3.73:1 Torsen to 3.55:1 Swap

[1958cyclist]

Swapping pumpkins in the S550 is rather easy. I've done several. You don't need any computer changes / program / tune to go from 3.73 to 3.55 gears. The wheel speed sensors detect the change with zero issues - if you have a manual trans. Auto will need some tune changes for shift rpm's etc. Speedo etc all dead on. I did my new (then) '17 V6 car swapped to 3.55 from 3.15. I swapped a 3.73 into a 3.31 geared car and swapped 3.15 from a wrecked V6 into a Procharged 3.73 PP1 car.

It's a quick job with the IRS Setup. I did this quick write up several years ago after swapping one:

- Remove the rear wheels
- Parking brake ON - break loose the rear axle nuts with a 1 1/4" 6 point socket and a breaker bar
- Parking brake OFF. Remove rear calipers - 2 14mm bolts on the back of the caliper - hang the caliper out of the way with zipties
- Remove the 8mm bolt holding the ABS sensor in the knuckle. Then pull the rubber bracket that hold the ABS sensor line onto the parking brake line bracket. Zip tie the ABS sensor / wire out of your way
- Remove the 10mm bolt that holds the parking brake cable bracket. Zip tie / arrange it out of your way as well
- I thread one lug on snug so the rotor stays when moving the knuckle around
- Remove the bolt where the knuckle mounts to the arm on top. 15mm if i remember - the nut you need to hold is 18mm. You might have to knock the bolt out -if so thread the nut back on flush and use a dead blow hammer so you don't screw up the end of the bolt
- Remove the TOP bolt from the vertical link (15mm)
- Remove the bottom lower front (faces to front of the car) bolt that holds the lower part of the knuckle / arm to the toe link. 18mm bolt.
- Remove the axle nut all the way. You can hold the axle still with a praybar looped into the wheel studs if needed.
- Whack the end of the axle with a DEAD BLOW hammer if needed to get it loose from the hub

TOSS The axle nuts - new ones are $3-4 a the dealer and come with loctite already applied

- Use a pray bar to get the top of the vertical link (you already removed the bolt) so the knuckle can clear and pull down / out on the knuckle while working the axle so you can get it out of the hub - it will come out - don't force it.

- Repeat all this on the other side

- Remove the exhaust. Stock comes out as one assembly, if you have a catback etc remove it. You'll need the h-pipe gone for easy driveshaft access

- Put the car in GEAR and crawl under and remove the 3 driveshaft to pinion bolts you can get too - 10mm - and I used a gear wrench with a hollow pipe on it for leverage - they are only torqued to 41 ft/lbs supposedly but thread locker AND Sealer make them fun to get out
- Put the car in neutral, move the driveshaft by hand so you can reach the other 3 bolts, put it back in gear and loosen remove the last 3 bolts.
- Rig up HEAVY zip ties / bungie or use a flipped over 5 gallon bucket to hold the end up the driveshaft so it doesn't fall. Push in a bit and it will disengage from the rear end pinion

NOTE THE ORIENTATION of your driveshaft. Mine had a factory yellow x / mark and it lined up with an indent on the pinion of the rear differential. I noted that and installed the new differential lined up the same way. Might have gotten lucky, but I seem to have NO vibration even at higher speeds

- Use a pry bar to pry against the differential to pop the axles loose. They will come out. There is a circlip that holds them in the rear end.

I didn't even remove my axles all the way, I fed them out until the rear differential had room to clear and left them propped on the suspension / top of the rotor / knuckle

- At the top of the differential remove the vent tube. It just a hose elbow that slips on a pipe nipple, but it can fight you

- Remove the lower pipe plug from the differential cover and drain the rear into a drain pan.

- Use a floor jack under the differential to hold it and take a bit of pressure off the (4) mounting bolts. They are 18mm - two towards front two towards rear.

- Remove the (4) bolts (the rear is heavy and awkward be ready). And lower the jack to remove the differential

- Install is literally the reverse.

- When seating the axles they will clunk / snap into place (when the clip engages) lube the ends with clear gear oil and hold them from outside the rotor area with two hands. Don't be afraid to push with some pressure.

- Loosely install all the suspension bolts you loosened until you are sure they are all in there. Don't be surprised if you have to jack up the rotor / knuckle with the floor jack to get it all lined up

- Reinstall caliper, parking brake bracket, ABS sensor etc.

Install the new axle nuts snug - I wait until last to torque them.
- Make sure the suspension is loaded (jacking it up from the lower arm) before torquing all suspension bolts to spec. If I remember correctly the lower front (Facing to front of car) knuckle / arm to toe link is 129 ft/lbs. The upper bolt is 79 ft/lbs as is the TOP Vertical link bolt
- Rear end mounting bolts are 129 ft/lbs

- Fill the differential (don't forget Ford Friction modifier)
- Don't forget the vent tube / hose

- Clean the threads / replace the (6) driveshaft to pinion bolts. Reinstall into the the two retaining clips (3 bolts per clip). Use blue loctite. Car in GEAR when installing 3, neural move driveshaft with hand to line up last 3, then back in gear.

- Torque to 41 ft/lbs in star pattern (like lug nuts) I had my son in the car for this step. Told him NEUTRAL, torqued one, had him shift to neutral, spun shaft, in gear, torque bolt - rinse repeat until all our torqued. Keep your helper in the car for the next step
- Tell them to hold the brake
- Tighten axle nut to 98 ft/lbs then add 45 degrees - Repeat on the other axle
- Install exhaust
- Install rear wheels
- Get off stands




As for cost / benefit....

You can find 3.55 complete pumpkins for $300-500. If you have to pay to change gears, you need gears $200-300 Install kit $150 - plus labor.

I'm thinking about doing 4.09 gears in my '19 GT 6 speed. Local Mustang shop will charge me $250 labor if I bring him the pumpkin, the Ford Motorsports gears and install kit.

Thanks much for the comprehensive step-by-step! How many hours would you guess this took you? I don't have access to a lift and I am way past doing these types of things on my back. More than likely, I'll have a shop perform the swap should I decide to move forward with it. My main focus for the diff at this point is having cooling installed, and though that while they've got this going on, why not make the gear change. Thanks again for blow-by-blow! Great to have a detailed insight into the process.

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[ctandc72]

Thanks much for the comprehensive step-by-step! How many hours would you guess this took you? I don't have access to a lift and I am way past doing these types of things on my back. More than likely, I'll have a shop perform the swap should I decide to move forward with it. My main focus for the diff at this point is having cooling installed, and though that while they've got this going on, why not make the gear change. Thanks again for blow-by-blow! Great to have a detailed insight into the process.

The last swap was done on jack stands. Jacking rails make it really easy to get the car high up on good jack stands. I pulled the car in around 8am or so and we drove it to get lunch a bit before noon. That included a few cigarette / coffee breaks.

 

[rtrdude]

Swapping pumpkins in the S550 is rather easy. I've done several. You don't need any computer changes / program / tune to go from 3.73 to 3.55 gears. The wheel speed sensors detect the change with zero issues - if you have a manual trans. Auto will need some tune changes for shift rpm's etc. Speedo etc all dead on. I did my new (then) '17 V6 car swapped to 3.55 from 3.15. I swapped a 3.73 into a 3.31 geared car and swapped 3.15 from a wrecked V6 into a Procharged 3.73 PP1 car.

It's a quick job with the IRS Setup. I did this quick write up several years ago after swapping one:

- Remove the rear wheels
- Parking brake ON - break loose the rear axle nuts with a 1 1/4" 6 point socket and a breaker bar
- Parking brake OFF. Remove rear calipers - 2 14mm bolts on the back of the caliper - hang the caliper out of the way with zipties
- Remove the 8mm bolt holding the ABS sensor in the knuckle. Then pull the rubber bracket that hold the ABS sensor line onto the parking brake line bracket. Zip tie the ABS sensor / wire out of your way
- Remove the 10mm bolt that holds the parking brake cable bracket. Zip tie / arrange it out of your way as well
- I thread one lug on snug so the rotor stays when moving the knuckle around
- Remove the bolt where the knuckle mounts to the arm on top. 15mm if i remember - the nut you need to hold is 18mm. You might have to knock the bolt out -if so thread the nut back on flush and use a dead blow hammer so you don't screw up the end of the bolt
- Remove the TOP bolt from the vertical link (15mm)
- Remove the bottom lower front (faces to front of the car) bolt that holds the lower part of the knuckle / arm to the toe link. 18mm bolt.
- Remove the axle nut all the way. You can hold the axle still with a praybar looped into the wheel studs if needed.
- Whack the end of the axle with a DEAD BLOW hammer if needed to get it loose from the hub

TOSS The axle nuts - new ones are $3-4 a the dealer and come with loctite already applied

- Use a pray bar to get the top of the vertical link (you already removed the bolt) so the knuckle can clear and pull down / out on the knuckle while working the axle so you can get it out of the hub - it will come out - don't force it.

- Repeat all this on the other side

- Remove the exhaust. Stock comes out as one assembly, if you have a catback etc remove it. You'll need the h-pipe gone for easy driveshaft access

- Put the car in GEAR and crawl under and remove the 3 driveshaft to pinion bolts you can get too - 10mm - and I used a gear wrench with a hollow pipe on it for leverage - they are only torqued to 41 ft/lbs supposedly but thread locker AND Sealer make them fun to get out
- Put the car in neutral, move the driveshaft by hand so you can reach the other 3 bolts, put it back in gear and loosen remove the last 3 bolts.
- Rig up HEAVY zip ties / bungie or use a flipped over 5 gallon bucket to hold the end up the driveshaft so it doesn't fall. Push in a bit and it will disengage from the rear end pinion

NOTE THE ORIENTATION of your driveshaft. Mine had a factory yellow x / mark and it lined up with an indent on the pinion of the rear differential. I noted that and installed the new differential lined up the same way. Might have gotten lucky, but I seem to have NO vibration even at higher speeds

- Use a pry bar to pry against the differential to pop the axles loose. They will come out. There is a circlip that holds them in the rear end.

I didn't even remove my axles all the way, I fed them out until the rear differential had room to clear and left them propped on the suspension / top of the rotor / knuckle

- At the top of the differential remove the vent tube. It just a hose elbow that slips on a pipe nipple, but it can fight you

- Remove the lower pipe plug from the differential cover and drain the rear into a drain pan.

- Use a floor jack under the differential to hold it and take a bit of pressure off the (4) mounting bolts. They are 18mm - two towards front two towards rear.

- Remove the (4) bolts (the rear is heavy and awkward be ready). And lower the jack to remove the differential

- Install is literally the reverse.

- When seating the axles they will clunk / snap into place (when the clip engages) lube the ends with clear gear oil and hold them from outside the rotor area with two hands. Don't be afraid to push with some pressure.

- Loosely install all the suspension bolts you loosened until you are sure they are all in there. Don't be surprised if you have to jack up the rotor / knuckle with the floor jack to get it all lined up

- Reinstall caliper, parking brake bracket, ABS sensor etc.

Install the new axle nuts snug - I wait until last to torque them.
- Make sure the suspension is loaded (jacking it up from the lower arm) before torquing all suspension bolts to spec. If I remember correctly the lower front (Facing to front of car) knuckle / arm to toe link is 129 ft/lbs. The upper bolt is 79 ft/lbs as is the TOP Vertical link bolt
- Rear end mounting bolts are 129 ft/lbs

- Fill the differential (don't forget Ford Friction modifier)
- Don't forget the vent tube / hose

- Clean the threads / replace the (6) driveshaft to pinion bolts. Reinstall into the the two retaining clips (3 bolts per clip). Use blue loctite. Car in GEAR when installing 3, neural move driveshaft with hand to line up last 3, then back in gear.

- Torque to 41 ft/lbs in star pattern (like lug nuts) I had my son in the car for this step. Told him NEUTRAL, torqued one, had him shift to neutral, spun shaft, in gear, torque bolt - rinse repeat until all our torqued. Keep your helper in the car for the next step
- Tell them to hold the brake
- Tighten axle nut to 98 ft/lbs then add 45 degrees - Repeat on the other axle
- Install exhaust
- Install rear wheels
- Get off stands
As for cost / benefit....

You can find 3.55 complete pumpkins for $300-500. If you have to pay to change gears, you need gears $200-300 Install kit $150 - plus labor.

I'm thinking about doing 4.09 gears in my '19 GT 6 speed. Local Mustang shop will charge me $250 labor if I bring him the pumpkin, the Ford Motorsports gears and install kit.

So on a 19 6speed going to a 3:73 from a 3:55 no tune or program needed ?

 

[ctandc72]

So on a 19 6speed going to a 3:73 from a 3:55 no tune or program needed ?

No programming difference - but why 3.73? I have a '19 6 speed GT and I plan on going to 4.09. Math wise it's perfect and I have talked to several people who have said it was hands down the best $ for $ mod they have done.

 

[rtrdude]

No programming difference - but why 3.73? I have a '19 6 speed GT and I plan on going to 4.09. Math wise it's perfect and I have talked to several people who have said it was hands down the best $ for $ mod they have done.

I have a roush supercharger and I think with the gearing of the 19 and a 3:73 will be perfect

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