1975 with TH400

[Carl H.]

Hi All,

I have owned a 1975 Convertible since 1985. The L48 (165HP) was always "too slow" At 33K miles, I had it taken out and had a 355 ci Blueprint 410HP engine installed (keeping old engine on a stand in garage) While engine was out we decided to send trans out for a refresh and shift kit. I thought all small blocks (1975 was SB only) had a TH 350 for the automatic trans. When it got to the re builder they said it was a TH400. I thought that the TH400 was only for the BB and there were no BB Corvettes for 1975. Of course it runs and shifts great.

The trans had NEVER been out of the car. All original until I changed the engine. My car was built the last week or so for the 1975 production run. Any idea how I ended up with a TH 400 instead of the 350? Would that make my car fairly unique?

With the auto trans, what rear end ratio would I have?

All comments and info is appreciated.

Carl
2019, 1975 and 1966

[Russ S.]

Re: 1975 with TH400

1975 used only the TH 400 for the automatic.

[Carl H.]

Re: 1975 with TH400

Thank you very much

[Tom B.]

Re: 1975 with TH400

76 was the last year for the Turbo 400 and you could only get it on L82 engines L48 had the Turbo 350

[Tom R.]

Re: 1975 with TH400

The THM 350 begins usage in 76 and 77 for the L48s. By 78, the 350 is used for both engines.

[Carl H.]

Re: 1975 with TH400

That is what I was kind of thinking from all the reading I did on the subject. But I have the L48. How did I end up with the 400?

[Joe L.]

Re: 1975 with TH400

That is what I was kind of thinking from all the reading I did on the subject. But I have the L48. How did I end up with the 400?

Carl------

For 1975 the only automatic transmission installed in Corvettes was the THM-400 regardless of L-48 or L-82. This was also true for all 1968-74 Corvettes with automatic transmission regardless of engine option.

[Tom R.]

Re: 1975 with TH400

But I have the L48. How did I end up with the 400?

The THM 400 is a "heavy-duty" automatic transmission and was a remnant of the high-performance years with cubic inches and horsepower. By 76, GM figured out that with less HP in motors, no need for the THM 400 and the 350 was adequate.

[Steven B.]

Re: 1975 with TH400

'77 L-82 came with TH400. Ordered mine Aug 6, 1976.