Exhaust bottleneck: downpipe, mids/cats, or mufflers?

[Thomby]

So on many turbo cars, a downpipe gives you the most performance gains, while mufflers/catback seem more popular because they change the sound and are easier to get to. I found a bunch of old threads about mids/cats not helping much, but not much talking about gains from just doing downpipes without a catback.

I really don't want a noisier exhaust, especially in the low-midrange, or any drone on highway trips, but I am considering downpipes to remove that restriction after the turbos, if I can get the quicker spooling and power/torque gains while keeping my stock mufflers (I have a '22 with the active valves). Does this make sense, and does anyone have sound or dyno comparisons for downpipes with stock mufflers?

I have plugs/intakes and run map 2 on 93 or 4 on E30. For reference, K8 shows +46hp / +62lbs-ft:
Kia Stinger 3.3T - Ultimate Performance HFC Downpipe + Mid Pipe Combo Kit

 

[turboAWD]

More than one person has had issues with oil blowing by the turbos after removing cats from the downpipe, due to reduced backpressure. IIRC, 400-cell HFC cats mitigate this somewhat, but I'm not sure it's 100% gone - search.

Also, more than one person has broken the turbo studs connecting to the downpipe while doing this, necessitating a much-longer-than-anticipated install / downtime / drilling out of a stud / etc. Even those who prepped with penetrating oil / blowtorch because they knew this was a possibility, IIRC.

IIRC, with a jb4, you'll also need EWG wires and to play with the map 6 settings if you change the pipes to avoid overboost, etc.

Finally, not sure what the gains are on an already modded car - that dyno plot for "gains" includes "FMIC" in the title - did they also swap in an FMIC there?

For me, the Stinger is my "not as modded as in the past" car, and I'm happy with map 3/4 on E30, although I might install the CPI kit to be able to run full E85 because it's less hassle.

Not saying you shouldn't do it, just be aware of the possible hassles.

 

[Thomby]

More than one person has had issues with oil blowing by the turbos after removing cats from the downpipe, due to reduced backpressure. IIRC, 400-cell HFC cats mitigate this somewhat, but I'm not sure it's 100% gone - search.

Also, more than one person has broken the turbo studs connecting to the downpipe while doing this, necessitating a much-longer-than-anticipated install / downtime / drilling out of a stud / etc. Even those who prepped with penetrating oil / blowtorch because they knew this was a possibility, IIRC.

IIRC, with a jb4, you'll also need EWG wires and to play with the map 6 settings if you change the pipes to avoid overboost, etc.

Finally, not sure what the gains are on an already modded car - that dyno plot for "gains" includes "FMIC" in the title - did they also swap in an FMIC there?

For me, the Stinger is my "not as modded as in the past" car, and I'm happy with map 3/4 on E30, although I might install the CPI kit to be able to run full E85 because it's less hassle.

Not saying you shouldn't do it, just be aware of the possible hassles.

Appreciate the insight, and I'm still considering next steps when I get used to the current power. Agree with your sentiment on not turning a nice sports sedan into a track day-only weapon.

But if the restriction after the turbo is that severe (1.25"?), and I can get reasonably strong gains without an obnoxiously loud or droning muffler, I'd certainly consider it. I just can't tell if the gains shown are with an open downstream exhaust, or because the stock downpipe is such a bottleneck.

 

[Daniel @ K8 Stinger Store]

We have sold a handful of the Ultimate Performance Downpipe Combo Kits and have seen successful and impressive results from the kit. Installation on the primary downpipe can be a little tricky as mentioned, so we recommend that you work with an experienced and professional installer to get this done. The primary and secondary downpipes will increase horsepower and torque a lot better than a catback exhaust would. Keep in mind, you will still need a tune such as a JB4 Piggyback Tune in order to avoid issues with this kit.

Feel free to message me if you have any questions or if you would like a discount on this product!

 

[Scorpion24]

More than one person has had issues with oil blowing by the turbos after removing cats from the downpipe, due to reduced backpressure. IIRC, 400-cell HFC cats mitigate this somewhat, but I'm not sure it's 100% gone - search.

Also, more than one person has broken the turbo studs connecting to the downpipe while doing this, necessitating a much-longer-than-anticipated install / downtime / drilling out of a stud / etc. Even those who prepped with penetrating oil / blowtorch because they knew this was a possibility, IIRC.

The examples you mentioned are from replacing the primaries ?

And not the secondaries(mid pipes) right?

 

[SRV]

More than one person has had issues with oil blowing by the turbos after removing cats from the downpipe, due to reduced backpressure. IIRC, 400-cell HFC cats mitigate this somewhat, but I'm not sure it's 100% gone - search.

This make no sense whatsoever and is just a coincidence.

 

[rpoffen]

This make no sense whatsoever and is just a coincidence.

Don't know. Been on a few meets with other stingers, including the Shasta run last year with the Washington state guys. AKAIK, only one in the group has catless primaries and secondaries, and he smoked like a bad diesel truck.

 

[rpoffen]

So on many turbo cars, a downpipe gives you the most performance gains, while mufflers/catback seem more popular because they change the sound and are easier to get to. I found a bunch of old threads about mids/cats not helping much, but not much talking about gains from just doing downpipes without a catback.

I really don't want a noisier exhaust, especially in the low-midrange, or any drone on highway trips, but I am considering downpipes to remove that restriction after the turbos, if I can get the quicker spooling and power/torque gains while keeping my stock mufflers (I have a '22 with the active valves). Does this make sense, and does anyone have sound or dyno comparisons for downpipes with stock mufflers?

I have plugs/intakes and run map 2 on 93 or 4 on E30. For reference, K8 shows +46hp / +62lbs-ft:
Kia Stinger 3.3T - Ultimate Performance HFC Downpipe + Mid Pipe Combo Kit

I have the MBRP catback, and it did make it louder. I just put Ark secondaries on a couple weeks ago, the 1/4 mile track near me has the first run of the season on Mar 1, so I will see if it improves things at all.
The secondaries did make it even louder, there is a slight drone just below 2K that is mostly evident as I am accelerating past 2K, doesn't really show up when just cruising at light throttle near 2K.

 

[GennyGuy]

So on many turbo cars, a downpipe gives you the most performance gains, while mufflers/catback seem more popular because they change the sound and are easier to get to. I found a bunch of old threads about mids/cats not helping much, but not much talking about gains from just doing downpipes without a catback.

I really don't want a noisier exhaust, especially in the low-midrange, or any drone on highway trips, but I am considering downpipes to remove that restriction after the turbos, if I can get the quicker spooling and power/torque gains while keeping my stock mufflers (I have a '22 with the active valves). Does this make sense, and does anyone have sound or dyno comparisons for downpipes with stock mufflers?

I have plugs/intakes and run map 2 on 93 or 4 on E30. For reference, K8 shows +46hp / +62lbs-ft:
Kia Stinger 3.3T - Ultimate Performance HFC Downpipe + Mid Pipe Combo Kit

I have a catless downpipe. I absolutely love the sound, but I would never mod it on this car again. You risk breaking the bolts, it's a pain in the ass job even when everything goes right, and you also are going to be no faster than the stock downpipe G70/Stingers now that @Avant has the record for the entire platform and is on stock DPs.

My advice is leave it alone expescially since you want the car to be quiet.

 

[Thomby]

I have a catless downpipe. I absolutely love the sound, but I would never mod it on this car again. You risk breaking the bolts, it's a pain in the ass job even when everything goes right, and you also are going to be no faster than the stock downpipe G70/Stingers now that @Avant has the record for the entire platform and is on stock DPs.

My advice is leave it alone expescially since you want the car to be quiet.

Do you have just downpipes (or downs+mids), or also aftermarket catback? If the rest of your exhaust is stock I'd be curious how loud, and if you've had any oil issues.

I'm not trying to have the fastest Stinger, but if the restriction is as tight as it looks, and this change is quieter than an exhaust, it seems worth considering.

 

[GennyGuy]

Do you have just downpipes (or downs+mids), or also aftermarket catback? If the rest of your exhaust is stock I'd be curious how loud, and if you've had any oil issues.

I'm not trying to have the fastest Stinger, but if the restriction is as tight as it looks, and this change is quieter than an exhaust, it seems worth considering.

Primary downpipe - catless
Secondary downpipe - stock
Midipe (X-pipe) - Stock
4th section/Mufflers - Muffler delete

Note: You should not run both primary and secondary catless as you will eventually have oil in turbo from not enough backpressure.

Not having a muffler causes drone. I hated it, but got used to it and don't care too much anymore. Car is relatively quiet to me but to others it might seem loud. At idle, an Elantra N is actually louder than my car. I might have a video somewhere with them both.

Cold start is definitely loud, but it puts a smile on my face every time. To be honest, I am probably going to make it louder with a change from the cat-back. I'm not sure what yet.

The exhaust mod is going to be about sound, you're not really going to see a gain on performance. Maybe something in a roll race, but if you do, it's by a hair.

 

[SRV]

Note: You should not run both primary and secondary catless as you will eventually have oil in turbo from not enough backpressure.

This is NOT true and wish you guys would stop posting this nonsense. Here's what a simple Google search returns:

10 Reasons Your Turbo Smokes or Leaks Oil

10 Reasons a Turbo Leaks Oil (and then smokes) Turbochargers are a fantastic way to add HUGE power to any powerplant, however, if the system is not designed properly you can spend a lot of time and money repairing turbos and chasing issues. Today we are going to cover why turbochargers smoke...

www.motionraceworks.com

Nowhere does it state a turbo leak is from lack of backpressure. Please stop parroting the same nonsense over and over and over.

 

[GennyGuy]

This is NOT true and wish you guys would stop posting this nonsense. Here's what a simple Google search returns:

10 Reasons Your Turbo Smokes or Leaks Oil

10 Reasons a Turbo Leaks Oil (and then smokes) Turbochargers are a fantastic way to add HUGE power to any powerplant, however, if the system is not designed properly you can spend a lot of time and money repairing turbos and chasing issues. Today we are going to cover why turbochargers smoke...

www.motionraceworks.com

Nowhere does it state a turbo leak is from lack of backpressure. Please stop parroting the same nonsense over and over and over.

No offense, but I'm more inclined to listen to the actual tuners of these cars instead of a random blog article. Every person and tuner that I've spoken to, involved with these vehicles at a high level, has warned against running fully catless for the exact reason I stated. A lack of backpressure. And iirc, there have already been turbos lost due to that exact reason. It's even posted on some websites.

 

[mbolan]

No offense, but I'm more inclined to listen to the actual tuners of these cars instead of a random blog article. Every person and tuner that I've spoken to, involved with these vehicles at a high level, has warned against running fully catless for the exact reason I stated. A lack of backpressure. And iirc, there have already been turbos lost due to that exact reason. It's even posted on some websites.

I wonder if your both correct. What if the catless down pipe allows the turbo to over rev from lack of backpressure. Not mentioned in the article but the same result as #5

5. Blown Cold Side Coupler Under Boost / Stretched Shaft: This little known fact can explain a lot of turbo issues. We see this several times a year. When a silicone coupler or Vband O-ring etc fail at large pressure levels and when the turbo is spinning high RPM, it can send a turbo shaft spinning from its operating range to double or triple. When the turbo overspins like this, the shaft can actually stretch. Knowing that the precision machined bearing shaft is ground and rides on a steel ring as a seal. It does not take much stretching to put the shaft out of tolerance which then creates the Exxon Valdez of oil leaks.

 

[AdrianG]

To add to this. Somebody else had given a pretty good explanation as to why the turbos can leak oil when going totally catless, it was due to how the turbos are constructed.
Also, just because that article doesn't mention this specific reason as a potential cause for losing oil out of the turbos, does not mean it cannot or does not happen, it was simply stating 10 potential reasons. It is not a definitive list.

 

[GennyGuy]

To add to this. Somebody else had given a pretty good explanation as to why the turbos can leak oil when going totally catless, it was due to how the turbos are constructed.
Also, just because that article doesn't mention this specific reason as a potential cause for losing oil out of the turbos, does not mean it cannot or does not happen, it was simply stating 10 potential reasons. It is not a definitive list.

I think construction of turbos is the most likely scenario.

David from Lap3, Terry from BMS, and Lozic himself have all mentioned the same warning, to not go full catless.

And for anyone who is not sure, you can't just swap on the catless primary without a tune or you will have overboost issues. Please heed the warnings from those that have spent the time on our platform and done the R&D. There is absolutely no benefit to them to tell us not to go catless, except sparing us from headaches down the road.

I wonder if your both correct. What if the catless down pipe allows the turbo to over rev from lack of backpressure. Not mentioned in the article but the same result as #5

5. Blown Cold Side Coupler Under Boost / Stretched Shaft: This little known fact can explain a lot of turbo issues. We see this several times a year. When a silicone coupler or Vband O-ring etc fail at large pressure levels and when the turbo is spinning high RPM, it can send a turbo shaft spinning from its operating range to double or triple. When the turbo overspins like this, the shaft can actually stretch. Knowing that the precision machined bearing shaft is ground and rides on a steel ring as a seal. It does not take much stretching to put the shaft out of tolerance which then creates the Exxon Valdez of oil leaks.

I read the article and believe that it alludes to the same conclusion. One of the ways you have it overspinning is by not having the right tuning solution. This can happen on stock ecu or an unqualified map.

 

[Avant]

Downpipes on pretty much every turbo car make power. Wasn't the case with my G70. I put Catless downpipes on and did a bunch of testing. I didn't get any faster when testing 1/4 mile times.

I didn't like the sound so I went back to 100% stock exhaust and then the car ended up running 10.79 with a little weight reduction (front passenger seat and rear seats out).

Muffler delete with Ultra Quiet Vibrant resonators sounds pretty good. Cheap and easy.

 

[Thomby]

Downpipes on pretty much every turbo car make power. Wasn't the case with my G70. I put Catless downpipes on and did a bunch of testing. I didn't get any faster when testing 1/4 mile times.

I didn't like the sound so I went back to 100% stock exhaust and then the car ended up running 10.79 with a little weight reduction (front passenger seat and rear seats out).

Muffler delete with Ultra Quiet Vibrant resonators sounds pretty good. Cheap and easy.

Thanks for the perspective -- so you're running full stock exhaust except for the muffler/resonator combo? Any before/after dyno or sound clip?

I've heard the "turbos need SOME backpressure" trope for years (decades?) and it always ended up being a misunderstanding of scavenging from 2-strokes or some other old wives' tale, but hearing multiple users report oil issues after dropping the cat restriction is a head scratcher. I wonder if it's just the overboost @mbolan and @GennyGuy mentioned. IE, if you add EWG wires and appropriate wastegate compensation at the same time, does it avoid the issue?

 

[Scorpion24]

Downpipes on pretty much every turbo car make power. Wasn't the case with my G70. I put Catless downpipes on and did a bunch of testing. I didn't get any faster when testing 1/4 mile times.

I didn't like the sound so I went back to 100% stock exhaust and then the car ended up running 10.79 with a little weight reduction (front passenger seat and rear seats out).

Muffler delete with Ultra Quiet Vibrant resonators sounds pretty good. Cheap and easy.

Why didn’t you like the sound of the Downpipes? Was it raspy?

 

[Avant]

Why didn’t you like the sound of the Downpipes? Was it raspy?

No. I guess im just old. To be honest our cars have the worst sounding TT V6 ever. I have not really heard a solid exhaust on these cars.