TH400 transmission question.

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Jun 16, 2019 | 09:08 PM

miked123

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TH400 transmission question.

I 1974 corvette with a TH400 trans. what is the max speed I can go with a 350 small block with a 3 speed transmission with out blowing my gears out?

Jun 16, 2019 | 09:14 PM

dmaxx3500

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From: chicago

7500rpm,and 185mph

Jun 16, 2019 | 10:05 PM

GTR1999

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From: Connecticut, USA

A stock 74 350 with 308 gear 120 mph.

TH400 has a 1:1 final gear, like the 4 speeds of the day. The trans is not limiting top end, your engine redline will. The gears just move the range up or down. TH400 were very strong but parts changed during the run. earlier 2nd gear parts are stronger then later ones but that is for abusive high power use. Which if you plan on doing you will soon find the weak links, part by part.

Jun 17, 2019 | 06:12 PM

HeadsU.P.

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From: Cool Northern Michigan

Just slightly less than a cop car.

Jun 17, 2019 | 07:06 PM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

Quote:

Originally Posted by miked123

I 1974 corvette with a TH400 trans. what is the max speed I can go with a 350 small block with a 3 speed transmission with out blowing my gears out?

You're not going to "blow your gears out." Nonsense.
A '74 350 will run out of rpm and power before any damage is done to the driveline. If you have a TH400 (or any other automatic with a 1:1 final drive ratio), 3.08 rear end, a stock engine (which poops out at 5500 rpm), 27-inch tall stock tires and 5% converter slip, the max speed you can theoretically achieve is 136 mph. If you have 3.36:1 rear end gears, the top speed, limited by engine rpm 5500, is 125. If you have the 3.55:1 gears, the limit is 118 mph.

The stock 1974 350 was 195 horsepower. I doubt you'll get to 136: The car will self-limit itself, due to power limitations, to something around 120. No gears are "blowing out" at 120 (or 136) mph.

Lars

Last edited by lars; Jun 18, 2019 at 04:40 PM.

Jun 18, 2019 | 10:41 PM

Motorvation

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From: Sandusky, OH

And the 400TH is not running any gears in high (direct, also know as third gear). A couple of clutch packs, forward and high, are coupled up and power runs front to back. Input shaft to output shaft. This trans can stand 750 HP in just about stock form. What kills them is low fluid level and mis-adjusted shifter linkage.

The first trans brake, manual valve body 400TH I built went in a 555 cu. in. 78 Malibu. Went 9.90's all day long. Awesome power....front wheels up on the first trans brake hit. The trans had the 36 element second gear sprag, stock type Raybestos clutch linings, proper clutch pack clearances, and a TCI valve body.

It will take all the HP your 350 can make. ENJOY!

Steve O.

Jun 19, 2019 | 01:38 AM

Goody

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From: Oak Harbor WA

If you stay at your max speed for as long as you have gas in the car I still don't think it would blow up..... 5k RPM hard launches on sticky tires at a prepped drag strip will blow it up but not max speed.... well unless that max speed is sustained over a significant period of time..... which isn't likely these days...