3.9L Cummins/Isuzu diesel swap in 89 Suburban

[87Skier]

I've got an 89 Suburban (picked up as a project vehicle, just unsure of the project) that has a motor that runs but I suspect will have bottom end issues in the next couple years. Sounds like a rod knock on startup, but could be anything. Already has top end issues.

Moving passed that, I thought this would make a good candidate for a diesel swap. I know the 5.9L Cummins are incredibly long and hard to fit in any vehicle and since this came underpowered from the factory, I thought putting the smaller diesel in would be a good compromise. I've been looking at parts and so far it isn't adding up.

V2500 Suburban (Square body style)
350 TBI engine
Turbo 400 (3 speed trans)
NP231 (chain driven, right hand drop) transfer case
4.10:1 front and rear gearing
235/85r16 tires

Currently at 60mph, the engine is at 2600 rpm. I am shooting for around 1500 rpm at 60mph. Even if I swapped in an OD transmission that still puts the engine close to 1850 rpm. I think it is still on the high side.

What are your thoughts on this? Which transmission should be used? 700r4? I've heard that 4bt's are a lot more expensive but could save you money in the long run since Isuzu parts are more expensive.

I would like to avoid changing out the gearing if possible.

[79c25]

4bt, nv4500, np205 passenger drop. Should put you around 1800 rpm in 5th going 60. I know in my old 12 valve I got about 20 mpg going 65 everywhere, I think that the 4bt will do a little better. But, that's if you wanted to go with a manual. As far as an auto the 4L80E should be right about the same. But I'm interested in this swap as I'm looking to do the same in my 84 K5. As a side note the 12 valve does fit in the engine bay but I wouldn't use a 4L80E behind a 12 valve personally. That's my $0.02.

[burnin oil]

You will want a little more RPM to keep that 3.9 happy. Basically it will be lugging around without any boost while trying to move that brick. My 95 Dodge spun 2600 RPM on the interstate and the rest of my commute was like going to a rock concert. She still knocked out mid 18s for milage in a 4x4. My dads 24v 5.9 gets just under 20mpg every tank, unless towing, running 4.10s and a 6spd in a 4x4. Don't let the RPM scare you to much. Just make sure the pumps gov springs are addressed so it doesn't go into defuel.

[87Skier]

Originally Posted by 79c25

4bt, nv4500, np205 passenger drop. Should put you around 1800 rpm in 5th going 60. I know in my old 12 valve I got about 20 mpg going 65 everywhere, I think that the 4bt will do a little better. But, that's if you wanted to go with a manual. As far as an auto the 4L80E should be right about the same. But I'm interested in this swap as I'm looking to do the same in my 84 K5. As a side note the 12 valve does fit in the engine bay but I wouldn't use a 4L80E behind a 12 valve personally. That's my $0.02.

Why not use the 4L80? Personal preference? I was thinking of using it because of the cost of finding a manual transmission and having to convert my auto to manual. I really like driving manual transmissions though.

[87Skier]

Originally Posted by burnin oil

You will want a little more RPM to keep that 3.9 happy. Basically it will be lugging around without any boost while trying to move that brick. My 95 Dodge spun 2600 RPM on the interstate and the rest of my commute was like going to a rock concert. She still knocked out mid 18s for milage in a 4x4. My dads 24v 5.9 gets just under 20mpg every tank, unless towing, running 4.10s and a 6spd in a 4x4. Don't let the RPM scare you to much. Just make sure the pumps gov springs are addressed so it doesn't go into defuel.

I didn't know that the 3.9L wanted more rpm. I figured it was pretty close to the 5.9L. I'm familiar with the basic process of waking up these motors, but I don't have wrench time on them though.

Also, is there any reason to avoid the Isuzu diesel? Would it cost me more in the long run? I think I can pickup the Isuzu for 1000 but I can't seem to find a Cummins for less than 2000.

And lastly, should I consider just swapping over to a 5.9L instead of the 3.9L?

[79c25]

Originally Posted by 87Skier

Why not use the 4L80? Personal preference? I was thinking of using it because of the cost of finding a manual transmission and having to convert my auto to manual. I really like driving manual transmissions though.

I don't feel that it's a reliable choice behind a 12 valve UNLESS you get it built to handle it. First generation 12s put out 400 lb ft of torque and if I remember right the 2nd generation put out 450+, the 4L80E is rated for ~350 lb ft stock. It might last a while if you are extremely nice to it though.it should do ok behind a 3.9, but once again,you have to be easy on it unless you get a built one.