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      01-16-2010, 09:12 AM   #1
eg3
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Exhaust mods in FI engines, backpressure, etc..

I've read some comments here talking about losing torque/power when the golf tee mod is done or the flap at the end is removed when using 50 exhaust tips. I posted some stuff on this but I think it's time to give some more detailed info. Here we go:

Torque - exhaust flow relationship on Naturally Aspirated Engines (not our engines):
  • The exhaust on any engine is created as a pulse, much like a heart beat. Every time an exhaust valve opens, it kicks out pressurized, hot gas out and these create a regular pulse that changes speed as you rev.
  • The MOST important factor on any engine is to get rid of this exhaust as quick as possible to make sure it doesn't accumulate anywhere. When it accumulates anywhere. If it does accumulate, it creates a "wall" of gas that bounces back the incoming pulse. THIS is called backpressure. That returning pulse goes back to the engine and causes the new pulse to be created less efficiently. THIS causes torque loss.
  • You might think that using an exhaust as free-flowing as possible would prevent the backpressure issues but there's one important problem here: The gases that come out of the engine fill a low volume when the revs are low and higher volume when the revs are high (more revs = flaps opening quicker = more gas out).
  • When a low volume of gas comes out of the engine and meets your 4" exhaust pipe, it tries to fill the new, larger space. This causes it to lose speed. When it loses speed, it causes the faster gas coming from the next pulse to hit against it, again causing that "wall" of gas. THIS causes the low end torque loss everybody mentions when they go with a freer flowing exhaust on a NA engine. Backpressure DOESN'T cause torque loss, the loss of SPEED causes it.
  • When you go to the higher rev range on an NA engine with larger-than-necessary tubing, the exhaust gases adequately fill the new, larger space and then you feel the power gain because the engine will be better flowing and more efficient now.
  • One may think that going with a smaller than stock size tubing will make the gases flowing faster and gain power/torque everywhere but this is false as well. When you go with a 2" tubing on a 2.5" setup, the gases will be flowing fast at low revs but at high range, when there're more gases coming out of the engine, it will bog up the exhaust and you'll lose power at the high range.
  • The BEST way to build up an exhaust (or mod small parts etc) is to make sure that you go with a large enough exhaust tubing & free flowing mufflers to keep the gases flowing fast at all rev ranges. This is hard to do since what you gain from high range will be lost at the low range with larger tubing and if you go with a smaller size, you won't gain anything. Also, keeping the muffler sizes not too large or not too small but keeping them free flowing and making sure you don't have any switch from say a 2.25" piping to a 3" piping on a flange is important. When you do such a switch, it will be an instant increase in volume that gases need to fill and will create backpressure

On Force Induced Engines (Turbos mainly):
  • When you add a turbocharger to your engine, you basically add a "gas blender" right at the first exhaust gas exit point. As many of you know, a turbo is a combination of two turbines joined by a common shaft. When the exhaust gases turn one, the shaft turns and it turns the intake turbine as well and that feeds up pressurized air to the engine.
  • The important factor here is that the exhaust gas out of the engine gets "chopped" right away at the turbo. The pulses are no more and thus the relationship mentioned above. Instead, you have gases that needs to be removed from the exhaust so that new gas can come in and backpressure is not created.
  • Since the pulse is gone now, there's no worry about losing gas speed. You just need to get those gases out there and this can be done by using a larger tubing. The gases will fill and flow with their own speed and It WON'T cause you torque loss unless you go with an extreme size.
  • Same thing applies for the mufflers as well. The better flowing muffler you use, the more power you get.

On our FI engines:
  • The flap that we have on our exhaust tips are there to block sound at low revs. It's NOT there to help you gain torque. If you think you lost torque removing that flap, you either got something blocking that tip more than before or you have another problem.
  • Especially for those who're replacing 35 exhaust tips with 50 tips: If you exhaust shop used crush bends, not mandrel bent piping, it WILL cause backpressure. Esp on 2.25" piping, when you use tight crush bends on small spaces, the effective pipe diameter will go BELOW 2". That will block gases and you'll lose power/torque. Don't fool yourself blaming the flap removal in that case.
  • The best power we'll gain will be from the downpipes. They are the first exhaust points after the turbos. There, the gas will be hot (more heat = better flow) and it will need to exit out quickly. A good aftermarket downpipe setup will give you more power than you can get with an entire exhaust setup after downpipes.
  • If you're replacing your rear muffler, replace the resonator as well. As you get away from the engine, the flow will be worse as the gas cools off. Keeping the stock resonator but replacing the rear one will give you more sound than power.
  • We have a dual exhaust setup. With this, I mean it's dual from engine to the rear muffler. Only the rear muffler is acting as a container. Replacing it with TWO smaller mufflers (like in Dinan setup) will give you more power (and more sound) than using a single large muffler (like in Supersprint setup).

This forum needs sections, stickies and better moderation. The X6 is a very capable CUV. It will response very well to mods and there are very good aftermarket companies that serve the 3 series community now. I understand that many here think modding = ricing = losing prestige. That's not true. You don't have to "rice up" or "soup up" your X6. Even adding some springs, replacing resonator and adding an intake will keep it prestigious still and make it sound and perform much better. If the reason you got your X6 is because it's a sporty SUV that is NOT meant to carry people around and that you like this fact, you can make it slightly more sportier. In my honest opinion, it's a high riding, large CUV in its stock status that handles corners well but doesn't feel like it. It goes fast but it doesn't get you excited about it. Just my .02

Final note on voiding warranty: You have a legal right to ask the dealership to prove that a mod caused the problem. Say if you have aftermarket wheels and have vibration problems, they won't touch it and with good reason. Go to the place you got the wheels from and to the tire shop that mounted/balanced them. A dealership can NOT blame the aftermarket wheels for a problem in brakes. Same goes with other stuff as well.

Ege
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      01-16-2010, 09:26 AM   #2
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eg3,
A couple of months ago I talked to the gent, see below link, who runs AR Designs. He was looking for a X6 to develop down pipes for. I think he mentioned 3" diameter. No one makes them but there are a couple of companies out there that make them for 335i's. They would be free but the car has to spend 5 days in Denver to get it done. I'm on East coast. I'd hoped to be able to help but it would be a 9 day trip for me. Perhaps there's someone closer?

http://www.ardesign.info/
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      01-16-2010, 09:37 AM   #3
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If someone really wants to get some HP wack off your catalytic converters and put in a Riss Racing Oxygen simulator in your ECU. I did it with a 328i and gained 14HP, it was dynoed before and after. You'll void your waranty but I guarantee you'll also get an increase in sound. FYI, Riss does not make an oxygen simulator for the 328i, no one does. You'll get a light you'll never be able to extinguish if you try it in that car. Conversely, if you could see the guts of a Cat you wonder how the engines run at all.
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      01-16-2010, 11:22 AM   #4
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looking to replace exhaust tips and increase growl

picked up a '10 X6 35i with perf. package. I really don't like the exhaust tips. what the best way to give dual pipe look and most imp increase the exhaust sound, to a deeper growl. your article was great. I need specific names of product for South Florida region.
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      01-16-2010, 11:22 AM   #5
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Quote:
Originally Posted by scottjays View Post
picked up a '10 X6 35i with perf. package. I really don't like the exhaust tips. what the best way to give dual pipe look and most imp increase the exhaust sound, to a deeper growl. your article was great. I need specific names of product for South Florida region.
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      01-16-2010, 12:17 PM   #6
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Quote:
Originally Posted by scottjays View Post
picked up a '10 X6 35i with perf. package. I really don't like the exhaust tips. what the best way to give dual pipe look and most imp increase the exhaust sound, to a deeper growl. your article was great. I need specific names of product for South Florida region.
There are not many aftermarket products for the X6 yet. From the exhausts, Dinan, Supersprint and Hartge have quad pipes but all are for the 50, which requires the parts of bumper around the tips to be removed. Also, all of these exhausts will cost you a min $1200 just for the parts and that's for a rear muffler only. If you don't mind getting the bumper cut in a body shop, I suggest getting 2 smaller Magnaflow mufflers, 2 x dual stainless polished tips and get a custom exhaust done. That way, the exhaust will cost you $400 max, labor for the exhaust around $200 and labor for the bumper another $200-300.

Also, take a look at other threads. You'll see the Dinan product and Skywagon here has the Supersprint. It's best to keep this thread a "technical" one.. There are other threads for product info.
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      01-16-2010, 01:35 PM   #7
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For what it's worth, the Supersprint system does not require any cutting of the bumper to install their plumbing through the bumper including the tips. The tip it's self is a large oval shape, at least for the 3.5. Anyone that want's a picture please PM me with your e-mail address. Some day soon I'll break the code on how to post a picture here.
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      01-16-2010, 02:08 PM   #8
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Quote:
Originally Posted by Skywagon View Post
For what it's worth, the Supersprint system does not require any cutting of the bumper to install their plumbing through the bumper including the tips. The tip it's self is a large oval shape, at least for the 3.5. Anyone that want's a picture please PM me with your e-mail address. Some day soon I'll break the code on how to post a picture here.
thanks for all the replies, I called my dealer cancelled the '10 X6 35i and reordered 50i
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      01-26-2010, 01:38 PM   #9
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Would changing exhaust on e70 X5d amplify the diesel engine rattle ?
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      01-26-2010, 07:08 PM   #10
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Originally Posted by MIMINO View Post
Would changing exhaust on e70 X5d amplify the diesel engine rattle ?
I'm not experienced with the diesel engines but I'd think it would since a freer flowing exhaust would increase the engine noise, thus the diesel rattle noise, especially at idle and low rpms. At higher rpms, I'd think the booming engine sound would mask the diesel noise.
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