Project SRT-4 is turning into a more serious driver's tool every month. With decent stick, big power and monster brakes, it's getting easier and easier to hunt EVOs and WRXs in this low-budget, big-boost econobox. This month, we add to Project SRT-4's arsenal with better handling and improved grip from polyurethane Prothane suspension bushings and motor mount inserts.
If you've been following our this project, you know the SRT-4 was in serious need of help in the suspension department. As we added power and increased grip, the little Mopar's chassis started asking for help. And as more testing and more time took their toll, it started screaming. This month, we find a solution. And uncover a side effect.
Suspension Bushings Mopar's Stage 3 coil-overs improved handling considerably, as did the addition of a Progress Group rear anti-roll bar, but further refinement required removal of the rubber which keeps the chassis from returning real feedback to the driver. Several symptoms have made this obvious. First, wheel hop under hard acceleration became the norm every time full throttle was used in first, and sometimes second, gear. This was especially true while cornering where most of the torque was trying to exit through one tire as our car still has an open front differential--something we intend to fix soon. Second, during brake testing, the SRT-4's rear wheels exploded into wheel hop under maximum deceleration--another sure sign of bushings which aren't up to the task.
Finally, the car never really felt connected. It could be driven quickly but it wasn't as precise as we'd like. Part of that, we suspect, lies in its economy car roots, chassis rigidity and other factors that are a product of its inexpensive platform. But a bigger part of it was bushings that couldn't come close to handling the load the SRT-4 was putting through them. Prothane's bushings have cured most of those problems by allowing the SRT-4 to maintain the alignment settings we select even under the increased loads the car now generates. But with the bushings come a few caveats.
Anybody who has installed polyurethane bushings before probably has some horror story to go with the experience. And if you'd asked us before this project how hard it is to install bushings, we'd have recommended against it. But installing them in the SRT-4 went remarkably smoothly and didn't require the car sitting on jack stands for days on end. Still, you should be prepared for some serious labor; it's not a job you'll want to tackle without the right tools. On the SRT-4 those tools include: A shop press and a big enough socket selection that you'll have the right one when it comes time to press out the huge bushings on the front control arms (22mm or larger) as well as the front bushing on the rear trailing links; a torch (oxy-acetylene or propane); a small cylinder hone and a wire wheel on a drill or grinder. And, of course, the appropriate tools to disassemble and reassemble the front and rear suspension.
Our bushings were installed by the techs at Under Pressure Fabrication and Distribution in Westminster, Calif. Wonder twins Mike Callicchio and Mike Holden performed the job from start to finish in about five and a half hours. If you're planning on doing this at home, keep in mind they had the benefit of a lift, air tools and prior experience with bushing installation torture sessions.
The rear suspension came apart first as it's the most complex and contains 12 bushings, plus the anti-roll bar bushings. We didn't swap those because our Progress anti-roll bar already had poly bushings at the end links. The bushings on the trailing links are easiest so start there. The trailing link pivots at the chassis on a bushing with a pin through it. This bushing must be pressed out. The other end of the trailing links mount through the upright and are located on either side by two large doughnut-shaped bushings. These bushings are easily removed by hand when the trailing link is removed from the upright.
Luckily, the stock bushing, which mounts the trailing link to the chassis, is so soft, it pressed out very easily, pin and all. The Prothane bushing is then pressed in place. The pin, which is centered in the bushing, is held in place by two snap rings. Seating the snap rings properly required using the press to remove preload on the ring and allow it to snap fully closed (see photos at left). Remember to lubricate everything with Prothane's supplied grease during assembly.
Now it's time to get out the torch and the gas mask because the rest of the bushings in the rear suspension must be burned out. Mark the lateral links when you remove them so you know where they go when you reassemble things. The two rear lateral links are identical, but face different directions when installed. You'll need to reuse the eccentric sleeve housed in the rear inner lateral link bushing (see photo at left). This bushing is unique as it's used to set rear toe so it's critical that it's not lost or damaged. Remove the eccentric bushing from the lateral link and clean it up with a wire wheel so it can be reinstalled into the Prothane bushings. The rest of the sleeves will be thrown away as the bushings are burned out.
There's no real science here--light the bushings on fire and let the rubber burn away. This makes a big enough mess, not to mention a lot of fire on the larger bushings, that we had to perform the task away from the UPFD building. We suggest you avoid doing it in your driveway unless you live in Kentucky.
Cleaning the remaining rubber out of the lateral links is easy if you use a cylinder hone and hand drill like the wonder twins. Once most of the rubber bushing is burned away, cool the link and use the hone to resurface the housing for installation of the urethane bushings. With ample use of lube, all our bushings and sleeves slid into the lateral links by hand. Pay attention to the Prothane directions, which show an exploded view of the rear suspension. Prothane supplies washers that must be installed in several critical positions. These would be easy to miss if you're not the wonder twins.
With only two bushings each, the front control arms are fairly straightforward. Both require burning out the old bushings, but the sleeve from the larger rear bushing must be reused. Follow the same prep procedure here that you did for the eccentric bushings in the rear (see photos on page 212). Use the hone on the housing and clean the sleeve with a wire wheel. The wonder twins were able to press these bushings in by hand as well.
The front anti-roll bar bushings couldn't be simpler. The bar bolts directly to the subframe and uses simple, doughnut-style end link bushings (which are now polyurethane instead of rubber). These bushings aren't the same diameter as the surface they seat against on the anti-roll bar. This means that when they're tightened against the bar, they deform and probably don't do much more than their rubber counterparts, but we installed them anyway. The bar in Project SRT-4 was covered in rust because DaimlerChrysler was too cheap to paint it. A few seconds with a wire wheel smoothed the mounting surface so we could re-install the bar without sliding the bushings over a rusty surface.
Engine mount inserts Prothane's engine mount insert kit might just be the best $35 you ever spend on your car. If your SRT-4 is making more than stock power, you have to get these. Our car was a wheel-hopping monster in first and second gears before this installation virtually eliminated the problem.
The inserts simply fill the voids in the stock upper and lower torque mounts with polyurethane. The lower torque mount is held in place with two bolts while the upper mount uses four bolts. Neither actually holds the engine up, so they only take about 15 minutes to install. There's only a very slight compromise in noise and vibration at high engine speeds--the rest of the time the inserts are imperceptible. Before installing these inserts, using full throttle in anything other than a straight line had the inside front tire hopping with axle-destroying force. Now, thanks to the unfortunate lack of a limited-slip differential, there's only wheelspin. But we'll take wheelspin over axle tramp any day. Grip is slightly improved, as the SRT-4 does accelerate better, but there's still work to be done. Bottom line? A huge improvement.
The Verdict Initial impressions are very good. The SRT-4 has much improved steering feedback and its dynamics are more predictable and much, much sharper. However, it's now a far more tail-happy machine. Turn-in is immediate and any throttle lift is met with instant rotation. This is good and bad. If you're experienced and prepared to deal with oversteer in a front-drive car, it's lots of fun. And, on the right road, it's probably pretty quick. Get it wrong on a freeway on-ramp though, and things could get ugly in a hurry. For track use, we'd probably want to relax the chassis slightly and slow its reactions with less aggressive turn-in/rotation. You have been warned.
We suspect this characteristic is a result of the lack of compliance in the rear trailing link bushings on the upright, which Dodge designed to flex as the rear suspension goes through its travel. The stock rubber bushings were extremely compliant and moved easily as the suspension traveled through its range of motion. The Prothane bushings are very stiff and it's likely their addition has effectively increased the rear spring rate. Several options are available as a potential fix. First, going back to the stock bushings at this location might cure the problem without compromising the sharpness added by the rest of the kit. A larger front anti-roll bar might also help--as would a smaller rear bar. As of this writing, we're still enjoying the car's newfound character, but we'll probably make some eventual effort to calm it down Instrumented testing doesn't really tell the whole story. Our previous skidpad and slalom numbers stood at .91g and 70.4 mph, respectively. With the Prothane bushings there's some improvement in lateral acceleration to .92g, while slalom speed drops to 69.5 mpg--the expected result of a more rear-bias handling setup.
Next time we'll tackle the Magnaflow side-exit exhaust we've been meaning to show you for months. And, with any luck, we'll install the Quaife limited-slip, which has been talking up space on our floor since we got the car. n