Large GMC Van, possible improvements?

[Big Dave]

A van is a subject I’ve thought about a good bit. Vans are supremely utilitarian vehicles and like you many van owners love them. They are a bit of a challenge to improve MPG but you can get a decent percentage improvement because they are such egregious gas-hogs.

Some stuff you can do. First is aerodynamics.

If you have any luggage racks, lose them.

You can reduce frontal area by slamming the van. Your van is essentially built on a c-10 chassis and I thing both the front and rear ends are the same. There are plenty of slam kits available for C-10s of your vintage. Between 3 and 5 inches of slam leaves you with a roadable vehicle. I reduced my F350 4” in front and 6” in back and gained 1.0 MPG.

After slamming the van, an air dam begins to make sense. Either that or making the bottom more slippery.

Lose the 235s. Get 195s or 185s Get LRR tires. Air them up rock hard. I run 25% over the sidewall pressure rating. Get flat hub caps or Mooneyes. Follow up with fender skirts.

If you want real MPG, you’ll eventually have to do something with the back of your vehicle. The more you can reduce the trailing area the better but there is one proviso, you cannot make the reduction too sudden or you will cause the boundary layer to separate. My fastback cover is too steep at 17 degrees. A better angle is 12 to 14 degrees. That is a 1 in 5 slope. A 1 in 4 is fourteen degrees. The ultimate boattail is basjoos’ but his car is a lot smaller. You’d wind up with a boattail twenty feet long. Best compromise is to make a tail fairing that is 12 inches long and tapers in 3 inches from each side, top and bottom. Make sure the transition is smooth and gentle.


Mechanical stuff

Forget the Gear Vendors. Too expensive. Maybe $5000 installed.
A big improvement could be in the cards by an engine swap to a diesel. A 6.2 or 6.5 GM diesel is nearly a bolt-in for your GM van. Another one that will work is a Cummins 4BT3.9. Same power (125 HP) a bit heavier, but it is a better engine. If you are really adventurous and resourceful, a M-B 617.952 engine will fit easily and they make a fairly durable automatic. If you substitute in a diesel consult a performance transmission shop. Us Ford guys like Brain’s Truck Shop in Arkansas, but I’m sure there are equivalents for GM. Tell them you are building a very high-torque low RPM setup. You will rarely be revving over 1800 RPM and generally more like 1600. You need a tranny that can handle 400 ft-lb as a steady diet. Most automotive automatics cannot take this as they cannot move enough ATF to the cooler at low RPMs. This results in spectacular automatic transmission failures. Parts and ATF scattered down the road. Ask me how I know this. That is the reason I am a stick shift loyalist.

Check out which axle and ratio you have. I suspect you have a GM 12 bolt with a 3.73 ratio. Hit Rick’s Ring and Pinion Randy's Ring & Pinion and see what R&P kits you can get. I’m sure you can get a 3.08 and pretty sure you can go down to 2.73. Even cheaper, raid a junk yard for the axle out of a 94-96 Buick Roadmaster Limited. Either way you would be reducing your RPM from 2000@70 MPH to 1450@70 MPH. This isn’t enough to lug the GM or Cummins engines. Those engines make staggering low-end torque.


The aero stuff is cheap and the mechanical stuff is expensive but they both work.

[Kalanco]

If you have any luggage racks, lose them.

I have a very small, likely aerodynamically designed, low rack of sorts. I don't know what to do with it or where I'd put it if I took it off. I do admit to only using it once, though.

You can reduce frontal area by slamming the van. Your van is essentially built on a c-10 chassis and I thing both the front and rear ends are the same. There are plenty of slam kits available for C-10s of your vintage. Between 3 and 5 inches of slam leaves you with a roadable vehicle. I reduced my F350 4” in front and 6” in back and gained 1.0 MPG.

I'm not sure how accurate that statement is. I have a G25 van, not a C-##. Nevertheless, I'm sure they do make coil spring lowering kits, and I've seen a leaf spring lowering kit.

Lose the 235s. Get 195s or 185s Get LRR tires. Air them up rock hard. I run 25% over the sidewall pressure rating. Get flat hub caps or Mooneyes. Follow up with fender skirts.

I'm a bit afraid of making the tires much smaller than the stock 225's(I thought larger tires reduced rpm's at a given speed?), as a lot of places in town have some pretty severe dips and bumps(I scrapped the back of my trailer hitch thing driving out of my workplace 2 nights ago). Also, 25% over the sidewall pressure rating? I could see that in a car, but when each tire is holding 1250 lbs of weight?

If you want real MPG, you’ll eventually have to do something with the back of your vehicle. The more you can reduce the trailing area the better but there is one proviso, you cannot make the reduction too sudden or you will cause the boundary layer to separate. My fastback cover is too steep at 17 degrees. A better angle is 12 to 14 degrees. That is a 1 in 5 slope. A 1 in 4 is fourteen degrees. The ultimate boattail is basjoos’ but his car is a lot smaller. You’d wind up with a boattail twenty feet long. Best compromise is to make a tail fairing that is 12 inches long and tapers in 3 inches from each side, top and bottom. Make sure the transition is smooth and gentle.

Do you have a picture? If I had one, I could probably wrap my mind around this concept a bit better, although this doesn't factor in my rear doors.

Mechanical stuff

I think the largest engine they put into this van was a 5.7L(350). They might have put a 6.2, but the van engine bay is pretty cramped as it is, I don't know how they would have made it fit. Also, while it only generates 250 ft-lbs of torque, it makes about 210 horsepower, far more than your 125hp estimate.

Final ratio, is actually a 4.1. The lowest I'd be willing to go would be a 3.08, as I'd expect a 2.73 to take me about 25 seconds to get to 60.

[Big Dave]

Check out the garage and the aerodynamic forum. Basjoos is the poster child for aerodynamics.

6.2/6.5 engines will fit anyplace a 350 will go, although you'd have to forego the turbo in a cramped van engine compartment. The Cummins is smaller, turbo and all and can be turnedup to make a lot of HP if you want.

We all have to balance acceleration vs MPG. Roll your own. GM built a 6.2 Suburban with a 2.73 and allegedly got 30 MPG, but it was real slow. 3.08 might be a lot more do-able.

MPG is like any other parameter of performance. The old hot-rodders' saying of: "Speed is just a matter of money. How fast you wanna go?" applies. MPG mods are just a matter of money. How much MPG you want