Upgrading rear gears

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Falcon_LS

Falcon_LS

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My '01 Grand Marquis is factory equipped with a 3.08 conventional rear end, and I'm in the process of upgrading to a locker 3.73 with an aluminum driveshaft and an extended tail shaft housing.

I do have one concern, however. I have a slight flare when shifting from 3 to 4, and I reckon the it's probably my OD band. To be in the safe, instead of rebuilding the unit, I am considering a built 4R70W next year (capable of handling 500 RWHP) before my engine upgrades.

For the time being, I'm wondering if there be any issues with the transmission giving up after upgrading to a steeper rear end? Would I better off leaving this to after the transmission upgrade? Thanks!
 
Well, a higher numerically ('shorter') rear end actually unloads the trans a bit. Shorter gears mean more revs, but less load.

What usually kills the trans is the way you drive after the change!

Should make a HUGE difference.
 
Thanks very much for the input!
thumbsup2.gif


Originally Posted By: SteveSRT8
What usually kills the trans is the way you drive after the change!


Could I possibly ask what you mean by that? My car rarely sees WOT, but speeds up to 100 MPH is a daily occurrence.
 
My old Chrysler had a 727 transmission,which is a very durable unit.However,it had terrible 2.45 gearing (obviously to get economy out of the big block V8),when I bought it (at 69K) there was a noticeable slip in reverse,to the point the rear would hop up and down.When I junked the car at 95,000 miles the trans was gone...I am convinced that with more reasonable gearing (3.08/3.21) that trans would have lived a better longer life.A classmate had a 75 Fury/RoadRunner with the same drivetrain,but with 3.21 gearing,at 90K miles there were no trans problems at all,and the car could easily chirp the tires,forward or reverse.
 
Originally Posted By: Falcon_LS
Thanks very much for the input!
thumbsup2.gif


Originally Posted By: SteveSRT8
What usually kills the trans is the way you drive after the change!


Could I possibly ask what you mean by that? My car rarely sees WOT, but speeds up to 100 MPH is a daily occurrence.


If you are seeing 100MPH on a daily basis, you may want to keep the gears you have..
 
Originally Posted By: Bluestream
Originally Posted By: Falcon_LS
Thanks very much for the input!
thumbsup2.gif


Originally Posted By: SteveSRT8
What usually kills the trans is the way you drive after the change!


Could I possibly ask what you mean by that? My car rarely sees WOT, but speeds up to 100 MPH is a daily occurrence.


If you are seeing 100MPH on a daily basis, you may want to keep the gears you have..



+1...you're going to see very high cruising RPM with 3.73 gears...
 
Originally Posted By: SteveSRT8
He has a true overdrive in his 4 speed.

His top end will still be excessive, and his highway rpms won't be that bad.


Maybe...

He can calculate what his new RPM at 100MPH will be if he wants. My Ford explorer has 4:11 gears, and my BMW has 4:27, and they both Rev way to high at 70MPH, and it costs me a lot in gas!!!
 
I used to play around with Crown Vics. If you see 100mph on a daily basis? how high of mph are you running? 120, 130? Can I also assume the car has had a tuning change and a top speed increase?

The issues with the Vic is the LONG drive shaft length and high rpms. This is the one reason the Police Vic have there Top speeds lowered over the years to avoid this. The vibrations at high speeds will destroy the tail shaft bushing. 3.27 rear 129mph, 3.55 119 on the current cars. The older ones had 3.55 with a 129 top speed but ford lowered that also to avoid tranny damage.
 
Originally Posted By: Ojustracing
I used to play around with Crown Vics. If you see 100mph on a daily basis? how high of mph are you running? 120, 130? Can I also assume the car has had a tuning change and a top speed increase?

The issues with the Vic is the LONG drive shaft length and high rpms. This is the one reason the Police Vic have there Top speeds lowered over the years to avoid this. The vibrations at high speeds will destroy the tail shaft bushing. 3.27 rear 129mph, 3.55 119 on the current cars. The older ones had 3.55 with a 129 top speed but ford lowered that also to avoid tranny damage.

He is getting a new drive shaft built, so it could be made and balanced more precisely than an OE ford driveshaft.
 
Thanks for all the input.
thumbsup2.gif
As far as the RPMs go, the vehicle tops out at 205 km/h (128 MPH), verified by a GPS unit. With the 3.08 rear end, this is about 3,613 rpm. With a 3.73 rear end, the engine will see 4,387 rpm at 128 MPH. Not too much of an increase.

That said, it rarely sees anything that high. I have a bit of a drive to work on an open road, which is why I hit 100 MPH where permissible. But with the stock suspension setup, even that is pushing it.

Originally Posted By: Ojustracing
Can I also assume the car has had a tuning change and a top speed increase?


I haven't done any tuning yet. But I will be doing quite a few upgrades, one of which involves stage 2 heads and camshafts along with a couple of other bolt ons, and that is when a tune will be required.

The car is an export model though, which is why it has a 3.08 rear end and a different tune from Ford (the US models top out at 106 MPH IIRC). Starting 1998, US models started to come with a 2.73 rear end, unless the optional Handling and Performance Package was chosen. Most of the HPP models came with a 3.27 rear end, although I believe at one point 3.55 was available. This is of course for civilian models. Having said that, there are quite a few people I know running 3.55s with the steel driveshaft.
 
Originally Posted By: SteveSRT8
Most people want to flog on it after a gearing change, as the run through the gears is a whole lot more fun!


Thanks for clarifying that!
laugh.gif
 
They only had a 129 limiter with 3.55 gears for one year, 2000. That year Ford had Alcoa cook them an Aluminum Metal Matrix driveshaft. They did this because they pulled some power out of the tune going from the 1999 model year and needed to make it up somewhere, so ALL 2000 CVPIs got 3.55 gears. 1999 PCMs are known to be really aggressive and ping in moderate temps on 87 while the 2000s are dogs because they went all the way the opposite direction. Then they got the PI Heads/Cams/Intake in '01 and they went back to offering 3.27 and 3.55 with the lower limiter for 3.55 since they went back to a standard alum shaft.
Here is something I found and saved.
Quote:

WARNING: You may want to write this down, and I want to be clear in noting these are critical speeds,
and a 10% safety margin should be deducted.

- These speeds calculate the critical speed for the transmission as well, this is based upon the
4R70 transmission with the tail housing in a CVPI. *If you do not have the extended tail housing
these calculations will not work and will exceed your critical operating parameters and your
transmission and driveshaft will explode. So again if you do not have the shorter police length
shafts these will not work for you!!

Maximum Critical Speed (225/60/16)
--------------------------------------
CVPI Aluminum Driveshaft 4.10 = 116mph
CVPI Aluminum Driveshaft 3.55 = 133mph
CVPI Aluminum Driveshaft 3.27 = 145mph
CVPI Aluminum Driveshaft 3.08 = 154mph
CVPI Metal Matrix Driveshaft 4.10 = 133mph
CVPI Metal Matrix Driveshaft 3.55 = 154mph
CVPI Metal Matrix Driveshaft 3.27 = 167mph
CVPI Metal Matrix Driveshaft 3.08 = 177mph
 
Last edited:
For some reason this person didnt do 3.73 calculation. Critical speed with regular Alum shaft is 127MPH with a 'safe' limit of 114MPH.
What you need is the MMX shaft. Critical speed with that is 146MPH with a 'safe' speed of 131MPH or just over you factory limiter.
 
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