UEL header issues

[Weston_Duyck]

So I bought a Borla UEL header and it fits a good but my stock cat won't work with it so can I just delete the cat and leave everything back from that stock and have enough backpressure on a N/A?

[evolutionmovement]

Oi, this keeps coming up. You never want back pressure. What people regard as power differences comes from pulse tuning. Back pressure is always detrimental to efficiency as it is, by definition, restriction.

Brief explanation of what goes on: when an exhaust pulse leaves the cylinder, it travels down the pipe until it reaches a different volume and reflects back to the source (smaller, bigger—it doesn't care). At a certain small range of rpms, those reflected pulses make it back to the cylinder and bounce back off the valve just as it opens for a second pulse, creating a lower pressure zone in its wake which helps to pull the new pulse from the cylinder (this is called scavenging). One of the considerations of proper design of a performance exhaust is accounting for the preferred rpm range that best suits the customer requirements in adjusting the lengths of the pipes to the individual cylinders upstream of any collectors or volume changes. The longer the pipe, the longer it takes the pulse to get back to the cylinder, the lower the rpm range it is best suitable for and vice versa. (This also applies to intake design, but in reverse, helping to push air into the cylinder at a certain rpm range.) When the exhaust pulses hit the collector, they reflect back and don't really care what was there at the volume change. For the collector, it's more about blending the gases from the various cylinders as smoothly as possible to get them out and so that they don't hit each other and cancel out scavenging gains at the preferred rpm. Down stream of that point, there is less energy in the gases and changes have much less effect. Then it's more about getting rid of it and that's more a function of trying to spec the diameters and curves to match speed and pressure to the engine output and the desired rpm range while also accommodating the chassis. Back pressure is one of those persistent bad car myths. Don't believe marketing departments or people who take their advice from them. Actually, if you care, don't even listen to me—google pulse tuning and scavenging and learn far more than I can explain briefly or even more than I know.

As far as the cat goes, that's up to your budget and how much you care about the environment and the legality of your exhaust. The added restriction of the cat over a straight pipe will be pretty insignificant, so the performance effect shouldn't be a concern.

[subaru_styles]

or you can find a stock cat section from an impreza

[Weston_Duyck]

I'm going to see how much a muffler shop will do it for and if not I'll look for an impreza cat. What year would mount up fairly well.

[subaru_styles]

if u get an imprezza cat section i think my buddy still has an exhaust that will mount to it and fit a gen1 legacy.

[Weston_Duyck]

What kind of exhaust?

[subaru_styles]

custom catback with a borla mufller i think. he used it when he owned our 94 which had an impreza header & cat section on it

[Weston_Duyck]

Hmm well if it can fit up I might be intrested.