N54Tech.com - International Turbo Racing Discussion
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DinoNac DinoNac is offline
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Default Custom exhaust creates too little back pressure? - 09-20-2013, 02:01 PM

I had a mechanic tell me at BMW that my Meisterschaft GTS 3.5" exhaust may be reducing back pressure enough to hinder turbo performance. My understanding was that the school of thought was generally the less resistance the better when it comes to exhaust. I do now have the meth injection kit from BMS installed so I'm assuming that if this was the case, that it is no longer because of the meth. I am getting higher boost levels now throughout the power band, but previously I was getting lower psi #s than I had expected and was told that it simply was becaus if the n55 turbos limitations. Can anyone clarify if too little resistance with exhaust is a hindrance to the turbo? I've heard of exhaust gasses being ported back to increase back pressure because of boost creep, is that basically what he was talking about?


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Eric SS Eric SS is offline
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Default 09-20-2013, 02:07 PM

My old school thought on Turbo motors has always been that the turbo itself creates enough back pressure. This is from years ago so maybe things have changed.


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Pitch Black Pitch Black is offline
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Default 09-20-2013, 02:34 PM

This does not apply to turbo cars in a way.

Natural engines need the right amount of free flow and back-pressure. The turbo inherently causes that if you really think about it. Its essentially an NA engine with a turbocharger on it. The turbo wheel spins and causes turbulence in the exhaust flow which causes back-pressure. So your exhaust does not need to cause more than the turbo already provides.

EDIT: If you do an oversized exhaust, your exhaust noise will be ridiculous.


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Eleventeen Eleventeen is offline
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Default 09-20-2013, 07:37 PM

Quote:
Originally Posted by Eric SS
My old school thought on Turbo motors has always been that the turbo itself creates enough back pressure. This is from years ago so maybe things have changed.
I fully agree. No need for any additional back pressure, especially with the tiny restrictive turbine housings on these turbos.




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Xshadows Xshadows is offline
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Default 09-21-2013, 05:54 AM

The less back pressure in a turbo car the better. Turbos hold the back pressure just fine. Sounds like old school mechanic with a BMW cert. All I know is I wouldn't let him touch my car.
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mikew2069 mikew2069 is offline
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Default 09-21-2013, 06:36 AM

This topic seems to come up more and more recently... I recently put a Catback on and ever sense, there seems to be just a bit of lag in throttle response but that could be a placebo effect from the noise...
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Default 09-21-2013, 08:53 AM

I installed a dual quad Magnaflow exhaust on my 535 last year and had to take it off in a couple of weeks.
My gas mileage sucked, the throttle response was terrible and it felt like it lost 20 hp. Droned like a bitch too.
Put the stock exhaust back on and the car came back to life.
I couldn't believe it.
I think that systems like Meisterschaft and Dinan have done alot of research and testing so they should be ok but the custom Magnaflow system I had installed definitely affected the car in a negative way.
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SneakyDeeky SneakyDeeky is offline
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Default 09-22-2013, 03:13 PM

Interesting, my custom quad Magnaflow exhaust didn't impact performance at all. It didn't help or hinder... it just gives the exhaust a much more enjoyable note.








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MrNeef MrNeef is offline
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Default 09-22-2013, 05:46 PM

The magnaflow for the e60 535i drones sooooooo bad at 2k


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Default 09-23-2013, 07:21 AM

The entire premise of backpressure being useful for anything other than emissions is wrong, regardless of turbo or NA. On NA you can build exhausts that actually help scavenging, which means the exhaust actually builds a partial vacuum to pull the waste out. As long as your cam timing and the rest of your setup and tune match, this is always superior to any backpressure. The point of the exhaust is to remove burnt gas as completely as possible, but in that endeavor if you can scavenge enough to draw fresh aircharge in, you can further increase VE.

Short answer he's just regurgitating wives tales brought about by old cars that slap long tubes on without tuning/building for them.

Edit- turbos don't need backpressure either per say. They they need exhaust velocity, which happens to be influenced by pressure ratios, but it's not the same thing. You can somewhat control this aspect with different sized turbine housings, but this is still completely helped by eliminating restriction after the turbine and increasing exhaust velocity (rid of all those cross *****!).

Last edited by v8bait; 09-23-2013 at 07:28 AM..
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Default 04-13-2019, 06:52 PM

I know this is a thread revival but it doesn't seem to be mentioned in case anyone is wondering in the future. The stock tunes in these cars essentially limit your power output through various calculations and parameters including engine load. With a more free flowing exhaust the turbo can work easier and more efficiently at which point the ECU can pull psi to bring it back to the same calculated power output. This is why these cars roughly make the same power when stock at different elevations. If you bump it back to the same psi as before or increase the load tables, you would make more hp. Obviously aftermarket tunes is a different story.


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