1994 Diesel Suburban Build

[aardvarcus]

On the aero mods front, I have some very serious plans to streamline this beast. However in the immediate term, I need to address the worst aero offenses as I will not have time to begin major bodywork in the immediate future.

My short list of low hanging fruit is as follows.

Remove stock fender flares (as the tires do not stick out of the bodywork).
Swap stock SLE/diesel bumper for the smooth/flush stock bumper off my parts truck.
Install conveyor belt style air dam.
Reinstall stock skid plates.
Swap to 235/85R16 Michelin Defender LTX from 255/85R16 Cooper Discoverer (they are more aero in addition to lower RR).
Install tire air deflectors to deflect air around tires.
Duct grill to radiator.
(Edit) Remove Roof Rack.

I am open for other quick to implement ideas to get me by until I can do a full AST-II-esque rear roof taper.

[slowmover]

The OEM roof rack removal won’t make enough difference, IMO. Not compared to carrying a ladder, etc, even one time. I think its’ utility is part of the vehicle description.

The rest is a lot easier emotionally if nothing else. Needn’t hold your breath any longer.

[aardvarcus]

Was able to fill the tank this morning to get a hint at the actual MPG, went just over 100 miles and filled at the same pump, resulted in 21.5 MPG. I don’t think that is a terrible starting point, EPA on a K2500 Diesel Automatic Pickup (closest thing EPA had to this vehicle stock) is 16, so that’s 37% more.

Yesterday had just a bit of time to work on the Suburban, got the very important cupholder modification installed. Started to pull of the fender flares, but they are bolted through the sides of the body, not just the fender liners, so I will have to rethink my approach on that as I will have holes to fill. Did manage to pull the front bumper license plate holder and cosmetic jut outs, not as good as a flush bumper but a starting point. (See picture)

I think I am going to start a thread in the aerodynamics subforum to discuss some of my new ideas for aero modifications, I will keep this thread for the actual build and non-aero modifications.

[oil pan 4]

2 words:
Air dam.

[ctmaybury@yahoo.com]

21.5 is a great start. For comparison my 97 1500 5.7 auto burb was able to get 18 to 19 on the highway. I have topped 20 mpg a few times with my 2500 98 dodge ram diesel with the 5 sp ( same transmission your using nv 4500). I press the skinny pedal too hard to post good numbers though. With aero mods, I won't be surprised to see you in the 30s.

[funkhoss]

I'll share two things I've found with my Caprice station wagon that may be helpful with your project. I know it's been a while since I've updated the wagon thread, and a lot has changed since I last posted. I'll hopefully be ready to share everything on that build in the next month or two...

In short, my wagon is now permanently a "3/4 ton station wagon." It's got a 14-bolt rear end, 4:10:1 gears, heavy duty brakes, springs, and other suspension parts, LT tires, and it will soon have an NV4500 transmission too. So, there's a lot of similarity between our two vehicles. I found that the heavy duty components didn't have too much effect on fuel economy, and it wasn't worth swapping back and forth between a lighter duty, higher geared axle (as was my original plan). I can still get close to 45 MPG under the right conditions.

However, there are two things pertaining to transmissions and tires I've discovered that may be helpful to you.

Originally Posted by aardvarcus

Overdrive is 0.73, gears are 4.10s. RPMs are around 2100 @55, 2600 @70. With the low end torque of the diesel first/low gear on the NV4500 is useless on the street so I launch in second, unlike my old gas 350 truck.

My plan is either to stick with the NV4500 (but have it rebuilt as the 3rd gear syncro is on its way out) and convert the differentials to 3.42 (tallest stock gears) or switch the transmission to the (I’m sure very overpriced) RSG Tranzilla TR 6060 4x4 with it’s 0.5 double overdrive.

The overdrive ratio of the 5.61:1 first gear NV4500 is actually 0.745, not 0.73. I know that's not a huge difference, but most of the information you find online about this is incorrect.

Here's what I've discovered, though: you can actually swap in a 5th gear set from an early GM (6.34 first gear) NV4500 into a later (5.61 first gear) NV4500. I haven't done this yet, but I've got all of the parts and plan to do it soon. Doing so will give you (in your 5.61 transmission) a 0.643 overdrive! That's basically like going from a 4.10 rear end to a 3.55 rear end, in fifth gear--without losing any of your lower gearing. And, swapping the fifth gear set in a NV4500 is actually a pretty simple operation.

I think that setup would be a much less expensive and more satisfying/useful transmission in a large vehicle than a TR-6060. Even if you had a 6060 with the 2.97 first and 0.50 overdrive, that's only a 5.94:1 overall ratio spread. With the NV4500 with 5.61 first and 0.643 overdrive, the overall ratio spread is 8.71:1. I also have the feeling that the first four gears in the 6060 are so closely spaced that you'd probably end up skip shifting a lot, anyway.

Originally Posted by aardvarcus

Swap to 235/85R16 Michelin Defender LTX from 255/85R16 Cooper Discoverer (they are more aero in addition to lower RR).

If you are considering buying new tires, I'd highly recommend that you look into Nokian's LT offerings. Nokian makes all of their tires very LRR, including their LT models. Since they also sell tires in Europe, you can look up their tires' European RR rating and compare them to other manufacturers. In the LT market, they tend to have much lower rolling resistance ratings than pretty much any other manufacturer. I have a feeling that a set of Nokian Rotiiva HT's would have less rolling resistance than the Michelins--and they would also be cheaper.

I have Nokian Rotiiva HT's in the 215/85R16 size on my wagon, and I'm very satisfied with them. I didn't notice a significant drop in fuel economy when switching to them from P-metric Bridgestone Ecopia (LRR) tires. They also handle well and have excellent wet traction. The tradeoff is that they will wear more quickly than the Michelins. As you probably know, when it comes to tires there is a balance between rolling resistance, wet grip, and treadwear, and only two of them can be really strong at without the third suffering. Nokian tires (including the ones--both summer and winter models--I've run on my Metro) tend to have very low rolling resistance and good wet grip, but also wear more quickly than some other tires I've used.

I don't know how much driving you expect to do with his vehicle. If you plan to do a lot, then treadwear might be more important to you. But if you don't plan to rack up the miles quickly, I have a feeling that a set of Nokian tires would probably result in better fuel economy than the Michelins.

I hope this helps!

-Funkhoss

[aardvarcus]

Oil Pan,
As you wish. It is lower than necessary for minimum drag, so I will trim it back. Easier to make it shorter than longer so I erred down with the initial construct. Welded plates to the back side of the bumper, then through bolted the conveyor to those plates with another plate like a sandwich. I wish it went wider, but with the angle it sits at I think it is still kicking the air past, not into, the tires. Hard to tell this from the photos.

[aardvarcus]

Funkhoss,

Thanks for your detailed post. That is interesting about the NV4500, I did all the legwork for a very similar 5th gear swap on my Tacoma, even ordered the gear from Japan, but never ended up pulling the transmission to do it before that project fizzled. My NV4500 is actually the early Chevy low geared one, with the 6.3 first. I was torn on swapping, because of the amazing gear spread, I was only considering the other Tremec because it would work better without regearing the axles. Your insights on the regearing of the fifth in the NV4500 have made me seriously reconsider that strategy. I wonder if a NV4500 rebuilder would consider putting me together a late chevy NV4500 with the early fifth gears, because mine definitely needs to be rebuilt or replaced.

Thank you for your insights on the tires, but I had already purchased them (caught them on sale a while back). I know the Michelins aren’t the best LRR, but they seem well rounded. I will consider other brands when they wear out though, the thought to look them up on European sites is a good one.

[19bonestock88]

Doing work! Does it seem to help highway mileage? Does it drag on everything you drive over?

[funkhoss]

Originally Posted by aardvarcus

I wonder if a NV4500 rebuilder would consider putting me together a late chevy NV4500 with the early fifth gears, because mine definitely needs to be rebuilt or replaced.

I would think you could talk a rebuilder into doing that. Another option would be to find a 5.61 version from a junkyard, in decent shape, and just swap in the fifth gear from the transmission you already have.

One more thought: I don't know how crazy you want to get with gearing, but it's possible that you could set this rig up with a 2.93 final drive. The 14-bolt full-floater that your Suburban has was never available with gearing that high, but the 8-lug 14-bolt semi-floater was. You can find 2.93 14BSF gearsets in 94-96 Cadillac hearses and limos. So, you'd have to swap your 14BFF for a 14BSF--but the semi-floater is plenty strong and it should be a direct bolt-in swap.

The front gears would be a little trickier. The 9.25 IFS was never officially offered with anything higher than 3.42. However, The 9.25 IFS and 14BSF are very similar internally--they use the same carrier, bearings, and yoke. So, there's a very strong possibility that you could just put a 14BSF gearset in the 9.25 IFS. You'd be running the gears backwards, but with as little as you would use 4wd, that would probably be OK. Some front differentials are designed that way, anyways. Thus, it's possible that you could put a 2.93 gearset in both front and rear of your Suburban.

If my Caprice with a 5.7 can handle a 2.14 rear end with a 0.73 overdrive and 30.5" tires, your Suburban with the 6.5 should definitely be able to handle a 2.93 rear with a similar overdrive and 31.7" tires.

-Funkhoss