How are people liking 4.09 gears in the MT82?

[Superspirit]

4.09s are awesome with the mt82d4

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[wadewolf76]

The only way to make 4.09 gears play nice with '19 and up cars (rev matching downshifts, gear position sensor in the MT82) is to purchase this from Ford Performance:

https://performanceparts.ford.com/part/M-9603-M8B

This allows you to change the Ford Performance tune to 4.09 gears.

The issues with 4.09 gears (really anything lower than 3.73 gears - the lowest option from the factory) seem to be limited to '19 and up cars when it comes to the MT82 equipped cars. '19 and up got rev-matching and the transmission also got a gear position sensor.

You CAN get your factory tune modified to play nice with the 4.09 gears. You don't even have to change anything in the the tune but the settings involving 4.09 gears. Any good tuner familiar with the newer S550 should be able to do this pretty easily.

Downside - you'll have to purchase a device to upload the tune to your car, and either pay for a tune or if you have local tuner, they will adjust JUST the settings needed for gear ratio usually much cheaper than a full tune.

Or you can buy the HP Tuners software / hardware + the credits you need to modify your tune (you need 4 credits for the 18-21 Mustang)

https://www.summitracing.com/parts/hpt-m02-000-04

A tick under $500 and you could make the changes you need to run 4.09 gears. The benefit to doing this way - if in the future you decide to add modifications or get tuned (E85 etc) you only have to purchase the tune file from the tuner you choose - you can use the HPTuners device to upload the tune to your car.

How often does that ford performance parts package go on sale I wonder?

 

[Superspirit]

How often does that ford performance parts package go on sale I wonder?

Message Benny at levitown Ford for a group but discount code. Cuz it doesn't go on sale

 

[RonsterGT]

The ONLY way I could see a 4.09 being useful is the '350

I own a ‘16 GT PP1 and a ‘22 GT PP1. Both came from the factory with 3.73 Torsen diffs.
Both cars are blown with Roush blowers.
When I first had the ‘22 blown, the ‘16 was still quicker.
I had the FP 4.09 diff gear installed in the ‘22 and it was, for all intents and purposes, identical to the ‘16.
The 4.09 is useful for more than just the GT350.

 

[H@mmer]

I own a ‘16 GT PP1 and a ‘22 GT PP1. Both came from the factory with 3.73 Torsen diffs.
Both cars are blown with Roush blowers.
When I first had the ‘22 blown, the ‘16 was still quicker.
I had the FP 4.09 diff gear installed in the ‘22 and it was, for all intents and purposes, identical to the ‘16.
The 4.09 is useful for more than just the GT350.

lol. umm...4 years later?

 

[Bulldog9]

Update....I've called 3 Ford dealers over the past few weeks asking for an installation cost of the 4.09's. All of them said they'd check and let me know but none called back. I wonder if it's just bad luck or if they don't want to do this for some reason?

Also, did some calculations... 7000 rpm in my 3.73 car in 1st is 47 mph, with 4.09's it would be 43 mph. In 2nd it's 73 mph vs 66 mph, and in 3rd it's 108 mph vs 98 mph. So in terms of 0-60, you wouldn't have to add an extra shift if you went to 4.09's, but you might in the 1/4 mile. Also, at peak 420 lb-ft of torque at 4600 rpm, the force applied to each tire patch would increase from about 2200 lbs to 2400 lbs, which would be the equivalent of increasing the peak torque from 420 to about 460. So the 4.09's seem like a pretty good mod if it's reasonably priced.

NFN, but in my experience 99% of dealers can do little more than plug and play factory parts and maintenance. You need a good custom speed shop with experience in modern Mustangs, or a highly capable indy shop.

Not sure when Ford changed the ratios in the Getrag, but my 2020 Bullitt with 3.73 gears is much longer geared than I expected. They messed with the ratios and 4th is a 1:1, so 5th & 6th are 'overdrives'. Fine for me as my dig and track days are over, but if I was racing, I'd want to go with a higher rear ratio. 4.09 seems to be common and easy bolt in.

 

[Mikepol2]

NFN, but in my experience 99% of dealers can do little more than plug and play factory parts and maintenance. You need a good custom speed shop with experience in modern Mustangs, or a highly capable indy shop.

Not sure when Ford changed the ratios in the Getrag, but my 2020 Bullitt with 3.73 gears is much longer geared than I expected. They messed with the ratios and 4th is a 1:1, so 5th & 6th are 'overdrives'. Fine for me as my dig and track days are over, but if I was racing, I'd want to go with a higher rear ratio. 4.09 seems to be common and easy bolt in.

LOL I posted that 4 years ago... ended up doing it the following July, was my favorite mod on the 2018 GT.

 

[RonsterGT]

lol. umm...4 years later?

Yeah, 4 years later...what's your point? Lol.

 

[Angrey]

After reading the resurrection, there was a lot of activity after my posts and some pretty wild discussions about big gears in the rear.

I want to caution anyone who's considering swapping rear ratios.

1) It's expensive. The ring/pinion set and the installation kit/parts (gaskets, lube, shims, crush collar, etc) are all pretty usual suspects compared to the old 8.8. However, the Super 8.8 install is much more complicated and actually requires special tools. It's not something that many shops have even done, let alone are experienced and proficient at it. You're probably going to get quoted somewhere between $1000-$2000 to do the swap. You'll fall over in your chair (as I did) but once you understand how involved it is, the prices make more sense. You're going to want to go with someone who's actually DONE the swap before (Super 8.8 IRS).

2) Changing rear ratios solves some problems but creates others. I saw discussions about 4.88. Sure it will wake up the car. It'll also add noticeable RPM hike to cruising speeds in your final ratio.

I resolved this by changing my 6th gear to a .50 ratio. Now, with 4.09 gears, I can cruise at 77mph at 2,000 rpm.

If you "ALLEGEDLY" like to cruise faster than that, a car with 4.56 (or 4.88) would be bordering on absurd. I don't have active exhaust anymore, so maybe it's more of an issue for me, but to put things into perspective, if you're the type of person that likes to cruise at 90 mph, with 4.09 gears, a typical .63 final overdrive, typical 26.4" street tires, you're talking about the car singing at 2950 (nearly 3,000 rpm). If you go with 4.56, that same 90 mph cruise is at almost 3300 rpm. If you were to go full retard and go with 4.88, that same 90mph cruising speed would be over 3500 rpm. Or put another way, in order to just drive 70 mph, with 4.56 you need to be pulling 2550 rpm.

As someone who used to wear ear muffs in my first mustang (with strange looks from people) I can tell you that a long trip at 3k rpms is miserable. You can't hardly even listen to the radio without having it full blast and it's murder on your hearing.

Before my Magnum swap, I'd have to literally slow down to about 55mph just to have a phone conversation.

The second aspect is that 4.09s were about as aggressive as I would go if you're naturally aspirated making decent sauce on Cup2s or P4S tires. The car would spin on the 2-3 shift and would waggle quite a bit on the 3-4 shift. Anything more aggressive would simply make the car spin city without significantly changing the suspension setup/grip. Boosted with gobs of torque, it would make things even more ridiculous.

I guess what I'm saying is that I wouldn't go bigger than 4.09s if you're naturally aspirated, unless you want a full on race only type car. If you're boosted, even 4.09's are too much without some sorta torque management to limit it from hazing the tires over.

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