Replaced HF Cat with Straight pipe

FWIW, today I replaced the HyperFlow Cat on my 328 with the HF test pipe. I could not tell any difference in sound between the cat and the straight pipe just as I could not tell any difference between the oem Cat and the HF cat when I replaced the oem with the HF.

Perhaps if there was a way to A-B test the sound immediately, switching between them, there may be some difference but driving the car with the oem and then the HF, I could hear no difference and again driving with the HF and then the straight pipe, I could hear no difference.

There was absolutely no difference that I could tell in how the engine acted with any of the three.

As I said in an earlier post, I replaced the oem with the HF because I ASSUMED (wrongly) that the oem was probably clogged due to its age. It wasn't. I replaced the HF cat with a straight pipe because my car is "historic" and no longer has to pass any emissions inspections in MD. Since my "slo-down" light system had quit functioning, I didn't want to have a CAT with no way to monitor it for overheating.

I could tell no difference in exhaust smell at all between the cat and no cat. There is a rich smell at cold idle, just as there was with the cat. Once the engine is warm there is no rich odor - again just as with the cat.

If you do not have to pass an emissions test, I would recommend a straight pipe over a cat to avoid any issues with potential overheating cats but there is no difference in performance and if you want a louder exhaust (I didn't) the straight pipe makes no difference that I can tell.

I bought the HF straight pipe from Ricambi. Yeah, it costs a lot more than just having one made up because it's chrome and has the fittings for the O2 sensor AND the overheat thermocouple. The Tcouple serves no function with the straight pipe but I prefer the elegance of having a fitting as opposed to tying/clamping the thermocouple out of the way.

THis added later: I just realized that for accuracy and comparison purposes I should (re)state that my 328 is running the more advanced Base (euro) ignition timing, not US ignition timing.

 

From my understanding, the euro exhaust has the two header outlets each feeding into a separate muffler input. THe US system has the two header outlets joining to feed into a single Cat and on to a single muffler input. So replacing the Cat with the straight pipe does not change the system in to the euro setup. The euro system would be more like a true dual exhaust although the same single muffler is used (at least it looks like the same muffler.

 

I was just referring to the 328 - The US 328's had a single Cat. I don't know anything about the 308's/what they used. But I would ASSUME that any "slow-down" light set up that could differentiate between banks 1-4 and 5-8 has to have dual cats. There would have to be two cats/two thermocouples to activate the two different lights. Maybe only the 328 had a single cat?

 

" but in Colorado, 1982 vehicles will always need emissions checks"

Is it an actual emissions test or just a "visual" inspection? In some states they don't do a test of the emissions on cars of a certain age but they check to see that all the emissions equipment is in place. If that's the case, you could "gut" the cats and have the same thing as a straight pipe but look completely stock. There's a guy here at the marina that does that with his old cars - though I've forgotten what state his cars are in. Here in MD, once a car has an "historic" plate it is not subject to any inspection at all which is nice. Though to register it the first time in MD it had to pass a very rigid technical inspection. In fact, I don't know if the PPI on the car was as thorough as the required MD inspection. But it's a one-time deal and is not required periodically.

 

"But aren't historic has limited mileage that can be driven per year?"

In MD there is no mileage restriction HOWEVER the law says it can only be driven to events and NOT for daily driving. However, in practice there are several beat-up trucks around that are used for delivery service that have "historic" plates. Nobody I know who owns a vehicle with historic plates pays any attention to the "rules" and I have not yet heard of any issues at all.

One of the folks here at the marina is a State Trooper and he said that there's NO chance that anybody in law enforcement is going to stop you re a historic plate no matter how many times they see you.