I am swapping my 1971 350/270 horse into my 84 trans Am LG4 this weekend. I am going to run the same carb on a holley dual plane high rise manifold, an XE262 cam, and Hedman shortie headers and Y-pipe going out a gutted cat and hooker cat back. Any potential problems to watch for? I know Im going to have to tune the CCC (need to pass emmisions), but what about stuff like the flex plate? I heard the knock sensor is a problem. What are some common problems inherent in this swap and how do I solve them. I know that the dipstick and stock exhaust manifolds won't fit, but I used that to justify buying headers .

 

As far as the flexplate bolt pattern is concerned, they should be the same cause they're both 2 piece rears but I think they had two different sizes, 168 tooth and 153 tooth. The 153 is just 13 inches in diameter and the 168 is just over 14 in diameter, so you don't have to go counting teeth, lol. But anyway, I had this problem when I did my swap. Took out a 91 305 and was putting in a 350 with a 2 piece seal. So I went to the parts store and picked up a flexplate for a 2 piece, got it home and it was too big. So I had to go back and swap it out for a 2 piece with 153 tooth which was pretty easy. And believe it or not, the motor I have turned out to be a 92 year with a 2 piece if you can believe that.

 

I don't know much about the Holley intake, but you need EGR provision - does it have it? Also, are you getting the headers with O2 and A.I.R. provisions? You need to, since you say you have to pass emissions (and the CC carb needs the O2 sensor). I don't know how gutted cat and emissions are compatible, but you might squeak by (a high-flow cat would be a better option).

I didn't think the '84 LG4 had a knock sensor. Are you sure about that? If you do have one, there seems to be varying experiences with a 305 knock sensor on a 350. You should be able to get 350 sensor & module, though (if you currently have provision for one, that is). GM lists the P/N's in the HO 350 conversion kit parts list.

Use your current flex plate if it is in serviceable condition.

 

The manifold does indeed have an EGR provision, and I bought the manifold used for $75. The money I saved went towards a brand new paint job. As for the headers, I didnt get the ones with smog provisions, and here's why. My smog pump was seized anyway, so that got chucked. my mechanic said they are pretty much worthless anyway, and said dont bother it wont make much difference. As for emmisions you ask? Car went clean through with the 305 and no smog pump and a new EGR valve. We have no visual in CT where I live. As for the cat, I have two. A real one and a gutted one. I can just play swap swap when emmisions comes around. Thanks for the help guys, will being swap on friday night and hopefully have it all running by tuesday. My poor father, his truck lost its 454 to his 67 camaro, now its losing its original 350 to the firebird, and its getting the LG4. Was his idea not mine. Of course that perfectly good 402 in the garage needs a home...

 

At the very least, the computer will want an O2 signal. If you can pass without the rest, more power to ya!

 

I totally ignored the O2 sensor in the car. Is it possible to weld one in the headers? There is no way I am spending that extra $100 on "legal" headers when I dont even have a smog pump. It doesnt look to hard to do, I may want to take it to a reputable shop instead of doing it myself though so i dont mess up the headers. What exactly does the O2 sensor do, and could I just mount it anywhere? Honestly I'm clueless on the purpose of this thing.

 

I'm no expert but here goes....
an o2 sensor I think is there for good purpose.
it monitors the exhaust gas for unburnt fuel...
thus richening/leaning out the motor for proper fuel management

 

Sorry, been away for awhile. Taxes done now, so...

The function is to keep the mixture proper. Too much oxygen, the computer richens it up. Too little oxygen, leans it out. When the sensor isn't sending a signal, the computer will richen the mixture.

The sensor should go in the collector of the header (usually on the driver's side). The fitting is available from Jegs. It should be welded in by someone who can weld a thick piece to sheet metal (MIG). Most any competent exhaust shop should be able to do it for you - have it done when the headers are out.