[CandyRedRC46]

You still have the factory 320i 65mm mid pipe?

[floridaorange]

Yep...Fortunately I have a reputable shop nearby with great pricing that will take care of that.

[floridaorange]

The exhaust mod isn't listed under supporting mods on BM3 - so not sure if less back pressure is required or not. Wouldn't want to overspool the turbo.

Also my axleback is 2.5" - fitted to the 3" magnaflow muffler.

[CandyRedRC46]

Back pressure is never a good thing or needed.

[rjd_F30]

^ +1 Back pressure is not necessary and is a detriment to performance.

[floridaorange]

So you're saying bigger is always better when it comes to turbos exhaust systems??
That's not how I understand it... but I'm not professional mechanic or tuner of course.

[rjd_F30]

Originally Posted by floridaorange

So you're saying bigger is always better when it comes to turbos exhaust systems??
That's not how I understand it... but I'm not professional mechanic or tuner of course.

Pretty much. Turbos use back pressure to spool to a degree, so with no back pressure you'll drop a few psi but the turbo will spin more freely. Generally, the less back pressure the better. It's a very common myth that engines need back pressure.

[sspade]

Not debating, just sharing...

I am also under the impression that some back pressure is needed and is healthy for an engine/system running optimally.

[rjd_F30]

Originally Posted by sspade

Not debating, just sharing...

I am also under the impression that some back pressure is needed and is healthy for an engine/system running optimally.

Many people are . I think where people get confused is on how back pressure relates to exhaust velocity. The purpose of an exhaust is to get those gases out of the combustion chamber as quickly as possible, and back pressure is a detriment to this. However, because you want those gases to escape quickly, you want a high exhaust velocity, which often coincides with back pressure. If you have a really narrow exhaust, you'll have really high exhaust velocity and really high back pressure. If you have a really wide exhaust, you'll have low exhaust velocity and low back pressure. You want something in between, not because you want back pressure, but because you want exhaust velocity. So people will say you don't want to just dump you exhaust for max flow because you need back pressure, but what they're really saying is that you don't want to do that because you want exhaust velocity. The reason we accept back pressure as a part of the system is because it is virtually inseparable from exhaust velocity, so we accept it as a negative side effect of creating a system that fosters velocity, not back pressure.

So, yah, for optimal performance on a normal engine, you don't want to just dump the exhaust out of the DP or headers, not because it needs back pressure, but because you want some exhaust velocity. I hope that makes sense.

[CandyRedRC46]

Exhaust turbines operate via pressure differential.
i.e. high pressure on one side and low pressure on the other.
A turbine will be most effient while there is as little pressure on the outlet side as possible.

You will need optimal pressure and velocity on the inlet of the turbine (cylinder head exhaust ports and turbo manifold, which is totally out of the scope of this topic/thread), but on the outlet of the turbine, exhaust gas velocity is of little concern. The only concern on the turbine and back, should be minimizing back pressure. So you will want to go as big as realistically possible on the turbo back exhaust, which due to clearance constraints means 3.0" turbo back. Any bigger and you will be rubbing hard parts and increasing noise/drone, with no added increase in performance....

Now on an NA engine, this is different. Exhaust gas velocity, scavenging and back pressure is very important through the cylinder head exhaust ports, header, collector and exhaust system. You will want velocity as high as possible, with out hindering pressure/flow. This will not be possible at all rpms, so it will need to be tuned to your desired powerband (and your ability to tolerate high noise levels lol).

An exhaust system with ideal velocity for a given engine at low RPMs, will have too high of exhaust gas velocity at higher RPMs, in turn creating too high of backpressure at higher rpms, reducing torque/power output. Vice versa, an exhaust system with ideal velocity for a given engine at high RPMs, will have a low exhaust gas velocity at lower RPMs, in turn reducing the scavenging effect at low rpms and reducing low rpm torque.

[sspade]

@rjdnyy224
CandyRedRC46

Both of those explanations provide quite a bit of insight. Much appreciated.

So far, Ive removed my cat restriction... plan is to throw MPE on this spring/summer (along with BM3).

That will leave me with only a stock resonator remaining which I will most likely keep depending on sound.


In theory this should make my exhaust "free'er flowing."

[floridaorange]

Is 2.5" not enough on my 320i? Thanks for the detailed info - very insightful indeed!

[CandyRedRC46]

Originally Posted by floridaorange

Is 2.5" not enough on my 320i? Thanks for the detailed info - very insightful indeed!

Ideally, at stage 2, I would want to be at least 3.0" O.D. from the turbo back.

[rjd_F30]

Originally Posted by sspade

@rjdnyy224
CandyRedRC46

Both of those explanations provide quite a bit of insight. Much appreciated.

So far, Ive removed my cat restriction... plan is to throw MPE on this spring/summer (along with BM3).

That will leave me with only a stock resonator remaining which I will most likely keep depending on sound.


In theory this should make my exhaust "free'er flowing."

You've already taken care of all the freeing up by getting rid of the cat, but it'll make a hell of a difference in sound. You'll have the same exact setup as me which is cool . Catless DP, stock resonator and MPE.

[sspade]

Originally Posted by rjdnyy224

You've already taken care of all the freeing up by getting rid of the cat, but it'll make a hell of a difference in sound. You'll have the same exact setup as me which is cool . Catless DP, stock resonator and MPE.

Shweeet!

My cold starts are horrible currently... as is the CEL im staring at...

I need BM3 bad!

Waiting for someone to offer up a used license for $350.

That honestly appears to be current fair market value. Ive actually seen them go for less!

[floridaorange]

I was done waiting. Got what I felt was a great deal 👍

The cold start delete is the best!

[sspade]

Originally Posted by floridaorange

I was done waiting. Got what I felt was a great deal 👍

The cold start delete is the best!

I know, I know...

If I was in Florida (or SoCal), I'd already have it too.

Been in full ski mode the past couple months.

Gotta pick my spots, everything fun is super expensive

Only about a month or so left and the focus will be back on the bimmer full time!

[rjd_F30]

Originally Posted by sspade

Shweeet!

My cold starts are horrible currently... as is the CEL im staring at...

I need BM3 bad!

Waiting for someone to offer up a used license for $350.

That honestly appears to be current fair market value. Ive actually seen them go for less!

Yah you do, both of those will be fixed

$350 for a used license would be a great deal, hopefully you can find it!

[liveevil]

Originally Posted by CandyRedRC46

Ideally, at stage 2, I would want to be at least 3.0" O.D. from the turbo back.

Is the bigger 3inch from 328 a direct plug and play? I have the ER catted downpipe and Eisenmann backbox. Thanks!

[CandyRedRC46]

I am not sure what your definition of plug and play is, the factory cat back is one entire solid piece, you would have to find an entire 328i catback, cut it at the axle back and then use your axleback or see if you could find the entire 328i mid pipe for sale, already cut in the right spots. Either way there isn't much plug and play for these cars so you will need to get creative, beyond the downpipes....

[liveevil]

Thank! I got it. Basically I need to find one whole piece of 328 exhuast after the downpipe. I am trying to stay stock looking (without additional welding/clamps) so I can pass the annual inspection.

[VSlowBoyLX]

Originally Posted by brofessorpham

I too, own a 320i. The amount of mods that I have done can be epitomized by my signature. I haven’t put my car on a dyno, but if I had to guess I would say that I’m running 250-260 hp to the wheels easily.. and that is a conservative estimate. The mods actually let the car run smoother due to fuel ratio/timing manipulation, increased boost, increased airflow via intake and catless downpipe. I have even observed slightly better gas mileage than factory’s rating of 34 mpg on hwy (I get 35.) In regards to JB4, I would stick to map 5 due to the dangerous amount of boost that can be pushed into the engine... 11:1 compression ratio doesn’t help either.

Can I know what you did to accomplish this?