[cmg5461]

Originally Posted by csu87

put a 4" dp and exhaust off your turbo and let me know how it works. It will affect the flow characteristics of the turbo.

No it won't. Not negatively at least.

Why don't you prove to me that it will affect it negatively?

[Mike@N54Tuning.com]

Everything I have seen points to catless downpipes making much more power over catted. Even ar design who sells both and pretty sure they make more money on the catted versions will tell you that catless >> catted >> stock downpipes.

Mike

[BQTuning]

Originally Posted by csu87

... Hence why the ETS dynos of a 2.5" exhaust showed improvements.

Good question, I would like to see the schematics of their mufflers that will probably answer a lot of questions or just complicate things more...lol .

[csu87]

Originally Posted by cmg5461

No it won't. Not negatively at least.

Why don't you prove to me that it will affect it negatively?

i dont really care that much to go through the testing. Just going off what I have seen in the past with other turbod cars.

Either way, I think we both agree that removing the 2nd cat is not going to lead to lower torque.

[shadow191]

No it won't; the bigger the pressure differential between pre and post turbine the more energy transferred through the shaft. So you really can't go to big with an exhaust on a turbo setup. At some point, a bigger exhaust won't do anything as the pressure differential can't get bigger. But at no time will too big of an exhaust post turbo cost you power. Anything that minimizes that pressure differential like a pipe that's too small or a cat is going to cost power.

Based on this thought, catless downpipes would make less power than catted ones and we all know that's not the case. There is even a power difference between the 3" downpipes and 2.5" ones even though they all taper down to 2.5". Simply because the 3" section allows the exhaust to expand more and there is more pressure differential.

Even with a N/A motor, it's not loss of "backpressure" that causes low end torque loss, it's loss of scavenging. With a turbo, there isn't really scavenging.

I only have one anecdotal data point that's relevant. A long time ago, I built a turbo setup for a car. With the OEM secondary cat in place and 2.5" exhaust, it made 270whp at 5.5psi (7lb wastegate spring). We then built a 3" exhaust and replaced OEM cat with 3" test pipe. 2.5" downpipe wasn't changed. Car immediately made 301whp and it hit 7psi on the same spring. No tuning changes, just exhaust. And I think it was an additional 20ft/lbs torque. We also pulled the exhaust and ran it open downpipe, it picked up another 5-6whp. Later on, a friend bought the setup and modified the downpipe to 3". I forget the numbers, but he picked up a few hp by going to the larger dp.

Originally Posted by csu87

put a 4" dp and exhaust off your turbo and let me know how it works. It will affect the flow characteristics of the turbo.

[cmg5461]

Originally Posted by csu87

i dont really care that much to go through the testing. Just going off what I have seen in the past with other turbod cars.

Either way, I think we both agree that removing the 2nd cat is not going to lead to lower torque.

So you're going to offer your opinion without backing it up? That's not how you debate my friend

[cmg5461]

Originally Posted by BuraQ

Good question, I would like to see the schematics of their mufflers that will probably answer a lot of questions or just complicate things more...lol .

BuraQ, why do I gain 30wtq from catless downpipes, but lose 30wtq from catless midpipes?

[csu87]

Originally Posted by cmg5461

So you're going to offer your opinion without backing it up? That's not how you debate my friend

I did back it up. "Just going off what I have seen in the past with other turbod cars."

I am not going to test the theory. I am not a scientist

[cmg5461]

Originally Posted by csu87

I did back it up. "Just going off what I have seen in the past with other turbod cars."

I am not going to test the theory. I am not a scientist

What do you mean by "seen"?

Read on forums, concluded from trends by multiple people on forums, remember from a test from someone else, remember from a test you did?

No offense, but I generally take other people's word with a grain of salt. Many times I have heard people say, "It's better because butt dyno," but I'm the type of person who wants to test theories with an actual dyno.

Testing has been drilled into me from FSAE, as EVERYTHING needed to be tested, reviewed, verified and documented. We were drilled on every change we made and why we made it.

Can you please expand on what you have seen?

Not trying to be harsh or anything, just genuinely interested!

[C.Pop]

Honeywell/Garrett states that the less restriction in a turbo exhaust the better, period. This isn't debatable, it's been known/established for a long time. I agree that his testing shows otherwise, I question the validity of the comparisons -there are a ton of variables to account for, no way would I assume it had anything to do with cats.

[csu87]

Originally Posted by cmg5461

What do you mean by "seen"?

Read on forums, concluded from trends by multiple people on forums, remember from a test from someone else, remember from a test you did?

No offense, but I generally take other people's word with a grain of salt. Many times I have heard people say, "It's better because butt dyno," but I'm the type of person who wants to test theories with an actual dyno.

Testing has been drilled into me from FSAE, as EVERYTHING needed to be tested, reviewed, verified and documented. We were drilled on every change we made and why we made it.

Can you please expand on what you have seen?

Not trying to be harsh or anything, just genuinely interested!

A few years ago on a Genesis Coupe me and a buddy did some testing with no exhaust, huge exhaust and the typical 3" exhaust that everyone was using. We were unable to get good dyno results, but the no exhaust had a little more lag than the 3" exhaust, and same with the bigger exhaust. Really the only testing Ive done on this subject. Who knows why the lag, but we figured it was due to the exhaust.

I would do testing on the 335 if I could, but the amount of time it takes to remove/install DPs on the xi is way too much to justify curing my curiosity.

If Im wrong, Im wrong and thats cool. You learn things everyday.

[BigHoncho]

Originally Posted by shadow191

No it won't; the bigger the pressure differential between pre and post turbine the more energy transferred through the shaft. So you really can't go to big with an exhaust on a turbo setup. At some point, a bigger exhaust won't do anything as the pressure differential can't get bigger. But at no time will too big of an exhaust post turbo cost you power. Anything that minimizes that pressure differential like a pipe that's too small or a cat is going to cost power.

THIS! Thats the proper explanation for the actual physics that are going on here!

[Tim603]

Originally Posted by cmg5461

Oh, and 335is A vs 335is B is not apples to apples.

Apples to apples is using the same car. Get a stabilized baseline with the catted midpipes. Then compare a stabilized result with your catback removed.

Need I say to use the same calibrated dyno as well?

This

[stanlalee]

Originally Posted by BigHoncho

I appreciate your experiences and your feedback, I really do.

But I still think that the assumption is false... there ist simply no reason why less back-pressure should yield into less torque. So I think the solution is on the tune side. Maybe we will see some advances here in the future.

I myself also prefer 200cell secondaries for a couple of practical reasons (smell, smoke, drones).

I think you hit the nail on the head. This comes up quite a bit on platforms with tuning and small factory turbos. There's always a camp that swears even through verified dyno testing loss of tq with less restriction (whether it be cats, pipe diameter, cell count etc). Then there's always another tuner who sites tuning and claims no such findings after compensating with x, y and z tuning technique. At the end of the day got to put your money on the turbo experts and engineers who pretty much accept there is no such thing as beneficial back pressure past the turbine. Yeah I read maximum boost by corky bell back in the 90s when they started slappin turbos on Hondas lol.