[insane79]

Hi guys,
I just ordered my bmw performance exhaust for my 325i(2.5ltr 160kw)
its only the back box & not the mid pipe as its an e90 lci, i read online that only installing the back box will give me lesser performance? is that true anyone here with a 320/325/330i installed only the PE back box felt decrease in HP?

Thanks..

[Phil325i]

Nope. In fact I believe it improves breathing at the top end. But only a dyno would prove this...or not.

[gaza01]

no noticeable decrease in hp with mine.

[John320d]

Do you mean having a back box only does not give you as much increase as having a back box and mid pipe.

[insane79]

Originally Posted by John320d

Do you mean having a back box only does not give you as much increase as having a back box and mid pipe.

no i mean only PE back box vs stock back box..

[insane79]

Originally Posted by Phil325i

Nope. In fact I believe it improves breathing at the top end. But only a dyno would prove this...or not.

hmmn interesting.

Originally Posted by gaza01

no noticeable decrease in hp with mine.

you got the pe back box installed on your 325i?

[AnthonyR]

Sometimes you may notice a loss of HP as the NA cars rely on more back pressure than forced induction cars, therefore a free flowing system is better on forced induction.
Putting the car on dyno would be the best way to confirm

[insane79]

Originally Posted by AnthonyR

Sometimes you may notice a loss of HP as the NA cars rely on more back pressure than forced induction cars, therefore a free flowing system is better on forced induction.
Putting the car on dyno would be the best way to confirm

thats interesting info, well i got 5 days more for my PE to come from the dealer

[ade146]

Back pressure is a myth imho although you can change characteristics with exhausts. The tuned length has more of an effect on noise than performance. The manifold lengths are important for power. If someone does know how the back pressure myth started it would be interesting.

[two,_OH_five]

My guess is the "myth" started by being true

If you make a significant change to the VE of the engine the fuel calibration will be wrong which will lower tractive effort - power

For eg
Take a stock Mini classic with stock peashooter exhaust. If you swap that for a 2.5 inch straight trough and bolt on a K&N it will run like a dog until you rejet the carbs. The existing ones are simply not able to supply enough fuel to meet the increased airflow so the engine runs lean and makes less power

The same applies to all cars but to a lesser extent on modern EFI systems. With wideband lambda sensors and long term correction of calibration tables newer cars will automatically compensate for small variations in VE so will usually end up making more power
It also depends on the fuel injection system - map based will suffer more than maf as it relies more on correct VE tables for fiuelling

I'm pretty sure that BMW would have run sufficient time in the dyno cell to ensure there are no issues with their PE range

[ade146]

205 you are indeed correct but that isnt what I said. A drastic change in flow will require appropriate corrections to suit a new free flowing happy state. I guess most would know that. I stand by the fact that you do not need back pressure. I can imagine them at BMW making the latest high efficiency high output engine when someone pipes up "how much back pressure have we got guv". 100mb at 3k ? No zat is not enough lets hammer zee exhaust flat. Someone may well ask how loud is it though. Manifolds are different and can be very technical. But again it isnt back pressure they are trying to generate but vacuum at the correct timings. Only my opinion of course and people are quite welcome to generate as much back pressure as they want.

[insane79]

Thanks alot for the info, abit too much for me to understand but once i get the exhaust installed will update this thread

[two,_OH_five]

Originally Posted by ade146

most would know that

Originally Posted by insane79

HUH?

Gotta LOL at that one

Originally Posted by ade146

that isn't what I said

No. What you said was "back pressure is a myth IMHO" which is somewhat contrary to popular science. Given the thread title and context and guesstimating I read your comment as "the common belief that reducing back pressure loses you power is a myth IMHO"

I agree with what you say. Prof Heinz Wolf isn't standing in the dyno cell with his 4lb lump hammer until there is enough. But there will be some, (even if it's the bare minimum required for tuned manifolds) and when you mess with exhaust flow you do risk messing with the mapping inadvertantly and you do run a risk of losing power until the mapping is fixed

It wasn't intended as an attack, just a bit of explanation to correct my interpretation of your post as it were

[ade146]

No I didnt take it as an attack and I was really just having a bit of fun. I rarely write on forums of any type but it is on forums such as this that the myth is perpetuated. So lets say in a perfect world where the fuel mixture can always be corrected by the electronics you will always get more power with less back pressure. I can tell you when my car had a dpf it was reading over half a bar of pressure in the exhaust. Now it reads zero and is a happy free flowing but smelly beast. And so is the car