I have the GX exhaust manifold with the bolt-on y-pipe, and I've been recommended a 1.75" or 2" exhaust, but the end of the y-pipe is 1.5".

So then is there any point in having a larger diameter than 1.5" with this setup? If not maybe I should not use the y-pipe.

Generally speaking, if the engine cannot make over 100 hp, fitting a 2" exhaust system will slow it down. 1.75" is for 85-100 hp

On the other hand the larger pipe will sound tougher even if it slows down acceleration slightly

The Y-pipe helps with tuned flow, definitely use it in conjunction with a resonator at the stock location


The resonator has the biggest effect on performance, the muffler is just for noise reduction in this setup

Ok that's good info. The y-pipe I have, which I think is the original GX, is about 1.5" internal diameter. But I understand that exhausts are usually measured by OD (right?).

How would I determine the stock position for the resonator?
And what size resonator?
Cheers.

I thought they were measured I.D.?

For racing, measure from the exhaust valve down to the resonator (6400 RPM power peak, or distance for whatever RPM you've built your engine for). For a street car, flexibility is more important, so maybe longer to the resonator -- for a fatter torque curve, -- and most importantly wherever there is room under the body to fit the resonator

Take a look at that photo to make a starting point for a stock A12GX/A12T engine

For reference, the internal diameter is 1.5 inch: