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Discussion Starter #1
Just curious. What is the top speed of a 4.6 bird with only mod being a driveshaft (for safety) and a tune? Also same question with 3.73's.
 

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With stock 3.27's your looking at about 105 MPH.With a 3.73 swap you are somewhere around 92-95 MPH.
 

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Well you also need speed rated tires for it to be safe, but assuming you have those, I would say about 140 or so. The 3.73s probably wouldn't make a difference in top speed cause you wouldn't run out of gear until about 165, so if anything you might go a little faster with the 3.73s cause at any given speed you would be higher in the rpms and most likely making more hp at the same vehicle speed.
 

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95xbird said:
With stock 3.27's your looking at about 105 MPH.With a 3.73 swap you are somewhere around 92-95 MPH.
He said with a tune, so we're disregarding the stock speed limiter
 

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Discussion Starter #5
Cool. Thanks for the quick reply.
 

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95xbird said:
With stock 3.27's your looking at about 105 MPH.With a 3.73 swap you are somewhere around 92-95 MPH.
Why does this seem so low?

I have a Chrysler 300M with 3.89 gearing and it has a top speed of about 140-145 MPH (drag limited).

MadMikeyL said:
The 3.73s probably wouldn't make a difference in top speed cause you wouldn't run out of gear until about 165, so if anything you might go a little faster with the 3.73s cause at any given speed you would be higher in the rpms and most likely making more hp at the same vehicle speed.
WooHoo! Someone who "gets it!" I salute you sir! I can't tell you the number of times I have had to explain this.
 

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MadMikeyL said:
He said with a tune, so we're disregarding the stock speed limiter
I misread.
 

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130-140 sounds right. I just watched a vid on streetfire of a Crown Vic Police Intercepter that looked older (prob non pi heads) do 130mph and was still pulling.
 

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DScruggs said:
Why does this seem so low?

I have a Chrysler 300M with 3.89 gearing and it has a top speed of about 140-145 MPH (drag limited).



WooHoo! Someone who "gets it!" I salute you sir! I can't tell you the number of times I have had to explain this.
I also have had to explain that more times than I can count. What's really scary is I work as a mechanic at a dealership, and I have had to explain this to guys who have been working on cars for 10+ years.

As for why the top speed is so slow, the computer cuts off fuel at 109 because in stock trim the tires and driveshaft are not capable of sustaining above that speed. With a tune to shut off the speed limiter (or cutting the speed sensor wire to the computer on cars without the computer controlled trans) the car is actually capable of much higher speeds. Even the weak 3.8 will do about 125 with the speed limiter disabled.
 

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I understand the tire issue but what is the driveshaft issue and is it somewhat peculiar to the MN12?
 

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its simply not meant for high-RPMs and will start to sling around (literally).....its first order resonance occurs slightly above the cut-off speed.....in case you are not familiar with undamped oscillating systems, if left to resonate for too long, the amplitude of the vibration may cause self-destruction because it is not being controlled....this is because of the particular combination of material type, mass, length, and diameter of the driveshaft
 

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guitar maestro said:
its simply not meant for high-RPMs and will start to sling around (literally).....its first order resonance occurs slightly above the cut-off speed.....in case you are not familiar with undamped oscillating systems, if left to resonate for too long, the amplitude of the vibration may cause self-destruction because it is not being controlled....this is because of the particular combination of material type, mass, length, and diameter of the driveshaft
So what he means is don't go that fast with a stock driveshaft. Get yourself a nice one piece unit like a Dynotech or something along those lines w/ a high-speed balance and some decent u-joints and have fun.

Jay
 

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A stock engine with a tune, 3.27 rear end and composite driveshaft will hit the brickwall we call 'air resistance' at about 155. It'll hold it, but just won't pull more...

Trust me. ;)

One thing often overlooked when going fast, is slowing down. Slowing from about 130 to 70(difference of 60mph) is a WHOLE lot harder than 60 to a stop. If you're not running an enhanced braking system, you're asking for potential trouble.

Peter M
 

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At one point in time I had my 96 marquis up to 132..... at which point the motor refused to pull any more. I had the pcm from a pi. I only did it once and never plan to go that fast again. The 2.73 gears really made highway cruising nice.

I would have to imagine the bird would be able to do the same.
 

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Oh yeh... I had a mmx drive shaft in there too... making things a lil safer.
 

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Discussion Starter #16
Peter M said:
One thing often overlooked when going fast, is slowing down. Slowing from about 130 to 70(difference of 60mph) is a WHOLE lot harder than 60 to a stop. If you're not running an enhanced braking system, you're asking for potential trouble. Peter M
A very good point... And exactly why I'm already running 13" duel piston pbrs and cobra rears.
 

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about the driveshaft issue....I had mine shortened, and rebalanced....it has seen 130+ many times now.
No issues.
 

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I'm surprised that no one has mentioned torque converter balooning. The TC was another reason for a speed limiter.

So... So far we narrowed it down to:

Tires
Driveshaft
Brakes
TC
How about some LEGAL stretch of pavement? LOL
 

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95LX said:
I'm surprised that no one has mentioned torque converter balooning. The TC was another reason for a speed limiter.
As has been mentioned many times before, the ballooning of the TC is rpm-based, not speed-based (for it to balloon, you would have to spin the engine higher than the rev limiter will allow).

-Rod
 
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