none of the lazy sloppiness that I don't enjoy about 4L60's and 4L80's.  It really elevates the whole driving experience for me - I'm a manual snob, but a really good automatic in a truck works for me.
  
• I wanted to tow, and I like having a lot of transmission in reserve.  By which I mean that I don't enjoy the feeling that I'm really running a transmission on the ragged edge of heat or torque capacity.  Every road trip I took in my E38 750iL, I couldn't shake the feeling that the trans was going to let go when I was 300 miles from nowhere, and I hated that.
  
• Building a 4L80E to actually handle 800-900 ft/lb of torque at like 1800 RPM, and also tow, is EXPENSIVE.  Like $8000+.  I already had a good 2WD Allison available to me for "free" and to beef it up a little won't be nearly that expensive.
  

So, to sum it up, I'm stubborn, and I really want to make this work.  This video gave me a much better sense of what I'm up against.

Welcome to the forums!

That is one heck of a project!
There is another member here who did a diesel swap.

Welcome to the club Great minds think alike - https://www.impalassforum.com/threads/what-did-you-do-to-your-wagon-this-week.260225/post-12102686

Curious to see how this progresses - everything you've faced/shown so far is familiar, but there is a lot more in your future as I'm sure you already know.

I'm glad to share any knowledge/tips/tricks/lessons learned/problems faced/etc

Great choice for a powertrain.  However, you will have to do some major modification to the trans tunnel and the firewall to fit the Ally in and to accomodate the rear turbo and exhaust plumbing.  Actually bought the Cadillac specifically to fit in a rear turbo 6.5.  The engine doesn't sit as far back and under the firewall as the wagons (longer engine compartment )

What are you towing?  The reason I ask is that in the early years, and maybe as late as 2010, the Duramax was available in the commercial vans using a beefed up 4L80 and a slight power reduction in the engine tuning. From what I have heard, it actually made better mileage then with the original 5 speed Allison. Know you want the Ally but the 4l80 can be built to handle the torque for far less than 8 grand.  More like 3 or 4 grand.

Another consideration would be the tow rating on the Buick.  If you plan on towing anything over 5,000 lbs, regardless of the powertrain, you will exceed the manufacturers rating unless you live in a jurisdiction that allows you to change the GVWR of your car.  Big fines in many areas and worse, if you are in an accident, your insurance company may refuse your claim.  This was one of the reasons I went to the Fleetwood.  Had a tow rating of 7,000 lbs.

Was pulled over twice with this combo, even though the combined rating was within the specs.

https://2img.net/h/i156.photobucket.com/albums/t5/convert2diesel/CarTrailer2-1.jpg

Will be following your progress with interest.

Bill

Fix Until Broke wrote:

Welcome to the club Great minds think alike - https://www.impalassforum.com/threads/what-did-you-do-to-your-wagon-this-week.260225/post-12102686

Curious to see how this progresses - everything you've faced/shown so far is familiar, but there is a lot more in your future as I'm sure you already know.

I'm glad to share any knowledge/tips/tricks/lessons learned/problems faced/etc

That's fantastic! I had seen the 6.5 swap on this forum during the research phase of my project but not yours - I can't believe there's someone else out there as dumb and stubborn as I am!

I have so many questions for you - but let me start with "what did you do for the oil pan and steering?" I've already committed to a specific direction on this, but I'm really curious as it looks like you kept FAR more B-Body chassis in your swap than I did.

Then I'm curious about the electrical side in general terms - I see you kept the truck steering column, did you do a "standalone" conversion on the engine and trans yourself? Thankfully I have an LB7 so I'm pre-CAN, which I'm hoping will make my life a TINY bit easier.

As you can appreciate, there's not a short answer to your questions . I'm glad to discuss in detail, but probably best done over the phone sometime.

You can e-mail me at my username (no spaces) at gmail and we can exchange contact info.

I'll try and briefly answer your questions below...

Oil pan and steering - Custom upper and lower engine oil pans along with crossmember modifications to get the engine low enough so the stock hood closes and the turbo fits under the firewall. This was one of my criteria - looks stock.

Steering is largely stock - only issue that I over complicated (see username) was the center link. There's a crankshaft where the OEM center link wants to be and one solution is to make a "U" shape of it and leave the rest alone . The upper oil pan modifications will allow for full steering travel with stock geometry. There are compromises with this choice though that are lengthy to discuss.

I went with a 6L90E so didn't have to do near the tunnel modifications that you have done (practically none - Just a little hammering by the tailshaft - no cutting, no welding). I do have a ~1.5" body lift (2 jr size hockey pucks) which is all for the transmission clearance. It helps under the hood, but not required.

Wiring - As far as it knows, it's still in a 2012 G4500 chassis with all control modules intact and functional. Everything shows up on the scanner, etc. Only persistent codes are for the mechanical cooling fan control which was deleted. There was a LOT of integration between 1996 and 2012 wiring to make this happen. CAN is no big deal and didn't drive any particular decisions in my case. Having a complete functional running/driving donor vehicle is really helpful - not essential, but highly recommended!

Very cool. I daydream about doing that but I updated my dream to an all aluminum 6.2 LS with the 6L80e. In my dream (never actually planned to do it) I wanted to swap the entire rear end and front suspension to get better ABS and update a bunch of electronics.

With the right gearing it would tow and haul butt.

Love that you followed your dream to reality.

Well, now that we've established that the engine and trans don't really fit under the body, it's time to see if I can mock up some engine mounts and get the drivetrain pushed lower in the chassis.



It didn't work very well.






As I previewed at the end of the video, I decided to abandon the idea of keeping or modifying the B-Body front suspension. The lower control arms come in too far and interfere with my ability to shove the engine down as far as I'd like, they are too narrow to use the side-mount approach from the truck, and as FixUntilBroke mentioned, the steering arm goes right through where the oil pan wants to be.

So I did the obvious thing - I bought C5 Corvette suspension and adapters to mate it to custom frame rails. I'm committed now - this is going to make the project MUCH more complex and include much more fabrication, but it creates an enormous amount of additional space, and also takes quite a bit of weight off the front end (which I'm more than putting back via the heavy drivetrain).


I decided on this course quite a while ago - so I was committed before seeing FixUntilBroke's version! Obviously it can be done by making different compromises - I may well regret choosing this path, but for the moment I like what the Corvette front end is gaining me in terms of real estate and geometry for the front end.


I actually bought front and rear suspension because the price was too good and I have a future Corvair-based project in mind for the rear setup - but the Duramaxster is only going to get the front suspension.


That being said, the sofa-like ride and "handling" is still important for me in the final product here - I'm expecting that I might need airbags or some similar method of getting some extra spring rate in the front for the weight. Anyone with experience adding weight to the front of a Corvette and still having good ride quality?

Well, the day job has taken up a lot of my youtube time lately, but I did get the next installment up just now - chopping the Roadmaster frame in half to convert the front half of the frame to a Corvette!





The key for making this project doable for me was interface adapters to simplify the fabrication, especially for someone who didn't have the whole Corvette handy to take measurements from.  You know a product is good when their website looks like this: C5 and C6 Corvette Suspensions Interface Adapters :p

In truth, this is a really high-quality product and I'm impressed.  And I'm just a retail customer, no affiliation or compensation even for the potentially dozens of views I'll be getting for his products, lol

Overall this phase of the chassis work is really intimidating to me, since there are so many ways to screw it up, but it's coming together and I feel good about the new set of compromises I'll be making.  

Fix Until Broke - you obviously found a way to make it fit with just a small body lift - I wasn't willing to lift the body and just didn't feel like I could make it work in the stock chassis the way that I wanted to.  Your way is definitely smarter but I'm optimistic that I'll eventually make this way work.  Famous last words!

Looking good - I like those adapters for the C5 subframe. That will be pretty slick!

I don't think there's a better/worse, smarter/dumber, etc way to go about something like this - The fact that we both did it is proof enough that we're not that smart to start with .

Anxiously awaiting the next installment!

Wow buddy... quite a project you have going on. Admire your persistence with this, I would have thrown in the towel many times by now.
A shame chopping up the perfect rust free frame though.. would have paid 2G for that alone any day.. but too late now I guess.. LoL
Keep up the spirit, looking forward for the next chapter.

Klaus, it's been hard for me to cut up a rust-free wagon as well! But it just made more sense to start with a non-running rust-free example, saves a few hundred hours of rust repair that I can spend on fixing my chassis fabrication mistakes instead

A couple of updates!:

Part 6 - Squeezing the Duramax in with the C5 front cradle



Part 7 - Engine mounts that might actually work!

A couple more updates!




I didn't realize there was a hidden VIN stamped on top of the frame rail, though I assume most of you here already knew that!

Installed a class III hitch from KDS Performance's DIY kit. Very happy with this setup from Michael, thanks to this forum for helping me find it!

And now for a rear axle upgrade!



At some point I decided that I didn't want to risk scattering the GM 10-bolt in the middle of nowhere with my family on board when I gave it the boot and sent 800-900 ft/lbs of torque through it.  I also priced out what it would take to beef up the 10-bolt and upgrade it to an LSD, and frankly it wasn't a lot more expensive to go to a much stronger aftermarket axle. So, custom Ford 9" to the rescue!





This was a pretty big undertaking, and with several hurdles with parts sourcing (unsurprising these days, I suppose).  You guys all know that the axle is wider on the wagons than the sedans (or just about anything else you might find!) which also made it challenging.  As usual, I decided to do everything in just about the hardest possible way...

Update! Fitting a steering rack has been quite an ordeal. The C5 front cradle made it much more reasonable to fit the engine for width and height, but the steering rack was trying to occupy the same space as the oil pan, and oil pump, and front pulley, and some other things that I kinda needed on the Duramax.

Not sure if I've shared these or not...I went as far as grinding the excess material off the oil pump relief valve to gain an extra ~3/8"



How much the upper oil pan was shortened...


You might have already figured this out, but the upper oil pan isn't really aluminum.  It's more like 50% aluminum, 40% $h!t, and 10% air.


It will clean up, but it takes a lot of patience!

Thanks for those pictures! I don't have a TIG welder or the skills to use one, so I've planned to hire that part out, but I haven't gotten it done yet. I've cooked it in the BBQ a couple of times to try and get it cleaner for that process, but it looks like it will still be quite a challenge!

More steering challenges worked through in today's video - Part of my goal with the videos is to not sugar-coat anything, but instead to show all (ok,nearly all) of my mistakes along the way and be realistic about the challenges of doing a project at this scale with limited time, tools, skills, and intellect. The steering rack has been a great example of this - getting a location for it was difficult and I really had to butcher the subframe, and then devising a way to bolt it in wasn't easy either.



Then, I had to relocate a hydraulic fitting on the rack, and that really exposed my lack of knowledge. I couldn't find much as far as exploded diagrams or solid info on the Corvette steering racks, so I went the old fashioned way and just dived in. That cost me 2 racks, because my first method of moving the fitting was totally wrong-headed, and on my second attempt, I was too focused on the modifications to even bother checking that the C6 rack would bolt in the same way as the C5.



However, as usual, pure stubbornness eventually won out, and I think I have a workable solution:

Update - the chassis is "done" and off to powerdercoating!  In this picture, the transmission crossmember is in place, but the front subframe has been removed.  The C5 Corvette front subframe is all aluminum, so I'm not going to powdercoat it.

My local blasting and powdercoating place gave a special price if you could get your chassis in before Thanksgiving, so that they could do a bunch of them together.  I wasn't really ready, but it made a good deadline to shoot for, so I scrambled and got a few things completed.



I had to build a transmission mount for the Allison, which I fabricobbled by chopping the stock B-body transmission crossmember in half and then splicing in a chunk of 2x4 steel tube and some plate to strengthen everything.

I also learned, thanks to some commenters on the GM Longroof forum, that I needed to thicken the edge of the receiver tube that I made to help it live longer.  After spending a few late-night, last-minute hours doing so, I tested with an actual 2" draw bar - and it didn't fit inside my receiver tube.  Something I should have tested a LONG time ago.

It turns out that the ID of the 2.5"  1/4" wall tube I was using was 2.02", and a standard receiver tube is 2.07".  That 50 thousandths is a big deal, it's the difference between fitting and not fitting.  So, I had to cut off the receiver tube I made, and weld on a pre-made one.  Something I should have done from the beginning, since it saved me making a thickened edge or having to drill the hole for the pin.

I fully expect to find more issues like this as I get closer to 'real' assembly, and that I'll end up having to grind off some powder coating to make changes for some of the remaining details like fuel/brake lines, parking brake cables, whatever breaks when I hit it with 900 ft/lbs of torque, etc.  But for the moment it feels like progress, and I'll take it!

More progress, be it ever so slow. I got the LB7 Duramax re-assembled with new headgaskets, injectors, and other bits, and got it placed into the freshly powdercoated chassis. As usual, there were plenty of self-imposed difficulties when put the heads on the wrong sides, didn't seal the injectors properly, and got a few other details wrong.



And HOLY CRAP there's a lot of parts on these engines. I've done a few GM gas V8s and a top end assembly job is a few hours of cleaning and a couple hours of wrenching. Admittedly I had extra time in this since I made mistakes that had to be re worked, but I had at least 40 hours into the cleaning and assembly of the top half of this engine.

i admire you tenacity on this undertaking,. i did see a 426 hemi in a frog eye sprite one time.

I think if i attempted this i would use the v6 benz diesel or the ford td6,. interesting thread.

A belated update - should have some more updates on this project over the next few weeks.

I needed a functional braking system since I now have Corvette front end and a custom 9" Ford rear axle with a generic GM Metric "Hot Rod" disc brake setup in the rear.  I also wanted to use the hydroboost setup from the diesel truck to save space vs. the vacuum booster, since my engine compartment is full of Duramax now.  Here's how I "solved" these issues - at least for version 1:

On to the fuel system!  In the theme of doing everything in the hardest possible way, I cut up the back end of the wagon to create a space for a massive filter and lift pump arrangement for the diesel.