enough back pressure?

[white91wagon]

yea i have a 91 legacy N/A with a glass pack in place of where the muffler is. It runs good and sounds great, but i wanna make sure I'm not going to eat up my valves and stuff. It idles pretty low in tell its warmed up to.

[Buffman]

Back Pressure stuff is a bunch of myth crap. You're always going to have some form of backpressure even with straight pipe. Unless you're running like 2" of exhaust off the heads you're fine.

[93forestpearl]

Backpressure is a misnomer for the hotrod guys that do not really understand what is going on. In an N/A application, you are trying to maintain a certain exhaust velocity, which has inertia and helps pull exhaust out of your cylinder, and pull new air/fuel charge into the cylinder. The wrong size of piping can screw that up. Old guys with mullets call it backpressure.

[Buffman]

I call that Scavenging.. I doubt unless you changed the pipe diameter to something huge you won't have a problem.

I wanted to keep most of the exhaust on my legacy, but it's all rotted. SO it's all coming off. I have free access to (limited quantities) of scrap stainless steel piping from work. I would have liked to stay with 2", but work's 2" pipe is 2.25" ID. But since it's thicker tubing it bends really weird in and cuts down on the ID. I would think I should be fine with a 2.25 on the N/A 2.2L?? Cats are going to get replaced to with straight pipe, and probably just plan a single muffler out back.

[evolutionmovement]

Yes, back pressure is always bad. Scavenging, that is the low-pressure wake from exhaust pulses from previously-fired cylinders passing by the presently-exhausting cylinder helping to pull the exhaust out of the latter cylinder, happens within a narrow range of rpms that is tuned to happen along the rpm band by using different pipe diameters and lengths. The Subaru stock exhaust is tuned for the lower end to match the heads and intake. Intake design, incidentally, works much the same way, but instead of scavenging, the intake pulse reflections (pulses caused by the closing of the intake valves travel up the intake, hit a different volume, and reflect back. In our cars, the primary reflections come from the pulse hitting the plenum chamber) help to push/pull air into the cylinders (sort of a supercharging effect). With exhaust, the reflections are of lesser effect due to the greater lengths and shaping of the pipes.

On either end, when pulses hit different sized volumes (pipes or chambers), some of their energy is reflected back to the source. This is also a contributor to the effects for good or bad. Variable exhaust and intake try to spread out scavenging effects over a broader range for an increase in efficiency and power (at some rpms, these beneficial effects become negative ((back pressure)), either partly depriving the cylinder of air or creating resistance to the evacuation of waste depending on whether your talking intake or exhaust). Longer pipes between cylinders mean pulse reflections take longer to make it to make it back to the engine or longer to travel the effective distance between cylinders for scavenging. This is beneficial for lower rpm as the time between cylinder firing is longer. High-rise intakes on old muscle cars were designed for this purpose.

As for what size would be best, that's a matter of trial and error that I don't know. You want to try to match the pipe volume to that of the cylinder size. Too small would create more resistance, too large would cause too great a reduction in the energy of the pulses. Every point of diameter change will cause reflections, but the further away they are, the lesser their effects.