blandry23

Ok, so as you all may know that after mazda dumped the thermal reactor bit they replaced it with a catalytic converter. To be more specific two pre cats and one cat. I have read that early catalytic converters are very restrictive with high back pressure. But i have also read that decreasing back pressure will decrease low end torque, actually shifting the torque curve to a higher rpm. I was explained that an engine is like a pump and that if the air can flow out faster than the air can flow in faster which to me makes no sense because at any given rpm for example at 1000 the same amount of air is being pulled into the chamber and is being pushed out regardless of how much pressure is in the exhaust. To make a long story short my questions are as followed.



1. Would decreasing back pressure really make me loose that low end torque or is back pressure only something that is needed for two-stroke engines



2. If back pressure makes a difference, how many of my cats (if any) should i take catalyst out of to attain a healthy amount of back pressure as i have been told that too much back pressure decreases the longevity of the engine (I have been contemplating buying a new cat but i don't have to pass emissions anymore and i have also been thinking of buying a new exhaust but money is an issue. after all i am a college kid and i don't have a constant flow of money into my pocket).

mazdaspeed7

Backpressure is never a good thing. The engine has to fight backpressure to expel the exhaust gasses, and high backpressure leads to more intake contamination during port overlap. People typically confuse backpressure with velocity, which is where that myth comes from. If you put 3" header tubes on a small engine, it will lose lots of bottom end compared to a proper exhaust, and will probably make less power as well. But its the lack of velocity that is the problem, not backpressure. Conversely, small pipes will have high velocity, and if they are too small, can have high backpressure as well. So people who dont know better tend to assume since small pipes make more low end, its the backpressure. Its not.



Put the most freeflowing exhaust you can find on. Most commercial exhausts for a rotary make for a significant performance gain, esp if the old cats are clogged or not flowing well.

j9fd3s

the rotary in particular doesn't like backpressure, as there is no exhaust valve.



the 81-83 cars had the pellet style converters stock, which suck. upgrading to the more modern brick style is a nice upgrade for those

blandry23

My stock exhaust would qualify as a correct exhaust correct? Minus the catalyst or old cats of course. I do want to buy a whole new exhaust eventually but do not have the money to do so (Prices ive seen for full exhausts range from $700-1000). If i could work off of the stock exhaust and just decrease the back pressure that would be great but if not it would help a lot if you knew where i could find a cheaper set if there are any out there. And thank you for the input guys it has helped clear things up a lot. I didn't want to take away all the back pressure and end up having the opposite effect that i wanted.

mazdaspeed7

The stock exhaust is restrictive, for the sake of cost, quietness and emissions. You can do far better. Racing Beat and Mazdatrix both offer exhaust components that will bolt up to stock components, so you could start with a cat replacement, and then later add headers and/or catback if you wish. Or for less money, you could probably have a muffler shop do something for you. But be prepared to buy flanges beforehand and take them with you if you dont want your exhaust to be welded together everywhere.

blandry23

what would be the best size pipe to use for the 12a. Racing beat ranges from 2-3 inch pipes. And also, they have a pipe that just takes all the cats out of the equation. My only question about that is that i live in NY and i think they have adopted the california laws so would that mean that im subject to a visual inspection every time i get the car inspected?

blandry23

a better question would be would gutting my cat decrease the velocity of my exhaust? cuz if i can gut the cat i will just so i can pass visual inspection if necessary.

mazdaspeed7

It will help with flow, but not as much as a cat delete pipe. You will need a cat to pass the sniffer test part of emissions though.

blandry23

ok. i'll probably keep my cat and not gut it just in case to bolt on during inspection. i don't know if they do that stuff where i live cuz i read up on it more and they said that in Cali they only do smog tests and all that extra stuff in urban/more populated areas. I live in a suburban area but you never know. It might help that my dad is a cop and the place where i get it inspected is where he takes all the crown vicks to get inspected/fixed and repaired so they might give me leeway about it. I did try taking my cat off today and the bolts wouldn't budge. I used pb blaster and let them soak for a while and even tried an air powered rachet that does 50 ft/lbs torque. Woulda tried an impact wrench but couldn't find the conversion from 1/2 to 3/8 so the socket would actually go on it. Have any suggestions on how to free them?

mazdaspeed7

If you gut the cats, you wont pass an emissions test.



Youll need a lot more than 50 ft lbs to get those bolts off. Definately an impact gun or at least a 2' long breaker bar. Those exhaust bolts sometimes are so seized up the stud will break before the nut comes loose. So Id be prepared to replace some studs too.