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3.90 in a 392 Scat pack A8

72K views 167 replies 37 participants last post by  SHAWN A. 
#1 · (Edited)
:grin2:

Just completed my swap from the original 3.09's to the 3.90 rear that comes in the 6 speed. Installation was pretty straight forward. It was a direct swap from the original rear and everything just bolted in.
Straight from speedlogix. $1062 before shipping.

1. Remove exhaust. Being able to just unbolt the hangers from the car make it easier. It takes a bit of hammering to get the exhaust pipes to slide out.
2. Unbolt the 8 Torx bolts from the driveshaft
3. Remove the 4 bolts that hold the differential to the car
4. As you lower the differential the half shafts will slide out. Some may need assistance from a pry bar to get the axles to pop out.
5. Repeat in reverse order
6. Make sure to fill with gear oil (Most of mine leaked out during shipping). It has a drain and fill plug on the back that take an allen wrench to take out (I believe 8mm). I went with RP 75w-90, it holds about 1.2 quarts of oil.

The whole install took me about 5 hours total. Because my help had to leave halfway through. I drove a couple hundred miles for a break in period before really getting on it, but boy does this thing launch now. It feels like it has another 100ft/lbs off the line. It launches so hard in fact its spinning my wheels in my 305/45r18 MT ET Street r drag radials.... on the street. couple of hard launches moved one tire almost 2" on the wheel. Explains why I was getting a wobble around 100mph

Before my swap my best 0-60 time on the street was a 4.2. After was a 3.8 after just two tries. I'm sure with cooler weather and some more practice I could knock a few tenths off. I have yet to take it to the track. I want to drive a few hundred more miles before I track it.

As far as mileage and drive-ability, its not bad at all. My mileage has actually stayed about the same. As I mostly drive city with a little highway mixed in I still average about 17mpg. My Highway cruising speed went from 2000 at 80mph to 2500 at 80. It's also surprising that I don't get that differential whine like I used to. The RP may have something to do with that. In town it does shift a lot, but the 8 speed is pretty smooth in regular mode and it's not too much worse than the 3.09s. I'll have to make a few videos of normal/WOT driving. I'll for sure have track times soon as well.
 

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#2 ·
HMMMMM......drooling my brutha!! I posted up a thread about a stall and someone told me that the 3.90 would be a better option. Post up on your track runs and how you do. Due to my back and neck issues I would have to pay someone to do this. What do you think a shop would charge for the install?
 
#4 ·
Not sure. With a lift and proper jack this install would only take a couple of hours. This rear is a good 70lbs so it was a little rough getting it lined up with the rear bolts. They didn't give you much room to reach your hand in there. the rear bolts wont even come out without cutting or beating the floor of the trunk.
 
#3 ·
Very interesting! Great work and keep us posted!!!
 
#6 ·
It's got me a little worried about the stock driveshaft. Anyone know the limits of a scat pack driveshaft?
 
#8 ·
Did you have to do anything with your tuner to have the PCM/TCM compensate for the new gear ratio? I'd think shift points would be off if left alone.
 
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#13 ·
Curious to hear your thoughts on streetability a month or two from now. Also your track impressions.
1st gear must be pretty steep with the A8.
I had a 3.91 in my Yellow Jacket with the A5 and 3500 stall and it was good, although extended highway use had it taching a good bit more.
I've considered a 3.55 in my Scat Pack, but currently we're pretty limited in our ratio choices with the ZF rearend.
Another alternative is the 3.70 from the Hellcat M6, amd may consider that down the road.

Skullnuts. It took me about 3 hours on a lift to do mine in the Hellcat.
My bid who is an SRT tech can do them in 2. Cost should be around $300 labor and some fluid.

One of the nice things so far with the A8 trans is it's ability to accept different ratios with no reprogramming.
 
#14 ·
3.91 in the A8, man I couldn't even imagine that lol. The torque in first gear would be ridiculous!!

But 3.91 is a big jump from the stock gear, with how short first is in the A8 I would be a little worried about something breaking if you somehow manage to hook. I know the rear ends are beefy in the scat packs and SRT, but you're talking about almost 33% more torque going to the wheels.

And with how much torque these motors put out, even the stock gearing on the A8 is somewhat questionable. If you have a high revving 4 cylinder, I can understand the desire for such short gearing. But a v8 hemi doesn't necessarily need that with all the low end torque it puts out.

I am curious on how well a 3.91 performs in the 1/4 mile. Although it is probably a hoot to drive, I question how effective it is performance wise.
 
#25 ·
Exactly this is another thing to consider as well. It is also possible that you might end up in the middle of the powerband in a different gear, which is not as good as crossing the 1/4 at the very top of another gear.


This is partially the reason why some gear changes in the older A5s actually resulted in a slower time. You would end up crossing the 1/4 at the bottom of 4th vs. the top of 3rd with the stock rear end.
 
#23 ·
Given the overdrive gear in all modern transmissions, it makes plenty of sense to go to a 3.90 gear. Steep? Not really. The overdrive will bring the cruising revs down just fine. If you do some math you'll see the difference at 80MPH between the stock gears and the 3.90's isn't much.

There's no need to do anything to adjust for the gears in terms of the PCM....that is because the PCM doesn't care what gears are there. It takes its information from the actual axle speed sensors (and trans output shaft sensor). I learned this back when I changed gears in my old Dakota. The PCM will 'know' the gears are different...but it will adjust itself automatically and instantly.
 
#26 ·
So I just need to change the rear end from 3.09 (automatic by stock) into 3.90 , and that would do the magic. So the initial gears will be quickly hit automatically and help to get better 0-60. However in greater speeds , it will become normal . right?

Now in case I goto any dealership for other repair , will i still have my warranty?
 
#35 ·
My Scatpack A8 shifts into 5th gear at 99.5 mph and 5th gear runs up to about 130 mph with standard gears and tires. It's not too bad for the 1/8 mile since I hit 94/95 mph but for the 1/4 mile I'm hitting 118/120 mph so that's not good. Guess it would be hard to pick one rear end gear that would be good for both distances.
 
#38 ·
I race mostly 1/8th mile, but I would like to run quarter every now and then. I'm curious to see what kind of difference it makes. I know it will drop my 60ft. Just don't know until i take her back to the track. Hopefully the rain holds off this weekend.
 
#47 ·
Busted the bead down cleaned it and applied some hairspray. Did one more 0-60 on the way home and got a 3.7(Still with a little wheelspin). I haven't marked my tires again to see if they moved, but I will before I go to the track. If it doesn't rain tomorrow I will have track times.
 
#49 ·
1/8th or 1/4 Mr. North???
 
#50 ·
1/8th I'm about 2 hours from the nearest 1/4 track.
 
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