how much back pressure does my m30 engine need?

[bullyd]

hi all! ive just bought a remus s/s exhaust for my bimmer but it looks like the boxes on the system are straight through, i was under the impression that engines needed a certain amount of back pressure to run properly! ive already removed the cat so do i need to refit my centre box or will the car be ok with this 'straight through' system? to clarify, my exhaust at present consists of straight through pipes to oem rearbox and the new system looks as if the boxes have no gas resistance at all! i still have the centre section in my shed. any comments will be appreciated.

[bullyd]

bump
:p

[632 Regal]

you dont need any backpressure as long as the O2 sensors are placed where needed, usually in US cars without cats it will read dirty and the DME cant compensate. Just a guess.

[wingman]

Correct me if I'm wrong but my understanding is straight through gives you improved top end at the expense of torque due to lowered back pressure. If this is a wives tell please let me know.

[Erwin8r]

Correct me if I'm wrong but my understanding is straight through gives you improved top end at the expense of torque due to lowered back pressure. If this is a wives tell please let me know.Mostly true, but good exhasut systems are designed to provide the proper reverse-pulses for a good burn (and good power...). I'm pretty sure the Remus are of fairly decent quality and are not really just "straight through." The "box" is designed to resonate and exhaust the gases in a certain manner (otherwise they wouldn't bother putting it there...).

[Alexlind123]

How could the cats affect the O2 sensor if the sensor if before the cats?

[rob101]

i have heard that it is a wife's tail depends on what lengths of pipes you use on your headers, there is tons of stuff on the web about that, but i think in terms of exhausts that is what determines your power band. as you tune the lengths so that the shock waves from the supersonic closing of the valves reflect and arrive at the correct time to help with "scavenging" during the exhaust stroke or something like that, there is alot on intake tuning haven't seen too much detailed stuff on the principles behind exhaust manifold pipe lengths. But it is similar to setting the lengths of the intake runners (in that it uses the shock waves)

[632 Regal]

it cant.
How could the cats affect the O2 sensor if the sensor if before the cats?

[BigKriss]

The literature I've read suggests that the less back pressure the better. This is backed up by scientific study.

"Contrary to popular opinion, any exhaust back pressure harms performance. What increasing back pressure does do is dramatically quieten the exhaust."

http://img79.imageshack.us/img79/268/14uf1.jpg
http://img95.imageshack.us/img95/176/21by.jpg

Source (http://www.amazon.com/gp/product/0947216901/sr=8-1/qid=1142316874/ref=pd_bbs_1/102-4398692-1369744?%5Fencoding=UTF8)


you dont need any backpressure as long as the O2 sensors are placed where needed, usually in US cars without cats it will read dirty and the DME cant compensate. Just a guess.

[rob101]

The literature

Ah yes i have seen that book before, good book. I concur i also wonder, do people think it reduces low end torque simply because it doesn't choke the engine at high revs thus the peak torque occurs higher, yet low down torque remains constant but the written down "peak torque" occurs at a higher RPM? and if you look in the same paragraph it mentions the tuned lengths of the exhaust manifold, which for an NA engine is critical. and they will affect low down torque (compare for example 4-1 manifold vs 4-2-1 manifold for a 4-cylinder)

[genphreak]

Ah yes i have seen that book before, good book. I concur i also wonder, do people think it reduces low end torque simply because it doesn't choke the engine at high revs thus the peak torque occurs higher, yet low down torque remains constant but the written down "peak torque" occurs at a higher RPM? and if you look in the same paragraph it mentions the tuned lengths of the exhaust manifold, which for an NA engine is critical. and they will affect low down torque (compare for example 4-1 manifold vs 4-2-1 manifold for a 4-cylinder)After taking a look at my car a BMW mechanic told me to expect more consumption with my Free Flow exhaust.

After some thought, I tend to agree. A Motronic car (like my M30) was never designed with one in the first place, and let's face it; it would have been dead easy for BMW engineers to suggest an upgrade for (at least) the M5 variant, should a FF exhaust be a good thing on these cars.

But no, they stuck with the same old design (which is certainly not a cheap one either) all the way through the range...

So for the moment I have settled on the notion that free-flow is hardly a variable that was dialled into the chip parameters, let alone catered for by the control and sensor design... (a fact born out by my car being definately more thirsty than it should be, even now with everything that makes a difference pretty much attended to). FYI the M5 even had an upgraded ECU setup- so they really could have changed something on the exhaust (other than the tuned front section) if it needed it. I am almost positive that this combination of things means stock exhaust has to be damn good.

And this is one of the reasons why I am replacing my ECU (one day) and adding a knock sensor.

btw, top article Kristian- I used to have a 5.9L Cleveland and can attest to the results they describe on that car. Given that the were looking at many cars for the back pressure product they were looking to market, I can imagine they'd have at least found a few! It would be interesting to find out if the guys in Wollongong actually tested any early Motronic cars... if what I am saying is true, they really would have a market after all. I am sure they'd like to know that... ;) Nick

[rob101]

yeah the stock exhaust is pretty good AFAIK in bmws so don't bother changing it, because they knew what they were doing unless of course you are increasing the power significantly. I don't believe that a Free flowing exhaust will cause more fuel consumption on its own, probably a follow on effect from having more power. I mean how can you lose effeciency by reducing the resistance that the engine sees in pumping the exhaust out? by looking at the pumping losses themselves that should be clear.

[rob101]

actually there is a good book on this kind of stuff that the engine guys at uni used.
http://www.amazon.com/gp/product/0837603099/002-5373547-4985605?v=glance&n=283155 might see if i can track this down, engine design is cool unlike suspension design which messes with your head!:(