tater

iTrader: (10)

Looking to lower rpms on highway WITHOUT changing my TH350 to a 700R4. I have a built TH350 with only 3k miles and it runs great.

I know you can install 3 series gears on 2 series carriers by using a shim BUT is it possible to put 2 series gear on a 3 series carrier?

I have a 3 series POSI carrier. That's why I don't want to get rid of it. Lol.

Thanks Third Gen.

sofakingdom

iTrader: (1)

No.

THIMK about "ratio" for a minute.

.


.

.

,,,



,
...



,

OK:

We usually talk about the "ratio" of the tooth count, right? 41 & 11 is 3.73, 40 & 13 is 3.08, etc. Well, that's only PART of the whole deal. "Ratio" is NOT ONLY the ratio of the tooth count, BUT ALSO, the ratio of the DIAMETERS. Meaning, a 3.73 ring gear is 3.73 times as large as its pinion, a 2.73 ring is 2.73 times as large as its pinion, etc. BUT .... the ring gear is always the same size, in any given model of rear; so in the 7½" rear, the ring is ... 7½" in diameter. Which means, as the "ratio" gets lower, the only way that can happen, is for the pinion to get LARGER.

The reason there are different carriers, is because the factory chooses a location for the flange that the ring bolts to, for the lowest ratio they expect to use; then as they create successively higher ratios, the pinion gets SMALLER for each one, and the ring therefore has to get THICKER. At some point, they get to where the extra thick ring gears cost too much to make (they warp during heat treating... a yield problem with the process), so they create a new "series" of carrier, which moves the ring closer to the pinion, and start all over with thin ring gears, and step them up again through a range of ratios. Some rears do this more than once. AFAIK in the 7½" it only happens once; carriers are either 2 series, or 3 series.

In the 7½" rear, the ring flange on the carrier is about .400" closer to the pinion on a 3 series carrier, than on a 2 series carrier. Therefore for ANY given ratio, a 3 series ring gear is about .400" THINNER THAN a ring gear OF THE SAME RATIO except to fit on a 2 series carrier, would be. Therefore a 2.73 pinion's teeth will probably not even fit between a 3 series carrier and the HUUUUUUJJJJJJJJJE 2.73 pinion, let alone leave room in there for the flat part of a ring gear. And even if it did, the ring would be SO THIN, the flat part of the gear behind the teeth, would crumple up like tin foil the first time you hit the gas.

A "built" 350 is a dime a dozen. Nothing even remotely special about that. Last one I "built", was on the tailgate of an El Camino in my back yard. Good clutches, new steels, careful stack-up, shift kit, good modulator, governor work, and so forth. "Built" I guess. Ran great too. Big whup. I got 60,000 miles out of it and it STILL "ran great", as "great" as the day I built it, when I sold THE WHOLE CAR it was in for less than the rebuild kit cost. ($175 or so for the kit, if memory serves... I guess I devalued the kit by (a) installing it in a transmission and (b) installing the transmission in a car) It's long gone now.

Do the right and common-sensical thing and step up into at least the latter half of the 20th century. Abandon the 350 and learn to work with the 700.

big gear head

Simple answer to your question is NO. You can NOT put a 2.73 on a 3 series differential in the 7.5 10 bolt, no way. The farthest you can go on a 3 series is a 3.23.

Atilla the Fun

Torque converter can make a difference also. If you're running a stock 1200-stall that slips 200 RPM while cruising, you can replace it with a good aftermarket 2000 stall that slips only 50 RPM, and get a better launch. I know, I've done it. Well worth the investment.