Im speaking of the stock auto and manuals. How much power can they hold before they need to be upgraded? Im more interested in the 5-speed as they are just funner to drive

 

The later 700r4 will hold about 400 hp before anything needs to be done other than a quality rebuild.

 

the 700 wont hold anywhere near 400 hp, at 400 ft lbs if you drive it hard it wont last all that long.

More or less if the engine has any real mods, the trans is gonna need to be replaced/built up.

 

Mine was holding 335 ft/lbs at the tires in a FULLSIZE G20. I am always driving hard and pulling trailers. The previous engine was making 360 ft/lbs at the tires. 30K miles later it is still intact. If I could only get an engine to last that long. It is a stock rebuild on a 1989 G20 700r4 with a corrective shift kit in it. Built in my garage I might add.

 

I don't care to do the math but my 3600 lb car (with me in it) cuts 1.600 60' times and runs 12.00 in the 1/4. With spray it runs 10.90's. Trans is bone stock and has finally started slipping into 3rd gear. I am gonna freshen it tomorrow. Last freshen was 9 years ago. They work that well if assembled properly.

 

does installing a shift kit in a healthy transmission extend or increase the durability of the tranny in any way?

 

Increases the life of the clutches and bands. Mine is missing accumulator springs too!!! I'm sure its not the best on spyder gears in the rear end or U-joints though.