Stock Rear Section with Akra Mid-pipe Dyno & Vid

[Trier Germany]

Originally Posted by aus

That combo sounds GREAT.
How was it in the morning when you're leaving the house? Some get kinda boomy and are very loud at initial start up.

It sounded freakin mean on cold starts! And the loud echoes from mu concrete/brick garage. All my neighbors new when i left and came home from work

[slipstream]

Finally updated the original post with the "After" dyno graph and comparison graph that I received a couple of days ago. The Akra X-pipe and HFCs added 14.5 HP and 12.1 lb-ft of torque.

Having lived with the mod for a couple more weeks, I can report that the car is noisier at startup, but quiets down almost to stock levels within a couple of minutes. At cruising speeds, I can't tell a difference from stock. You have to be on the throttle or above around 4,000 rpm where it is definitely louder than stock and maybe a tad raspy.

My CEL came on after about 200 miles; no big surprise there. There is a defiinite hesitation when starting out from a complete stop. I'm not sure if this is the computer acting up because it is getting different sensor readings or what. I think I will end up getting a tune after I buy a new muffler.

Forum member Albert1028 has tentatively agreed to let me install his Vanguard exhaust sometime after the holiday just to try. I plan to post a review and sound clip from that, so stay tuned.

[mixja]

Originally Posted by slipstream

Finally updated the original post with the "After" dyno graph and comparison graph that I received a couple of days ago. The Akra X-pipe and HFCs added 14.5 HP and 12.1 lb-ft of torque.

Having lived with the mod for a couple more weeks, I can report that the car is noisier at startup, but quiets down almost to stock levels within a couple of minutes. At cruising speeds, I can't tell a difference from stock. You have to be on the throttle or above around 4,000 rpm where it is definitely louder than stock and maybe a tad raspy.

My CEL came on after about 200 miles; no big surprise there. There is a defiinite hesitation when starting out from a complete stop. I'm not sure if this is the computer acting up because it is getting different sensor readings or what. I think I will end up getting a tune after I buy a new muffler.

Forum member Albert1028 has tentatively agreed to let me install his Vanguard exhaust sometime after the holiday just to try. I plan to post a review and sound clip from that, so stay tuned.

Hesitation is likely the torque dip you get with this system around 2500RPM - it is quite clear on your before/after dyno as well. I was interested to see if you still get it with the stock exhaust system as the back pressure with the factory mufflers would be higher than the full Akrapovic system.

I haven't seen any reports of any tune fixing this hesitation - only downside of the Akrapovic system...

[Singletrack]

Originally Posted by mixja

Hesitation is likely the torque dip you get with this system around 2500RPM - it is quite clear on your before/after dyno as well. I was interested to see if you still get it with the stock exhaust system as the back pressure with the factory mufflers would be higher than the full Akrapovic system.

I haven't seen any reports of any tune fixing this hesitation - only downside of the Akrapovic system...

As a data point, I have the rear slip-on Akra with the fabspeed HFC/X and, if there is a hesitation, I cannot feel it. I think moving the cats away from the headers, or removing them altogether, is what causes the dip.

[slipstream]

Originally Posted by PencilGeek

Updates added to the DynoDB.
bmw.pencilgeek.org/DynoDB.html

Thanks, PG! Looking forward to seeing your Forum Wars appearance.

[stylinexpat]

Originally Posted by mixja

Hesitation is likely the torque dip you get with this system around 2500RPM - it is quite clear on your before/after dyno as well. I was interested to see if you still get it with the stock exhaust system as the back pressure with the factory mufflers would be higher than the full Akrapovic system.

I haven't seen any reports of any tune fixing this hesitation - only downside of the Akrapovic system...

ESS was supposed to come up with something to have this dip fixed but never did I guess. I was wanting to get one of these exhausts but decided on waiting as I did not want to be stuck having a dip and CEL on.

[Singletrack]

Originally Posted by stylinexpat

ESS was supposed to come up with something to have this dip fixed but never did I guess. I was wanting to get one of these exhausts but decided on waiting as I did not want to be stuck having a dip and CEL on.

IIRC, they lost access to the car in question. I believe it was a customer car with the setup...

[ChasenM3]

Originally Posted by Singletrack

The dyno operator was shifting way too early which is what made it sound so bad. The actual pull sounded like a race engine.

I think it's the stock muffler - it's tuned for the OEM midsection. My car sounds a little "off" with the fabspeed resonated xpipe and the stock muffler at low speeds and high load. The thing sounds absolutely amazing at 4k plus though.

I would guess you get similar, but not identical, sound with all the aftermarket xpipes.

Thanks for posting this for the forum!

I shift early on the run up to gear so we have no issues with the customers vehicles misalignment or jumping off the dyno.

Your statement is correct though, the sounds is produced because the engines revs are considerably low (under 2500rpm). In daily driving you wont have the same sound

Exhaust sounds great in person! I would know ;D
Nice to see your enjoying it!

Chasen

[SOCAL]

Nice numbers fore sure!!! I think it sounds great, I've come to the realization that is is difficult to make this motor sound even slightly bad. Each exhaust system has similar yet very different characteristics. I love hearing all these different combos. Nice work!

[Singletrack]

Originally Posted by ChasenM3

I shift early on the run up to gear so we have no issues with the customers vehicles misalignment or jumping off the dyno.

Your statement is correct though, the sounds is produced because the engines revs are considerably low (under 2500rpm). In daily driving you wont have the same sound

Exhaust sounds great in person! I would know ;D
Nice to see your enjoying it!

Chasen

Certainly didn't mean to imply that you were not operating the car correctly on the dyno. I concur that the way you were shifting is the safest : )

[mixja]

Originally Posted by Singletrack

As a data point, I have the rear slip-on Akra with the fabspeed HFC/X and, if there is a hesitation, I cannot feel it. I think moving the cats away from the headers, or removing them altogether, is what causes the dip.

Yeah I think so - although jonmartin has had a primary cat delete with the factory X-Pipe and can't feel any hesitation, I guess those crushed bends in the factory X-Pipe keep up the exhaust gas velocity and the secondary cats no doubt will contribute more back pressure than the HFCs on the Akra X-Pipe...

[Eugene-TAIWAN]

moving the cats up to primary cat position with 200Cel will not give you any problems with the so called ''dip'' in power. This I know for sure as I tried it out with my custom exhaust system which I had from before. but he size of the piping also has to do something with it. As the stock size is 2.5 inches and the akra is slightly thinner I have to measure it again I forgot but it should be 2.3 inches so it make the air goes through faster. And being such free flow and fast flowing exhaust I think its when you give it that sudden punch of the gas dips just for a little bit.

[mixja]

Originally Posted by Eugene-TAIWAN

moving the cats up to primary cat position with 200Cel will not give you any problems with the so called ''dip'' in power. This I know for sure as I tried it out with my custom exhaust system which I had from before. but he size of the piping also has to do something with it. As the stock size is 2.5 inches and the akra is slightly thinner I have to measure it again I forgot but it should be 2.3 inches so it make the air goes through faster. And being such free flow and fast flowing exhaust I think its when you give it that sudden punch of the gas dips just for a little bit.

So are you saying the Akra exhaust is actually thinner than the OEM exhaust?

If that's the case, then the dip shouldn't be attributed to loss of exhaust gas velocity, and IMO the change in back pressure is messing with the valve timing...

[Eugene-TAIWAN]

Originally Posted by mixja

So are you saying the Akra exhaust is actually thinner than the OEM exhaust?

If that's the case, then the dip shouldn't be attributed to loss of exhaust gas velocity, and IMO the change in back pressure is messing with the valve timing...

yes, but I think the stock cats has a big play in it as well.

I mean we have 4 cats compared to 2. And I think the stock cats are 400 cel each. That is a lot compared to 100 cel each.

[10M3MAN]

Nice sounds good

[mixja]

Originally Posted by Eugene-TAIWAN

yes, but I think the stock cats has a big play in it as well.

I mean we have 4 cats compared to 2. And I think the stock cats are 400 cel each. That is a lot compared to 100 cel each.

Sorry, I'm comparing the OEM X-Pipe w/ Primary Cat Delete to Akrapovic X-Pipe - so 2 cats in each instance.

All reports I've heard indicate you don't get the torque dip with the OEM/Primary Cat Delete setup, but you do with the Akrapovic X-Pipe.

At 2500RPMs, the flow capacity of either exhaust system is not an issue, it comes down to exhaust gas velocity and back pressure. If the Akrapovic exhaust diameter is smaller, then the exhaust gas velocity should be greater, which of course is better, so that rules out exhaust gas velocity as the cause of the dip.

That leaves back pressure - the Akrapovic system should have a lower back pressure even with a slightly smaller pipe, because it has a high flow 100CPI cat. My theory is that the lower back pressure causes intake charge to flow too quickly whilst intake/exhaust valve overlap occurs during the intake stroke, causing intake charge to actually flow straight through the cylinder chamber and out the exhaust (this is referred to as over scavenging). I wouldn't be surprised given the high RPM nature of the engine that BMW engineers tuned for more valve overlap at low RPMs to try and maximise the scavenging effect, hence any significant decrease in back pressure could cause over scavenging...