Deeper Sound With Active Autowerke Mid-Pipe!

[The Flying Scotsman]

I had the opportunity to have installed, one of the first Active Autowerke Mid-pipes last week.

Current modifications on the car:

-Active 8 Tuning Module
-Drop-in Filters
-AA Downpipes
-AA Transmission Tune
-AA De-catted Mid-pipe

As many of you know, The M4's stock exhaust is one piece from the Downpipes back. This means that the stock mid-pipe and rear muffler must be cut, if you are looking to retain the factory rear with a modified mid-pipe.

One of the benefits of keeping the stock rear, is that you can tame down the exhaust note when looking to fly under the radar. Now that the car is running with both sets of cats deleted, the exhaust system is noticeably a lot louder in sport + mode. (with the can flaps open)

Please see the pictures below which shows the AA mid-pipe (left) and how it connects to the factory rear (right).

Please be aware that there are slight modifications that must be done to your factory muffler, if you are to retain the use of it when installing the AA mid-pipe. This modification will include a 4" connecting pipe that will slip over the two pipes coming from the rear muffler and the mid-pipe. This connecting pipe needs to be welded to the factory rear muffler. The other side of the connecting pipe is clamped to the mid-pipe. This way, if you have future plans to install an aftermarket rear muffler, the AA mid-pipe will connect directly to the new rear exhaust.

First Impressions

I got in the car and went for a joy-ride right away. I noticed that the exhaust note has got a lot deeper. The high-pitched almost "chain-saw Honda civic" note has gone and has now been replaced with a deeper race car sound that I am much more pleased with.

I also notice that although the exhaust note is much louder than before(in sport + mode) , the stock muffler does a pretty good job at quietening down the tone when the exhaust flaps are closed.

Power

I have yet to put my car on the dyno ever since installing the Mid-pipe. However, I can say that the car is definitely pulling harder in the higher gears (3 and up). I will post some dyno numbers when I get a chance to visit my local dyno-jet.

[hdekerme]

Post a YouTube video when you get a chance. Video links aren't working.

[The Flying Scotsman]

Originally Posted by hdekerme

Post a YouTube video when you get a chance. Video links aren't working.

Picking the kids up right now. I'm gonna fix it when I get home.

[supremepower]

Originally Posted by M3*GaryShrig*M3

I had the opportunity to have installed, one of the first Active Autowerke Mid-pipes last week.

Current modifications on the car:

-Active 8 Tuning Module
-Drop-in Filters
-AA Downpipes
-AA Transmission Tune
-AA De-catted Mid-pipe

As many of you know, The M4's stock exhaust is one piece from the Downpipes back. This means that the stock mid-pipe and rear muffler must be cut, if you are looking to retain the factory rear with a modified mid-pipe.

One of the benefits of keeping the stock rear, is that you can tame down the exhaust note when looking to fly under the radar. Now that the car is running with both sets of cats deleted, the exhaust system is noticeably a lot louder in sport + mode. (with the can flaps open)

Please see the pictures below which shows the AA mid-pipe (left) and how it connects to the factory rear (right).

[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]

Please be aware that there are slight modifications that must be done to your factory muffler, if you are to retain the use of it when installing the AA mid-pipe. This modification will include a 4" connecting pipe that will slip over the two pipes coming from the rear muffler and the mid-pipe. This connecting pipe needs to be welded to the factory rear muffler. The other side of the connecting pipe is clamped to the mid-pipe. This way, if you have future plans to install an aftermarket rear muffler, the AA mid-pipe will connect directly to the new rear exhaust.

Please see the pictures below which shows the completed install of the mid-pipe:

[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]
[IMG]https://www.flickr.com/photos/133504...posted-public/[/IMG]

First Impressions

I got in the car and went for a joy-ride right away. I noticed that the exhaust note has got a lot deeper. The high-pitched almost "chain-saw Honda civic" note has gone and has now been replaced with a deeper race car sound that I am much more pleased with.

I also notice that although the exhaust note is much louder than before(in sport + mode) , the stock muffler does a pretty good job at quietening down the tone when the exhaust flaps are closed.

Please see the video below of some quick pulls in sport + mode:
[IMG]https://www.flickr.com/photos/133504.../in/datetaken/[/IMG]
[IMG]https://www.flickr.com/photos/133504.../in/datetaken/[/IMG]

Here are some videos of how the exhaust sounds during cold start:
[IMG]https://www.flickr.com/photos/133504.../in/datetaken/[/IMG]
Flaps closed
[IMG]https://www.flickr.com/photos/133504.../in/datetaken/[/IMG]
Flap open

Power

I have yet to put my car on the dyno ever since installing the Mid-pipe. However, I can say that the car is definitely pulling harder in the higher gears (3 and up). I will post some dyno numbers when I get a chance to visit my local dyno-jet.

Images aren't working

[The Flying Scotsman]

Originally Posted by supremepower

Images aren't working

Give me an hour or two to fix it when I get home. Was in a rush earlier...

[supremepower]

Originally Posted by M3*GaryShrig*M3

Give me an hour or two to fix it when I get home. Was in a rush earlier...

No worries! If you want, email me the pics and I can upload and repost to the thread on your behalf

[dmk08]

Looks good. You can click one link and scroll through his flickr posts.

[The Flying Scotsman]

Here are the photos:

[The Flying Scotsman]

And some more:

[supremepower]

Originally Posted by M3*GaryShrig*M3

And some more:

Sweet pics, saw the video too. That midpipe + downpipe makes a huge difference in sound!

[supremepower]

Video of open valve:

[Bluesummer]

Can we get more video

[The Flying Scotsman]

Originally Posted by Bluesummer

Can we get more video

Car is in the shop for 1-2 weeks. I will try and put something together when I pick it up.

[CaryTheLabelGuy]

This design slightly concerns me in regards to excessive back pressure.

Are there any before and after independent dyno numbers available (preferably a graph)?

[brianeck]

Sounds more like my 11' 335 with a full Dinan exhaust now and I mean that in only a positive note. Before any of you knock it, that sounded better then this car in its stock form and if it revved as high it would have sounded heavenly!

[S65V8]

Sounds a bit rattly.

[The Flying Scotsman]

Originally Posted by CaryTheLabelGuy

This design slightly concerns me in regards to excessive back pressure.

Are there any before and after independent dyno numbers available (preferably a graph)?

I don't understand what you mean about the downpipe giving too much back pressure, please explain?

I don't have a before Dyno pull, only after.

[The Flying Scotsman]

Originally Posted by SROC5

Sounds a bit rattly.

That may be the stock exhaust that you are hearing. Remember the active rear is not installed yet. I'm not very happy dealing with the stock flaps.

If you mean raspy, I disagree, as the active mid- pipe was engineered to cut out almost all rasp and drone, along with that tinny noise that you hear with the stock system.

[CaryTheLabelGuy]

Originally Posted by M3*GaryShrig*M3

I don't understand what you mean about the downpipe giving too much back pressure, please explain?

I don't have a before Dyno pull, only after.

Well, the stock exhaust is 2.5 inches on both sides, after the downpipes. So each bank (3 cylinders), is flowing through 3inch exhaust, necked down to 2.5 inch exhaust after the downpipe, all the way to exit.

With this mid-pipe, both banks of cylinders (6 cylinders) are having to squeeze into a 3.5 inch mid-pipe, the separate back out into dual 2.5inch pipes before the muffler. The mid-pipe would need to be close to 5inches in diameter to be equivalent to running two dedicated 2.5 inch pipes (one for each bank).

It would need to be tested in the dyno and datalogged to know if this will make a difference in back pressure or not.

[The Flying Scotsman]

Originally Posted by CaryTheLabelGuy

Well, the stock exhaust is 2.5 inches on both sides, after the downpipes. So each bank (3 cylinders), is flowing through 3inch exhaust, necked down to 2.5 inch exhaust after the downpipe, all the way to exit.

With this mid-pipe, both banks of cylinders (6 cylinders) are having to squeeze into a 3.5 inch mid-pipe, the separate back out into dual 2.5inch pipes before the muffler. The mid-pipe would need to be close to 5inches in diameter to be equivalent to running two dedicated 2.5 inch pipes (one for each bank).

It would need to be tested in the dyno and datalogged to know if this will make a difference in back pressure or not.

I understand your concern.

Here is my Dyno:

Active 8 module
Downpipe
Mid pipe
Filters

[MChat]

Originally Posted by CaryTheLabelGuy

With this mid-pipe, both banks of cylinders (6 cylinders) are having to squeeze into a 3.5 inch mid-pipe, the separate back out into dual 2.5inch pipes before the muffler. The mid-pipe would need to be close to 5inches in diameter to be equivalent to running two dedicated 2.5 inch pipes (one for each bank).

Actually if you consult the math to calculate flow rates, the 3.5" single pipe will flow *almost* as much as two 2.5" pipes.

(Had to look this up again):
Generally speaking, an exhaust will flow 115cfm per square inch (cross-section) and you want 2.2cfm of flow per horsepower.

A single 2.5" pipe has 4.91 square inches on the cross section, supports 565cfm, good for ~257hp. Double that is 1130cfm or 514hp.

A single 3.5" pipe has 9.62 square inches on the cross section, supports 1106cfm, good for 503hp.

Also note that this is of course "theoretical". I know guys running 4 cyl turbo cars pushing 600+ whp through a 3" exhaust. According to the math, they'd gain a decent amount of power if they were to increase their exhaust size.

Lastly, This formula is for naturally aspirated engines, I never found any material on how a turbo or other forced induction changes the formula, but the more energy that the exhaust gas looses, the cooler it and more dense it becomes, requiring less volume. So my guess is that since a turbo will drain a great deal of the energy from the exhaust, a turbo engine will not require as large of an exhaust pipe.

[CaryTheLabelGuy]

Originally Posted by MChat

Actually if you consult the math to calculate flow rates, the 3.5" single pipe will flow *almost* as much as two 2.5" pipes.

(Had to look this up again):
Generally speaking, an exhaust will flow 115cfm per square inch (cross-section) and you want 2.2cfm of flow per horsepower.

A single 2.5" pipe has 4.91 square inches on the cross section, supports 565cfm, good for ~257hp. Double that is 1130cfm or 514hp.

A single 3.5" pipe has 9.62 square inches on the cross section, supports 1106cfm, good for 503hp.

Also note that this is of course "theoretical". I know guys running 4 cyl turbo cars pushing 600+ whp through a 3" exhaust. According to the math, they'd gain a decent amount of power if they were to increase their exhaust size.

Lastly, This formula is for naturally aspirated engines, I never found any material on how a turbo or other forced induction changes the formula, but the more energy that the exhaust gas looses, the cooler it and more dense it becomes, requiring less volume. So my guess is that since a turbo will drain a great deal of the energy from the exhaust, a turbo engine will not require as large of an exhaust pipe.

This formula doesn't work for a turbocharged engine. You want as little back pressure as possible, especially with small turbine sections. The best option for a turbocharged engine is a dump pipe right off the turbo.

You are correct about the cooler exhaust gasses being more dense. However, your theory is backwards. As the hot exhaust gasses exit the turbine section, the are flowing at a much higher velocity than they will at the mid section. The exhauat gasses cool very quickly as they traverse the system. As they cool, they'll expand very fast. This expansion will be occurring right at the mid-section, where the exhaust pipe is necking down and merging. The merging itself is fraught with turbulence if not done properly.

This exhaust gas velocity issue is why it is best to ceramic coat your turbine sections and downpipes. This keeps the exhauat gasses as hot as possible for as long as possible, so they'll traverse the exhaust system warmer and at a higher velocity than without that few feet of ceramic coating.

So in closing, a turbocharged engine can tolerate a smaller diameter downpipe, but will need a larger diameter system as the exhaust gasses cool and slow down. Any merging and necking-down of the exhaust post downpipe can be detrimental to performance.

How much will this mid-pipe effect performance? There isn't a way to tell without some dyno testing and datalogging. I will say this though, it is a nicely made piece and sounds great!