I was just wondering why Porsche dumps the wastegate downstream of the catalytic converter. I was looking at some of the cat bypass pipes (like Fabspeed's) and I noticed that they designed their pipes to dump before the catalytic converter.

Would emissions be any cleaner plumbing the wastegate to dump pre-cat? Or does it make no difference at all?

I just figured that if Porsche engineers desiged it that way, there must be a specfic reason why it's done so I just want to understand.

Thanks.

 

You're right, there is a reason.

I cant remember it right now, but I believe it is better for performance/exhaust flow. It was mentioned at some point on here, you may want to try the archives.

Also the wastegate dump has its own cat (as you may have noticed).

 

Heh, I tried to do a search but I think I didn't word the search correctly. I recently passed the California smog test and I'm just wondering if plumbing the wastegate outlet before the main cat was a main factor in my 25mph NOx readings coming up so low.

I guess I should just be happy that I passed, huh?

 

Anyone just not plumbed it back in?

 

NZ951

Yeah.. sounds stupid when on boost. Did that about 5 years ago. Drove it around the block and went back to the muffler shop to put it back.

TonyG

 

I can't imagine it would even matter, unless you're getting boost on the 25mph dyno.

 

Could you elaborate on stupid? Any performance difference?

 

i cant find the thread either. the thread was about test pipes and where they dump the WG output back into the exhaust....and how there wasnt a test pipe on the market plumbing the WG back in the correct spot (further down the pipe). very good thread w/ multiple pics, tidbits from some turbo guru's book, etc...

ps searching for the terms: test pipe plumbing wastegate yeilds you every other thread on the turbo board.

NZ i think what tony is refering to in the sounding stupid comment was venting the WG to atmosphere. once it starts bleeding off boost i would think the car would sound like a tractor heh.

 

Tractor? But at cruise is normal yes?

 

Ah, good point! I wasn't even thinking about that. I wonder if there is a performance hit or degredation plumbing the wastegate exhaust before the cat IF I leave the cat in. I guess if/when I install the test pipe it should make little difference but if I leave the cat in, will it cause a noticeable increase in backpressure or loss of power?

 

iirc it has somthing to do w/ timing the exhaust pulses / flow.... was it Danno maybe who mentioned this in the thread w/ all the exhaust diameter discussion? i don't remember off the top of my head...

 

It's to reduce backpressure on the exit side of the valve. There will be much higher pressure before the cat than after. That backpressure could effect the operation of the wastegate, since most of them are designed not to have pressure on both sides. The dump tube also has a small cat in it, but I question it's effectiveness because it rarely gets to operating temp.

 

Ah thanks for the info Perry and I loved your "Ill Fated Rebuild" story. I had to stop working to read the entire saga. Ok, well that is an answer that makes sense to me so for now I'm running an aftermarket "hi-flow" cat through 3" pipe. Hopefully it won't cause too much backpressure for now, but I guess I can always install the test pipe for more free flowing exhaust if it does.

Regards,
Aleks

 

Your high flow cat will cause less backpressure than the OEM one... no worries for that. The issue is the amount of pressure from the outlet of the turbo hot side to the entry of the cat.

For example, the crossover sees 10-15psi on a stock car, and the downpipe usually sees 5-10psi. After the cat, it's only a few psi, so you can see this could effect how, and when the wastegate opens.

 

As far as the sound goes...I ran mine like that for a while to diagnose a leaking wastegate. While going through an underpass you could hear the "psss, psss, psss, psss" sound of boost leaking off of the wastegate while accelerating. When you let off the gas you're rewarded with an obnoxiously loud "whoooooosh". It's a little rice for my taste but for some it may work. I did also notice a slight bit of performance increase in relation to how quickly I was able to build boost. It was minimal but it did build boost a bit quicker.