cat vs catless

[jakedoza]

So, I emailed Cobb Tuning to see if they sold a Catless DP.. This is the email I got back..

Thank you for contacting COBB support!

We always recommend using a downpipe with a high flow catalytic converter, not only to help meet emissions standards, but also for performance and safety of the car. In many cases, using a catless downpipe can cause boost creep, or uncontrollable boost. This is typically something that tuning can not fix as it is a mechanical issue that can only be resolved with a catted downpipe, porting the wastegate, or by using an EWG with a custom tune. You should keep a close eye on your boost levels to make sure the car hits the proper boost target and tapers towards redline.

Catless downpipes will not offer a measurable increase in performance and for this reason we do not offer catless downpipes.
Please let me know if you have any additional questions!

Thank you,
Drew Gorrie


Going cat less causes boost creep?

 

[Alin]

There are other variables that aid in boost creep but from what ive seen, a catted dp has a more restrictive/controlled flow rate with the downside of not making as much power. A catless dp alongside other things will cause boost creep.

I advise safety over power.

 

[Batmobile_Engage]

Wouldn't a manual boost controller effectively control boost creep?

 

[Grinder34]

Wouldn't a manual boost controller effectively control boost creep?

Not if the WDC is already at 100%.

I'm no guru, but the way I'm thinking about it is this:

With a catted DP, there's a restriction in the exhaust, so you can only get a max of, say, 29 PSI boost. If your target is 20, your wastegate has to get rid of 9 PSI. That's ok, because in the example your wastegate can get rid of 10 PSI so its at 90% WDC. Now, if you remove the cat, you have no restriction, the exhaust can go faster and you can produce 32 PSI. But your target is still 20! That means your wastegate has to get rid of 12, but thats more than it can do (120% WDC isnt possible). Hence, boost creep.

I dont know if thats the right way to think about it, but it makes sense to me.

 

[War_Panda04]

Not if the WDC is already at 100%.

I'm no guru, but the way I'm thinking about it is this:

With a catted DP, there's a restriction in the exhaust, so you can only get a max of, say, 29 PSI boost. If your target is 20, your wastegate has to get rid of 9 PSI. That's ok, because in the example your wastegate can get rid of 10 PSI so its at 90% WDC. Now, if you remove the cat, you have no restriction, the exhaust can go faster and you can produce 32 PSI. But your target is still 20! That means your wastegate has to get rid of 12, but thats more than it can do (120% WDC isnt possible). Hence, boost creep.

I dont know if thats the right way to think about it, but it makes sense to me.

wouldn't a proper pro-tune compensate for the exhaust flow?

 

[Alin]

wouldn't a proper pro-tune compensate for the exhaust flow?

Keyword is proper. You are correct though. However mechanical issues will always override a tune.

 

[jakedoza]

soo, why isn't this a huge issue since most tuned STis run catless?

 

[TK-421]

Alright, let's first start off with this.

1 a 200 cell catted Downpipe will not flow as freely as catless yes that's true, but a 200 cell catted downpipe compared to a Catless Downpipe has very very very small margins of power loss. The stock catted Downpipe with 2 cats is horrendously restrictive. But a cobb or Invidia catted Downpipe flows just as well as a catted, being the only difference is having the cat creates a small amount of back pressure which helps reduce the boost creep issues that are common on the 2.5 dual cam subaru engines.

2 the reason cobb sells it as a catted with no option for catless is because they advertise all their products to be 50 states CARB OE legal. And in order to do that it has to pass the emissions requirements on the minimum level for each state that requires testing. Some states are different, PA for example is OBD II compliance, they don't use the sniffer test or have a visual inspection of the exhaust system. So technically you could go catless just as long as you don't have any active readiness monitors and a active CEL on the dash.

3 from a tuning stand point, if you want reliability and drive ability, you stay catted. The further you alter the car from manufacturers guidelines, the more unreliable the car becomes, granted going just catless will not affect the car to the point where it's unreliable as a daily. I myself owned a catless 04 STi back in the day and daily drove the wheels off that thing, not a single issue.

Running catless and having boost creep issues can result from many different circumstances, it's not just the fact that it's catless, it can atone to elevation, other supporting or unsupporting mods, the tune itself, temperature, I've seen plenty of catless cars boost creep like a SOB in the winter and underboost like crazy in hot humid summer months. It all boils down to 3 main things,

A- tune
B- elevation and surrounding weather effects
C- mods used in conjunction with said Downpipe

A lot of the boost creep issues I do see as well come from your typical wing wang ebay knock off stuff. Downpipes that have a cast aluminum Bellmouth section and then transition into the stainless steel exhaust piping seem to resist boost creep better than the other all stainless steel designed ones.

But my personal opinion on the matter is this, your car, your choice, is going catless going to be cheaper than going catted? Answer is the following

Up front cost of parts wise? Yes, but you will have to factor in a tune, how much will your tuner charge you to tune for that Downpipe? Sometimes the cost of the tune plus the Downpipe out weighs the cost of a catted Downpipe that follows cobb's guide lines for the AccessPORT. And your honestly not going to see any kind of significant gain that you would be able to feel by using one over the other. If your chasing dyno numbers then go catless. If you want a fun reliable car that follows cobb's guidelines for stage 2 stick with the catted Downpipe.

Personal preference folks.

 

[Hulakatt]

Dunno but I've had a Cobb turbo back setup on my 06 STI for 3 years now and I'm still impressed by everything about it, from the build quality sound to the look to the performance. I've had many oem parts not bolt up as easily as the Cobb stuff.

 

[Grinder34]

I think Cobb is a little like the Apple of the subie world. Maybe not technically the best, but everything just kinda works together in it's ecosystem.

 

[Batmobile_Engage]

I'm a big fan of Cobb's SS 3" catback, but I usually recommend a different downpipe such as the Invidia V2. The Cobb dowpipe despite being advertized as 3", necks down to 2.5" in the throat. I believe they do this to help control boost creep. However, a proper setup and good tuner can control boost creep itself without sacrificing smooth exhaust flow post turbo.

Also, I'm fairly sure that the Cobb downpipe doesn't have a wastegate flow divider like the Invidia. It's always a good idea to have a divider or partially divorced wastegate runner. Otherwise, the smooth exhaust flow from the turbine exducer is disrupted when the wastegate opens and exhaust begins to exit that port as well. Seperating the flows until they hit the throat of the downpipe is always best for spool and boost stability.

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