Alternative to exhaust "donut" gasket?
 

Does anyone have any alternative weld job idea or aftermarket coupler that can have a more secure connection?

I wonder why didn't Subaru just go the typical two-bolt flange with a flat gasket like other sections have. (header->overpipe and overpipe->frontpipe)

This design does not play too well with my frontpipe/catback combination and I've been struggling with coming up with a good and cheap solution, other than use the stock catback.

Why not?

Because this setup allows flex in the exhaust system, so it reduces the chances of failure at the flanges or exhaust cracking.

You want a donut gasket at a point in the system where it would have the most amount of movement. A solid exhaust system front to back would simply force the weakest part of the system to crack under repeated thermal cycles. The added benefit is also less NVH transmitted into the cabin from the exhaust.

Go look at a car from the 1980s into 1990s, their exhaust systems are designed pretty much how you want it... they also tend to fall apart every 2-3 years.

Some catbacks, like the Perrin, use a flat flange with a flex section. I use the perrin catback with a flat-flange JDL FP. The only issues I've had were a failure of the FP support bracket, which was caused when I went offroading during a track day.

Damn it. Ah well, if the hissing gets too annoying I will just have to throw the stock catback back on, and prepare to find a better fitting one. I like the Perrin idea, thanks for that.

What are the effects a random 2.36" pipe in a 2.5" exhaust system?

The new overpipe I have just discovered is not 2.5", it is 2.36". Is there any big concern with performance or even just a practical matter and principle? Sound difference? Car seems perkier on the throttle, but I'm 99% sure that's just a placebo effect that always happens after I install anything new on my car. (*changes wheels* "WOW, butt dyno is reading +20whp!")

I'd like to hope there isn't too big of a difference in just 0.14", but I have been wrong before.

header: 2.5"
overpipe: 2.36" <-
frontpipe: 2.5"
catback: 2.5"

Minor OCD tendencies tells me I should probably get that replaced to a 2.5" piping, but I literally just bought it.

Throw a walker 31355 gasket on the front pipe before the donut gasket. My exhaust has always been a couple mm off. This sealed it right up.

You'll be fine, I wouldn't worry about this unless you're FI trying to hit over 400hp.

my exhaust was leaking around the donut gasket. The old gasket material gets stuck on surface of catback, super small but still caused leak. Take it off, make sure the surface is clean of old gasket material, put the donut back in and tighten it but not over torque it.

The Greddy EVO catback installs to the front pipe doing away with the OEM donut gasket for a supplied Greddy flat gasket.