i race a 2002 pontiac sunfire with a 2.2 ecotec and in the intrest of removing weight and saving money, i removed my cat. will this effect my engine performance? and im not looking to spend money on an upgrade unless it appears stock. but the way i see it unless i upgrade the air filter (witch is against the rules) and port and pollish the head adding a bigger exaust wont gain me extra hp... right?

Sam Braun wrote:i race a 2002 pontiac sunfire with a 2.2 ecotec and in the intrest of removing weight and saving money, i removed my cat. will this effect my engine performance? and im not looking to spend money on an upgrade unless it appears stock. but the way i see it unless i upgrade the air filter (witch is against the rules) and port and pollish the head adding a bigger exaust wont gain me extra hp... right?

I deleated my cat and had a 2.5 header back ehaust made my car a little faster. Cel will be on of course unless you have hpt I suggest buying that first, you can set your car at 7000 rpms
Do a underdrive pulley cheap and easy up grade to add power

You might see a small gain because the exhaust chokes down some before the cat but its not going to be anything your butt dyno will feel IMO.

I got my cat removed from my car by some thieves. Did a header into a 2.25 inch cat delete into stock exhaust and I did notice a little more pickup but definitely still slow as hell.

well idk if its just a myth or not, but i heard that without that back pressure that the cat provides the car wont run right... i guess it dosent matter if what your telling me is correct.

If you have two o2 sensors in your car you might have a bit of issues with them both reading the same because one is after the car

cat doesnt provide back presure back pressure is bad in any situation the cat in some cases can actually help exhaust flow because a gas flows better hot and the cat heats the exhaust back pressure will cause a car to not accelerate at all on a small engine like ours half a psi can choke the engine out to the point of big driveability problems

jeremy spencer wrote:cat doesnt provide back presure back pressure is bad in any situation the cat in some cases can actually help exhaust flow because a gas flows better hot and the cat heats the exhaust back pressure will cause a car to not accelerate at all on a small engine like ours half a psi can choke the engine out to the point of big driveability problems

Its called a period.

All exhaust systems have back pressure. You need back pressure for exhaust scavenging. Anything in the exhaust system will affect back pressure. Pipe diameter, bend, changes in volume, baffling. I'm calling

Ive seen people LOSE power by adding cut-outs too far forward in the exhaust system. I've seen people LOSE power by going with too large of diameter of pipe. That back pressure creates velocity in the exhaust. You need velocity in any exhaust or intake system.

Brian is the only competent poster in this thread. Deleting the cat won't make a noticeable difference and the CEL can be taken care of with the antifoulers trick.

Did you knock the guts out of the cat or weld in a straight pipe?

Nope, I just cut the cat off and went racing. The exhaust is only about 3 feet long... btw ty for all the info

Sam Braun wrote:Nope, I just cut the cat off and went racing. The exhaust is only about 3 feet long... btw ty for all the info

Thats fun as hell. I may have bombed around in the country like that for a day.

No problem. Have fun!

how will the o2 sensor be affected by knocking out the guts of oyur cat?

running a 97 cavi 2.2. noticed an o2 sensor behind the cat was wondering how the car would pass smog with out it having a cat.... any way around that with out failing smog?

When I'm helping buddies with their exhaust systems I try to keep the scavenging up as much as possoble. Back pressure is sort of part of the equasion.

Most of the higher ecotec HP guys run 2.5"-3.0", but they had way more bolt ons then you probably do. Removing the cat if anything will shift you power band up slightly.

I would run an anti fouler to make sure the rear o2 still thinks it is working. Our cars like to add fuel if the cat is messed up.....which makes no sense at all. Our cars already run fairly rich from the factory.

I would get a tune, and either turn off or greatly reduce the torque management in the pcm.


Stockish ecotecs gain around 10hp with a good tune, and are much faster off the line with lessened torque management.


PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

I love the backpressure myth. Mike touched on it that it's "sort of" part of the equasion.

Simple explanation:

First, backpressure is bad, period. It doesn't help scavenging, it doesn't help velocity, except when the exhaust is oversized for the powerband you are looking for:

Velocity is maintained when the pipe diameter best matches the volume (we're not talking sound here, to clarify) of exhaust moving through it. The reason why too large of a pipe causes loss of lower end power is because of turbulence created when the volume of exhaust passing through it is much lower than the pipe is sized for. This is basic fluid dynamics. Now, the reason why a car has a shift in powerband when the cat is removed is because the cat slows the velocity (yes, through backpressure) so that the upstream pipe has less turbulence at lower exhaust volume. At higher RPMs, the backpressure is causing an overload in the upstream pipe, reducing the exhaust flow, and reducing the power output of the engine. Bottom line is that the desired power band of the engine needs to be taken into account when sizing the exhaust. Bigger is only better if you're running high RPMs or a turbo (the turbo adds tons of backpressure, so you want it to be able to breathe as much as possible at any RPM).

As for the second O2 sensor causing issues if you remove the cat, it's because the engine computer is expecting a specific range of input from that sensor, indicating a proper functioning cat in the system. Without the cat, the computer thinks the cat is bad, and will adjust the performance of the engine, as well as turn on the annoying check engine light. The better thing to do is to buy an aftermarket high-flow cat in the proper size for your engine. They weigh less than the factory ones, so you will still see weight savings, but it will flow better, allowing your engine to breathe better, and make more power, especially if you add other mods to it.

Ultimately, no matter what you do, you should get it properly tuned to see the most benefit out of the mods.

Yep, mr quick is right, some vehicles will actually stay in open loop when this is done...

Back pressure is no good. Velocity is what you want. Velocity = scavenging.


- Your not-so-local, untrained, uncertified, backyard mechanic. But my @!#$ runs

corey nagel wrote:running a 97 cavi 2.2. noticed an o2 sensor behind the cat was wondering how the car would pass smog with out it having a cat.... any way around that with out failing smog?

go to a junk yard and get a stock down pipe and cat off a car. if you were planning on using the stock piping and just straight piping it do it to the junkyard exhaust and keep the stock down pipe and cat in your garage or whatever(since you know it passes). when you need to pass smog throw on the stock one, go get your smog passed, then go home and put the other one back on. lol i have a couple friends who do this every two years where i live to get around it.

I should have explained myself better, but I didn't feel like typing more. My back pressure comment was towards moving around the power-band. After re-reading my post, i was not clear at all.




PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

Make sure you put a long resonator or you will hate the sound. Vibrant 18 resonators sound nice no rasp.