on most cars a cat back system usually cant affect performance at all, however the bravo/a range seem to get some of the exhaust back pressure post cat (from the cat back part of exhaust). this means you can both increase and decrease the overall back pressure to a certain extent by fitting different cat back systems. adjusting the overall back pressure can have an effect on performance.
how does back pressure effect performance? well to keep it simple its the same as the overlap effect. actually thats a bad example if i'm trying to keep it simple.
erm, bascially think of it like this.
if you increase back pressure you will have the least amount of compression lost during the intake stroke, and the ratio of intake to exhaust gases will be high, and this can give torque gains at low rpm (more grunt). however at higher rpm you will begin to get too much exhaust gas left in the cylinders causing a loss of cylinder pressure (and therefore horsepower).
on the other hand if you lower back pressure you will suffer from scavenging. this is where the low pressure allows some of the air/ful mix to leave the cylinder before ignition. this unburnt fuel hits the lambda which causes the fuel injeciton to reduce fueling, worsening the situation. when that happens you have less fuel in the cylinder to ignite, causing a loss of torque at low rpm. however at higher rpm the incoming air/fuel mix is drawn is very well by the low pressure in the cylinder and exhaust, and also the exhaust valve closes quickly (before any can escape), so the amount of air/fuel in the cylinder actually increases, increasing horsepower.
so you have the issue of low rpm torque vs high rpm power.
do not underestimate the difference a change of back pressure can make, it can ruin a car and make it undrivable.
i know that sportex for example tend to follow the traditional approach of reducing back pressure to get a high rpm power gain. this works well on many cars (and looks good for marketing purposes since the average boy racer doesnt understand an engine but he can read a bhp number), but on some engines it leaves them feeling a bit too flat at low rpm. this has been reported by several bravo/a owners who have fitted sportex cat back systems.
i dont know about other brands, but generally they will all try to lower back pressure. usually this wont matter since on most cars the back pressure is created pre cat, so anything after the cat makes little or no difference, however that is not true on bravo/as so we need to consider carefully which exhaust to fit. personally i would want to maintain the stock back pressure mostly due to the fact that other people have had bad experiences with lowering it. i dont know if increasing back pressure would be a good thing on any model, but i think it is definately something worth while experimenting with. i made a few different exhaust configurations for my 1.6 bravo last year and found it made a big difference, but none worked as well as the stock exhaust so i gave up.
if you fancy making a constantly adjustable back pressure device (basically a wide muffler with an adjustable valve inside) to fit to the cat, and then use a rolling road to get different power and torque curves at different back pressures, you could find the best possible back pressure for your car. that would be fun, and its something i'd like to do, but my days of experimenting with things like this are over since i dont have the time or money any more.
