78 Formula Firebird 400

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DasRottweiler

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Picked up this car in tough shape. A project car to say the least. Body is solid and most of the big stuff is there. Interior is shot, but has original buckets. Engine came on a stand and trans was in the trunk, a TH400 that had a long tail? Ran the numbers , vin, RPO codes from cowl tag, engine, and cylinder head codes. Car is a true Formula 400 auto. Engine code has it coming from a late 74 b-body , heads are 101cc 75 heads (5c coded) and trans appears to have come from a b-body car as well in that it has the longer tailshaft. Do the Formula 400 cars come with a TH350 normally? If so, can the TH400 be shortened and used as is done in a-body cars like my Skylark? The driveshaft that came with the car has a smaller differential yoke ,looks to be TH350. That's why I ask.
Curious what rear end the car has in it, but that will have to wait til spring , as I bagged up the car and stored it til then. Any info would be appreciated , as this is my first f-body build. Jim/Rott
 
Turbo 350 was the standard automatic on all Firebirds in 78. ( reguardless of motor)

If you build HP in the 400, then serious upgrades will be necessary to make the 350 trans live.

A stock Turbo 400 can easily take the abuse of the 400 motor.

Your rearend is a GM 8.5 from the factory.

https://summitracing.custhelp.com/app/answers/detail/a_id/2501

To change the lengths on either trans, the input and output shafts has to be changed, so a complete teardown is necessary.

Turbo 375 trans had the TH400 case and internals but with the Turbo 350 output shaft. "375" is cast on the tailhousing and it has 1 less clutch in each pack than the TH 400. Mainly a " low performance" trans.

Hope this answers your questions..............

Doug in P.R.:cool:
 
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And the rear end ratio is likely to be a 2 series (2.73 or even a 2.5x series), or if you are lucky a 3.23. Assuming it is OE.

Build that Poncho 400 with a great exhaust, give it a built turbo 350 or a 400, and some gears of at least a 3.23, or better yet a 3.73, and run some low 13s with some weight reduction!
 
Thanks for the replies , appreciated. I will have to make a decision regarding transmission choice, but that can wait . When spring comes I will examine the car thoroughly and decide what direction to go in. Would love to rollcage the car and nitrous the 400. I have to figure compression ratio and what cam is in the motor as it is a fresh rebuild. I'm sure I will have plenty of questions as the work progresses. I'll keep my fingers crossed on the rearend, one can pray it is 3.73 posi, can't one? Jim/Rott
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Yes one can! But it would not be an OE set.

I'd love to get rid of the 2.52 in my '68 Le Mans. Maybe this Spring....
 
Years ago I swapped out ring and pinion from a 3:08 to a 3:55, difference was fantastic.
 
I have a 1971 GM Corp posi rear w/ 3:42s that is going in my 70 455 powered Skylark build. I'm hoping the Bird has posi, that would save me time and money, fingers crossed. Can't wait to pull the car out and tear into it this spring to see what I got. I can only house and work on one car at a time in my garage , because my other project car is in the midst of a frame-off. Once I get the shell back on the frame, I can pull the Bird in and strip it in the other bay. Jim/Rott
 
melsg5 said:
Years ago I swapped out ring and pinion from a 3:08 to a 3:55, difference was fantastic.
Click to expand...

I'll take your 3.08 off your hands, even that would be a massive improvement from my 2.52! J/K, I'm probably going with either a 3.55 or 3.73. And a limited slip.
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Unfortunately when I said "years ago", I meant about 1972, dont have the old ring and pinion any more, plus it was for a 12 bolt and you probably have the 10 bolt.
 
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