1993 GT Mustang Need Help With Tire Pressure And Launching
I ran Nitto Drag Radials last year with my old set-up, basically stock 5.0 with 5 speed T-5. Launched at 3500, tire pressure 18 psi, 1.8 to 1.9 60' times.
This past winter, I had Ford Racing GT-40 b-303 cam motor installed with a new King Cobra clutch, and a T-5z transmission.
I purchased Hoosier Quick Time Pros (similiar to MT ET Streets) 15x9.5 . I've never run a tire like this before. I called Hoosier, they reccomended that I start at 14psi and go from there.
I started at 14 and they dead hooked. The car actually bogged. I launch at 4000 rpms. I let the clutch out as quick as I can while mashing the throttle. I've been moving the pressure up, trying to get a little wheel spin to eliminate the bogging. I got as high as 24psi and they stiil dead hooked and the car bogged. This past weekend I went up to 25psi and the tires spun like crazy.
I went down to 23psi and I blew the tires off again. I went down to 20psi and got a little spin in the first round of elimination. I left the tires at 20 psi and the next round I dead hooked and bogged.
I 'm trying to let the clutch out the same each time , but apparently I'm not. It's getting hard to stay consistant.
Should I be letting the clutch out quick , or trying to let it out slower so that it slips a little? I'd like to get rid of the bogging, It's making my 60 foot time times at 2.00. Where should I start as far as launch rpm, tire pressure, and clutch technique?
Any help would be greatly appreciated.
Thanks
Below is my best run and my car set-up.
Since you found that 20 psi can give a decent hook, I'd leave it there for now and concentrate on the clutch technique. I'd try about 200 rpm higher on launch and slip the clutch quick. If it bogs, slip it quicker or outright dump it, since you have slicks. You have 31 splines and an Eaton so it should take the launch. You might look into a 2 step so that you can have the exact same rpm at launch and not worry about that element. If it still bogs, raise the rpm again by another 100 rpm. I have friends that have similiar combos such as yours and they launch as high as 5000-5200 rpm and it took some work to find that balance.
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I think I've figured out the problem.
Correct me if I'm wrong.
The 1st gear ratio in my old T-5 was 3.35. (Rear gears 3.73's)
The 1st gear ratio in my new T-5z is 2.95. (Rear gears 3.73's)
I've dropped my 1st gear ratio by .40 with the new T-5z.
If I put 4.10 gears in the rear , I could get back .37 of the .40 that I lost when switching transmissions. I think that may bring my launches back the way they where. The 3.35 with the 3.73's really worked well with my car.
Does that sound right?!
I really don't think my motor has the power to get the car out of the hole running slicks with the 2.95 1st gear anyways. I know I could beat the crap out of it and launch at 5500 rpms, but I'd rather change the gear and save my driveline. I'm at the track every weekend, not just once and a while. So I'm putting a lot of passes on the car and I want it to last. I'm bracket racing , so I'm not seeing how fast I can go. I've done that in test and tune. I'm more concerned with having a consistant car. I've launched at 5200 rpms and it still bogs when it hooks.
I'm even thinking of going in the reverse direction with before changing anything.
3500rpm launch and tire pressure 25 psi.
Maybe I can get the car to rev up quicker after the launch with just a little wheel spin.
Any comments would be great!
Thanks
lots of it has to do with track prep and the heat of the track. imo. i usually wait awhile and let some cars go down the track and make sure its warm. if your the first one out there i think you would have wheel spin. also it depends how much compound they put down on the track.
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My car is very similar to yours in power and set up
I launch on MT slicks at anywhere between 4200rpm's and 5000rpms. No bog - lift both tires about 1".
I use 4.10's out back, have 3.35 first gear.(13.74/1 ratio in first gear)
Your 2.95 on 3.73 is only 11/1 ratio in first
You would need a 4.56 rear to match me with that tranny.
One of my friends replied when I asked him about doing the 2.97 or 2.95 ratio(world class ratio) and his reply was that if I wanted to have my car fall on it's face - then that would be a good recipe for achieving that.
That being said - if you go to 4.56's then you will beat the 3.35 transmission. That first gear ratio set-up makes ya or kills ya..
13.74/1 is quite violent(launches hard) but I would go for more like 14.4/1(4.30's) So plan according to what you want to achieve. With the 3.35 transmission you rev less on the hwy as .63 x 4.1 is less than .63 x 4.56's. It depends what you want..ther's always a bit of trade-offs. Good luck
My computer and fuel pump relay went 2 weeks ago.
With a new computer and fuel pump relay my car is running consistant 13.3's, best 13.2.
Now launching at 3800-4000 rpms, tires at 20psi.
1.8 60' times, consistant hooking.
I think the computer that was on it's way out had a lot to do with the car being so inconsistant.
When the time comes, I'm going for 4.10's rears.
I really like the way the T-5z's ratio from 1st gear 2.95 to 2nd gear 1.94 just keeps going. More of an even transistion from gear to gear. With my old T-5's ratio, 1st 3.35 to 2nd 1.33 , you'd get a hard launch with the lower 3.35 1st gear, but the 1st to 2nd shift wasn't as even as with my T-5z.
I really like the way the T-5z's ratio from 1st gear 2.95 to 2nd gear 1.94 just keeps going. More of an even transistion from gear to gear. With my old T-5's ratio, 1st 3.35 to 2nd 1.33 , you'd get a hard launch with the lower 3.35 1st gear, but the 1st to 2nd shift wasn't as even as with my T-5z.[/quote]
Second gear is the same practically wether you have world class or wide ratio(1.99 to 1.94) you are referring to 3rd gear with 1.33
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I really like the way the T-5z's ratio from 1st gear 2.95 to 2nd gear 1.94 just keeps going. More of an even transistion from gear to gear. With my old T-5's ratio, 1st 3.35 to 2nd 1.33 , you'd get a hard launch with the lower 3.35 1st gear, but the 1st to 2nd shift wasn't as even as with my T-5z.
Second gear is the same practically wether you have world class or wide ratio(1.99 to 1.94) you are referring to 3rd gear with 1.33[/quote]
No. I'm referring to 1st to 2nd gear. 2.95 to 1.94.
My old T-5 was 3.35 to 1.99 1st to 2nd.
Here's the ratio's T-5Z 1st 2.95; 2nd 1.94; 3rd 1.34; 4th 1.00; 5th 0.63 T-5 World Class 3.351.991.331.000.6753.15
My set-up works great now. Launching at about 5000.
Thanks!
1993 GT Mustang Need Help With Tire Pressure And Launching 
