do I need a cat?

mack6820 · 07-01-2007, 08:38 AM

Hey guys. I recently found out that my cat converter has been gutted out. This probably explains the rich fuel smell. My muffler shop says that if the cat is gutted, then I could be losing horsepower since the exhaust needs a certain amount of back-pressure. So, should I install a new cat?

pmajka · 07-01-2007, 09:10 AM

Needs some back pressure? They all say that.

are you sure its a cat and not a premuffler.

Yes, if it is a gutted cat, exhaust flow could be disrupted and rob you of performance.

mack6820 · 07-01-2007, 09:14 AM

Im sure its the cat, i was gonna have a cat bypass installed so when my mech pulled the cat off, it was hollow, hince no need for a bypass pipe.
The car pops alot on deceleration and has a bad exhaust smell when idling, like at a redlight. The mixture is set right.

mack6820 · 07-01-2007, 09:15 AM

sorry, I didnt mention its an 83sc with a 3.0L.

dbrisson · 07-01-2007, 11:52 AM

I have an 82 SC 3.0 with a GHL single in single out with the cat bypass pipe. The popping and gurgling are the natural sounds without the cat. I love the way it sounds and runs. I get a little more mixture smell but I am running a bit rich.

Set on the lean side the popping will start to decrease a bit, or at least mine settled down but I then couldnt get the idle correct, so I went back to the richer setting.

911nut · 911nut's Garage

I did a back to back with a gutted cat and a bypass pipe installed on my old SC. The butt dyno liked the bypass pipe a lot better.
As far as the rich mixture setting goes, once it's set (3.0% CO) make sure you leave the O2 sensor disconnected so the K-Jet runs open loop. This will also get rid of the annoying low speed surge from the lambda controller overshooting the correct fuel distributor pressure differential.

mack6820 · 07-01-2007, 01:44 PM

so let me get this right. Installing a new cat wont give me more horsepower? Also, I should disconnect my O2 sensor? Is the K-Jet the same as CIS?

bigchillcar · bigchillcar's Garage

shouldn't the decrease in total surface area in the bypass pipe, also called a 'test pipe', make up for the gain in volume of the cat without the honeycomb material that results in his car's symptoms? that's the whole point in making the bypass pipe a smaller diameter. don't know which year model your car is, but assuming it uses a chip, you might contact steve wong about having a custom chip made to help compensate for gutted cat or use of a bypass pipe.

911nut · 911nut's Garage

Quote:

Originally posted by mack6820
I should disconnect my O2 sensor? Is the K-Jet the same as CIS?

K-Jet is a model of CIS.
Let me clarify. You would need a Gunston or some kind of exhaust analyzer and a CIS mixture wrench. You would warm up the car, disconnect the O2 sensor and use the analyzer to set the mixture to 3.0 % CO. Then you would leave the O2 sensor disconnected.
If you don't want to do that or don't have access to an analyzer just leave the O2 sensor connected.

bigchillcar · bigchillcar's Garage

to follow up on paul's advice, the equipment ain't cheap for you to buy. just call around..virginia beach is gonna have a p-car shop with an exhaust gas analyzer and can easily make this adjustment. i found a shop once for my '74 and the guys, not being p-car familiar, just hooked it up and let me make my own adjustment with my wrench.

mack6820 · 07-02-2007, 03:24 PM

but what's the benifit of disconnecting the o2 sensor? more horsepower?
sorry if I seem needy, this is my first P-smile

911Mac · 07-02-2007, 03:49 PM

If you set the CO to 3% your AFR will be a little richer than 14.7:1, your engine should develop a little more HP. 14.7:1 is the AFR at which theoretically all the air and gas is burned, this is best for emissions, but not necessarily the best for top HP.
If you hook up the o2 sensor, the lamda control will try to lean out the mixture to maintain an AFR of 14.7:1, defeating your tuning efforts.

Mac

bigchillcar · bigchillcar's Garage

with the o2 sensor it's referred to as a 'closed loop' system in terms of the sensor's feedback to the computer, without it we call it running 'open loop'.

jasonlee · 07-07-2007, 10:52 AM

I run a cat bypass, get all the fun popping sounds on decleration. Sounds good to me. My mechanic has tuned the mixture to run without my O2 sensor hooked up. Then I just plug in the o2 sensor 3 or 4 days before emissions test and its all good. Worked last year.

mack6820 · 07-07-2007, 11:26 AM

Jason, was there a noticable horsepower change after unplugging the 02 sensor?

onlycafe · 07-07-2007, 12:39 PM

mack.
for each ten pounds of weight your car loses you will gain about one extra horse power, give or take. eliminate rear seats and sound proofing, ditch your spare and jack, go to an odyssey battery, get rid of your console and door pockets and you are close to one hundred pounds of lost weight , or about ten free ponies.
then you can start spending to get more.

VaSteve · 07-07-2007, 04:10 PM

I think the equasion is more like 100 lb/hp not 10 lbs/hp. Otherwise, when I dump my instructor, I'd be getting like 200 hp.

Joe Bob · Joe Bob's Garage

Personally....you are modifying a pig....it'll be a pretty pig, but still a pig.

Trash the stock heat exchangers and go with SSIs or early (73) Heat Exhangers. They are equal length, act like headers and still give you heat......run a 2 in and 1/2 outlet depending on how much noise you can put up with.....gutted sport muffs are fun but tend to pi$$ off the girlfriend and the local cops.

Adjust the mixture afterwards....as to the 02 sensor.....it's a toss up after the SSI mod.