Removing the Cat from aftermarket exhaust ?? gain in HP 15-30??
08-24-2001, 08:23 PM
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Removing the Cat from aftermarket exhaust ?? gain in HP 15-30??
is this true if u remove the Cat and replace it with a staight pipe u will gain 15-30hp. if so is it bad good i know its ilegal in some states ??????? thanks guys
08-24-2001, 10:23 PM
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more like 2-3HP, and it is illegal in ALL states
Corky Bell of the book "Maximum Boost" gives all kinds of techniques for making more power and he specifically mentions that you shouldn't bother taking off your cats unless you've done everything else first. It just doesn't make a noticable difference.
It is completely illegal in all states. If your car comes with a cat, it is illegal to remove it, even if it would pass emissions without it (a few cars do). Furthermore, it is illegal to replace it if it isn't malfunctioning. This technically makes it illegal to put a high-flow cat on. But no one cares.
Note that most high flow cats bypass at high flow rates and therefore are about as efficient as removing the cat as well as dumping as much hydrocarbons into the air also under moderate to heavy acceleration.
It is completely illegal in all states. If your car comes with a cat, it is illegal to remove it, even if it would pass emissions without it (a few cars do). Furthermore, it is illegal to replace it if it isn't malfunctioning. This technically makes it illegal to put a high-flow cat on. But no one cares.
Note that most high flow cats bypass at high flow rates and therefore are about as efficient as removing the cat as well as dumping as much hydrocarbons into the air also under moderate to heavy acceleration.
08-25-2001, 12:09 AM
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Not worth it. Minimal HP gains.
I'll give it to you that you will gain at a maximum of 10HP, but most likely you will gain anywhere from 2-5 at most.
I dunno how strict it is everywhere else, but here in CA the citation itself is $2500 for having no cats. Not worth the risk for so little HP.
I dunno how strict it is everywhere else, but here in CA the citation itself is $2500 for having no cats. Not worth the risk for so little HP.
08-25-2001, 09:07 PM
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You'll gain more than that. (long)
not to mention a lot less turbo lag. The first couple feet of the exhaust are the most important.
Because the way a turbo operates, as the air is coming off the turbo, it is spiraling, and very turbulent. On the 1.8T, the cat is located right off the turbo, this is great for quick light-up, but terrible for performance. The turbulent air slams into the cat, creating backpressure.
A turbo works on pressure differences. You want faster spoolup, you have two options, increase the pressure in the exhaust manifold, or decrease the exhaust backpressure. Which is easiest?
Anyways, back to the subject at hand - as the exhaust flows it straightens out, cools down, and takes up less volume. This means the first couple of feet should be at least as large as the rest of the system.
I plan on pulling the cat off, and relocating it under the car. I may try replacing it with a hi-flow cat, but I'm not sure yet.
A normally aspirated motor is a different story, it's worthless to pull the cat unless it's extremely restrictive.
Because the way a turbo operates, as the air is coming off the turbo, it is spiraling, and very turbulent. On the 1.8T, the cat is located right off the turbo, this is great for quick light-up, but terrible for performance. The turbulent air slams into the cat, creating backpressure.
A turbo works on pressure differences. You want faster spoolup, you have two options, increase the pressure in the exhaust manifold, or decrease the exhaust backpressure. Which is easiest?
Anyways, back to the subject at hand - as the exhaust flows it straightens out, cools down, and takes up less volume. This means the first couple of feet should be at least as large as the rest of the system.
I plan on pulling the cat off, and relocating it under the car. I may try replacing it with a hi-flow cat, but I'm not sure yet.
A normally aspirated motor is a different story, it's worthless to pull the cat unless it's extremely restrictive.
08-25-2001, 11:53 PM
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Much respect, but I'll take Corky's word over yours
I know you know a ton about turbos and you are very helpful to others, but I'll take Corky's word over yours.
I also don't think the question was about the precat, just the normal cat.
Do you feel the 12HP loss when the S4 migrated to the US was due to the precats?
I also don't think the question was about the precat, just the normal cat.
Do you feel the 12HP loss when the S4 migrated to the US was due to the precats?
08-26-2001, 11:17 AM
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I don't know a whole lot about pre-cats....
We don't have them on the 1.8T I assume they are just a cat, buy maybe not as restrictive?
Does Corky's book say anything about placement? If you stick a cat on the end of the exhaust, you may lose only a couple HP, but stick it right at the turbo outlet and you'll lose more.
Check out the link below for a dyno test that shows before and after a downpipe/cat removal. You'll notice the power comes on sooner, and there are gains all accross the board. Take Corky's word all you want, but this dyno plot says otherwise (14 HP, 15 ft lbs of torque)
A 12hp loss could be a lot of things, less timing because of the lower octane gas in the US, pre-cats, etc.<ul><li><a href="http://www.jwelty.com/18t/dyno-giac-apr3inch-dp.jpg">http://www.jwelty.com/18t/dyno-giac-apr3inch-dp.jpg</a></li></ul>
Does Corky's book say anything about placement? If you stick a cat on the end of the exhaust, you may lose only a couple HP, but stick it right at the turbo outlet and you'll lose more.
Check out the link below for a dyno test that shows before and after a downpipe/cat removal. You'll notice the power comes on sooner, and there are gains all accross the board. Take Corky's word all you want, but this dyno plot says otherwise (14 HP, 15 ft lbs of torque)
A 12hp loss could be a lot of things, less timing because of the lower octane gas in the US, pre-cats, etc.<ul><li><a href="http://www.jwelty.com/18t/dyno-giac-apr3inch-dp.jpg">http://www.jwelty.com/18t/dyno-giac-apr3inch-dp.jpg</a></li></ul>
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08-26-2001, 02:11 PM
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no, Corky doesn't mention precats, that's why I asked you
And this dyno plot would appear to be the normal (2.5"?) downpipe w/cat versus a 3" downpipe without cat. The gain could be as much or more from the downpipe upsize than the cat. I dunno.
08-27-2001, 09:08 AM
#9
you don't remove it... you just knock it out...
leave the cat body there and just hollow it out (for track use of course) 
this will avoid any citations and the car
will still appear to be legal.
HTH!
rich
this will avoid any citations and the car
will still appear to be legal.
HTH!
rich
08-27-2001, 11:03 AM
#10
Pre-Cats are just a smaller catalytic placed closer to the engine. The 1.8T doesn't have one, just
ughh the main cat mounted as close as it can go. The pre-cats are smaller in design to heat up rapidly but they are the same 3-way reduc/oxidize cat as the main cats usually located further down stream. Mainly there to help start up emissions - usually CO because NOX is a byproduct of higher temps - and stoich too rich or lean.