[Blue2]

I removed the secondary cats from my otherwise stock M2 Competition (just replaced the two secondary cats with straight pipe) and took short before and after revving videos for comparison. I'm pleased with it. The volume increased 10-15% and there's no additional rasp. The tone is a little bit deeper. It's a pretty subtle change, but I definitely here it when driving.

I know conventional wisdom says that there is no performance to be gained by removing the secondary cats because they are not restrictive, but the butt dyno feels stronger at higher RPMs. Picture of cat below. I'm no expert, but it looks restrictive to me!

Edit: I can't get the Vimeo tags to work, so you'll have to just use the links:



Before: https://vimeo.com/428322626


After: https://vimeo.com/428322644

[SEAT TIME RULES]

Why go this route when AA and others make a well designed and tested midpipe with Sec Cat delete...Unless you just trying to save a FEW BUCKS?

[Blue2]

Originally Posted by SEAT TIME RULES

Why go this route when AA and others make a well designed and tested midpipe with Sec Cat delete...Unless you just trying to save a FEW BUCKS?

Yes. For $140 and 30 minutes, it's a nice option.

[chmura]

You get more benefit from removing the reasonator. You should of done that as well.

[whoop_ass]

Originally Posted by chmura

You get more benefit from removing the resonator. You should of done that as well.

how so?
are you referring to performance or sound?
The stock system is very well designed and may as well be almost a straight pipe setup with the removal of primary and secondary cats.

[SEAT TIME RULES]

Almost all of us like to save a few dollars on mods so that we have more money for additional mods.

The OP's solution is simple but not as "clean" or tested as an AA midpipe.

My way of thinking after 30 years of modding cars was to add the AA Midpipe then go with a set of Wagner 200 cell HFC's and painting the OEM muffler black to hide it.

IMHO the cost of a full exhaust is a waste of my dollars, and this way I get max performance and way better sound for my money..

My build:
https://f87.bimmerpost.com/forums/sh....php?t=1696726

[TrentMeister]

Originally Posted by whoop_ass

how so?
are you referring to performance or sound?
The stock system is very well designed and may as well be almost a straight pipe setup with the removal of primary and secondary cats.

Stock system actually has a ton of backpressure

[whoop_ass]

Originally Posted by LBbluem2

Stock system actually has a ton of backpressure

Sorry buddy, I honestly don't agree with you.
if you had to take the secondary cats and primary cats out and run stock exhaust with Valve open and an aftermarket catback exhaust I doubt there will be even 5HP difference.

all the power is made from the primary cats and secondary cats.
the reason the AAW midpipe makes gains is because of the removal of the secondaries.

look at the design of the OEM exhaust and you can clearly see it's straight pipe when valves are open.

[TrentMeister]

Originally Posted by whoop_ass

Sorry buddy, I honestly don't agree with you.
if you had to take the secondary cats and primary cats out and run stock exhaust with Valve open and an aftermarket catback exhaust I doubt there will be even 5HP difference.

all the power is made from the primary cats and secondary cats.
the reason the AAW midpipe makes gains is because of the removal of the secondaries.

look at the design of the OEM exhaust and you can clearly see it's straight pipe when valves are open.

Here you go skipper

[G80BSM]

Originally Posted by SEAT TIME RULES

Almost all of us like to save a few dollars on mods so that we have more money for additional mods.

The OP's solution is simple but not as "clean" or tested as an AA midpipe.

My way of thinking after 30 years of modding cars was to add the AA Midpipe then go with a set of Wagner 200 cell HFC's and painting the OEM muffler black to hide it.

IMHO the cost of a full exhaust is a waste of my dollars, and this way I get max performance and way better sound for my money..

My build:
https://f87.bimmerpost.com/forums/sh....php?t=1696726

Can you post some exhaust note videos with the wagner 200 cell DP’s, AA mid and stock muffler?? I am thinking about the same setup. I ran AA mid with VRSF DP and Stock muffler but the rasp and smell was unbearable - at least for me!

[SEAT TIME RULES]

Originally Posted by S.111

Can you post some exhaust note videos with the wagner 200 cell DP’s, AA mid and stock muffler?? I am thinking about the same setup. I ran AA mid with VRSF DP and Stock muffler but the rasp and smell was unbearable - at least for me!

Sure I can do that...

No rasp, no smell...awesome sound with the exhaust flaps open at start

[G80BSM]

Originally Posted by SEAT TIME RULES

Sure I can do that...

No rasp, no smell...awesome sound with the exhaust flaps open at start

Great! Looking forward to it.
Thanks

[whoop_ass]

Originally Posted by LBbluem2

Here you go skipper

thanks very much for this but i am well aware of it.

Just a side note the reason they gained power is not because of the midpipe but because of the removal of secondary Cat, the midpipe mitigates.
Remove the secondary cat on your stock units and it will also gain power and less back pressure.
the akra also makes power actually and more power than the slip-on
WHY????
because the bulk of the gains come from downpipe and midpipe.

http://www.akrapovic.com/en/car/prod...70&yearId=5052

http://www.akrapovic.com/en/car/prod...70&yearId=5052


back to my point on the exhaust, remove primary and secondary Cats and valves open it's just about a straight pipe!!
might not sound the greatest but it will make the most power with the least back pressure.

go educate yourself on what the original stock exhaust looks like when the valves are open and how it flows.

[TrentMeister]

Originally Posted by whoop_ass

thanks very much for this but i am well aware of it.

Just a side note the reason they gained power is not because of the midpipe but because of the removal of secondary Cat, the midpipe mitigates.
Remove the secondary cat on your stock units and it will also gain power and less back pressure.
the akra also makes power actually and more power than the slip-on
WHY????
because the bulk of the gains come from downpipe and midpipe.

http://www.akrapovic.com/en/car/prod...38;yearId=5052

http://www.akrapovic.com/en/car/prod...38;yearId=5052


back to my point on the exhaust, remove primary and secondary Cats and valves open it's just about a straight pipe!!
might not sound the greatest but it will make the most power with the least back pressure.

go educate yourself on what the original stock exhaust looks like when the valves are open and how it flows.

It's got back pressure, obviously.

I agree that after the primary components of the mid pipe are removed it's basically straight.

Why you said you disagree with me. Idk.

[Bass_Masterson]

Not agreeing or disagreeing with anything posted thus far, but here's first hand experience to describe how modifying the stock system works.

My buddy (2020 M2C) initially only installed cat-less downpipes. The car was a bit louder, but nothing obnoxious. His next step was deleting the secondary cats and the resonator. They were replaced with separate straight pipes. With the valves closed, car was fairly quiet, not much louder than stock. With the valves open, car was pretty loud and sounded good (in my opinion). However, at load at around 1600 RPM, the exhaust developed a loud rattle based on the resonance in the exhaust. This annoyed him so he had only the resonator re-installed. The car is a little quieter/less aggressive sounding than before, but the rattle is gone.

On my 2020 M2C, I did the same thing at first. Cat-less down pipes, secondary cats and resonator delete (I thought I might get lucky with the valve rattle). At first it was fine, but after about 1000 miles the rattle got so bad to where it buzzed and rattled under almost any condition. So, I had the valves removed and the muffler completely gutted (I wanted to keep the stock tips and this was the cheapest option for now.) If I recall, even with the valves open, there is some muffling being done. Yes, stock it is a straight shot through the muffler after the open valve, but there are holes in the pipe with some dampening material around them. Once I had that removed, the exhaust is noticeably louder. Also has a more aggressive tone.

To summarize:
Cat-less downpipes = moderate noise increase
above + secondary cats delete = car is loud, but still reasonable
above + resonator delete = car is slightly louder and has more resonance/drone (may cause rattles/buzzing)
above + valve and muffler internals delete (totally straight piped) = very loud, very aggressive sound. No rattles/buzzing. Lots of drone (doesn't bother me).

I also disabled active sound (which removes the artificial burbles). Now it has a few natural cracks and pops. Sounds so much better with it off.