Exhaust advice - 2.5" down from 3

[dassaur]

The Euro 5 OEM cats are physically smaller than the Euro 4 OEM cats.
I remember reading somewhere that the Euro 5 cats are 600cpi compared to 400cpi on the Euro 4's.

I've looked inside the factory VF1 cats with a camera. There is no way they are 400 cpi, definitely more.

 

[RevNev]

I've looked inside the factory VF1 cats with a camera. There is no way they are 400 cpi, definitely more.

I don't really know what cpi they are, they've always said to be 400. We tested some VE cats (same as VF1) on a flow bench once, and they had some serious turbulence causing a restriction from the steep 60-degree entry angle more so than the cat biscuit itself.

 

[phantom0817]

Plenty of OEM Euro 6 (not 4 or 5) compliant cars out there pushing 600+HP, many are turbo obviously (typically V6 or V8 where each cat is flowing 300HP, but also plenty of 300HP+ 4cyl out there). So clearly it is possible to have a compliant cat that doesn’t pose a restriction. Don’t understand why these all these after market ones are 100/200 CPI. The only argument I’ve heard is size but if that’s the case these 300HP flowing Euro6 cats must be ginormous.

 

[panhead]

My W204 C63 (6.2 litre N/A) is Euro 4 compliant and my W205 C63 (4.0 litre Twin Tubo) is Euro 6 compliant.

Both cars have four Cats, two primary Cats at the engine pipes and 2 secondary Cats behind the transmission.

The W204 primary cats are ceramic 400 cell and the secondaries are made out of wire, not ceramic, and mainly act as a catch can.

I have no idea what the cell count is for the W205 but my understanding is it's similar to the 205.

A populator mod when having the cars tuned is just to remove the secondary Cats and replace them with straight pipes and some owners also go further and change the primaries for 100 and 200 cell Cats plus fit headers, all in the name of making more power or just making the car sound like sh&t.





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[RevNev]

My W204 C63 (6.2 litre N/A) is Euro 4 compliant and my W205 C63 (4.0 litre Twin Tubo) is Euro 6 compliant.

Both cars have four Cats, two primary Cats at the engine pipes and 2 secondary Cats behind the transmission.

The W204 primary cats are ceramic 400 cell and the secondaries are made out of wire, not ceramic, and mainly act as a catch can.

I have no idea what the cell count is for the W205 but my understanding is it's similar to the 205.

A populator mod when having the cars tuned is just to remove the secondary Cats and replace them with straight pipes and some owners also go further and change the primaries for 100 and 200 cell Cats plus fit headers, all in the name of making more power or just making the car sound like sh&t.

The S55 twin turbo inline 6 cylinder in the M3/M4 BMW's in production from 2014 to 2020 had a couple of different Euro compliances. The MY19/20 had particulate filters replacing the secondary cats and lost the carbon fibre tailshaft with the bulkiness of the particulate filters. There are 2 cats directly behind the turbos and removing them with catless down pipes makes 30hp at the rear wheels and removing the secondary cats makes another 10hp. I don't know what the dpi of the cats are but seems the cats at the turbo end have the greatest restriction and the secondary cats not so much.

 

[daves8]

My W204 C63 (6.2 litre N/A) is Euro 4 compliant and my W205 C63 (4.0 litre Twin Tubo) is Euro 6 compliant.

Both cars have four Cats, two primary Cats at the engine pipes and 2 secondary Cats behind the transmission.

The W204 primary cats are ceramic 400 cell and the secondaries are made out of wire, not ceramic, and mainly act as a catch can.

I have no idea what the cell count is for the W205 but my understanding is it's similar to the 205.

A populator mod when having the cars tuned is just to remove the secondary Cats and replace them with straight pipes and some owners also go further and change the primaries for 100 and 200 cell Cats plus fit headers, all in the name of making more power or just making the car sound like sh&t.





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How many friggen cars do you have?

 

[Draimond]

From my experience, exhaust volume is basically equivalent to exhaust velocity.

Any RPM with good efficient scavenging and flow will be very energy intensive to quieten.
The extractors almost act like a turbo pushing that gas through.
That's why expensive systems sound more peformant than back yard bits and pieces systems. Consistency helps a great deal to make it loud and fast.

Adding different larger diameter chambers reflects pressure waves back up the system, which acts like an air brake to settle things down.
Different geometries and diameters react on different frequencies.

I'm sure you could just restrict the system with a smaller outlet at the rear without hurting low to mid performance too much. I'd worry that it would change the exhaust note quiet a bit, not in a good way.

Reverse flow or s bends tend to kill sound quiet a bit.

There's resonance tubes, which look like an appendix, they're made with certain frequencies in mind and reflect pressure waves to neutralize particular rpm drones.

You could run 3" oval tube, (aka 115x42 cattle rail, $70 for 6 meters in gal) for the cramped areas.
Take a longer S bend over the diff as much as possible, not being afraid to reverse direction a little (long radius still if also possible).
Add long resonators as the crossover pipes.

There's a bunch you can do to change or suppress sound levels while maintaining pipe diameter and high rpm flow. Remembering that resonance chambers and resonators only work bellow their designed RPM.

OR ... Keep the 3" and upgrade the carpet and underbody with a few different sound deadening products

 

[RevNev]

I'm sure you could just restrict the system with a smaller outlet at the rear without hurting low to mid performance too much. I'd worry that it would change the exhaust note quiet a bit, not in a good way.

A good 2.5" exhaust will flow at least 580 flywheel hp naturally aspirated and takes a radically modified NA LS engine to use the volume of a 3" exhaust. I think 1 7/8" headers and 3" exhausts were initially aimed at supercharged engines and appealed to the "bigger must be better" tendency. Stepping down a 3" exhaust to 2.5" with an engine that doesn't make enough horsepower to use the 3" exhaust volume will be fine!

Furthermore with a long exhaust, the exhaust gas cools towards the end of the system and takes up less volume as many have discovered dyno testing, removing loud and drony 2.5/3" rear mufflers and refitting the smaller OEM 2.25" bimodals had no power loss with the valves open. Reducing pipe diameter at the rear of a long exhaust subject to a dyno test, is another usable tool to quieten a noisy exhaust and reduce drone.

 

[panhead]

How many friggen cars do you have?

I'd be too frightened to say in case my wife found out.




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[Ginger Beer]

I'd be too frightened to say in case my wife found out.




.

All my parts get shipped to work where I typically use our workshop area to install them

I do have an approved "slush fund" bank account that I use as well, which she has not questioned in 17 years of me having it, hopefully she has totally forgotten about it

The Minister for War and Finances has no idea the car blew the rear end a while ago and required the "slush fund" to take a substantial hit