exhaust options

[So Live]

Why will he lose power down low? due to loss of back pressure? FALSE

"Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V."

-uuc motorwerks

[matt]

Sigh.

[static]

Why will he lose power down low? due to loss of back pressure? FALSE

"Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V."

-uuc motorwerks

bud, you are a little confuseled

1. NA cars need not backpressure, but scavenging effect (search)
Factory tuned scavenging effect is tuned for low rpm and thus increases the volumetric efficiency at lower rpms (it's all about taking advantage of resonance)

2. freer flowing exhaust will increase overal volumetric efficiency due to lack of backpressure, but will also alter the resonance frequency that the stock exhaust was tuned for (to high rpms), that's why you loose torque down low with bigger pipes but gain up top

3. the OEM EMS is designed to adjust to changing VE to an extent (i heard a figure of 5-10% somewhere) by means of measuring the flow of air through MAF. But, if you will search hard on bf.c, you will find a post by JimC himself where he says that one would definitely need a retune if the exhaust system is significantly changed. (most likely refering to m5x engines with factory EMS with his own chips inside). Which is proven on practice by my salt-n-pepper'ed sparkplugs, JimC chip and a very-free flowing exhaust. So i wouldn't rely just on MAF, motronic and narrow band for AFR correction with significant mods.

Alex

[matt]

bud, you are a little confuseled

1. NA cars need not backpressure, but scavenging effect (search)
Factory tuned scavenging effect is tuned for low rpm and thus increases the volumetric efficiency at lower rpms (it's all about taking advantage of resonance)

2. freer flowing exhaust will increase overal volumetric efficiency due to lack of backpressure, but will also alter the resonance frequency that the stock exhaust was tuned for (to high rpms), that's why you loose torque down low with bigger pipes but gain up top

3. the OEM EMS is designed to adjust to changing VE to an extent (i heard a figure of 5-10% somewhere) by means of measuring the flow of air through MAF. But, if you will search hard on bf.c, you will find a post by JimC himself where he says that one would definitely need a retune if the exhaust system is significantly changed. (most likely refering to m5x engines with factory EMS with his own chips inside). Which is proven on practice by my salt-n-pepper'ed sparkplugs, JimC chip and a very-free flowing exhaust. So i wouldn't rely just on MAF, motronic and narrow band for AFR correction with significant mods.

Alex

:up:

I am sooooo glad somebody is paying attention.

[So Live]

Static, thank you for the information. Thanks matt for enlightening me...

Alright, so the ecu's will compensate enough for a really free flowing exhaust to not harm it right? The loss is just in torque but youd likely gain up top. Can you go too big? Whats an acceptable size for a dual exhaust on a M/S5x motor?

[87-325ic]

just think back the the days of the eta.

small intake, small exhaust, tons of low-range torque.

[bimerok]

just think back the the days of the eta.

small intake, small exhaust, tons of low-range torque.

eta had 166lb of torque, which is about ~92.5% less then a ton of torque.:D



Just messing with you :pimp:

[e30polak]

just think back the the days of the eta.

small intake, small exhaust, tons of low-range torque.

an eta...and tons of torque

[static]

Alright, so the ecu's will compensate enough for a really free flowing exhaust to not harm it right? The loss is just in torque but youd likely gain up top. Can you go too big? Whats an acceptable size for a dual exhaust on a M/S5x motor?

please re-read point #3 in my prev post :) .

yes, you can easily go too big on a NA car. With NA car you need reasonable exhaust gas velocity (and not just for scavenging). With going too big with pipes, the velocity goes down, gases cool off before they exist the exhaust creating a restriction (it's harder to push out cold gases than hot gases due to increased density). Big pipes also create more turbulence, which further harms the flow (laminar vs turbulent flow)
And finally, bigger pipes create drone (which is the sound of "bad" kind of pipe resonance :) ), which i personally hate.

[hoveringuy]

This will be clear to someone that has done the swap but not to me (yet)...

can an m50 manifold and downpipe be used with the stock e30 cat? I am looking at swapping and am looking for the easiest exhaust option until I get it running and can devote the time into a proper exhaust.

What would be involved in mating it?

Steve

[matt]

It's possible. I wouldn't bother.

Easiest option is probably THR Eyeball Arms and a stock e36 exhaust.

[So Live]

Hoveringuy, this is what my set up is but with a e30 325i bb triflo cat back. Seems to work fine.

It was pretty "custom" 4 welds per pipe so 8 total. And yes, I know im a shitty welder. Everything clears with about 1/2 inch clearance everywhere (subframe/sway bar + CAB).

[hoveringuy]

So you just modified the e-30 cat to fit the new downpipe?

That's what's what I was wondering about. How do you like it?

[So Live]

Yeah, basically thats what went down. I had the front 3 feet or so of e34 exhaust. I cut em up and tacked them together and thats what fit.

I guess its fine for a stock M50. but technically the e30 pipes are about 1.75" iirc. just cross sectionally speaking it should flow plenty. dual 1.75 = ~4.8 sq inch. single 3" = 7.07 single 2.5 = 4.9. A few people here were reccomending single 2.25" exhaust for these motors.

Its not my favorite way to go about it. Im sure it could flow a lot better. It is cheap and fairly simple if you can weld. Why not?

[N2MYE30]

I didnt think that there was need to make a new thread so maybe we can keep some of the exhaust topics here...bump.

This is my current situation. I am going to the exhaust shop soon and wondering how members are mating up their previous exhaust, such as mine below...


...to their 24V manifolds.


I have the e36 downpipe with the o2 sensor already but of course, i have no clearance. I do not plan on getting any sort of headers for this project.

Any members have pix on how they have their exhaust fabed up? BTW, I have a NVM50.