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Mustang "Stop The Hop" Kit And Diff Upgrade

Side view of metallic blue Ford Mustang GT with black wheels on a paved look concrete driveway.

Over the last couple of years, I've had a chance to drive (and work on) a couple of late model Mustangs and the team here at Kashy think they're great cars. In fact, my first experience of a new model Mustang was a road-trip from San Francisco to Los Angeles in a 2.3L EcoBoost convertible and, while there is plenty of power available, there is a certain 'je ne sais quoi' to the V8's. To the mustang GT, the Darkhorse, the Mach1 and all the other 'special variants'.

Actually, scratch that. Je ne sais quoi means an indescribable quality. And the quality that the V8 variant's have is simple, it's a bunch more power and noise.

However, while a lot of the Mustang GT's BIG character is derived from the V8 engine, there are quite a few 'problems' that have come to light, and that only seem to exist in the V8.

Some preface.

While you might not have known it, the Mustang GT is almost 100kg heavier then the EcoBoost, and it holds all of that weight over the front. This doesn't change the driving characteristics that much, but it does mean that the EcoBoost is has a lighter and more nimble feeling to the steering.

The real issues with the Mustang GT's that I've driven (and for whatever limited edition they're doing this week) are in the gearbox and rear suspension.

Photo of mechanic Lachlan Palmer holding 2 spanners and working on the rear brakes and suspension of a Ford Mustang GT.

I should 100% preface this with some conditions. First and foremost, I have not driven either variant on a track yet and we have only had short drives in each vehicle (maybe 10-15 minutes on moderately twisty roads).

Secondly, I have a lot of love for tiny front wheel drive cars (such as the Abarth we talked about a couple of weeks ago), which gives me a bit of a bias when trying to describe the handling characteristics of a big, lumpy American sports/muscle coupe.

Thirdly (and most importantly), I almost always tend to look at driving dynamics based on my tiny bit of experience in karting. This means I personally like a lot of push (a tight differential) and a little bit of looseness (or slide) in the rear and really pointed steering.

Photo of white and blue coloured Tony Kart (go kart) driven by Lachlan Palmer with the numbers 43 on the front driving through a corner. In  in the background is another go kart  and in the middle ground and sides is grass and bitumen.

So what are the driving dynamics I notice in the Mustang GT when giving it a proper boot full.

Well, under load and in a straight line it's pretty manageable, except for a significant feeling of torsional play through the rear end, especially from 1st to 2nd gear. On top of this, the gearing for the Australian variants is much too long. In fact, an 'unverified third-party participant' in our small survey of Mustang owners reported that second gear approaches 140Km/h.

However, the biggest problem for daily drivers is probably that torsional play in the rear. While it's somewhat tolerable, it leads to a fairly consistent feeling of unease. The reason for which becomes apparent when giving it just a sprinkling of 'enthusiasm' through a corner.

Now, there are plenty of things I could say about the Mustang GT's driving characteristics if we were left with it for just a couple of days, but with the time given, the most aggressively obvious flaw is the tendency to break loose on throttle/corner exist.

Now luckily, someone else has done a lot of the research into this (because I frankly didn't have the time) and has come up with the Steeda "Stop The Hop Kit" (or Whiteline Front and Rear Grip Kit).

What is the cause/fix for the rear subframe play in the Mustang GT?

One of the most important things that seems to cause this torsional play in the rear of the Mustangs' is the difference in size between the inside diameter of the rear subframe mounting bushes and the bolt shank size.

Rear subframe bush from a S550/FN/FM Ford Mustang GT showing the difference in size between the inner diameter of the bush and the outer diameter of the bolt shank. There is approximately 10-20mm of float on either side.

The general consensus from the likes of Steeda is that the factory bushings allow 10-20mm of play back and forth under changing loads, causing that unease and want to break loose.

To overcome this, Steeda and Whiteline (and a bunch of other Mustang enthusiasts) have come up with a brass insert or polyurethane bushing kit that replaces the factory slop and ensures the bolts fit snug against the inner diameter of the bushing.

Fitting the "Whiteline Rear Subframe Bush Kit" in the Mustang GT.

Now, the overall installation of the Whiteline "Mustang Front and Rear Grip Kit" was not bad, but it was definitely a challenge at times. The biggest problem was removing the factory bush to fit the Whiteline system, a process that involved many hours of hammering, a cold chisel and some selective vocal encouragement.

Photo of mechanic Lachlan Palmer laying on the ground after working on rear subframe bushes from a Ford Mustang GT. He is wearing a blue shirt and black jeans and has a set of florescent yellow ear muffs on. To the left of the photo is the subframe and a trolley used to move it.

Whiteline does suggest using a sawzall to cut the outer shell of the bush, but the risk of damage to the subframe is pretty high for beginners. Whiteline also suggest using a bush removal and install kit (which they'll sell you for over $700), but we tried a generic variant and the piece of 18mm threaded rod that came in the kit bent and snapped before the first one even moved.

The best method for us was (unfortunately) violence, and we found that pushing over the lipped edge of the bush and trying to fold it inwards was the best method to get movement, though there is probably a much better way to do it.

The basic process for the Whiteline kit is as follows:

  1. Remove the exhaust, propellor shaft, brake callipers and everything else that might get in the way of the subframe.

  2. Remove the subframe.

  3. Beat the old bushes out with whatever method seems most likely to work that day.

  4. Cry when the first attempt doesn't work.

  5. Attempt another method and repeat steps 2-4 until old bushes fall out of subframe.

  6. Install polyurethane bush kit and inserts and washers.

  7. Refit subframe to vehicle, and refit all accessories as needed.

  8. Don't crash on the test drive.

Whiteline rear subframe bush upgrade fitted to the rear subframe of a Ford Mustang GT.

For the Whiteline kit in particular the subframe needs to be removed, a far more involved process than the Steeda kit, but something that was fine for us anyway as we intended to replace the diff gears at the same time.

Is it worth doing other upgrades while doing the subframe is out on a Mustang GT?

While we were in the subframe and mucking around anyway, the owner of this vehicle also opted for a 3.73:1 diff ratio, a lovely little Truetrac LSD (to match the 3.73:1 ratio change), a Barton transmission mounted short shifter, a set Whiteline of front and rear swaybars and a front lip extender (to make sure it scraps with every speed-bump).

All this added to the set of factory forged wheels and lowering springs already installed.

Differential pinion wheel and crown set in white Styrofoam packaging.

So, what were my notes from the first test drive with the "Steeda Stop The Hop"/"Whiteline Front and Rear Grip" kit?

Put simply, it makes the back end feel a lot more reliable.

As we said before, the biggest problem in confidence came from the unpredictable nature of the rear handling and this managed to take that away entirely. The slight reduction in body roll with the sway bars gives the car a sporty feeling, but being on the softest setting allows just enough body roll to still notice the transfer of weight front to back and side to side.

Rear differential from a Ford Mustang GT removed from vehicle and sitting on a positioning trolley. The trolley is on a grass surface.

The combination of the Truetrac and Whiteline "Mustang Front and Rear Grip Kit" also make the car a lot more planted and pushy in the back, meaning the drive off of corners is easier to understand and that any slide or 'slip angle' was a lot more intentional and controllable.

In fact, I would go as far as to say that the original problem and this particular set of products might be the cause and (partial) solution to all those Mustangs' that were ending up in crowds a few years ago.

An added bonus to all of this is that the new differential gears have actually made the speedometer more accurate too.

Now, there's a lot more to come with this car in particular as we have to get the owners reactions to the first track day, but so far they report significantly more enjoyment, better grip and significantly worse fuel economy. They also say that is "much more like the car it should have been from the factory", so take that as you will.

Left hand rear half Ford Mustang GT (from the middle of the door back) showing the rear suspension and subframe removed. Underneath the car are the shock absorbers, wheel hubs and CV shafts.

If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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