Originally Posted by UseYourNoggin (Post 135222)

I'm saying whatever your stock size is, increase it by 1/4" diameter across. Increasing it more than that is just: stupid, stipid, stupeed, stewpide, stupide. If you had a V8 engine then 1/2" bigger would be fine, but you hace a 4 cylinder!

A small restrictive pipe would immediately fill with exhaust, and move at a considerable velocity, but excess gas would be left behind, which in the worst case scenario would actually remain in the combustion chamber and mix with the incoming fresh air, reducing power.
A large pipe would immediately absorb all the exhaust volume, but drop velocity to the extent that scavenging doesn't occur. Exhaust would still flow out, but would not be as efficent as it could be.
If the piping size is just right, the velocity is high enough so that it creates a suction effect at the cylinder head, and literarly sucks the exhaust out of the cylinder. This makes it easier for the piston to move up, and recovers a good amount of previously wasted energy."
"modifying the catback or mufflers will provide little or no gain, because all of the restriction is further upstream. This is verified by dyno's, as no one besides the various exhaust manufacturers have shown any gain whatsoever with any of the available catbacks on the market. (on owner dynos).
If anything, some of the catbacks are actually reducing overall power, because they're slowing down what little exhaust velocity there is, by the time it gets done going through the twists and honeycombs of doom."

For more great info from a famous mazda internet guru click here: Exhaust Technology (Sizing/Velocity) - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
That guy only made 11,747 posts, his name is Crossbow

Here is another great article from Magnaflow: MagnaFlow Performance Exhaust - FAQ, Frequently Asked Questions
Quote from Magnaflow: "To reduce additional backpressure, the OEM exhaust tubing can be replaced with mandrel-bent tubing of the same size or one size up from the OEM. As a general rule, you can enlarge the pipe diameter of your OEM exhaust system by 1/4 to 1/2 inch to increase your horsepower. However, any additional increase in pipe diameter is likely to decrease your performance; specifically, low end torque."

Originally Posted by tanprotege (Post 135211)

232fromtijuana:

I am not experienced in upgrades. It's all theory with me. But here is information on upgrading on the Protege. The 323 is the precursor of the Protege.

https://www.mazdaforum.com/forum/maz...pgrade+protege

By the way it is not necessary to upgrade a car to have fun at an autocross. An autocross novice like myself has no chance to find the limits of a regular car. It takes some auto crossing to get there.