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Discussion Starter #1
Hi all,

I want to put 3.73's in my 99 Cobra. I've heard that the half shafts sometimes break, especially when running slicks. Do I need to upgrade to something stronger, and if so, what, and how much should I expect to have to pay?

Thanks! :)
Tai
 

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If you run slicks on your IRS with or w/o gears... Your gonna break it soone or later, probably sooner. You could beef up your IRS with a php irs brace, and cobrar or 03 halfshafts etc... Or you could just get a rear assembly from a 99+GT

Not sure on the price, someone will chime in on that who has done these things.

And why 3.7.3's? If you chose them out of the blue, get 4.30's instead...That an't a Mach1, its a Cobra, to gear it effectively, your gonna need 4.30's or more
 

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4VMENACE said:
And why 3.7.3's? If you chose them out of the blue, get 4.30's instead...That an't a Mach1, its a Cobra, to gear it effectively, your gonna need 4.30's or more
The ideal ratio depends on the application. 3.73s are ideal for autox and good for road racing. The bigger ratios are for drag racing.
 

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Discussion Starter #4
Thanks for the feedback, the 03 shafts fit into the 99 IRS without a problem? That sounds like the easiest approach for me. I already have a PHP brace on it. I'm setting the car up more for road racing than the drag strip, so I don't want to go too steep.
 

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Allright, consider the 3.90's then...I don't care what your doing, 3.7.3s shouldn't be in an n/a cobra w/o a stroker kit and cams. Theres just no pull down there.

Yes the 03 halfshafts go right in.
 

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Aren't the 01 and 03 halfshafts 31 spline while the 99 halfshaft are 28 spline???

Henry
 

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HPLouis said:
Aren't the 01 and 03 halfshafts 31 spline while the 99 halfshaft are 28 spline???

Henry
Bingo...
i'm getting 4.30's should be a good compromise between 3.73 auto-x gears and 4.56 drag gears...
i still say CV joints are worthless for IRS on the cobra...
use titanium shafts with U joints like they make for the vettes and vipers..
~J
 

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Discussion Starter #8
Okay, so it's not a straight swap. Do you have to replace the pumpkin to go up to 31 spline, or is it something simpler? Would it be easier to just order an 03 Cobra IRS assembly, assuming I could find a place that sold them?

If not, what do 99 owners generally do?

Thanks for all the help!
 

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Black99CobraMofo said:
i'm getting 4.30's should be a good compromise between 3.73 auto-x gears and 4.56 drag gears...
Nope... for autox, anything over 3.73 / 3.9 will slow you down because you'll bump the rev limiter in 2nd, or you'll have to shift to 3rd, and either way you'll lose time.

For road racing most guys use 3.73 as well because you can still hit 100 MPH in 3rd gear. But you can get away with 4.1s on the open track; the extra shifting is not going to kill your times like it does with autox.

Either way, 4.1 or higher is going to suck for autox and be sort of marginal for road racing. But they might be great for drag racing. Again, it all depends on the application.
 

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you might want to check with ford racing they make a whole new pumpkin with 3.73s not sure if 28 or 31 spline but its like 600 bucks and is ready to bolt into the car so you dont need to have the gears put in and if it breaks you still have the stock one
 
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