3.23's or 3.42's???
 

I want to swap out my 3.73's now that the race season is due to start in march....gotta make sure everything is good for some turbo passes.

Should I go either to 3.23's or 3.42's???? What would load the turbo better and get me the best passes?

Stall is a Vig 3800, 4L60E.

Why not 3.08s or even 2.73's?

Is that the stock tranny ya got there? Hows it holding up on ya?

FI is so funny, I get ripped on for having 2.73s on my car and here is someone recommending to get them.....thats awesome :)

Of course, I would need more car info, but 3.08's or 2.73's shouldn't be out of the question. Take my car, for example:

3750 lb, 670 rwhp, should be good for 140 - 145 mph in the quarter.

25.5" tall tire, TH350, 10% converter slip.

Power peak is at 5800 rpm.

It tops out at ~145 mph at 6200 rpm with 2.73 gears. I know this because I ran it all the way to 150 mph on the dyno and the engine was tapped out.

Even with the 2.73's, it will still spin the Nitto's from 55 mph without even downshifting to 2nd gear.

I'd like a lil grunt from that a lower gear provides since our cars are dogs up here a mile high.

BUT it is interesting that engineermike is doing awesome with 2.73 as well as 2002 TA.

2002, stock tranny over 200+ passes still going strong...knocking on wood(Tax return is likely going to a TH350 swap)

How much more info would you need, what kind of specifics? I'll provide any info you need, but a quick run down can be gathered by glancing at my sig. My cam is a mismatch from my blower days (233/244 .578/.602) so it likes to spin high.

Keep the info coming:D

Sounds like you need more boost. . .

What is your expected mph in the quarter? What tire height?

You'll appreciate the 2.73's when you lose the overdrive.

Your cam is probably not as bad as you think.

15psi is my max for running at the track. I run on 28in ET Drags.
With the old S/C set up I was trapping 124-125 consistantly.
I want to break into the 10's(yes 10's pretty hard to do up here:lol: ) Best time with the s/c set up was 11.10@125
The turbo should put me comfortably into the 10's with the 3.73's but you never know.
J:D

Hearing this worries me a little, I have a vig 3000 in my th400 and currently have 3.50 gears in the 9". I'm looking to move to 2.70's though. I haven't run the car yet.

You ran out of rpm in 3rd at 150 with the 2.73s? I plan on spinning mine to 6500 if it makes power there. FWIW, when I go under boost the converter goes to 5200 and stays at just 11psi. I wonder if i'm going to need a tighter converter.

well using your 28" tall tire when you switch to your Th350 keeping the 3.73's you'll top out at 140mph @ 6000rpm, or 151@ 6500rpm cruising @2400rpm @ 55mph
using that gear would probably be the best on the track, though street it may not be as desirable.
3.42's would take you to be able to 152mph@6000rpm or 165@6500rpm.
while cruising @2300rpm for 55mph
3.23s would take you to 161 and 175 respectivly with a cuise of 2100rpm@55mph

id do the 3.42s if it was me.

Here is my favorite tranny/gear/tire MPH calculator. These are peak numbers and do not include the driveline loss or converter loss.

If you're not looking to make a parachute pass at the track then 3.42s will work very well for any automatic.

The 3.50s with a TH400 have too much grunt on the street, but the CD player can cover them on the freeway most of the time. :D

Rick

Did anyone understand Rick's last sentence?

Force, what rpm will you be shifting?

Dont forget about converter slip, I go through @ 138mph at 6500rpm with a 3.70 and 28" tall tires.. Naturally a lot depends on when your motor stops making power, if it dies at 6K then gear accordingly.

Your 1st gear with the 4l60 is 3.06, TH350 is 2.52 or something so when you switch transmissions your 3.73 wont be as bad as you think if it can pull the RPM on the other end..

FWIW, a t76 car that i tuned has a 3.42 rear with a th400. It's too much gear for it. It feels ok in 1st, better in 2nd and 3rd feels like you grabbed a 150 shot! It has a tight 3000 stall, goes to 5200 with 13psi. He's trying to go to 2.73's

So by reading all the post so far it looks like a 2.73 would be a good gear for an auto. But how about the 6-speed guys? Would a 3.08 or 3.23 be better? I'm sure the 2.73 would be a little low for us right?

very good thread by the way!

Hmmm... I'm not sure I understand it either. :)

I was reffering to the cruising RPMs on the freeway and the amount of noise that a person would experience. I can live with the noise level of a three speed tranny and a 3.50 rear. The kids get a little more noise back there than they would like some times though. :)

Rick

dont know how that happened, but double post.

i estimated about 4% converter slip on my numbers
maybe real word are different dunno?

Consider a 2.91:1 ratio for your 9". Look up the specs and compare the number of ring & pinion teeth and you'll see the advantage. You'll never break a 2.91, and you may be able to get a lightened ring used from a NASCAR.

FWIW I think you need a much tighter converter. When you swap to the lower ratio rear you may blow right through the converter.

I'm concerned about the converter and low gears like you mentioned. I'm in wait and see mode now since my car is not running right now. As for 2.91's it's an option, I know someone who works for a NASCAR truck team, my driveshaft is from one of their trucks LOL.

I'm not against a tighter converter since the primary use of my car is on the street. Tighter just means more streetable. I'm thinking 2600, but I have to see how this one works first.

Wouldn't that calculate out to more like 20% converter slip? (Seems like you need a tighter/better converter).