1991 GMC Sonoma Autocross Truck - A Tale of Independent Supension, V8 Power, and Scope Creep

Having read GRM for several years now, and having seen many an eccentric build come and go on the forums, I decided to take off on a fool's errand of my own. I've always been a big fan of modified mini trucks, especially the S10 and Sonoma, so it wasn't a hard decision when a roller came up locally for $1000 bucks. Looked like this when I picked it up (Sorry for the garbage pic, forgot to take any before I ripped it apart).

Clean examples are a bit tough to find in the midwest, so a Nevada truck at that price couldn't be turned down. Ended up offering $700 and flat towed it behind a buddy's truck back to my house that day, and started stripping off the front end down to bare frame. Getting married, moving into a new house, and overtime at work kept me busy for a month or so, but it's now time to start in earnest in hopes of getting it ready for challenge time. 

Goals for the project are improved handling, better power, and challenge legal budget, and while it might not be competitive at the challenge I think I'm in for a great time building this thing. So, to accomplish this, an LS truck motor up front, 3-speed manual transmission in middle, and independent rear suspension in the back will be located and made to fit. 

I chose an E39 touring to donate its suspension parts to the project. This was selected for its fairly sturdy rear differential and axles, lightweight aluminum subframe, and the self-containing nature of the wagon suspension. The E39 wagons lack the real estate for the struts mounting to the body, so the suspension looks like this:

As a result, a set of coil overs will make the whole supension assembly self-contained, and only need four chassis mounting points. Last Friday, I was able to pull a full BMW 528i Wagon rear supension and drivetrain assembly for $271 from a local u-pull-it junkyard. I found the importance of removing all the driveshaft bolts before trying to pull a rear end, it is very helpful. But soon enough, it was sitting next to the truck.

I have not started test fitting yet, as the stock 10-bolt rear axle is still in the truck, but a rough measurement shows the frame rail centerlines at the same width as the subframe mounting bolts, and the shock mounts as fitting snugly between them. So far, looks like a promising concept, and there will be much more to come.

Following for more LS swapped, IRS minitruck goodness.   I'm very jealous if that truck is as clean as it looks.   I was originally looking for a Pre-93 S10 or Sonoma when I found my Ram50, it's hard to find clean minitrucks in the Midwest. 

Are you trying to fit this underneath the bed without cutting or notching?

That's pretty cool!  I know early f-series guys use Jag rear-ends for much the same purpose, so the idea is solid

This is badass! With a pickup frame, it might be easy enough to weld upper spring perches to the frame instead of needing to switch to coilovers. I did something similar on a solid-to-IRS conversion using an Infiniti G35 IRS, which is very similar to your BMW unit using only 4 mounting points + spring perches.

Yep, excited to see how this plays out!  Very cool!

Schaweeet! I hope turbos are involved.

interested to see how this comes together.  minitruck fan here too!  

In reply to Ram50Ron :

At first glance it looks like if the axle hoops get straightened out, everything will fit under the bed. 

In reply to maschinenbau :

Good to know, I think that the spring perch location might be tricky, because it's at the same width as the subframe mounts, but good thought! Very well may end up working.

In reply to Robbie :

Long term yes, NA LS power is the plan for the first iteration.

Neat! I’m going to keep this in mind as a future mod on my S10.  I love how self contained the IRS is.

Having built and completed in an S10 I never felt the rear suspension holding me back. However Keep On Keepin On! You may have found the new hot setup for pro touring s10s 

In reply to Dusterbd13 :

If the truck handles well enough, I'm gonna throw stupid power at it down the road, and the Eastern European drift scene has big power BMW parts down to a science as inspiration. 

whiskey_business said:

I chose an E39 touring to donate its suspension parts to the project. This was selected for its fairly sturdy rear differential and axles, lightweight aluminum subframe, and the self-containing nature of the wagon suspension. The E39 wagons lack the real estate for the struts mounting to the body, so the suspension looks like this:

As a result, a set of coil overs will make the whole supension assembly self-contained, and only need four chassis mounting points. Last Friday, I was able to pull a full BMW 528i Wagon rear supension and drivetrain assembly for $271 from a local u-pull-it junkyard. I found the importance of removing all the driveshaft bolts before trying to pull a rear end, it is very helpful. But soon enough, it was sitting next to the truck.

I have not started test fitting yet, as the stock 10-bolt rear axle is still in the truck, but a rough measurement shows the frame rail centerlines at the same width as the subframe mounting bolts, and the shock mounts as fitting snugly between them. So far, looks like a promising concept, and there will be much more to come.

This is the most important thing that I've learned all year. Or maybe since childhood.

Dusterbd13 said:

Having built and completed in an S10 I never felt the rear suspension holding me back. However Keep On Keepin On! You may have found the new hot setup for pro touring s10s 

How much tire can you fit under a S10 front and rear with stock suspension design?  It looks like the leaf spring limits tire size, at least for LS powered S10.

I fit 275/40/17 with no trouble. Used spacers and 17x9.5 vette wheels.

Zombie post, what ever came of this thing?

So glad you asked! After a lot of debating, the BMW IRS route was not feasible for a few reasons.

1. No LSD options. To get an LSD, the M5 diff/axles/subframe would need to be used. They cost a lot of guala. 
2. Subframe I bought was absolutely trashed from stock power and tire. Cracked and tweaked welds for control arm mounts. God only knows what would happen with more grip and abuse. I have some parts still, but scrapped the majority.
3. Frame shape. While the S10 rails line up nicely with the subframe width, to keep wheelbase the same the subframe has to mount below the C-notch. To maintain ride height/geometry, it has to hang 8 or more inches below the frame, making the mounts large and flexible. Could be fixed with back-halfing the frame and welding in straight rails, but because of 1 and 2 above, not a good option.

Big post coming tomorrow with the direction I ended up heading last weekend, just didn't want to leave Dusters comment unanswered!

Sneak peak. . .

Now you’re going down the road of what I would do

Everything is better with wide 5s.

Wide 5s + Street Car = Ultimate Win

Here I was thinking you dodged a bullet with that IRS, and ultimately having mismatched wheel bolt patterns. 

That doesn't seem to have improved. LOL

Like the others I'm excited to see this project.  As these older mini trucks age and become "trendy"  once again its fun to see the new projects.

I've been trying to keep myself from looking at old Chevy Luvs for this very type of build. At least with threads like these I can live vicariously.

What is that peeking out in the rear view?

I'm part of a Michigan statewide circle track classifieds page on Facebook Marketplace, older roundy-round cars seem to be an anomaly of nice race-ready parts for basically scrap price. I think this is due to availability and frequent rules changes making cars obsolete/illegal, but it all works in my favor.

Behold my latest purchase!

A little dodgy on the details, but this was a fairly competitive asphalt late model car at Berlin Raceway in western Michigan. Chassis is made by Port City Race Cars as an offset car, so not much use for turning both directions, but this thing is a goldmine for high-dollar parts. You can see the Sweet Mfg. power steering rack, adjustable aluminum tie rods, height-adjustable coilovers, and Port City control arms/spindles pretty well. Just those are likely worth more than the $1000 purchase price new. . .

Interior has some decent parts too, auto meter gauges, Longacre switch-panel, MSD box.

Add in the fuel cell, plexiglass wings, AN-8 fuel lines, filter, and rear end that came with this thing, and this thing is really looking like an awesome value proposition.

And never fear about mismatched wheels, because all four are 15x10 Bassett wheels with R800 Hoosiers, and are mounted on Winters Wide 5 hubs. Not pictured (hidden behind massive hubs) are 4-Piston Rear Wilwoods and 6-Piston front Outlaw billet calipers, grabbing some ~12 inch rotors. Theres a Tilton pedal box in there somewhere too. . .

Borrowed a coworker's truck and trailer (thankfully the car seats aren't mine. . .) His cars must all run, because there was no winch on the trailer, but an hour with some ratchet straps later and the car got up on the trailer.

En route back to the garage.

And the eagle has landed.

Unfortunately, the collapsible column did not come with the car, so the front body work had to come off so that tires could be manually steered.

More to come this weekend. Having a small build party this Sunday to get the ball really rolling. Hoping that the two cars will mostly become one. . .

Keep me in mind for one of these chassiseseses  over summer, i stupidly cut the one up i got for $600 and have another desire to use one