Very Good Exhaust tech link

[JBrady]

I enjoy all HP automotive applications and frequent other forums. Did a search for the Subaru WRX and ended up finding this company, Cobb Tuning. Their downpipe for the turbo Subaru's is second to none and they have a six page description of what they did and why. It has pictures and details including great info on the differences in catalytic converter construction, performance and durability.

Highly recommended reading.

Remember, these are turbo engines and exhaust flow requirments and response characteristics differ from non-turbo engines. I ran a 3" system (designed and tig welded myself from mandrel) on my old TBird 2.3 turbo engine even though it only made about 250hp (pretty good for the 80s). I also intend to test 2.5" vs 3.0" for my 300+hp 1999 LS400 engine. While the low end response on the turbo engine was enhanced by the 3" pipe on my non turbo 1UZFE it may be hurt... hence the need to test. This is for the center single pipe section. I will use 2.25" for the dual sections. I would actually try 2.125" if available but going with 2.25" as best compromise.

Here is the link:

http://www.cobbtuning.com/wrx/wrx-downpipe.html

 

[Pro]

that work is second to none. nice find there J and thanks for sharing.

i would wonder why they didn't provide a seperate pipe for the wastegate gasses to join the main exhaust further down the dumppipe with that much research...? maybe it was to retain the factory look.

by splitting the wastegate gasses away from the main exhaust turbine and introducing them even just 10cm (about 3in) further away it reduces turbulence at the turbo outlet and gives way for a nice, smooth exhaust flow.

certainly a procedure i will be looking at.

 

[LEXUSV8NZ]

pro240c,
If you are considering this setup (which is very good) I would recommend that you kep them seperate for as long as you can 20cm (8") at leat and more is better, also considering flaring the pipe after you join the waste gate pipe into it(from what I hav read/researched this semms to be the best way)
Im sure J Brady can give you better advice than I can on this

Logan

 

[Pro]

oh yeah, the longer the better for sure, but even a small staggering of the outlets can make a big (well...noticeable) difference.

thanks for clearing that up

 

[JBrady]

pro240c,
If you are considering this setup (which is very good) I would recommend that you kep them seperate for as long as you can 20cm (8") at leat and more is better, also considering flaring the pipe after you join the waste gate pipe into it(from what I hav read/researched this semms to be the best way)
Im sure J Brady can give you better advice than I can on this

Logan

Thanks to everyone for their kind words.

The answer is that everything is a compromise that depends on your goals...

Here is additional (and very good/recommended reading) from Cobb. Here is the link for their exhaust theory:
http://www.cobbtuning.com/tech/exhaustdesign/index.html

Here is an excerpt:

Divorced Wastegate - Keeping the gasses from the turbo outlet and wastegate separate until farther back in the system is an attempt to combine the advantages of not collecting the gasses and the real world. Combining them far back is closer to optimal than collecting them closer to the outlets. It is also critical to power production and spool-up to join the pipes smoothly and avoid turbulence. The disadvantages are that you add a lot of cost and complexity. You have big temperature differences on each pipe and that makes for a system that can crack. Putting in flex or expansion joints helps, but adds even further complexity and yet another part to fail. With all of the exhaust systems we have tuned with on the dyno we have seen that it is generally harder to bring boost on as quickly with these types of systems as compared to the bell mouth type systems. Perhaps it helps the wastegate function too well. Also, we have had a few situations where the splitter caused problems allowing the wastegate to function properly by not allowing it to open to its full extent, or even open at all. That caused either boost spiking, or no control over boost what so ever. Since the wastegate could not function the turbo ran as if it did even not have one, and the poor turbo just ran whatever boost it could make uncontrolled. The fix was not hard, but the least amount of stuff to go wrong the better. I know that I would not be happy having to pay for someone to install the exhaust only to have another place diagnose the problem, remove the exhaust, repair the part, and re-install the exhaust.

 

[LEXUSV8NZ]

Or just vent it externally... No problem

Logan

 

[Zuffen]

Externally vented can be annoying and is in breach of Australia's EPA laws.

Whilst I don't pay too much attention to the finer details of the Law as regards legality of my car, I do know blow off valves are pretty easy for a copper to detect. The defect notice is a pain.

There are also times when you don't want to look like a "Rice Boy" and not have your car fart every time you shift gears.

 

[LEXUSV8NZ]

Waste gates not blow-off valves

Logan