I'm looking to buy a new cat and decided to measure my current one and hte Y-pipe. The cat is 3" on both sides, and mates to a 3" Y-pipe with 2.5" collectors. The car does have stock manifolds.

This wasn't stock, was it? I was unaware my car had an aftermarket cat and Y-pipe on it already. The Y-pipe isn't mandrel bent, but its not too bad looking.

Anyways, will I be able to buy any headers that will bolt up to this Y-pipe? I haven't decided on 1 5/8" or 1 3/4" yet, so everything is an option still.

What would be the best headers that could bolt up to this (if any)? Motor will be an LT1-headed-and-intaked 355 with a CR somewhere between 10 and 11 with a 222/232@.050, .479/.501 cam. Custom 3 or 4" (haven't decided yet) intake with SD conversion and K&N filter and a 3" SLP 2otl setup. Would I be better off with 1 5/8" or 1 3/4"?

 

No header bolts up to the stock y-pipe. Even if your y-pipe is not stock it has been tailored to connect to your stock manifolds. Therefore you should treat it as a stock piece. Most header kits come with a y-pipe. Some are better than others but all are better than stock.

 

My car is an 89 GTA and it came with a 3" cat. The Y pipe is also 3" inch at the cat end, and 2.5" at the manifold end. This is all factory.

Sounds like what you got is what the car came with from the factory.

 

Hmm okay, everyone told me that my 305 shouldn't have come like that. Then again, the direct-fit cat I just bought is 3" inlet and says its a stock replacement for my motor/year..

I know its a widely debated topic, but am I better off with 1 5/8" headers or 1 3/4" headers for a 350 with LT1 intake and heads flowing around 260int/190exh and a 222/232 cam? Price *does* matter though, and the 1 5/8" Hedmans are very attractive at $230 shipped for new ones including the Y-pipe. If these will be fine on that motor, then I'll go with these. I'm hoping to push about 400hp at the crank with this setup..

 

One thing about these cars, is that you never know what strange option they end up with. My 89 GTA has the WS6 option, but did not come with the big front brakes. It also came with the aluminum driveshaft. The rims aren't the standard GTA rims. They are the older 16 inch rims you see on the mid 80's TAs. The build date on the door is Oct. of 88, so it was built pretty early for being an 89'.

If those are the same Headman headers and y pipe I have, the y pipe is a bit small. 2 1/4" tubes with a 2 1/2" collector. The y pipe comes with a 2 1/2" to 3" adapter to connect to the 3" cat. I think that is a little small for 400HP. BUT you might want to get some other peoples opinion other than mine before deciding.

 

Ah, I do remember that about these headers. Probably since they are so cheap.. I'd like 2.5" collectors and a 3" cat connection.

Anyways, yeah my TA has the WS6 package, no big front brakes (that was NOT part of the WS6 package), the JG1 aluminum driveshaft, the 16" black mesh rims. The options on these cars are pretty weird. I guess the ones to look for were the Formula 350 WS6 cars with the 4 wheel discs..