2019-up 4.09 Rear Gear Swap

K4fxd

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Scooter MGee

Scooter MGee

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Car is gone.......moot point.
 

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Sweetface

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Do you guys with 4.09 gears think a 4.88 would have been a better choice to net You the results that you were hoping for?
Don't hold me to this , but i think i read somewhere that with the difference in transmission gearing that a 2018 up with 4.56s is comparable with the 2015 - 2017 with 3.73s
 

Sweetface

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Yah, I've had Kooks LT's and the JLT and I wasn't impressed either way. That was with an A10. Im sure it pulled harder and made more up top but I dont spend alot of time up there and it "seemed" to have a more negative impact on lower RPM response/torque. I'm talking under 3k here.

I wouldn't undo the gears either.

I think we have similar set-ups . LTHs , E85 and 4.09s. all together make for a really fun N/A steet car.
 


Tucker80

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Scooter MGee - Would you recommend the shops you mentioned in your post? I'm in Cincinnati and looking for a shop recommendation to get the gears swapped in my 2021 M6
 

Angrey

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Few things.

1) The Super 8.8 is different than the 8.8. I'm assuming you've asked the shop that's doing it if they've done it before and have the special tools necessary? It's more complicated than the 8.8 swap (hence the additional costs)

2) Which ring/pinion set are you going with? I was one of the first to do the swap out. US Gear makes the gear sets for FRPP. They have a couple different options which I "THINK" they've incorporated into their FRPP offerings at this point. I had to buy a set of FRPP and send them off to Axletech (division of US Gear) to get their isotropic polishing/finish. I think the FRPP sets now come standard with that finish, but you should check/verify. The lightning finish reduces break in time and supposedly makes the gears run cooler.

US Gear sold the "rights" for their 4.09 to FRPP (meaning they can't sell them to you directly, have to get them from FRPP). They sell a 4.10. While the multiplication is virtually the same, the teeth ratio is different (I think one is like a 9 on 45 while the other is an 11 on 39 or something like that).

In discussions with US Gear, they indicated that while bigger teeth are stronger, smaller teeth have more "mesh" and contact with incoming and outgoing teeth so they get a more contribution so they indicated they would both be sufficiently strong. They also indicated that with proper installation, neither set should whine any more than the other.

3) If you're looking to replicate the ratios of the old MT and a 3.73, then a 4.56 would be closer (as posted in the gear analysis above).

4) Be cognizant of the idea that if you ever go boosted, lower multiplication is going to help with driveability and traction. So while a 4.56 might be the right gear to pep the car up N/A, it will probably be a handful not to roll the tires in every gear once you add the torque and power of a turbo/supercharger.
 

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4) Be cognizant of the idea that if you ever go boosted, lower multiplication is going to help with driveability and traction. So while a 4.56 might be the right gear to pep the car up N/A, it will probably be a handful not to roll the tires in every gear once you add the torque and power of a turbo/supercharger.

Absolutely. Not to mention -- even on an N/A car, it is not actually faster with deeper gears. It will feel faster, but it is not actually faster.

As a matter of fact, the car will probably actually dyno lower numbers and trap speeds in there 1/4 will likely be the same or slightly slower.
 

young at heart

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Absolutely. Not to mention -- even on an N/A car, it is not actually faster with deeper gears. It will feel faster, but it is not actually faster.

As a matter of fact, the car will probably actually dyno lower numbers and trap speeds in there 1/4 will likely be the same or slightly slower.
Really?

If this is true we sure wasted a lot of $$ on 4.56s and 4.88s back in the day. And they were also quicker than 3.55s In the 1/4. Maybe we’re talking about the difference between quicker and faster?
 

Cobra Jet

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I think some of the misconception with the "not impressed" folks is this:

Just installing a rear gear set isn't the only part in the overall equation. One HAS to consider their transmission (auto or manual) internal gearing to make a proper choice as to which rear gear sets would be beneficial. Not just blindly throwing in a gear set and expecting a drastic change in "pulls".

As was stated above too - one also has to determine how the car will be used now and in the future. Meaning, if N/A is the now, but the soon to be future sees forced induction, the rear gear choice AND transmission gears play a bigger role for final results.

Going from a 3.55 to a 3.73 is hardly any negligible difference, the same is true going from a 3.55 to a 4.09 as some have illustrated before me - and that reason can be attributed to the transmission type AND the associated internal gearing found in Owner A vs Owner B vehicles.

I had posted this below thread a while back that listed the internal gearing for the 10R80 found in the then new 2021 Mach 1. Also see the hyperlink within this thread that contains the other charts for the 10R80 2018-2020 internal gear ratio data:
https://www.mustang6g.com/forums/th...1st-time-10r80-gear-ratios-are-listed.159312/

I never mapped anything for the 6R80, MT82 variants or TREMEC variants - as the internal specifics in the mentioned thread(s) were related to and only for the 10R80 discussions of the time.

But those with calc skills can apply X-rear gear ratios to the 10R80 internal ratios to figure possible +/- gaps in what would be any beneficial gains between shifts and the end result.
 
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K4fxd

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6R80 1/4 mile is either 3.55 if you want to trap at redline in 3rd or 4.88 if you want to trap redline in 4th.

7800 RPM redline 27 inch tall tires.
 

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Very surprised to see guys saying with the swap to 4.09s they didn't notice any difference. On my 06 GT with the stock 3650 Tremec, 4.09s really woke that car up. hard on the throttle you had to really be ready to catch second gear because first went by so fast it would catch you off guard. It was also nice that it would pull in 5th gear for everyday driving. you didn't have to downshift if you wanted to pass somebody unless you really wanted to rip their doors off... lol.

Best mod I ever did to that car the only con that I found to it was you had to give it a lot more gas taking off in first gear to not kill it and if you were driving on any kind of wet surface, you had to start out in second as it would just spin like crazy in first even with little throttle.

The car was definitely faster and the everyday driving pleasure of the car went way up going from 3.55 to 4.09s.

I get the overall gearing in the S550 is probably a lot different but I still would think youd notice a huge difference going to 4.09s. My experience was night and day.
 

Superspirit

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Very surprised to see guys saying with the swap to 4.09s they didn't notice any difference. On my 06 GT with the stock 3650 Tremec, 4.09s really woke that car up. hard on the throttle you had to really be ready to catch second gear because first went by so fast it would catch you off guard. It was also nice that it would pull in 5th gear for everyday driving. you didn't have to downshift if you wanted to pass somebody unless you really wanted to rip their doors off... lol.

Best mod I ever did to that car the only con that I found to it was you had to give it a lot more gas taking off in first gear to not kill it and if you were driving on any kind of wet surface, you had to start out in second as it would just spin like crazy in first even with little throttle.

The car was definitely faster and the everyday driving pleasure of the car went way up going from 3.55 to 4.09s.

I get the overall gearing in the S550 is probably a lot different but I still would think youd notice a huge difference going to 4.09s. My experience was night and day.
I agree completely. that has been my experience also
 

 
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