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I have a 4" shaft for my 91 Cougar, only cause I plan on ridiculous speeds with that car when it finally gets done. Fitting it without rubbing required poly mounts for the rear diff, trans, and engine to prevent movement, and removing the metal shield on the fuel tank in the driveshaft area. I wouldn't advise going to 4" unless you are hitting critical speed of the 3.5" shaft, which would be in the 140mph range with 3.73 gears.
 

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What is the critical speed of the stock shaft with 3:73's?
106mph. It would be the same as stock regardless of gearing. The crimped stock two-piece is a consumer grade, get Grandma to church on Sunday, POS. I wouldn't push it to hard.

I have a 4" shaft for my 91 Cougar, only cause I plan on ridiculous speeds with that car when it finally gets done. Fitting it without rubbing required poly mounts for the rear diff, trans, and engine to prevent movement, and removing the metal shield on the fuel tank in the driveshaft area. I wouldn't advise going to 4" unless you are hitting critical speed of the 3.5" shaft, which would be in the 140mph range with 3.73 gears.
Thanks Mike. Yeah, I remember someone cutting their 4" in half by not removing the metal shield on the fuel tank.
 

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I have a 4" shaft for my 91 Cougar, only cause I plan on ridiculous speeds with that car when it finally gets done.
Why settle for ludicrous speed, when you can go plaid? :)
 

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I'm aware of the clearance issues with the 4 inch shaft. I'll definitely be taking my time to make sure it's not rubbing/contacting. I wanted to run the 3.5 inch but dynotech did not reccomend it for my build.
I have 4.10 gears, will be installing a built 4v that should make about 650rwhp and turning 7000+rpms The car will spend a healthy amount of time on the track. Steve at dynotech highly recommended the 4 inch shaft.

For a milder combo not seeing hard track use, I'm sure the 3.5 is the best solution. I would have preferred to stay with the 3.5 to prevent issues.
 

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In that case, yes the 4" shaft is appropriate.

What is the critical speed of the stock shaft with 3:73's?
It isn't that easy to calculate. If it were a regular steel driveshaft, it would probably be about 125mph, but with the crimped together 2-piece design, that adds weight in the middle of the shaft, which is going to significantly lower the critical speed, and even then, it could be lower still if the crimps have managed to loosen up over the last 25 years. So basically if your car starts vibrating at speed, let off, or it will break ****! Ask me how I know!
 

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106mph. It would be the same as stock regardless of gearing. The crimped stock two-piece is a consumer grade, get Grandma to church on Sunday, POS. I wouldn't push it to hard.
Is this the reason why the ECU has a set speed limit of 105MPH?
 

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Is this the reason why the ECU has a set speed limit of 105MPH?
Correct. One more mile per hour and the car will self destruct beginning with the driveshaft. Very scary stuff! Living on the edge! :p
 

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The shaft will spin faster at 106mph with 373s than 327s though.
25% faster I think, according to my mustang that has 373s but no corresponding speedo gear change.
So I think the critical speed would be lower than stock gearing.
 

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The shaft will spin faster at 106mph with 373s than 327s though.
25% faster I think, according to my mustang that has 373s but no corresponding speedo gear change.
So I think the critical speed would be lower than stock gearing.
Is the construction method of the driveshaft for the Mustang the same as it is on the MN12 / FN12? Two piece crimp with rubber damper inside the crimp?
 

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I have no idea, but regardless of the construction, it would still spin faster with 373s than 327s to achieve the same vehicle speed.
 

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The shaft will spin faster at 106mph with 373s than 327s though.
25% faster I think, according to my mustang that has 373s but no corresponding speedo gear change.
So I think the critical speed would be lower than stock gearing.
So, 105 or 106 MPH - 25% ( 25 MPH) = ~ 80 MPH Critical speed of stock DS with 3.73 gears.

As I said, consumer grade POS.
 
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The shaft will spin faster at 106mph with 373s than 327s though.
25% faster I think, according to my mustang that has 373s but no corresponding speedo gear change.
So I think the critical speed would be lower than stock gearing.
ACTUALLY more like 14%.

Math says: 373/327 = approx. 1.1406 .

So the critical speed becomes 106*327/373, or approx. 93 MPH.

RwP
 

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So, 105 or 106 MPH - 25% ( 25 MPH) = ~ 80 MPH Critical speed of stock DS with 3.73 gears.

As I said, consumer grade POS.
All this DS talk has me curious.

In my build I'm going with the S197 TR-3650, so I needed to get a custom single piece aluminium 3.5" drive shaft. I'm also doing 3:73 gears. The gearing of this transmission is as follows:

Gear Ratio TR-3650
1st 3.380:1
2nd 2.000:1
3rd 1.320:1
4th 1.000:1
5th 0.670:1

What would be the critical speed of this driveshaft be? What engine RPM would that be at? Knowing my driving style, I'll be doing some "drive it like you stole it" driving when given the chance.
 

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I wonder if the 95 mustang gt came with 308s, because gps shows the speedo is 25% out all the way through various speeds with 373s
 

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ACTUALLY more like 14%.

Math says: 373/327 = approx. 1.1406 .

So the critical speed becomes 106*327/373, or approx. 93 MPH.

RwP
Thanks Ralph!
 

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Upon further research, my 1995 mustang gt came with 273 or 308 before the 373 were installed, so its apples to oranges I guess.

ACTUALLY more like 14%.

Math says: 373/327 = approx. 1.1406 .

So the critical speed becomes 106*327/373, or approx. 93 MPH.

RwP
Thank you.
That is plenty fast enough, 150kmh in our language.
 

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Discussion Starter · #40 ·
I am seriously considering a purchase of one of these fat sticks and I was wondering...
Who might be running one of these in an MN12 car and wants to share their experience of using it.

Who may have installed one in an MN12 car and wants to share their experience installing this part.

It's a relatively high cost part as a percentage of total build cost. So was the converter, and so was the differential, they are all about a grand before I get done fixing all my mis-steps or so it seems.

I know the 3.5" 94-98 mark shaft fits, but I also know it rubbed the back hoop when it first went in...a 3.5" stock shaft came out of that spot...and I had to shim the hoop.

It's a good deal on a used shaft but I am sure I heard people say it was an awful fitment issue. But I am not 100 percent certain they were the exact same Dynotech aluminum shaft.

It would truly suck to buy it and be unable to use it...or return it or worse. I have had bad luck in the past with driveshafts.

Considering that we are clueless and trying to go way faster than the car was designed for to begin with, I figured I should check again.
With my luck I could be chasing smoke forever trying to get NVH free with a shaft that can never work right in my car.
If you have one, please point me to your story so I can do the right thing myself. MN12 people are genuine enthusiasts and drive the heck outta their cars. Nobody is getting fat flipping Clinton era Coupes, Obama decimated the fleet and managed to drive the price down further. It ain't about the investment, it's about right now living the deal. Driving.
 
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