Turbo car gear ratios for drag strip, I need some advice!

[speedfreak426]

I need some decision making guidance here guys...


Some background info:
I have a 1977 Toyota Celica that I swapped a 92 turbo supra engine and R154 manual transmission into.

Rear axle is out of a '68 big block Dart, a 8.75" with a 3.91 gear.

I built myself a Megasquirt system for the car, all assembly and tuning is and will be done by my lonesome.
Currently I only have a rough tune, about 1 hour into making the volumetric efficiency map, and I just made an educated guess on timing, because I was running out of dyno time and I just wanted to make some shakedown runs at Milan.

Well, I ran a 11.51 @ 122mph, not too bad for not being tuned yet! It should have a lot left in it once I get the tune all set, and up the boost some more. It is set up for 2 step launch control, but I didn't use it on these runs. Running around 16psi right now, and I plan on 20+ once I get the roll cage in. So it will be somewhere in the 10 second range. (and 30+MPG, I just thought i'd throw that in there :naughty1: )


Well, at the strip, I ended up breaking the Mopar differential, which is the last thing I thought I would break!
It is a cone style limited slip, and I busted off one tooth off of the one side gear, luckily I was still able to drive it home!

This is pretty much a strictly street car, but I do like to take it to Milan once or twice a year.

More background info / cliffnotes:
-I am crossing the finish line at 6300ish rpm's (6250 oem redline) in 4th gear @ 122-123mph
-Safe to spin up to 6500.
-current 3.91 rear gear
-235-60-15 MT DR's (26.1" dia)
-R154 Transmission gear ratios:
First Gear: 3.250:1
Second Gear: 1.955:1
Third Gear: 1.310:1
Fourth Gear: 1.00:1
Fifth Gear: 0.753:1
-broke the diff, looking to upgrade and change gear.

some pics of what I have:

[video=youtube_share;T-O8x3sk8A0]http://youtu.be/T-O8x3sk8A0[/video]

So, finally on to my questions:

Main question- at the drag strip I don't want to use an overdrive gear correct? I assume the max I should be using is 4th. Since I am already at redline in 4th I am under the impression that I should change the diff ratio since I plan to go faster next year. There are not a lot of options for the 8.75" out there under 4.30:1
Since this is a turbo car I would think keeping the car in boost as long as possible would help my et's, or I could be wrong and it will slow me down.... what do you guys with some turbo car knowledge think? any suggestions?
My options are- 1: buy another 3.91 gearset and shift to fifth. 2: 3.73 gearset will put me at 6250 rpm at 130mph in 4th, I don't know how fast I will get this thing going in the future, but I only want to upgrade once.
3: 3.55 will be 136 @ 6250 4: I can buy a 4:30+ gear and get it up into 5th gear, but I don't know what would happen with spooling the turbo and the time lost from an extra shift.

Does anyone have any real world experience with turbo cars and changing ratios at the strip? any input is greatly apreciated!

I need to buy a new carrier, looking at the Eaton Truetrac torsen diff and a few others.
Has anyone had good experience with the truetrac? It seems like it would be pretty awesome for a street car.

 

[te72]

Going off the Norbie calculator (http://akb.norbie.net/Public/Gearing.aspx), the 3.73 Supra differential gets you to about 135 or so in 4th at 6500 rpm. From what I've seen, that will put you in the mid-high 10's, at least in a Mk3. Now, for a 2300ish lbs Celica, I can't say for certain. I would almost suggest an even lower ratio than the 3.73 on account that you MIGHT end up replacing a turbo with something bigger, making more power, etc... and if you only want to upgrade once (smart man), I like giving myself headroom on most things. I mean, running something to its ragged edge has it's potential risks anyway, so I like to leave a bit of reserve in a car's potential.

Now for the disclaimer... you already know that lowering your final drive ratio is going to hurt acceleration, spool time (due to being at a lower RPM at the same speeds as before), in exchange for a potentially higher top speed. Trouble is, are you going to make enough power to make up for that minor loss in mechanical advantage? My money is on yes, particularly if you're going to be launching on boost with that two-step. I say upgrade to that 3.55, which puts a max speed in 4th at over 140mph, and don't look back.

Horsepower and racing are a bit of an addiction though, you may find that once you hit that 140+ in the 1/4, you're bored with it, it's too easy, whatever. Then you're going to look into a more powerful turbo, then once the 9's get boring, you're going to get irritated by the time lost to shifting, then you're going to upgrade to an auto (assuming you have no interest in racing around corners), etc... it's a nasty bug to get bit with.