Right now I'm plannin on getting an nx kit sometime hopefully soon. I have 3.42's in the car right now. And I feel as if I'm leaving some on the table NA. What would you recomend for gears for a nitrous fed A4. Basically I'm undecided on what I wanna do. Prolly should set it up for NA since thats what it'd be most of the time. Shifts at 6400rpms, and peak power is a 6200. But I'd assume whenever I'd go to the track I'd wanna spray it. Can anyone help a severly undecided person out. Thanks

Joe

 

What size (diameter) tires are you running? What do you trap at in the 1/4 right now (mph and rpm wise)? On a 26" tall tires, 3.42's are going to be perfect for nitrous (maybe too perfect). On a 28" tall tire, 3.73's would be an optimal nitrous gear.

 

I have 3.42's in my car now. I like them for the street, and the car runs good NA and on the dope at the track. I could use 3.73's though. I'm crossing the line a little low in the RPM range. I say if you can get traction go with the 3.73's. According to the online calculator with a 6,500 redline, 3.73's, and 26" tires you can trap 135 mph. So I may up to the 3.73.

 

Well thats the thing. Last time at the track was the first time I have been to the track since the gear swap, tranny rebuild, and the 2800 vigi. I have 15x10 pro stars and running a 26x11.5x15 et street for traction. The pcm hasn't been reprogramed for the 3.42's so I'm not real sure where the rpm is at while crossin the line. But I was trapping about 109. Thats about a mph less with the stock stall but the weather was a lot worse in the summer. The only thing I worry about is how it'll run off nitrous.

Joe

 

You have to account for the slip of the torque converter when performing such calculations.

With a 6500 rpm redline, 3.73's and 26" tall tires with 0% torque converter slip, yes 135 mph is possible. However, assuming 7-8% slip, that would be more like 7000 rpm at 135 mph. If he wants to stay under 6500 rpm, 124 mph is going to be tops in his 1:1 gear.

Now if he wants to stay under 6000 rpm on the juice, 115 mph is about as fast as he will be trapping assuming 7-8% converter slip. So you can see how you can run out of rpm pretty quick on the spray...

Now if he has a lockup converter its a different story.

 

I was going by my car. I have a lockup switch for the converter so I don't have any slip in 3rd. But I see your point about the slip and higher rpm with the converter not locked. I'd say try the 3.42's first on the nitrous. See how it runs and what rpm your crossing the traps at. Then decide if you want or need 3.73's. For my car I think 3.73's will be better NA and on nitrous.

 

I feel 3.73's would help out a lot NA. I have a vigi 2800 lock up stall for a lt1. I don't think NA my car has enough ***** to get a better 60' then the 1.77's it cuts. I launched off idle and at about 1500 and 1.77 is all it'd do. Peak power is at 6400 and I don't think its anywhere close to that as I'm going through the traps. I don't think I'd wanna spray the car all the time just go get it to go fast. Does that make sense?

Joe

 

Since you didn't reprogram the PCM the tach should still be right, your speedo is what is going to be off.

 

I would try the 3.42's first.
I have 3.23's and have a very similar set up to you
I ran a 11.77 @117.7 with bottle pressure at 900 and the kit not working properly.

You should make some nice times with your car and a 100 shot

 

i would stick with 3.42 i had 3.73 and 4.11s in my car on the nos but the 3.42 i ran my best times with a 150 shot 3200 stall 24# inj 52 mm tb msd hpp nitto drag radials cai mass air shift kit i ran a 11.27

 

Hey Joe man, I wouldnt be suprised if your car went faster on those DRs you and TJ have layin around than it does on slicks, just because they are not as tall. Worth a try maybe?

I'd go with a 3.73 on the gas or NA with your car, particularly on a 26 inch tire...on the street, it will have more ***** NA with the bigger gear, and on the track on the gas and a 26 incher it will put ya right where ya wanna be in the traps. JMHO.
BTW, this is Drake on Buss's name...oops...

 

Bias ply drag tires grow considerably at high speed. Figure at least 1" for traps in the 120mph range and more if you go faster. I am using a TH400 with 3.42's and a 27" tire. The converter stalls at 4,200, so it's fairly loose. Even so, I go through the traps at 140mph without hitting my rev limiter at 6,600rpm.

Rich Krause

 

I have roughly the same setup as the thread starter. I trap around 109 and have 3.42 gears. I have a stock stall and shift at 6000-6100.

How high would I have to shift if I trapped 125 (roughly a 150 shot)? I looked around for the formula to calculate it, but I couldn't find one that included converter slip and have no idea how to figure in tire expansion.

Just curious if I would run out of gear without shiffting higher?

 

When I ran my best time of 11.63 on a 175 shot, I was on ET Streets (26x11.5, which actually measured more like 27" tall standing still; and they expand a little at speed) and 4.10 gears. It was perfect for me then. I was going through the traps at about 5700 rpm or so.

The trade off is when driving n/a...the 4.10 gears a a little much. Fortunately for me, I dont do much tollway/expressway driving, so it wasnt uncomfortable for me to drive at 60 mph with those gears. As long as I kept my foot out of the throttle, the fuel mileage wasnt too bad either. Just my opinion...it worked for me, and I plan on going back to 4.10 gears once I get a stronger rear. If not, I'm going to go with 3.73 and drag radials.

 

Every converter will be different. But the increased rpm will be roughly proportional to the increase in trap speed. IOW, you will be revving ~15% higher at 125 vs. 109mph. Significant tire growth will occur only if you are using a bias-ply race tire and will lower the rpm a bit, but increased converter slip with higher hp will tend to increase it. Probably cancel out, more or less.

So, just multiply the rpm through the traps you see now by ~15% and you will have your new rpm.

Rich Krause