View Full Version : Cats or Not

Alan K
09-11-2003, 09:30 AM
I am starting the process of completely changing out my exhaust, from the exhaust port on the heads to the tip of the pipes. Are Catalytic converters nessacary to facilitate the scaveging the cyclinders? I would rather have the long tube headers feed directly into the resonator. If the cats can be eliminated from the exhaust system, are there any precaution I need to be aware of?

09-11-2003, 09:57 AM
Cats have nothing to do with the scavenging process. They are for emissions control only. The only way they effect scavenging is to reduce it through backpressure.


Alan K
09-11-2003, 12:44 PM
Not quite true. It's true the cats are for emissions control. However, There has to be some backpressure from the exhaust "system". That is one of the reasons that the exhaust tubes are shaped/twisted in the manners that they are. How much backpressure may be needed by the EEC to sense exhaust gas composition (O2) and regulate fuel/air mixture is what my concerns about the cats are about. Anyway, pastera, a simply yes or no would have sufficed. Any one that doesn't know the reason & function of the cats shouldn't have a wrench in their hand. Precautions about how I treat my machine have keep me from damaging (head gaskets?) this bird.

09-11-2003, 02:21 PM
Just having headers bolted to the heads causes backpressure, not much but some. Back pressure is not totally needed for a good exhaust. And most vehicle, the less you have(up to a point) the better it will work. especially on forced induction engines. Less back pressure means better airflow through the motor making the SC more efficient. If you have straight pipes from the headers back, no muffler and no cats, there will be plenty of backpressure for your engine. And back pressure is not needed for a/f readings. As long as your O2 sensors are functioning properly, the only thing requires is exhasut gasses to be flowing by the O2 sensors for them to read correctly. As far as the reason exhasut tubes are bent twisted the way they are, it's usually for routing reasons, not to create backpressure. Get rid of the cats, and you will be happy. As you said, if you don't know the reason or function behind the catalytic converters, you shouldnt' be screwing with your exhaust. If you have read anything about performance vehicles, you will know most run better without cats installed for less back pressure.

09-11-2003, 02:29 PM
On a tuned set of headers the twists are there to allow all of the header tubes to be the same length. This causes the pressure pulse from the exhaust to take the same time to arrive at the collector for each cylinder. As the high pressure pulse moves through the tube, it draws a vacum behind it. If the tube is long enough, there will be a still vaccum at the intake valve upon opening. When the pressure pulse exits into the collector, expands and looses velocity, the vaccum behind it draws some back into the header tube creating a reverse flow pulse. When perfectly timed this pulse will effectively stop scavenging at the point where raw fuel is about to enter the exhaust port.

The HEGO sensors do not need backpressure to work, but they do need heat. If you exhaust flows so well that they are not heated to operating temperature, they will not work. This would be the only reason for some 'backpressure' to make them work, to reduce exhaust velocity so that the HEGOs can get up to temp.


09-11-2003, 03:30 PM
Nothing 'in the way' can be be neuteral. If it don't help, get rid of it (if you can get away with it). Fair disclosure also makes me mention that I am an owner of both volumes of "101 things to do with a dead cat" and am a dedicated dog lover (Satan had a cat, a dog's only purpose in life is to love and be faithful).

VIC WARUSZEWSKI 1990 5sp SC 178,000 miles
origional head gaskets, cat free/magnaflow/flowmasters
10% pulleys, Cold air, K&N, Ripper and countless
number of Michlin 235/60/16 on the back

We'll have some high times, and live them low.
Get some fast food and eat it slow.
I'm hungry when I get up, and sleepy when I eat.
Only time I feel right, I got the road beneith my feet