3.08 or 3.42's?
 

Which would be better for the track? Running a TH350 & 28" radial on 15lbs of boost. Car currently has 3.90's & on 10lbs of boost it's hitting rev limiter (6900rpm) just before the traps, which was 129mph. Im limited to these two gear options with the HD Eaton posi in my Strange 12 bolt. Car is 3600lbs with me in it , thanks!

if the track has good prep the 3.42 is a good choice.

3.42's should give you 147mph at the same 6900 rpm, if everything else remains equal. If that's not enough, guess you have to go to the 3.08. For the track, I always like to have the gear that I run out of just past the traps. If the car isn't hooking, change something else, dampers, spring rates, IC, tires, tire pressure, converter, boost off the line, etc. but gear to run out of motor, just after you run out of track.

What 60' and how many pounds are you leaving on? Following this, th350 turbo ls1 and I'll be doing a gear swap from 3.73 also

Sort of sounds like converter slip is adding to the issue.
With 3.42's, 28" tires I cross at 6600 just shy of 150mph

The taller gears will make that slip increase.

Might be. I wanna say my shift extensions were around 800rpm. Not sure what that translates to converter slip %.

LOL, was leaving in vacuum with an impressive 2.5 60ft. I bet it didn't go positive pressure til almost 100ft out. Got a 2 step & trans brake going in the first of the year though, should make it a very different animal then.

I'd say the tune needs some work. Even leaving in vacuum, you should be able to make a good amount of boost well before the 100 foot mark. I run a th400 with a 3.08 gear in my Nova with a 28' tall tire and an s475 on a 5.3. I could sneak through the 1/8 in second around 6800 rpms at about 116 or so. That's figuring in 10% slip. Unless the slip is way off, don't get too hung up on it. Keep the engine where it makes power, it'll be quicker and faster. Some can't afford a custom gear set to keep the starting line ratio down, so we need to use higher gears. But, as stated, you'd gear according to your finish line rpm desires and then adjust everything in between.

Your at 12.5 % slip and that will get worse as you add power or take gearing out of the car. Probably going to need to tighten it up.

This works well to calculate your ratios
https://www.summitracing.com/experta...tio-calculator
This assumes 0 slip. So putting in your numbers, 3.90,6900,and 28", your current combo with no slip will go 147.44 according to this calaulator. Since your getting 129, you have 18.44 mph of slippage. 18.44/147.44 = 12.5% just like Vince said. No you know how he got it. Vince is also correct, as you drop lower numerically in gear, you will increase torque load on the converter, and it will slip more. I don't think there is any easy way to calculate how much more though. Your on the verge of blowing through your converter. Not crazy bad. I had a friend who added like 800 hp to his car and ran it without changing the converter, (went from a 400 hp NA car to an 1200hp turbo car) and had zero shift extension at all. Just ran like a CVT about the red line, or slightly above. It still made an 8 second pass though.

Thanks fellas! Yeah that makes sense as the converter was spec'd for my car when i was only planning to do a 150 shot. Got hooked on going turbo instead. Being the cheap ass i am, i just decided on trying the converter & cam i had. Cam seems ok for what it is, but the converter will get sent off for a billet cover & re-stall.

My car has 3.90s and a turbo 400 with 28" tall tires. I've gone 147 mph in the 1/4 @ 7000 rpm. Fix your converter first.

What effect would going to taller gear 3.08 or 3.42s have on your set up?

I see 2 conflicting suggestions:

1. Run taller gears to put a load on the turbo

vs

2. Gear so that you maximize rpm right at the end of the 1320

CyberGrey, those are not conflicting suggestions, it is up to the user to find the balance between those two philosophies for his/her specific set-up. You want to load the turbo as soon as possible while still being able to get traction, and you also want gears that keep your engine where it is making power all the way out the back. It's about balance, not conflict between the two suggestions.

Right now I'm limited on mph because of my gearing, my car doesn't make power past 7000 rpm. I'm going to install either a 3.40 gear or the ever elusive 3.20 pro gear if I can find one. I plan to trap 160 mph so regearing is needed. Taller gears will cause more slip in the converter, if it's too much just tighten up the converter.

That's good to hear. My goal for the car was to drive it to the track, run a 9.99 or faster & drive it home. If tightening up the converter will get me there, then i may not even mess with the rear gear. Thanks for the heads up!

The car will be its fastest if you get the converter tightened up so that you can run the 15lbs of boost with minimal slip. Then only take gear out of the car if you can't reach the finish line without being in the limiter.

You should have no problem doing that I drove to the track and ran a 9.17 on 16 psi and drove home on my 3.90s with the old mt 275/60-15 drag radials. I don't like to go much over 60 mph but I've cruised at 70 mph for a bit. My converter is stupid tight though only 1-2% slip.

Good info.