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I have a '66 gto, but has a '71 400 in it. I found a deal on '66 rebuilt 389. It came out of a catalina. My question is other than not having the exact gto engine code stamp on block, what is the major difference in motors as far as performance from a catalina 389 to a gto 389? Can I just get the 093 heads and bigger cam, and it run like a YS 389? Thanks in advance for your replies.
Neil
 

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Yes, you can add the GTO spec heads and cam and it will be the same except for the block code. I would seriously consider driopping the compression if you're going to drive it much by installing custom dished pistons for about a 9:1 CR.
 

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The Catalina motor should be 325 HP, compared to 335 HP for the GTO motor. But the GTO motor is lying, more than that. The carbs would explain the 10 HP. I had the cat motor for my 66, put a 69 400 in it, I figured if it wasn't the right motor, what's the difference.. 389 sounds cool, but 400 with 2.11 valves are better flowing heads. 71? motor may have hardened seats for unleaded gas. Also, the 71 engine is already closer to the correct 9:1 compression you are looking for without being a later smog motor, one of the last good ones. 400 pistons are cheaper than 389.
 

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Jetstang makes several excellent points about using the 400 you have over the Catalina 389. It would definately be the cheaper, more powerful, and more practical way to go. 389 does sound "cooler" than 400, but unless you find a year correct, GTO coded 389, it will not add any real value to the car. If I were in your spot, I'd stick with the 400....JMHO. Years ago, I installed a 389 out of a '65 GTO into my own 428 equipped '65 GTO to make it "more correct". I never regretted it, and 389's run very hard, indeed.
 
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