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Discussion Starter #1
I have been doing some big time research into the 3.73 swap. From the looks of it, to get the best setup possible you need:

3.73s with all other hardware
Full rebuild kit
Driveshaft 3.5?
Torque Converter

I am already bumping around the converter by shifting over 5500RPMs with my tune. SCT's standard tune has the bird shifting at 6500RPM, which I know is too high for the converter. It also sounds like you need a Driveshaft, otherwise you get the vibration at highway speeds.

Money is not the issue for me and I realize its a pay to play kind of thing, but could I use 3.55s and keep my driveshaft and TC as is? I run 3.08s now, even 3.55s would be better then what I have now. It would be nice to shave .5 of my 1/4, but maybe the 3.55 wouldn't be enough to do that.

What do you think?
 

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you could probably keep your stock hardware with the 3.55's but it wouldn't be as much fun :) ideally with your car (i have a 94) the transmission should be swapped if you have any kind of miles. there were upgrades in 96,99,and 2001 for the transmissions with each series being better. I have a rebuilt 2000 mustang tranny in mine with a high stall ford torque converter, i'm running an aluminum 1 piece driveshaft, 4.11 gears and posi with 275/60/r15 rear balognes. It is fun even with a mostly stock motor.
 

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1994 Cougar XR7 DOHC/5-Speed
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If your shifting at those rpm's already, you need a converter now, gears won't make a difference because the engine is spinning it too fast as is. The driveshaft won't be too much of an issue even with 3.73's.

If your already buying gears go 3.73, 3.55's are a nice change from 3.08's but are a waste.
 

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Discussion Starter #4
I suppose I can modify the tune to shift at 5000K. I was just watching Trunk Money's videos (http://forums.tccoa.com/showthread.php?t=113203) and he was shifting in about the same place I am shifting at right now.

I was just trying to avoid opening a can of worms with the driveshaft. If they vibrate at 70MPH as some of the others have experienced, that would drive me crazy. SCP is saying that the Driveshafts in the 3.5 size isn't for the track, but how in the heck are you supposed to get the 4.0 size in? It seems that a lot of people have opinions on the whole driveshaft thing.

I want all of the info before I start buying parts, that is why I am asking the N00b questions. I have been searching the heck out of this place the last couple of days.
 

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1994 Cougar XR7 DOHC/5-Speed
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I get driveshaft vibrations at 70 with the 3.55's (granted my toasted u joints arent helping) so if thats your worry with 3.73's, 3.55's are just as bad.

I honestly can't think of a single person using the 4" driveshaft, theres plenty of real quick mn12's out there running the MMX 3.5 or even 93 mark shaft
 

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I run the 4inch SCP shaft. it was a tight fit but a have no rubbing issue's at all. As for the gear swap you might as well go with 3.73's and do the converter at the same time.
 

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Discussion Starter #7
Check out this thread from 2002:
when a driveshaft is spun faster than it's balenced rpm, will start to bend. this is a VERY small bend, but at 6000rpms, it becomes enough to make the entire car vibrate, or even break drivetrain componets.

the stock drive shaft is only good till 5500rpms. to make sure it wouldn't fail, ford decieded to limit the car below this speed..at ~4300rpms or 105mph. If you are keeping the stock driveshaft and getting 4:10 gears that would be 84mph!
Thread found here: http://forums.tccoa.com/showthread.php?t=1933

The poster above had a good argument. My friend and I were doing some math and in almost all the cases we talked about (60MPH, 90MPH and 100MPH), we are talking about less then 800RPMs between a 3.08 and a 3.73. The higher the speed, the more the RPMs went up of course. Its all really interesting stuff. With 3.55s, I would imagine that the difference would be even lower.

One question, since the driveshaft is turning at higher RPMs, couldn't one just simply re-balance the stock unit?

Just trying to get some info. I really appreciate the help! The search really helps, but you can't just type in 3.73 into the search box, its too short. :)
 
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I would recommend 3.73s, I had them and they were a nice change from the 3.08s. They were also very very comfortable for street driving. As for your shift points, a stock 95 4.6 is just making noise past approx. 4700 rpm anyway. My 4v is set to shift at that rpm.

Some people seem to have luck with the 93 mark shaft, others do not. Mine was fine with the 3.73s. The stocker was tolerable with 3.73s, but far from perfect. I haven't driven past 70 mph with the 4.30s yet though, I don't expect good results.

Balancing a stock two piece shaft probably won't help. The biggest factors are its length, diameter, and rigidity. It is like a jump rope flailing around under your car. At its next harmonic (resonant frequency) it is like two jump ropes with a node in the middle.
 
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Discussion Starter #10
How was the 3.73s past 80MPH or so? I want to at least finish a 1/4 without vibrations
 

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I have 3.73's and would never go back. Modular motors have minimal cubic inches and torque. They need RPM to run best. I see fuel economy gains around town that offset the minor loss on the highway. Keep it at the speed limit and you'll brag about your economy. Try to run 90 and you'll guzzle fuel.
Alan
 

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Discussion Starter #12
Thanks Alan. I keep going to your site wishing I could swing one of those torque converters. :)
 

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I have 3.73's and would never go back. Modular motors have minimal cubic inches and torque. They need RPM to run best. I see fuel economy gains around town that offset the minor loss on the highway. Keep it at the speed limit and you'll brag about your economy. Try to run 90 and you'll guzzle fuel.
Alan
X2 My around town average is close to 24 MPG in the city it used to be 21.5ish with 3.27's. even on the highway it's around 27 MPG drivin 70 ish which is slow and everyone's passin me lol I don't mind I still get where I'm goin. For me personally I expected too much out of just adding gears I was glad I grabbed a converter to get the beast off the line for sure.
 
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