4.09 Gears in 2019+ M6 questions

WildHorse

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tire size and gear ratio
I found it really doesn't really do anything. I had to change it all in the BCM. I changed the tire size/gear ratio in my device and it threw U???? (can't remember now). Once changed in the BCM (with FORscan), it was right as rain.

 
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Vlad Soare

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You need to find another tuner. From 19 on the gear ratio must be changed in the pcm. In 18 it would learn from bcm. They changed that in 19. Several threads on here with horror stories because they didn't get the proper tune. One guy had to have Lund redo his tune 3 times.
There's an option in the PCM, whether to use the PCM-stored ratio, or to learn it from the BCM. My 2020 is set from the factory to learn from the BCM. The same goes for the tire size.
Here's how my factory configuration looks:

Untitled.png


And here are the possible choices:

Untitled1.png


Incidentally, the value stored in my PCM is wrong. My actual final drive ratio is 3.55, not 3.31. Apparently, since they had set it to learn from the BCM, they thought that was enough and couldn't be bothered to correct it in the PCM as well. :cwl:
 
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NeverSatisfied

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Based on this, I wonder if I could at least get away with a 3.7 torsen in my ‘21 base Gt?
 

mangosmoothie

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Does anyone know what it would take to get the mach1 tremec working in the 19+ MT82 cars? I find the gear spacing on the MT82 D4 to be the problem, not the rear end ratio. The tremec lands about 300 rpm higher in each gear at redline if my math is right. Which is not insignificant, esp with a centri blower. 5000 vs 5300 is like 50 whp on my setup.

The clutch assembly is the same between both cars so it should just bolt on. Mach 1 has same rev match and what not so I'm guessing the connectors are the same and it'll plug in. But what would need changed in the tune and is it even possible to change that?
 

Jaymar

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There's an option in the PCM, whether to use the PCM-stored ratio, or to learn it from the BCM. My 2020 is set from the factory to learn from the BCM. The same goes for the tire size.
Here's how my factory configuration looks:

Untitled.png


And here are the possible options:

Untitled1.png


Incidentally, the value stored in my PCM is wrong. My actual final drive ratio is 3.55, not 3.31. Apparently, since they had set it to learn from the BCM, they thought that was enough and couldn't be bothered to correct it in the PCM as well. :cwl:
Do you know if the max and min shown are still enforced even when set to learn from BCM? That could be the very problem that others have referred to.
 


Vlad Soare

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I don't know, but I think it's very likely that they are enforced. Actually, I suspect they are there precisely for this purpose, namely to check whether the BCM-stored value is plausible, and to raise a DTC when it isn't. Otherwise there would be no point for them to exist.
 

WildHorse

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I don't know, but I think it's very likely that they are enforced. Actually, I suspect they are there precisely for this purpose, namely to check whether the BCM-stored value is plausible, and to raise a DTC when it isn't. Otherwise there would be no point for them to exist.
BCM stores the actual ratio/tire size. PCM expects it to fall within a certain range. If the PCM goes outside that range (gear ratio + tire size) , you get communication error(s).
 

ZeroTX

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That's with the cold air that comes with the kit. And factory traction loc diff. My wife has a pp1 with torsen and we both hate it! Would never own a torsen diff myself I much prefer the Trac loc diff. Opinions vary but that's mine.
Can you detail what are the differences and why you hate the torsen? I had a torsen in my 4th gen Camaro Z28 and never had an issue w/ it, but I wasn't really into performance driving, just street.
 

NeverSatisfied

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Can you detail what are the differences and why you hate the torsen? I had a torsen in my 4th gen Camaro Z28 and never had an issue w/ it, but I wasn't really into performance driving, just street.
having driven both heavily on the street and track, I would say torsen hands down. It is definitely a little clunky when ice cold on the street, but absolutely launches the car on corner exit. Trac loc has a limited shelf life when fighting to turn a couple 200tw 305’s on a regular basis.
 

ZeroTX

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having driven both heavily on the street and track, I would say torsen hands down. It is definitely a little clunky when ice cold on the street, but absolutely launches the car on corner exit. Trac loc has a limited shelf life when fighting to turn a couple 200tw 305’s on a regular basis.
Ok. For a guy like me who will never be 'tracking' the car? ... I just want to make two black lines when I burn out, haha.
 

NeverSatisfied

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BCM stores the actual ratio/tire size. PCM expects it to fall within a certain range. If the PCM goes outside that range (gear ratio + tire size) , you get communication error(s).
I just wonder if there is enough wiggle room between tire size and rear ratio in the bcm to still fall within learning range on the pcm?
 

NeverSatisfied

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Ok. For a guy like me who will never be 'tracking' the car? ... I just want to make two black lines when I burn out, haha.
Lol you can lay black stripes with either. torsen will be a little finicky when you pull out of your driveway but become silent. Trac loc is always silent but may have a limited number of 2 black line strips it can lay.
 

smoke_wagon_6g

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I see you have a 2018 but this info is flat 100% wrong for the 2019+ manual cars with rev match. For those cars, a tune IS REQUIRED.

This seems like an important clarification for anyone who might be searching this thread in the future.
I'll just take that post out actually or it'll confuse.

I just can't keep up, even within one generation so many changes happen from year to year. I had some arguments with a 2017 owner when I explained my 2018 had larger engine displacement, different transmission ratios, direct injection and so on.

Seemed like too much to believe so I had to be wrong.
 

ZeroTX

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Seemed like too much to believe so I had to be wrong.
Except that all of those things are true :) Which is why I find it a silly when I see a Boss 302 intake in a Gen3 Coyote. It ain't a 302!
 

wa9njm

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For anyone who's done the 4.09's in a 18+ car how bad of a mileage hit did you take?

I already get worse mileage in the Non-pp '21 than I did in the PP '16 I got rid of, with current gas prices I don't want it to get too much worse. But the ridiculous gear ratios are killing me.
 

 
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