TCCoA Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
2,853 Posts
Discussion Starter #1
Right now, my tbird's governer is set at 108MPH.
Having not pushed this car to this speed, I'm curious to find out how exactly the Tbird implements the speed governer on this car?

Q: Is it a fuel cut out, is it some kind of rev limiter, or is the speed governed in some other fashion I'm not guessing here?

Q: Perhaps more important to me, is the governer assert itself gradually (i.e.: your car slows down as you approach the governed speed) or is it an abrupt change (which would be NO fun at the track if it occurs at an inopportune time)?

I know the solution (for my needs) is to replace the speedo gear as an ECU reflash is outside our 24 Hours of Lemons budget. However, before I spend any additional cash, I just wanted to get more data so we can make an informed decision as a team about our potential choice in gears.

-g
 

·
Super Moderator
Joined
·
10,910 Posts
A: Replace the driveshaft before driving fast; the limiter is to keep things from coming apart.
 

·
Registered
Joined
·
4,311 Posts
A: Replace the driveshaft before driving fast; the limiter is to keep things from coming apart.
.. yeah, like the Tires.

With a 3.27 rear end ratio, critical speed is more up to 135 mph, and vibrations may be noticed around 95 mph ( 70% of critical, first bending on a micro scale ). If you put in some 3.73's, the critical speed is much lower and I would definately recommend the different shaft. So it really depends on how fast Top speed the OP plans to go on the straights and gear ratio's.
 

·
Super Moderator
Joined
·
7,620 Posts
A: Replace the driveshaft before driving fast; the limiter is to keep things from coming apart.
The driveshaft has absolutely nothing to do with the speed limiter. The limiter is there due to the tires and only due to the tires. The driveshaft can cause a vibration at speed, which would be unpleasant and could eventually wipe out your tailshaft bushing in the trans, but it will never be a safety issue. For some reason, this myth has been propagated on these forums for years without any evidence of a single driveshaft falling apart. The speed limiter exists only due because of the tires. If the driveshaft was the reason for the speed limiter, then the SC, which has the exact same driveshaft but has H rated tires, would have the same speed limiter, but instead it has a higher limiter or none at all depending on the year and transmission.

Gunn, as for your original question, the speed limiter is accomplished with fuel cutoff, and it is pretty abrupt when it cuts in, but it will hold that top speed pretty steadily and won't cause you to lose control of the car or anything like that. Also you will have full power right up to the point when the limiter kicks in, and then it will cut out until the speed drops by 1 or 2 mph, then it will be full power again.
 

·
Super Moderator
1997 Thunderbird 4.6, 1998 Mark VIII LSC
Joined
·
9,454 Posts
Mikey, first I have to say that I do definitely agree that the limiter also must be due in part to the tires like you're saying, but I also trust what Jerry writes in his thesis about the driveshaft. He has a whole page written about driveline critical speed and the part it plays in the speed limiter. http://www.tccoa.com/articles/tranny/transmission/page17.shtml He doesn't reference broken driveshafts, moreover he references broken transmission extension housings, cases and the all too well known vibration issues. Might it be possible that the speed the stock tires are rated at has anything to do with this "driveline critical speed" he writes about?
 

·
Registered
Joined
·
4,311 Posts
Mikey, first I have to say that I do definitely agree that the limiter also must be due in part to the tires like you're saying, but I also trust what Jerry writes in his thesis about the driveshaft. He has a whole page written about driveline critical speed and the part it plays in the speed limiter. http://www.tccoa.com/articles/tranny/transmission/page17.shtml He doesn't reference broken driveshafts, moreover he references broken transmission extension housings, cases and the all too well known vibration issues. Might it be possible that the speed the stock tires are rated at has anything to do with this "driveline critical speed" he writes about?
I was under the impression that article was written for 3.73 or numerically higher rear end gear ratio's ... ( post number 5, I was just talking about this .. )
 

·
Super Moderator
Joined
·
7,620 Posts
Well Jerry was an engineer for Ford, and perhaps Ford thought the speed limiter had the added bonus of protecting the transmission from someone who drove everywhere at a steady 120+ mph, but then you would think that guy would have just bought the SC, which has the exact same driveshaft and no speed limiter. As far as a street car goes, for the occasional blast up to top speed and back down like what most people do with a street car, the driveshaft simply will not ever be an issue. I've hit 120mph in an MN12 with 4.10s and a stock driveshaft, and while the vibration and harmonics get annoying, telling people that they need to upgrade their driveshaft before they delete their speed limiter with a tune is something that has been repeated on this forum for years, and it simply is not necessary, nor will the driveshaft cause any safety issues due to speed. Driveline critical speed does exist, and the article is very accurate as far as what it is, and why it is a good idea to upgrade your driveshaft if you are going to change your rear end ratio, however the safety issue is the tires, not the driveshaft.
 

·
Registered
Joined
·
2,853 Posts
Discussion Starter #10
Well Jerry was an engineer for Ford, and perhaps Ford thought the speed limiter had the added bonus of protecting the transmission from someone who drove everywhere at a steady 120+ mph, but then you would think that guy would have just bought the SC, which has the exact same driveshaft and no speed limiter. As far as a street car goes, for the occasional blast up to top speed and back down like what most people do with a street car, the driveshaft simply will not ever be an issue. I've hit 120mph in an MN12 with 4.10s and a stock driveshaft, and while the vibration and harmonics get annoying, telling people that they need to upgrade their driveshaft before they delete their speed limiter with a tune is something that has been repeated on this forum for years, and it simply is not necessary, nor will the driveshaft cause any safety issues due to speed. Driveline critical speed does exist, and the article is very accurate as far as what it is, and why it is a good idea to upgrade your driveshaft if you are going to change your rear end ratio, however the safety issue is the tires, not the driveshaft.
Just so all of you know, I'm not some 18 yr old hoon out to endanger your wife and kids on the highway. My questions were directed because i'm building my tbird for the same application and MadMike here -- a 24 Hours of Lemons track car.

While you would care for a road car, Noise-Vibration-Harshness (NVH) is the least of my concerns. Safety is a concern as I don't want this car to encounter a weekend-ending event. It's good to know that should something break, it won't be anything critical (for me). It also helps to know that Madmikey has pushed a driveshaft well beyond the limits I will encounter with my lowly V6.

As for the tires, I've got that covered. My selected tires (Toyo Proxes 4) are rated for 95V or 1521LB and 149MPH. At 3500Lbs race-ready and 140HP (15 years ago), I'm not approaching either extreme (even if my car is front heavy).

-g
 

·
Super Moderator
Joined
·
10,910 Posts
I've wasted two stock driveshafts from 80 mph driving everyday for ~8 mos. (different cougars, both 96)

Both died with the same overall symptoms, but I spun the last one inside itself. :) Thought I'd blown the tranny, lol.

V-rated tires and an overhauled suspension are two other important things, IMHO.

YMMV.

:)
 
1 - 11 of 11 Posts
Top