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To remove or not to remove CATS

T

topgearsvt

CEG'er
Joined
Apr 2, 2011
Messages
53
Location
Findlay Ohio
So i was told the other day that removing the main catalitic converter can actually reduse HP and leave you with an annoying sounding exhaust!!! I was told this is because it takes away the back pressure that your engine can use to create more HP! What do you guys think? has enyone ever tried it or had experience with this? Has anyone dyno proven this theory?:confused::shrug:
 
topgearsvt said:
So i was told the other day that removing the main catalytic converter can actually reduce HP and leave you with an annoying sounding exhaust!!!
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Hey topgearsvt; i can definately agree with this. You may get a lot of rasp at about 3500rpm. Don't think de-restricting the exhaust reduces power though.

topgearsvt said:
I was told this is because it takes away the back pressure that your engine can use to create more HP!
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again, don't think back pressure increases power. Headers are the way to go and a high volume (sport cat) to give max power and no added noise. ...G.
 
I was wondering the samething i wanted to do the same an maybe make my own headers for my svt with the o2's i was told without the o2's the mpg can be worst , i also dont want the car to sound like i have a fart cannon on the back i like the stock sound it sounds mean not like some jap rice burner thing but wanna get the cats off without losing that mean sound
 
Someone can correct me if im wrong, but I understand (from what i have heard) that backpressure reduction in NA cars can reduce torque. I think this is just low end torque though.
 
Max Power18 said:
Someone can correct me if im wrong, but I understand (from what i have heard) that backpressure reduction in NA cars can reduce torque. I think this is just low end torque though.
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i think this explains the two properties that some folks confuse;

Really it shouldn't be called back pressure because back pressure = loss in performance.
Basically; a "proper-free-flowing" exhaust exhibits properties which give it a "scavenging affect". Your exhaust has areas of low pressure (a vacuum) that helps suck exhaust out of the cylinders. This is only achieved if the exhaust is the correct size which allows gas to be accelerated as it exits the engine, hence creating a low pressure in the exhaust. If your exhaust is too free flow/large, it loses it's ability to affectively accelerate the gas in the exhaust and cannot create a low pressure to "scavenge" the gases from the cylinders. Headers are very important in this. Smooth bends, no cat, but proper exhaust diameter (not too big, not too small) also help in the scavenging effect. If you've ever felt your exhaust, you'll know it pulsates. Think of each pulse as a ball of energy leaving the engine via the exhaust. Trailing each ball, is a vacuum, which in turn helps suck the next "energy ball" out of the cylinder. If your exhaust is too large, you lose the "air seal" for the energy balls and cause turbulence(and turbulence causes back pressure as well), too small and you get back pressure.

hope that helps .... G.
 
gorman said:
i think this explains the two properties that some folks confuse;

Really it shouldn't be called back pressure because back pressure = loss in performance.
Basically; a "proper-free-flowing" exhaust exhibits properties which give it a "scavenging affect". Your exhaust has areas of low pressure (a vacuum) that helps suck exhaust out of the cylinders. This is only achieved if the exhaust is the correct size which allows gas to be accelerated as it exits the engine, hence creating a low pressure in the exhaust. If your exhaust is too free flow/large, it loses it's ability to affectively accelerate the gas in the exhaust and cannot create a low pressure to "scavenge" the gases from the cylinders. Headers are very important in this. Smooth bends, no cat, but proper exhaust diameter (not too big, not too small) also help in the scavenging effect. If you've ever felt your exhaust, you'll know it pulsates. Think of each pulse as a ball of energy leaving the engine via the exhaust. Trailing each ball, is a vacuum, which in turn helps suck the next "energy ball" out of the cylinder. If your exhaust is too large, you lose the "air seal" for the energy balls and cause turbulence(and turbulence causes back pressure as well), too small and you get back pressure.

hope that helps .... G.
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best description iv heard ever. not even wiki was this simple
 
I agree, even I can understand that, and I am an electrical engineer. Fluid sciences never made much sense to me.
 
Well I have a 3L with MSDS headers, no main cat, stock resonator, and stock mufflers and it sounds terrible. I don't think I lost power but I didn't really gain any. The first chance I get I am going to buy a high flow cat with a magna flow resonator and mufflers.
 
if you had more power with a cat, race cars would run them. i personally took them out of both of my SVT's when i built the exhausts. definitely had gains
 
When I installed my exhaust I didn't have any cat's; MSDS headers, Trubendz y-pipe w/test pipe & magnaflow resonated cat-back. I found that from around 2800-3400 rpm it had really bad rasp and made my car sound like a rice rocket. I then installed the cat that came with the y-pipe in place of the test pipe and it sounded way better, didn't really notice any power loss. Even if I did lose power, it would have been worth it to get rid of that annoying rasp.
 
DonKingKong said:
When I installed my exhaust I didn't have any cat's; MSDS headers, Trubendz y-pipe w/test pipe & magnaflow resonated cat-back. I found that from around 2800-3400 rpm it had really bad rasp and made my car sound like a rice rocket. I then installed the cat that came with the y-pipe in place of the test pipe and it sounded way better, didn't really notice any power loss. Even if I did lose power, it would have been worth it to get rid of that annoying rasp.
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X2 and i'm going to run one (cat) with the vortec too ....G.
 
Dead on gorman. Prove it to yourself. Put a hand directly over exhaust on a big twin bike as close as you can. When the pulse blows your hand away, a fractional second later it sucks it back.

I've actually measured vacuum in a well designed race exhaust.........necessary back pressure is a myth straight out of the '50s. Sometimes it helped on misdesigned systems.

Using sonic wave design for pipes CAN sometimes result in needing a SMALL SHORT back pressure 'pulse' if tuning very hard on a 4 stroke race motor, its' effect is similar to the reverse cone used on expansion chambered 2 strokes. After the long drawn out vacuum pulse is used to pull intake harder at overlap time, one can use a small positive backward pulse to stuff intake back in at the exhaust closing. It results in a mild supercharge effect.

'Someone can correct me if im wrong, but I understand (from what i have heard) that backpressure reduction in NA cars can reduce torque. I think this is just low end torque though.'

Absolutely not. Say dual exhaust. You almost always get a torque increase by putting a crossover pipe between the two sides. Guess why that works.
 
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