Downpipe?

A Down pipe is for turbo application, we have no such thing. The next best thing for our cars would be a long tube header.

Down pipe can also refer to the pipe going from the exhaust manifold to the rest of the exhaust, common term if referring to short tube headers.

Just wait for a header, as you can replace the stock manifold and pipe at the same time.

A long tube header on these cars should give about 15-20 whp for $600-800, not that bad. Of course you get even more gains with an ecu tune.

I wouldn't hold my breath for 15+ hp from a header without tune on a 4 cyl car, 'maybe' with.

I thought that downpipes helped with catalytic converter flow?

Depending upon the setup of the header, it may delete your CAT(s). That is where most of the HP gain from a header comes from.

im thinking it replaces one cat as we have two ???

Yes a header only replaces the CAT found on the downpipe, the one under the car can be changed out for a test pipe or a higher flow cat.

You have to remember that todays cars usually have 2 O2 sensors -- the second one lets the ECU know if the cat is functioning. If you delete the cat you will get a CEL/SES light on the dash for cat/conv efficiency loss.

ForteSX2.4 said:

You have to remember that todays cars usually have 2 O2 sensors -- the second one lets the ECU know if the cat is functioning. If you delete the cat you will get a CEL/SES light on the dash for cat/conv efficiency loss.

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Resistor mod and/or a washer spacer will fix that.

Some well constructed headers, such as Thunder's for the Mazda 3 includes a high flow cat and O2 bungs that will avoid a CEL. So it all depends upon the manufacturer.

But yes just using some antifoulers to move the O2 sensor out of the exhaust stream is all it takes to avoid a cel.