you could just get the magnaflow direct fit piece, fresh pipes at a bit larger diameter with a brand new cat all in one package.

 

careful... if you start jumping up the dia. be careful not to lose the positive back pressure... you dont need your exhaust sucking air...

 

good point

 

I'm surprised you are having problems locating a Y pipe - several of the online retailers list them. If you have an Autozone or O'Reilly near you they should also be able to get one for you.

Interesting...could you please explain how altering the diameter of the Y pipe is going to cause the exhaust to "suck air"?

 

Upgrading pipe size can increase scavenging, the effect where exhaust gas is pulled from the cylinders. If increased too much it has a negative effect, just like if the pipes are too small and restrictive. Pipes have to be sized for engine output to produce power efficiently.

 

I understand, and agree that you can increase the scavenging effect by increasing (and/or adjusting the length of) tube diameter, but I have never seen it cause a system to "suck air". I'm not saying it can't happen, just that I have never seen it. Having seen several different exhaust systems modeled on a computer, the only negative effect noted by using too large diameter of tubing was a reduction in velocity. Once that threshold was reached the diameter was reduced slightly, so I suppose if you continued to increase the diameter you might begin to "suck air", but I imagine it would have to be a ridiculously large increase to make it happen. The only scenarios I am aware of that will literally cause the system to "suck air" into it is a leak or insufficient tubing length at the outlet of the muffler. This can happen no matter what diameter of tubing is used.

 

88Vic is spot on. If you push the dia. and lose back pressure the negative pressure will draw out the gases in the combustion chamber. It makes a vacuum and sucks the air out. It can also make your intake work harder if it's scavenging too much. Sucking air through the system faster.

It sounds like you are already up to speed on scavenging though.

 

Yup, depending on the engine & what/how it's made -scavaging is the all important issue a lot of people get messed up by. For a reason unknown to me, some engines actually perform better with backpressure. To me who or why would you want it? If you can 'tune' the exhaust to PULL spent gasses from the cylinders you'll get a more efficient burn & more power/efficiency.
Just my thoughts

 

I used to work on Harley's and I've seen several people put large diameter pipes on and then wonder why the low end power is crappy. On dirt bikes, having a tuned exhaust can make a very noticeable difference in performance. Same with car engines. The larger diameters are great for top end performance, but the bottom end suffers.

 

And bottom end - mid is what our vehicles usually see. Top end/high revs aren't a blazers DD lifestyle.