Now, if it goes about stock Y-pipe in M54. Put this # 18301733901 into your realoem, and tell me what you got.
Tried it, nothing came up. I googled the part number, and it looks like whatever it is, it came on M52tu equipped cars, and not M54 cars. There is NO CROSSOVER ON M54 cars.
OK, once again about pipe merging. I hope, it will be the final proof to confirm I'm right about my pipe diamater ratito theory.
Check this -> http://www.supersprint.com/USP00bmwE4626.asp
Supersprint offers 63,5mm pipes for headers back exhaust. Now, you can also choose something like this:
what do we get than? 76 / 63,5 = 1,1968
I'm not sure I follow. You posted an image of a SuperSprint Y pipe and measurements compared to a twin pipes. Are we comparing cross sectional area? It's hard to follow your post, and furthermore, what does it prove?
Iceman00, could you explain the relation beetwen stepped headers and merging several pipes into one (creating Y-pipe). I agree that stepped headers work. Stepping helps in cylinder scavenging. But why than, for example, supersprint do not offer whole exhaust stepped, but just the headers? That's because it's only benefical when gases are travelling very fast, which happens just at the cylinder head.
You are arguing in cricles with no real point. I've already established that stepped headers are used to broaded the powercurve by keeping exhaust gas velocity, without to much of a sacrifice to higher rpm power. This is not case with Y back exhaust, and as explained before, there is nothing to be gained with an exhaust after the Y/merger. You can lose power by creating a disruption in flow (backpressure) by sizing the pipe to small, but you can't go to large. (That is how a exhaust cut out works)
Now about the situation farther from the head.
You have a pipe, 10 feet long, 2" dia. Is it going to flow more water if you make one end of the pipe 4" ?
Answer is no. Because you still have a 2" restriction on the other end, which just can't flow more.
So, what is the point of using big pipe diameters at the end of exhaust? None.
Doing so just hurts low end torque, due to no backpressure.
And for now, I donít want to judge anything from AA and BW graphs, unless we have some proper info about dynoíed cars.
I'm at work, but I have quite a few that I can post up. Keep in mind, we are arguing theory here. I have supported my theory with opinion and testing of many others who are more experienced and educated on this subject than I am.
That being said, I'm not sure what you're arguing, that a single 3 inch pipe is too large?
For low end torque you need a system with proper backpressure,
For high end torque you need big flowing system,
Back pressure is never good. Exhaust gas velocity might be the term you are looking for.
With stock setup you get nearly optimal balance for wide torque curve. Nearly because they are limited by emissions, noise levels and costs.
The stock exhaust configuration has nothing to do with Emissions, and everthing to do with packaging.
And what AggieE46 has? Definitely a big flowing exhaust. Low end torque is down, but where is his top end? Nowhere, because he is now limited by stock head. Thatís why bigger bored throttle didnít help either.