catless downpipe

What are the downfalls of running a catless downpipe?

I've always heard that it's the smell, but I can smell my catted dp after a pull too so?

some people have boost creep from the lack of back pressure

None:) Never had boost creep, but it requires a tune, plus you may fail inspection.

 

Hi flow cat or gutted cat is the way to go.

will it work w a AP STAGE 2 tune?

no problems, and yes, it works with the AP map.

will it work w a AP STAGE 2 tune?

 

I used a COBB catted down pipe and a Cobb non cat up .pipe and a Cobb stage II tune.

 

That was 100k miles ago. No problems, but then I don't race the Subie. The car has the original turbo.

 

I just change the oil (dino) every 2000 miles. Checked the turbo screen at the last change, clean as a whistle.

 

I know I have left power on the table, but then I kinda like longevity:)

 

The most important thing on these motors is to look at the torque curve. Just because you can get 7000 rpm, is no reason to do it

my car is babied I have 10k on the new EJ257 shortblock and rebuilt heads. I love my LGT!!

What about the difference between a divorced/non-divorced pipe? Should the divorced be use on a stock turbo?

I have catless. No complaints here. Since there is more fuel making it out the exhausts then you can get some afterfire (Not always a bad thing ). Also can get some pops and crackles from downshifts, again nothing to complain about.

 

I've never noticed the smell. I'm sure if you do if you get beside the exhaust after driving but otherwise nothing.

 

What about the difference between a divorced/non-divorced pipe? Should the divorced be use on a stock turbo?

 

A bit of debate goes on about this. Some say divorced separates the gasses coming out of the wastegate from hitting the gasses coming out of the turbo. This is supposed to lead to less back pressure and better performance.

I don't know the process design behind cars that well but I'm a process engineer and I'd say there would be next to no difference between having divorced or not. Any back pressure created by the wastegate gasses would be well driven downstream by the turbo gasses. You'd need some massive flows from the wastegate to get any back pressure.

 

I suspect it's just a marketing gimmick to make one product seem better than others.

some people have boost creep from the lack of back pressure

 

Never had this. The less back pressure behind a turbo the better. N/A cars benefit from back pressure as it can help "scavenge" the gasses out of the piston chambers. Forced induction cars don't need this.

You would think thats is true! I wasnt sure if the exh from the turbo would have any neg affect on the air being released from the wastegate

The velocity of the gas coming out of the turbo will create a vacuum effect for the gasses coming out of the wastegate. Any gas from the wastegate will just get sucked into that stream of gasses whether or not it's divorced. Just changes how close this happens to the turbo.

Thanks for your advice...

Originally Posted by Nightmaresmk http://legacygt.com/forums/skynetim/buttons/viewpost.gif

some people have boost creep from the lack of back pressure

Boost creep from lack of back pressure? Then surely that would be a tuning issue. Wastegate controls boost, period.

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Boost creep from lack of back pressure? Then surely that would be a tuning issue. Wastegate controls boost, period.

 

 

There are tons of info on boost creep, but to make it super simple:

 

When the wastegate is fully open, it will flow "X" amount of exhaust through it and around the turbine, which in turn limits how fast the turbine spins, which in turn limits how fast the compressor spins, which in turn limits boost.

 

The problem occurs when the ECU/boost solenoid reaches the target max boost number and the wastegate is fully open, thus bypassing as much exhaust around the turbine as the stock system is physically capable of doing. However, because of a much free'er flowing exhaust due to being catless, it is possible for the engine to breath that much better that the amount of exhaust being produced is more than the "X" amount that the wastegate can bypass around the turbine.

 

This extra exhaust continues to pass through the turbine, which in turn causes it to spin faster, thus the compressor spins faster, thus making more boost that continues to "creep" higher and higher, totally uncontrolled.

 

So, boost creep is based off of physical limitations of the factory internal wastegate. Once the boost solenoid is at 0 wastegate duty cycle(wide open), and your boost is still climbing, tuning will not help at that point.

 

So yes the wastegate does control the boost...until the wastegate can't control the boost no more

 

FWIW, I had 3" catless exhausts on my old Legacy Sport Sedan as well as my old WRX and never had a boost creep problem at stock or near stock boost levels.

I think that would be a tuning issue.

 

Any turbocharged system is limited by the compressor efficiency - the compressor will reach a limit at which it cannot pressurise any more air through the system, and because a piston engine acts as a batch kind of system them there is a limit at which the exhaust gas volumes will reach.

 

The only time you would expect to see more exhaust gasses than the wastegate could handle would be if the tuned WGDC was not designed for the exhaust gasses (ECU controlled as you mentioned), or there was a mechanical issue with the wastegate. Any turbocharger system should be engineered to the compressor limits.

 

Boost creep will happen with an oversized turbocharger but the bigger, resulting, problem would be compressor surge.

 

The lack of back pressure on a turbo will help mitigate any boost creep by allowing freer flowing gasses through the wastegate allowing it to do its job. A lesser pressure differential on either side of the wastegate will restrict it further.

So yes the wastegate does control the boost...until the wastegate can't control the boost no more

Most definitely... replacing the wastegate or porting the existing would suffice in this aspect... still down to the wastegate though

For the smell alone, I'm a huge advocate of running a catted system. Not to mention potentially throwing codes.