Performance Upgrades: Turbo and Rear Bushings

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A few weeks ago we took the Suburban to see family in Ohio. The highway mileage was pretty good at 25 mpg, but this trip highlighted two shortcomings for the vehicle's current setup.

The Garrett GT2259 has an ideal compressor map for a 3.9 liter diesel in the rpm range I'm using.

First, the Suburban had plenty of power at speed, but short uphill highway ramps made me uncomfortable. I especially don't like the rather large step from 3rd to 4th gear in these situations. After the engine has been wound up to the point of diminishing returns, I make that 3-4 shift and find myself at some rpms that are too low for the turbo to produce much boost. The vehicle is accelerating, but s-l-o-w-l-y. The stock, non-wastegated Garret that came with this late-80s Isuzu 4BD1T is designed so that it never needs a wastegate. It never spools up enough to produce "too much" boost (so, it's weak by definition!). It has a roomy case on the turbine, too, so that by the time it reaches the max boost of 12psi, it is at a point where the driver will be reaching for the next gear.

I've been staring at turbo compressor maps for awhile, and I've settled on a Garrett GT2259.  I fed Garrett's Boost Advisor some information about my engine's displacement and this turbo was the best recommendation it made.  I told it some things about my engine's displacement, intercooler assumptions, my "mid-range" and "max" rpms, and my target of 200 hp.  If I reach my targets, this turbo will put out 22 psi, putting the pressure ratio at 2.6 on the map shown here.  The corrected air flow will vary with rpm from 17-22 lb/min -- which puts it right in the upper-middle part of the map.  Some of the other recommendations I've heard for this engine (including others made by Boost Advisor and some on 4BTSwaps who recommend the Holset HE341) would put these points off the left side of the map, on the wrong side of the "surge" line.  This map is somewhat similar to the HX30 maps, but it's stretched out to include higher pressure ratios and corrected air flows.  The HX30 is known to provide 20+ psi on these engines as low as 1500 rpm, but flow will choke at around 2500 rpm and the turbo won't deliver any more air.  I think the GT2259 is a better match than the HX30 for onroad use, and I've found a wastegated, water-cooled, remanufactured turbo on Ebay that's meant for use on a Hino truck.  When it arrives, I'll start my next step in the diesel portion of this project.

The second shortfall with the Suburban was its handling.  I've driven a number of large vehicles over the years, including Special Transit buses in Boulder, CO during my college days.  I know better than to expect sports car handling in something as hefty as a Suburban, but it shouldn't handle like a fish, either.  At low speeds the vehicle is fine.  At highway speeds, the vehicle slops around with the rearend moving left and right without immediately changing the direction of the vehicle.  It forces the driver to think further ahead and make more predictive corrections to keep the vehicle straight.  In a crosswind, or when passing semis, these behaviors make me uncomfortable.  I want this thing to be drivable by my wife, too, so that we can switch off on long trips, so the handling needs to be fixed.  I intend to fix the handling and do the turbo upgrade before our trip to Colorado this autumn.

With my initial build, I replaced the rusted-out shocks on the vehicle with some upgraded, larger-diameter units -- so at least I knew that the axles wouldn't be hopping around as I go down the road.  I suspected that with 152,000 miles on the odometer, the original rubber bushings might be worn out, but I thought I'd see how it handled, first.  Now that I knew the answer, I ordered a stack of polyurethane bushings online and started installing them this weekend.  I got the rear end done and got started on the front end.  I was hoping to do the whole thing, but I ran into some things that slowed my progress.  I'll cover the front end in another posting.

In advance, I'll apologize for the photos in this post.  My old camera died and I wanted to take better pictures than my camera phone will allows.  I also don't want to expose my family's "good" camera to the metal chips and other conditions that probably killed the last one.  So, I bought a waterproof Vivitar, thinking that being waterproof would also keep metal chips out.  I should have paid more attention to the reviews of photo quality, and this one is going back to Amazon!  It took multiple pictures of any scene and external lighting to get any useful pictures out of this thing.  I should have just used my Android to take these, as it provides better quality with less fuss.  When a phone takes better pictures, money has obviously been wasted on the purpose-built camera!

After the vehicle was properly supported, the rear axle was unbolted from the leaf springs and set onto smaller jackstands.

First, I put my floor jack under the Suburban's rear differential and supported the vehicle's weight while I loosened the lug nuts.  Then, I lifted the whole rear end as high as the jack would allow.  I put my larger jackstands under the rear frame, and slowly set the vehicle down onto them.  I removed the rear sway bar and end links, so that they'd be out of the way and could also receive some upgrades while they were off.

I removed the rear wheels and thought about the forces on the bushings from the weight of the hanging axle.  I realized that I would actually have to remove the springs to use my press, so I pulled the U-bolts that attach the rear axle to the springs on both sides.  I supported the axle with the jack and settled it onto a small jackstand on each side.  I didn't want to mess with the brake lines, so I just let the rear axle sit on the stands during this work.

I did both sides, but please note that all photographs and discussion here are on the right-side assembly, in order to avoid any confusion.  Please also note that I recommend doing one side at a time on almost any component disassembly and reassembly efforts -- because you can always consult the properly assembled components on the other side to avoid errors.

Because the bushing sleeves had rusted onto the bolts in a few places, some of these grade 10.9 bolts needed to be cut.

I unbolted the front pivot and the point where the shackle meets the frame at the rear.  For the shackle, I was able to use a tie-rod tool to push the bolt out.  On the front pivot, I couldn't fit the tie-rod tool, and smacking the bolt with a hammer / 2x4 combination wasn't budging it.  When I rotated it, I found that the bushing's inner sleeve was rotating with the bolt, so I knew I had a problem that I'd seen when doing a suspension lift on a Cherokee a couple years earlier.  These bolts pass through a sleeve that goes through the bushing's ID.  There's apparently never any grease on these bolts, so after a few years the bolts freeze to the sleeves -- making them nearly impossible to remove.  I borrowed a friend's reciprocating saw and wound up cutting this bolt (I had to cut two of them on the vehicle's left side, as well).  These grade 10.9 bolts are pretty hard to cut and will dull a bi-metal steel-cutting blade very quickly.  So, I used a flat blade with carbide grit attached to it.  These are usually sold for cutting tiles and cast iron.  I learned this trick on the previously-mentioned Cherokee effort.  It takes awhile to cut through, but it works!

Using a hydraulic press to remove old bushings.

After that, I pulled out my super-fabulous Harbor Freight hydraulic press and pushed out the old sleeves and bushings.  This sounds simple, but these leaf springs are heavy.  In the picture, you don't see that the other end of the spring is supported by a wooden sawhorse.  I started off using a plastic, folding sawhorse, but the weight was too much for it, and I didn't want to drop the spring on the concrete again.  That's not the kind of excitement I'm looking for!  I had to use a combination of spacers, plates, sockets, and socket extensions to get these out.  I unbolted the shackle from the rear of the spring and used the same technique.  If I did it again, I'd apply some heat with a torch ahead of pressing--as this technique gets the rubber to unbond from the steel shells and has been very helpful on the front end!

After I got the bushings out, I used the reciprocating saw to make cuts in the shells so that they could be removed.  The polyurethane bushings are designed to be used without shells, in this case.  It's important to be very careful on how deep these cuts are.  I nicked the spring steel in a few places, but I used a rotary tool to smooth/round out the resulting grooves--in order to avoid the progression of stress cracks at these points.

Inserting polyurethane bushings -- mostly by hand.

With the shells removed, I cleaned up the springs, wire-brushed areas with corrosion (especially inside the eyes), and hit all the external areas with rust convertor followed by some black spray paint.  When the paint was dry, I greased up my bushings and squeezed them into the spring eyes.  They are a tight fit, but I could insert them most of the way by hand.  I used a clamp to ensure they were bottomed out.  Next, I started the sleeves by hand, but had to use a C-clamp to get them fully inserted.

For the shackles, I realized my kit contained 1 3/8" shackle bushings, rather than the 1 1/2" ones I'd need.  Rather than wait another week to get this job done, I chose to go with the OEM style rubber bushings from Napa.  These included shells and took more force to insert.

I then bolted the shackle onto the rear side of the spring, after consulting the components on the other side of the Suburban to see (1) which side of the leaf spring was forward, which wasn't immediately obvious, and (2) which way the shackle should point.

These leaf springs have plastic parts between the ends of the springs where they meet, and I can see that these parts rub on each other as the springs are loaded and relaxed.  I put a coating of Boeshield on these areas, thinking that the waxy/oily substance may provide a bit of lubrication and help inhibit further rusting of the components (I spray this stuff all over the place under the Suburban to prevent rust, and it also makes a good chain wax for bicycles).

Reinstalling leaf springs. After the front eye bolt and the shackle bolts were all inserted, it was time to torque everything down.

I then carried the heavy spring assembly back to the Suburban and put it in place.  I set it on top of the axle perch, being careful not to crush the brake lines nearby.  I bolted the front eye in first, using grade 10.9 M14 x 120mm bolts with a 2.0 thread pitch.  Unhappy with a purchase from an online GM seller that still hasn't arrived (after over a week), I sourced these from Utterback Supply in Indianapolis (I really like doing business with these guys, and would link to them if they had a site!).  I didn't torque them down right away, because it's best to get all of the bolts in, first.  I bolted up the shackle in the rear, then I torqued everything down.  I also noticed that I'd forgotten to apply rust convertor and black paint to this shackle mount, and did that next.

At that point, I jacked up the axle on that side, aligning the hole in the perch with the bolt on the bottom of the spring, and reinstalled the U-bolts.  After double-checking my torques, I was done with that side.

While I was replacing bushings, I also chose to replace the rear sway bar bushings with polyurethane units.  I won't go into much detail on this, but the kit included right and left sway bar bushings with new brackets and grease fittings.  The end links also got new polyurethane bushings.  I spent a lot more time on the leaf spring bushings, but this upgrade probably makes more of a difference in keeping the Suburban's rear end well-behaved.  I've found that polyurethane sway bar bushings and end links will do a lot to flatten a car's attitude in a corner.  Stock rubber bushings are often too soft in this location.  Much of the process in replacing the Suburban's bushings was just a do-it-once maintenance activity.  The rear sway bar was bolted in after everything else was reinstalled and was the last step before setting the vehicle back on its tires.

The rear sway bar also received polyurethane bushing upgrades.

I should point out that I used marine bearing grease on all of the polyurethane bushings.  This is a bluish-green silicone-based grease that won't wash off and won't react with the polyurethane.  I've used it in a number of applications and it does a good job in keeping the squeaks at bay.

--Just because a vehicle is large and heavy, doesn't mean it has to handle dangerously!