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2010 2011 BMW 5 Series Forum F10 F10 Technical Topics Engine, Transmission, Exhaust, Drivetrain, ECU Software Modifications What size piping for custom exhaust system on 550i
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      11-02-2015, 02:13 AM   #1
saleen751
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What size piping for custom exhaust system on 550i

I bought the Megan supremo axle back system for a m6 and I'm going to have it modified to fit my 550i. I'm also going to redo all the piping and delete all the resonators. My question is what size piping should I do from the Down pipes 2 1/2 inch or 3 inch. My plans were to do a complete 3 inch system but don't want to lose back pressure or power. Do you guys think the 3 inch will be fine?
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      11-02-2015, 08:05 AM   #2
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the 550i already has 2.75 piping, BUT with alot of bends, maybe you can have a shop redesign it to 2.75 but with very slight to no bends, thats what I was thinking of doing as well.

I wouldnt suggest 3" unless you have aftermarket downpipes with a tune, because you will loose back pressure for sure.
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      11-02-2015, 08:10 AM   #3
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Losing back-pressure on a turbocharged car is a good thing. Go with a dual 3".
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      11-02-2015, 10:05 AM   #4
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Quote:
Originally Posted by NickG_TechniqueTuning View Post
Losing back-pressure on a turbocharged car is a good thing. Go with a dual 3".
yea but he might get some lag, some people have gone with 3" and no tune and have felt the lag
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      11-02-2015, 10:31 AM   #5
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I have the full M5 exhaust at 3" and definitely feel some lost pressure. You need a tune to get it back to feeling stock-y and better.
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      11-02-2015, 03:09 PM   #6
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Quote:
Originally Posted by 550Reasons View Post
the 550i already has 2.75 piping, BUT with alot of bends, maybe you can have a shop redesign it to 2.75 but with very slight to no bends, thats what I was thinking of doing as well.

I wouldnt suggest 3" unless you have aftermarket downpipes with a tune, because you will loose back pressure for sure.
The turbos are all the back pressure you need, anything after them is irrelevant with regards to being "too big". I chose not to do 3 inch merely due to the cost over 2.5 inch (2.75 isn't a common size in america unless you want to spend huge money, and to my knowledge there aren't any 2.75 inch mufflers, either a stretched 2.5 or a necked down 3). In addition there isn't a study in the world that proves you can go "too big" on exhaust.

As far as when you NEED 3 inch, thats a different story, and also one of calculations regarding at which point is it worth it to pick up 7-10 horsepower at more than double the cost of 2.5.
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      11-02-2015, 03:11 PM   #7
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Quote:
Originally Posted by 550Reasons View Post
yea but he might get some lag, some people have gone with 3" and no tune and have felt the lag
They didn't feel any lag, what they felt is a sharper increase in power as the turbos spooled up faster further up the power band. The same thing happens when I install larger cams into domestic cars... even on paper its proven there isn't a single loss of horsepower or torque anywhere on the graph, however the change in power in other areas increasing more quickly gives the feeling of "lost low end". Its all mental and id bet any amount of money you could put dual 4 inch on a car and still not lose any spool time nor torque.
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      11-02-2015, 03:23 PM   #8
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Quote:
Originally Posted by saleen751 View Post
I bought the Megan supremo axle back system for a m6 and I'm going to have it modified to fit my 550i. I'm also going to redo all the piping and delete all the resonators. My question is what size piping should I do from the Down pipes 2 1/2 inch or 3 inch. My plans were to do a complete 3 inch system but don't want to lose back pressure or power. Do you guys think the 3 inch will be fine?
To answer your question I have to first ask you a few to qualify the proper diameter. What you have to understand about exhaust is, bigger is always better, but there is a point of diminishing returns where you just don't see gains other than certain areas. For me, after finding out its unlikely that I'm going to get a custom tune for my car, that ruled out aftermarket turbos and downpipes, so I just did dual 2.5. Its more than sufficient for 500 rear wheel horsepower, and until about 10 years ago most 800+ cars ran dual 2.5 inch (Lingenfelter vettes to name one). The other thing is the actual cost of steel... its about double for 3 inch vs 2.5 and typically carries much more labor as fitting larger piping in smaller areas takes more bends. I reused as much of the factory piping as I could after removing all the pinch areas and resonators, then added 2.5 inch mufflers and an x-pipe. I picked up a noticeable difference in power and faster spool time with harder spools due to getting rid of the restrictions, but I'm sure with 3 inch it would have been even better, I just couldn't justify the cost...

If money is no object, go 3 inch without a doubt... if you're like me and would rather have really darn good, but not quite the best, but save a bunch of money, go 2.5. If you plan on doing turbos, downpipes and really turning that car up, then 2.5 will work, but you will be leaving power on the table. By the book, dual 2.5 is a bit on the small side for a twin turbo V8 reaching near 500 rear wheel horsepower, but for the 5-10 horse I might gain, it wasn't worth it to me.
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      11-02-2015, 06:17 PM   #9
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Back pressure is not needed at all; it always hurts with making power. I did hours of research on exhaust systems years ago. The problem with going with too large of pipe, and losing torque because of it, is actually a result of increased back pressure. Naturally one would think "how is going with a larger pipe causing more back pressure?" The answer is that when at low RPMs, the air inside the exhaust pipes is flowing slowly to the point that it is cooling down inside the pipes. Cooler air is heavier. So when you slam on the gas and the engine is trying to rev up (and increase the flow rate of the exhaust gases) it is having to accelerate a large volume of dense, slowly moving air, resulting in less torque. Once the engine is spun up to high RPM though, most of the time it can overcome this issue and a drop in hp is not observed versus the smaller diameter pipe. So, there is a sweet spot of pipe diameter with every car. Pipe insulation is always good, as it keeps the exhaust gases hot and therefore lightweight.

This principle of less pressure always being better is also the reason a car can make more power with a very short run of exhaust pipe connected by an x-pipe as opposed to open headers (the x-pipe creates a scavenging effect).
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      11-02-2015, 10:17 PM   #10
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Thanks guys didn't expect so many responses. What tunes are you guys running? I'm planning on getting the bms stage 1. My car is a 2011. I'm planning on doing down pipes, possibly the custom intake I saw on here and the exhaust. What tune do you guys recommend? I want to get the most power out of it. I'm located in Los Angeles so if you guys know a better route to go like a custom tune please let me know.
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      11-02-2015, 11:21 PM   #11
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Quote:
Originally Posted by saleen751 View Post
Thanks guys didn't expect so many responses. What tunes are you guys running? I'm planning on getting the bms stage 1. My car is a 2011. I'm planning on doing down pipes, possibly the custom intake I saw on here and the exhaust. What tune do you guys recommend? I want to get the most power out of it. I'm located in Los Angeles so if you guys know a better route to go like a custom tune please let me know.
Everyone will say a different piggy back. I like my ESS E-tronic.
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