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164 Posts
Discussion Starter #1 (Edited)
I bought a 1990 Mustang LX V8 last year for a toy. It's a nice ride, had 105k on the clock, and was owned by a former KY State Trooper. It was bone stock when I bought it, with no modifications whatsoever.

Being the gearhead that I am, I can't leave anything alone for long. I installed a new set of FMS 4.10 gears in the rear, along with a new FMS 31 spline differential and a new set of Alloy USA 31 spline axles.

Along with that went a FMS aluminum driveshaft, and a Hurst SST shifter for the T5.

For the suspension, I went with a set of Strange coilovers in the front along with new caster/camber plates and lower a-arms. Tubular upper/lower control arms were installed on the rearend.

A Bassani X-pipe going through Dynomax Super Turbos was used for the exhaust.

OK, now for the part that this thread was intended for: the C4 install. Whoever owns the car in the video below gets credit for my wanting a C4 in my Fox Mustang.

YouTube - 408 Mustang Road Test (5-105 mph) No Traction!

I've always owned 4 & 5-speed cars, so I wanted to walk on the other side of the fence and see what a fast shifting automatic was all about. Hearing the car in the video above going through the gears with a screaming Windsor under the hood made me a little excited.

I started doing some research and found that a C4 swap was relatively easy on a Fox-body Mustang. So I started ordering parts.

1. Flex-a-lite trans cooler #3818-6.
2. Cooler hoses with -6 AN fittings.
3. Hurst Quarter Stick shifter, reverse manual valve body.
4. JW Ultra Bell, SFI Bellhousing.
5. New SBF C4 block plate.
6. Holcomb Motorsports Hurst shifter installation adapter for Fox Mustangs.
7. TCI SBF SFI flexplate.
8. ARP flexplate bolts.
9. Transmission mount.
10. Steeda driveshaft spacer.
11. C4 transmission.
12. Hurst reverse switch.
13. Lokar flexible dipstick.
14. Stock stall 26-spline 10.5" torque converter.

For #11, I found a local C4 on Craig's List. It is a 1972 trans, which is case filled and has a 26 spline input.

I was in possession of it for about 30 minutes as I dropped it off at my trans guy's shop (Wolford Transmission in Hodgenville, KY) right after I picked it up. He tore it down, cleaned it up, then assembled it with Alto clutches, extra wide bands, a new hardened input shaft, a reverse manual valve body, a billet servo, and a deep pan. My next step for the Mustang is to build a 408-410W, so I wanted a trans that would easily hold the power.

When I got it back, it looked like this:

With a butt-load of parts in front of me, I put the Mustang up on 4 ramps so I could easily get under it and work. I yanked out the T5, the bellhousing, clutch, and flywheel. (It still had the factory clutch in it and it didn't look half bad.) I then pulled the pilot bearing out, fixed a couple of oil pan drain plug leaks, and installed the new block plate and flexplate.

Out in front, the Flex-a-lite trans cooler went in pretty slick. Not sure about other years, but my Mustang has a front core support that vertically runs right down the middle of the condenser. It sits pretty close to the condenser as well, so I just mounted the trans cooler vertically on the passenger side of that support. I ran the hoses (with -6AN fittings) up on the inner fender near the smog pump hoses and then down between the trans tunnel and the bellhousing.

That cooler is roughly 7x15x3/4.

Up inside the car, the shifter went in. Holcomb Motorsports adapter is meant to mount a Hurst Quarter Stick or B&M shifter in place of the factory 5-speed shifter without having to hack up the console.

However, I had to do a lot of hacking on the shifter adapter. I ended up having to space the front of it up and notch the back of it so that it would sit without the shifter handle hitting the CD player. I ended up getting some good clearance, but I decided to move the CD player down a hole.

I then cut the top 1/3 of the factory shifter boot off and tucked it down. I plan to have it professionally sewn later on.

Some carpet jute insulation went in around the adapter plate to keep outside noise out of the passenger compartment.

I had to order a Hurst reverse switch as the shifter only had a neutral safety switch.

I had removed the clutch pedal completely and of course I wanted to keep the reverse lights working, along with the neutral safety switch so some moron (namely me) couldn't start it in gear.

There are two switches that connect to the factory clutch pedal. One is a clutch safety switch and I believe the other disengages the cruise control so you don't stab the clutch while running down the interstate and cause the engine to bounce off the rev limiter.

Armed with some advice from a good buddy, a schematic, and a multi-meter, I got it working by doing this:

1. Jumpering both switches on the clutch pedal.
2. On the 4 prong switch in the trans tunnel, I connected the red/blue and white/pink wires in line with my neutral switch on the Hurst shifter.
3. On that same switch, I connected the black and purple/orange wires inline with my Hurst reverse switch.
4. There is a 2 prong switch under the car in the trans tunnel also, which is a neutral switch for computer KOER diagnostics. I left it open for now.
5. I confirmed that the backup lights worked and the engine would bump over (starter has to be mounted and grounded solid).

Once all of that was done, it was time for the trans to go in.

I'm 5'6" and 140lbs totally sopping wet, but I managed to slide the transmission under the car, and up p on a floor jack. I put a quart of ATF in the converter and installed it on the trans. Up goes the jack, and the trans is mated to the engine.

For the transmission crossmember, I simply cut the welds on the tubes so that the crossmember could slide, and just pushed it as far forward on the tubes as I could. The C4 transmission mount barely made it, but the stock member works fine. I ended up eliminating the exhaust hanger plate that is sandwiched between the trans and the trans mount.

The shifter cable was then installed and I mocked up the driveshaft. The driveshaft turned out to be a little short; about half the slip yoke was sticking out of the transmission. A little more research and I found that this was a common problem and that Steeda made an adapter to go in between the driveshaft flange and the pinion yoke.

In goes the driveshaft:

At this point I was ready to fill the trans and fire it up. Between the trans with the deep pan and the trans cooler/lines, it took 8 quarts of ATF (plus the 1 quart in the converter).

I can report that this has been one of my favorite projects. I absolutely love the C4 and how it performs. Shifting is lightning fast and hard.

For anyone that is thinking about taking on this kind of project, I say jump in with both feet. It's very straightforward and it's not hard at all. The C4 in combination with the 4.10 gears is a little rough to get used to and I think I may swap them out for some 3.73 gears, in addition to my wheel/tire swap that I have planned this year.

Now, here are some videos of my own:

YouTube - BackingOutOfDrivewayC4

YouTube - Testing the C4



78 Posts
Great info and article. Just one question.
Does the swap require switching the ECU?


296 Posts
In the event you jump to a windsor engine you may want to go with a 3.55/28" tire combo. For me to run 60 requires about 2600 rpm. I am acutally thinking about dropping to a 3.27 gear

159 Posts
Hey iam thinking of doing the t5 to c4 swap also in my 88 mustang, my new 393w is almost done so i need to decide soon! I do go to the strip once a week and i do drive down the street also. I already have a c4 core and a t5 core beiing a 5speed currently. I know i will like the auto at the track but i like 5speeds so much on the street. My question to you is do you still enjoy your c4 on the street? And am i going to loose all highway use (my car has 410 gears)? I need help have been going back and forth with my decision for almost 2 months! Thank you.
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