Replacing cat with straight pipe in 328?

A properly designed test pipe will have provisions for those sensors (hint: avoid Stebro). The sound will be louder. You may want to look at a modern cat replacement like Hyperflow instead.

 

The O2 sensor needs to be present in the exhaust stream when using either the cat or the testpipe. The thermocouple needs to be present when using the cat, but, for the testpipe, it doesn't matter -- if the testpipe has a fitting for the thermocouple, use the thermocouple to "plug" the hole; if the testpipe doesn't have a fitting for the thermocouple, just neatly zip-tie the thermocouple and its wiring to the chassis (without a cat, it is impossible for the exhaust stream to overheat so, even if the testpipe has a fitting for the thermocouple, it really isn't doing anything functionally).

 

Legality...it's a Federal violation to tamper/remove emissions equipment...

 

I see you are in Canada, so disregard my statement...

But the modern high flow cats certainly pose no penalty on performance....it'd be a good idea to update an older expired one anyway they don't last to infinity....it's a chemical process after all...

 

Precisely....

 

The straight pipe will do nothing at all performance wise. It will eliminate the potential for a fire caused by an overheating CAT (caused by a faulty ignition/fuel deliver system). The cat will eliminate the "fuel" smell from the exhaust.

Assuming you are not legally required to have the Cat, take your choice.

 

What Mike says above is true, unless you have a partially blocked cat, in which case you'll have a much better running car with a straight pipe. I've seen several partially plugged 3X8/mondial cats.

There is a bit more noise with a straight pipe.

When you're looking at which straight/test pipe to buy, make sure the one you do end up buying has bolting flange mounted such that the pipe actually extends past the flange by a half inch or so. Why? Because the tapered compression gaskets need some internal support otherwise they just collapse in on themselves. The stebro straight/test pipe doesn't have the extra half inch and it's a problem.

 

This is exactly what I did Steve, thermocouple on the test pipe is not needed.
If test pipe doesn't provide fitting for the thermocouple it's ok to leave it out.
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Nothing wrong with what you've done, but you are really just "storing" the thermocouple by attaching it to the testpipe outer surface -- regardless of how much unburned fuel is present in your exhaust stream, it won't ignite, so the temperature of the testpipe will never reach the temperatures that the thermocouple is designed to detect. You can just as easily use some zip ties to attach it to a chassis member and "store" it there.

 

Are you sure there is more than one sensor on your cat.? Like you, I have a Swiss Market 328 and there is no O2 sensor because there is no frequency valve to vary fuel mixture. I only had a thermocouple. I replaced my cat. with an Ansa straight pipe and simply tied the thermocouple down in a convenient location. Incidentally, the Ansa straight pipe does have bungs for thermocouple and O2 sensor if you desire to use them (exhaust temp. guage, A/F mixture guage).

It has been quite a while but I do not recall the exhaust being much louder at all with the straight pipe. I'm sure you will NOT notice any performance increase by eliminating the cat. unless yours is really plugged up from age/damage. Adding a Tubi, which I also did, will certainly make it louder.

Having said all that.... mine is 1988 Swiss Market and yours is a 1989. Maybe they added an O2 sensor and frequency valve to the final version. Or maybe I'm not reading your profile correctly. You have written "1989 Euro 328 GTS Swiss with cat conv." It really should be classified as a "Euro" or a "Swiss." Not both. They are not the same (except from the geographical standpoint of Switzerland being in Europe

PS. my car is in storage in Europe while I'm currently in California so emissions laws/tests over here do not come into play.

 

The hyperflow that I have has the fittings for both the thermocouple and O2 sensor so I hooked them both up. The cat I had was shot and the "test pipe" was half the price of a new cat. Since there are no car inspections here in Florida, what the hell. Little louder and I get more of an exhaust smell when stopped in traffice, ect. other than that not much difference than when the cat was on the car.