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upgrading downpipe without going stage 2

9649 Views 9 Replies 6 Participants Last post by  why2kmax
I just bought a 2018 WRX and still breaking in the engine. I'm starting to plan out my upgrades. I immediately want to go stage 1 with these planned parts: cobb accessport, sway bars, michelin PS4S tires, upgraded intake, invidia R400 catback, and some kind of downpipe.

My worry is that with an aftermarket downpipe, will I need a more aggressive tune, putting me in stage 2? And if the tune is a requirement, will that hurt my fuel economy if my driving habit stays the same?

As for specific DP's, any recommendations? I'm in california so will needed a catted one. Not sure how strict our emissions testing is. Lastly, will this DP upgrade change have any impact on the exhaust sound/noise of the invidia R400 catback?

Thanks in advance!
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you'll have to get the car tuned for a downpipe im afraid.

also you dont have to smog your car for 6 years after a new model. no cat other than oem will pass smog im afraid.

anyways, welcome to the forum and feel free to hit up the mods if you have any questions!
...and if we're ignoring CA state laws, you can look to Cobb for the JPipe as well since you're shopping the Cobb AP. A protune is the way to go. Don't skip the tune. You can get a catted one. They even have a resonator option as well if you want to reduce the drone. The JPipe addition will increase sound volume and you will decrease economy. To what extent, I'm sure varies.
ah so a downpipe upgrade is the fine line between stage 1 and stage 2?

Can I use the Cobb accessport to flash a OTS stage 2 map to hold me over until I upgrade more parts and do a custom pro tune?
ah so a downpipe upgrade is the fine line between stage 1 and stage 2?

Can I use the Cobb accessport to flash a OTS stage 2 map to hold me over until I upgrade more parts and do a custom pro tune?
Yes, a JPipe is considered stage 2. You CAN use an OTS map, but it's not perfect and you have to make sure the tune matches the parts its intended for. Read the map notes. You don't want to put just any JPipe on and assume the Cobb OTS map is OK for it. Call them or shoot them an email if you have questions about their tunes. They WILL get back to you fairly quickly.
Obligatory the moment you put any engine mod or tune on the car you can plan on bbn paying for any engine related failure out of pocket. You can flash back to stock, but they are easily able to tell, you can remove bolt on parts and it's still obvious.

To your question. Stage 2 doesn't mean anything. To some it's just an exhaust, to some its intake exhaust, to others it's even something different entirely. Per cobbs definition Stage 2 is down pipe and tune. 2 plus is down pipe, Intake and tune. Stage 1 is just a tune and stage 1 plus sf is intake with tune.
Thanks guys! It looks like I'm going straight to stage 2+ from the get-go, according to Cobb's definition.

Now for fuel economy, I'd like to elaborate more on this concern. I know it's easier to be heavy footed with more power which results in worse milage. But assuming I'm driving exactly the same from before and after downpipe/stage2 upgrade, shouldn't fuel economy increase, or at least stay the same since upgrading the downpipe means engine breathing easier? STI has lower MPG than WRX with same driving habit simply because STI has a larger engine. But for this upgrade, everything is the same except swapping out for a more efficient part, so it just seems counterintuitive to me that MPG would drop. What's the logical that I'm missing here?

The reason for this concern: although the slight bump in HP is much appreciated, I also have to commute in this car, so majority of the driving is on the freeway where the extra HP will be of little use. If possible, I'd prefer not to burn extra fuel on the freeway if my driving is the same as before. On local, I'm perfectly ok gunning it at the cost of some extra gas, where the HP bump can be realized.
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..assuming I'm driving exactly the same from before and after downpipe/stage2 upgrade, shouldn't fuel economy increase.. ?

Yes.. that would be my expectation (and has been my experience). If nothing else, the tune should be a little more on the lean side than the stock tune, although still pretty rich. That in and of itself will improve fuel efficiency (economy implies the cost of fuel, not the amount used). You should also find that you don't have to open the throttle as much to accelerate at the same rate as before (although this doesn't correspond to pedal due to the stock throttle mapping). I was able to get excellent fuel mileage in my 02 WRX with significant engine mods and a larger turbo.. if I tried.

That said, improved fuel efficiency is not what the vast majority of people experience. Fuel efficiency is HEAVILY effected by driving style. At least in a manual transmission car, I can virtually double the fuel mileage simply by changing my driving habits.
When I had my secondary air pump removed, tgv deletes installed and corsa installed I had a new tune done. My fuel economy increased from about 19 avg to about 23.

I'm not saying that is normal or expected, but it should give you a little better economy until you get into wailing on it. More air more fuel
Another note of caution, if you mess with the intake you MUST have a tune specifically paired with that intake. Otherwise you are asking for it to blow up.

my 17 WRX FA20 saw ZERO MPG increase with a cobb catback and Zero when I added the MAP Stage 1 tune.
Exhaust made seemingly no difference other than noise.

contrast that with
My 18 STI with the same Cobb Catback and stock tune is up 2-3 MPG since installing the exhaust. Noticeable difference in driving dynamics. Car revs more freely, less backpressure when coasting down, car is much more fun to drive.
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