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Discussion Starter · #1 · (Edited)
I've posted a little about my diesel Corvette in the past but never really did a build thread. All the details have been over on Blogspot. Well, it's no fun posting it over there because there's no real interaction with other diesel nuts. I figured I would close out the blog and start posting the story here.

How it all began...

I'm not sure why it got into my head to have a diesel powered Corvette. I remember looking online at nearby cars for sale. I saw a Corvette for what I thought was a pretty cheap price (around $2500). That just sort of got the wheels in my head turning. Although I never was all that interested in Corvettes (not really a "sports car") I figured it might be a fun toy for a while.

I've been a diesel fan for years. Owned 5 different diesel trucks and have always been impressed by the power and potential for more power from a turbo diesel motor.

Back during the gas crisis in the late 70s GM engaged Detroit Diesel to design a fuel efficient diesel motor to power GMC and Chevy pickups. To save GM from doing a bunch of expensive retooling the 6.2L diesel was designed to bolt right in place of the existing gas motors. It used the same bellhousing bolt pattern and motor mount locations. This modular approach makes swapping out a gas V8 for a diesel relatively straightforward. The original 6.2L was rather gutless but it would return fuel economy in the mid 20s. Not bad for a full sized pickup. in 1992 GM increased the displacement to 6.5 liters and added a turbo. This was done to make more power to compete with Dodge and Ford. Even so, the 6.5L turbo diesel is no powerhouse.

So after much shopping around I found a 1980 Vette on Craigslist. I paid about $3200. The car was complete and unmodified but old and somewhat neglected. I figured I'd drive it for grins and make some initial measurements to see what difficulties I might run into trying to shove a 6.5L motor in place of the stock small block 350.

 
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Discussion Starter · #2 ·
What's the point?

Most folks that hear about this wonder why? I could just build another hot rod Vette with a big block or stroker mouse motor but why? There's lots of those and anything I did like that wouldn't be unique. Plus, it would be a gas sucking monster which would take away a lot of the fun of driving it.

The Corvette is about 2100 lb lighter than a full size pickup. In stock form the 6.5L diesel is rated at 190hp and 380 ft. lb. of torque. Same HP as the gas motor but with 35% more torque. That should motivate the Corvette at least as well as the gasser does now. As a plus, the slippery shape of the Vette should allow it to get great fuel economy. I'm not doing this just to save gas money but I think it's reasonable to expect fuel economy in the mid 30s or better. Most economy cars are severely lacking in style. This will be different!
 

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I'm subbed to this project. This should be fun to watch. Have you thought about a Duramax.
 

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Discussion Starter · #4 · (Edited)
No, I never seriously considered a Duramax. The 6.5 was a really tight squeeze.
Plus, the 6.5 uses the old standard bellhousing bolt pattern and motor mount locations.
 

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Discussion Starter · #5 · (Edited)
The donor truck

New power for the Corvette is coming from a 1992 GMC 1 ton C3500 pickup with 209,000 miles.


I bought the truck for $1700. The motor is not original but is a "Goodwrench" crate motor. No telling how many miles are on the motor but it starts and runs good. 1992 & 1993 6.5L diesels had mechanical fuel injection. Later models have computer controlled electronic fuel injection. Using a mechanical injection motor will make the swap a lot less complicated. I drove the donor truck around for a couple of months just to make sure there weren't going to be any problems. It turn out to be a nice little truck. OK, it's not little but after driving my 4-door Dodge 4x4 the GMC 1 ton feels like a mini truck. It will turn around in the street. You can park it in a regular parking space. Sweet! It's almost a shame to take it apart. But, it is this truck's fate to give up its motor for a higher purpose.
 

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Discussion Starter · #6 · (Edited)
Driving the old Corvette

The late 70s and early 80s were definitely a low point for Corvette design. This is not the overpowered hooligan car that some people imagine. This car has the L48 350 cu. in. motor that GM rated at 190hp and 280 ft. lb. of torque. It motivates the car OK but isn't surprisingly fast or really all that exciting to drive. People like the look of it though and driving it around is kind of cool in an 80s American car sort of way.

I drove it for a few months and tinkering with those things that need attention - lights, stereo, etc. Other things that need repaired will wait until I'm deeper into the project - power steering cylinder, suspension, etc.
 

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Discussion Starter · #7 · (Edited)
With the motor out of the truck and on the stand I can start taking measurements to see where the biggest challenges are going to be.



One thing I knew from the start was the 6.5 oil pan was going to have to be modified. The diesel holds a lot more oil than the gas motor and the oil pan is a lot deeper. The Corvette doesn't have much ground clearance to start with so the oil pan has to be shortened.

(edit: This turned out not to be necessary. Further along in the project it will get an unmodified oil pan.)

I measured how far the oil pump protruded down into the pan and marked a cut line around the sump. Off to the bandsaw! There's a little bit of a thrill to be had taking perfectly good expensive parts and running them through a saw. Now you're committed! It worked out fine. I cut a piece of diamond plate to fit the oil pan bottom and welded it in place. I made it about 1/8" too short so a quick trip to the hydraulic press put a dimple in it right below the oil pump body for clearance. The pan is now about 1 1/2" shorter than stock. With this modification and some mods to the motor mounts to carry the motor a little higher I should have about the same ground clearance as 350 gas motor.

 
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Discussion Starter · #8 · (Edited)
I figure the Diesel Corvette is going to be a bit of a curiosity. Folks are going to want to look under the hood to see that big ugly diesel lurking in there. That means it needs to be made presentable. The oil pan is already looking pretty so I figure I better pull the valve covers and intake manifold and give them a good cleaning and a coat of paint.



To get the valve covers off you have to remove the fuel injection lines. The get the injection lines off you have to pull the intake manifold. Here's pics of the injection lines. The lines have to come off in the right order or you can't get a wrench in where you need to. The same goes for putting it back together. Check out the cobbled up (rubber) fuel supply lines. That will get tidied up as well.

 

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Discussion Starter · #9 · (Edited)
It is not a thing of beauty but at least it's presentable. The top half of the intake manifold is a snorkel that connects to the turbocharger that will be mounted to the right hand side of the motor. I'm going to have to modify all the turbo plumbing to get the turbo to fit in the tight space under the Corvette hood. That top snorkel will have to either be modified or (more likely) replaced with a fabricated part. The same goes for the water outlet and thermostat housing. It looks like it is going to be a little bit too tall. I'll probably have to make a replacement that doesn't stick up so high.



 

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Discussion Starter · #10 · (Edited)
There's a bit more to do to the 6.5 before it's ready to be dropped into its new home under the Corvettes hood. Even so, it getting to be time to get the 350 gasser pulled out. There will be plenty of tidying up to do in the Vette engine bay.

The view under the Vette hood. Ughh... The first thing to do will be to remove the radiator shroud and intake snorkels so I can get in deeper with a tape measure. I want to keep the car driveable for a little while longer.

 
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Discussion Starter · #11 ·
Which Transmission?

Certainly not the TH350 that is in the Corvette now. To keep the diesel motor happy at freeway speeds it will need something with an overdrive. The GMC pickup has a 4L80E which is a very robust transmission. It's also quite a bit larger and longer than what's in the Vette right now. It would probably be possible to stuff the 4L80E into the tunnel but that looks like a really tight fit and a lot of trouble. Plus, I would probably lose about an inch of precious ground clearance.

The 700R4 would be a good choice. It has a nice low first gear for good off the line response and a nice tall overdrive for relaxed freeway cruising and good fuel economy. Then there's the 4L60E which is a computer controlled version of the 700R4. I would have to use a stand alone aftermarket controller which complicates the installation a little bit and adds to the expense. One nice advantage to the 4L60E with aftermarket controller is all the shift parameters can be dialed in without digging in to the valve body. The controller will also support the use of paddle shifters. I would like that a lot.

For the time being I can get a lot done without having the actual transmission I'm going to use. I'll leave the TH350 in there for now and bolt the 6.5 to it (no torque converter or flex plate). That will give me an accurate motor position and I can work on plumbing for the turbo, modifications to the thermostat housing, wiring, fuel supply, etc.
 

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Discussion Starter · #12 · (Edited)
Out with the old (350)

There's nothing stopping me from pulling out the old 350 gas motor. Everything else I want to do with the 6.5 can be done just as easily with it in the Vette as on the engine stand.



I made a simple brace to hold the TH350 transmission in place while the 6.5 gets fitted to its new home. A piece of channel is bolted to the frame
rails and runs under the transmission.

 

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Discussion Starter · #13 · (Edited)
Front pulley clearance

The 6.5 diesel is almost 2.5" longer than the small block 350. This means the front pulley is going to end up over the crossmember rather than just behind it. One solution would be to cut a piece out of the crossmember and weld in a patch with more clearance.



Another solution would be to make spacer plates to raise the motor up at the motor mounts. Raising the motor would also give a little additional ground clearance.

The downside to raising the motor will be decreased clearance under the hood and decreased clearance between the bellhousing and firewall.

Here's a drawing for a motor mount spacer plate. Because it will sit at a 45 degree angle a 5/8" thick plate will raise the motor about 7/8". I'll probably make both modifications. If raising the motor runs into too many unseen difficulties I can remove the spacer plates. If I've already modified the crossmember I won't have to pull the motor back out to do it later.

 
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How is raising the motor going to effect drive line angle? Been a long time since I have been under one of those vettes so I don't recall what was used for u-joints.
 

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Discussion Starter · #15 ·
Raising the motor 7/8" at the motor mounts would change the angle by about 1 degree.
 

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I think in a lot of ways a 700 is weaker than a 350. The 3/4 clutch in the 700's isn't very big and running in 4th its got quite a load on it. The low 1st is nice and with the diesel its nice there is not a vacuum modulator to deal with.

A 400 with a 9" tail was only an inch or so shorter than a 4L80. Not sure about the diameter difference, I don't think it was that much bigger and the 400 did fit that body style. You already have an 80 and the controller for it?

The Vettes had a independent rear since 63 I think so its a little different game drive shaft wise than most cars being that the diff is bolted to the frame and does not really move. I don't remember if these still had U joints or if they used the hockey puck looking things like the European cars did.
 

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Here is a video of a guy that did it. Seems like I found the video a couple years ago after finding a blog or forum the guy did on the build. No idea where that was it was too long ago for me to remember the details. I think it may have started with the guy just being bored and throwing a 5.7 diesel in there and then kinda evolved from there. It could be the wrong guy too.

https://www.youtube.com/watch?v=sAWbD9io6bg
 

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Discussion Starter · #18 ·
A 4L60E (or 700R4) can be built to take the torque. Remember, this isn't something that is going to be towing a backhoe to a job site.

While some guys have shoehorned a 4L80E into a C3 body it required some bandsaw work to the bellhousing, mods to the trans tunnel and makes accessing the bellhousing bolts extremely difficult.
 

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Discussion Starter · #19 · (Edited)

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I was wondering if the bell was a bit fatter on the 80. I suppose its likely a little fatter near the back too and I suppose they didn't leave any extra room anywhere in a vette.

If I have time later I will try and dig around and see if I can find the write up I remember reading. It had a lot of details about modifications and such. At the time I actually did poke around looking to see what I could get a vette for to do something similar.
 
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