89IROCZZ4

I met a guy off the board who was willing to deal, so here was the (bulk of) swap:

My 700R4/10 bolt (w/3.73 gears)/Vigilante 9.5" 3200 converter

for:

built TH350/Moser 9" (w/4.11 gears)/Dacco 10" 3500 Converter

I just bought the car a few months ago (1989 Firebird) and ran a best 11.09 @ 122 on 26" drag radials with a 1.5 60' time.

Here are a few of my questions: I am sure that the torque converter is a bit inferior to my previous one, but is it worth changing back to a Vigilante? I know with the 4.11 gears I am going to be running out of rpm, because the car was setup to cross the traps perfectly at about 6100 rpm or so, but is there anything I can do to make the car best utilize the 4.11 gears? If I were to keep the 4.11 gears would it get me a better 1/8 time? I've got a lot of questions with the new setup, so I just would like some direction, because I have heard lots of things about the new gearing, bigger jump from 1st to 2nd in the TH350 over the 700R4. Any advice welcome!

Stephen 87 IROC

The 700's 3.06 first gear and 3.73 ratio give a 11.41 starting line ratio. The TH350's 2.52 and 4.11 gears give a 10.35 final ratio. There won't be as much torque off the line so you won't be coming off the line as quick but then it also depends on how the new converter works. The plus side is that the TH350 has a better gear ratio compared to the 700R4's. You'll be able to pull longer in first gear and when doing the 1-2 shift, the rpms won't drop as much. By staying in first for a little bit longer, your finish line rpm may be close to the same rpm as before.

As for converter choice, it's hard to say. 2 different converters, same size and stall speed are still not the same. Internally, converters can be very different. Without knowing the multiplication factors of each converter it's hard to say how they'll react. The Dacco may work well with your combination or it may not. The only way to tell is to try it. Converter rated stall speed is only an estimate. The same converter will stall at different speeds behind 2 different engines. The only way to know what speed it stalls behind your engine is with a transbrake.

If you feel you're going to run out of gear with the 4.11's, then bringing the shift points up a couple of hundred rpm if you can. The other option is to get taller tires. MT ET drag 28 x 9 are actually 27.3" tall. That little bit may be all it needs.

89IROCZZ4

Thanks Stephen. I have been reading your stuff from thirdgen.org too so I appreciate your advice. As far as bringing it up a couple hundred rpm, what do I need to kon about the motor? It is 11/1, has forged pistons, Speed-O-Motive 1-piece rear main crankshaft (cast) and GM LT1 Forged powdered-metal connecting rods w/ ARP bolts.

89ProchargedROC

wow, 11.09 on motor is pretty good...what kinda heads and inches does that motor have?

and what size drag radials are you using? 26" just tells us the height

89IROCZZ4

Yeah, I said the height because I figured that was the only necessary information for help. The drag radial is 275. It is a 383 with AFR 210's.

Stephen 87 IROC

Bringing it up a couple of hundred rpms really depends on the engine and where it builds power. Theoretically you should be shifting around the peak torque and going through the traps at or just above your red line. Where the engine builds power mainly depends on the camshaft. Running low 11's at 122 mph tells me you're building a lot of power already. I'm guessing you should be shifting in the 6600 to 6800 range. A SBC likes rpms and can easily go to 7000+ if built right.

I've played with shift points before. Keep track of the shift points in each gear. If the rpms climb too high, you can feel the car nose over as the torque curve starts to drop. Not every gear has to be at the same shift point. According to Dyno/Drag2000, I should be making my 1-2 shift at 6200, the 2-3 shift at 6700 and go across the finish line at 7356. Of course I've never had a chance to try it but feel safer doing both shifts at 6700 rpm.

engineermike

That motor valve floats at 6200. Shift points are 6100 and it crosses the line at 6100 with 3.73's and 26" tall tires.

28" tall tires or taller gears are the only way to remedy the too-short 4.11 gears.

Mike

Ken95Z28

Also he will be running a non-lock up version compared to a lock up version that he use to have. There might be more there just by going to a non-lock up converter (no internal clutch in the converter) less mass and drag on the converter.

Put that sucker in there and see what it does.

engineermike

The non-lock-up converter SLIPS at full throttle. The 122.7 mph was run with the converter locked-up in 3rd. A looser, less-efficient, non-lock-up converter will give up 2 - 4 mph in the quarter over the tighter, more-efficient, lock-up Vigilante converter that was in the car.

Maybe valve float at the top end won't be a problem with the losses from the converter slowing the car down.

Mike