That 78 400 engine had 7.6:1 compression, 180 HP @ 3600 rpm and 325 ft lbs @ 1600 RPM
Your stock engine is a 305, 9.3:1 compression, 230 HP @4600 RPM, 300ft lbs @ 3200 rpm
It would be cheaper and easier to swap in a chevy small block (350, 400, 327, etc)
The 78 trans am used the old pont. engines which are very diff from chevy v8's.
Your 89 uses a chevy v8 (305 or 350)
The motor mounts, transmission bolt pattern, exhaust manifolds, etc are diff on the pont and chevy engines. The accs brackets like alt, PS, etc are diff.,
And your changing from Newer model to older model is one thing,
but your going from a Small block Chevy 350 TPI to a Big Block motor
Yes, 1st know if you are going to swap in a 78 400 its the size and weight of a Big Block. I to recommend swapping in a LT1 or LS1 or LS2 easyer and less work and get more Horsepower.
If you decide on the 78 Pontiac 400 .
You need a Transmission also, because the bolt patterns are drifferent on the bell housings.
Will need to make motor mounts and make or modify a transmission crossover. Will be changing driveshaft length and also will need different radiator.
Of course your wiring will be changed to almost nothing over the emissions system of the TPI injected motor.
I could go on about the 400 swap but its more of a hands on. And its been done before. But was a 474 butler motor that got installed. with Pro street setup.
I recoomend LT1 if you want to just bolt in HorsePower to what you have. Build a 500 to 680 hp LT1 and put in a HD or Jegs clutch might even get to run your 3.08 or 3.31 rear gear thats all ready in your cars rear end. But I would set it up with at least a 3:42 rear gear for sreet / strip or 3:73 rear for more of a strip car less cruising.
Next if you have the cash I would go for LS1 or LS2 with 6 speed. Yes 6 speed.
With almost no mods LS1 with 6 speed with lay down 340 HP on the ground. LS2 will put down 390 to 405 HP on the ground. With intake exhaust cam upgrades can easy put down 450 to 485 HP AND EVEN MORE >>>
OOOH I almost forgot.. LS1 and LS2 you still get great fuel mileage over 18 to 19 just cruising around town and up to 22 highway cruising with the 3:42 or less rear gear. Less if you keep you foot in it. The 78 400 depend on how you build it will only get 12 to 16 mpg. or less alot less.
Just remember to have fun.
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