Straight pipe back from cat - BMW Forum - BimmerWerkz.com
3-Series (E36) Chat relating to the BMW 3-Series from 1992-1999. Autodoodad Specific models include: BMW 316i, BMW 318i, BMW 318iS/ti, BMW 320, BMW 323, BMW 320, BMW 324, BMW 325, BMW 328.

 
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#1 (permalink) Old 08-31-2004, 10:22 PM
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I was wondering if anyone has tried to run dual straight pipes back from the cat. Is it way to loud and is it a retarted idea?

thanks,
sam
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#2 (permalink) Old 09-01-2004, 08:00 PM
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Not if you live on a farm and drive a pickup truck! Save that for the good 'ol boys. The bimmer says HELL NO!
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#3 (permalink) Old 09-01-2004, 09:14 PM
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my bro cut his cats out and did straight pipes... it sounded pretty damn good but ther was no backpressure and he ended up putting on high flow mufflers

~*Teg*~

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#4 (permalink) Old 09-01-2004, 10:21 PM
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I've thought about taking my cats off, de-stuffing them, and putting them back on. When it's inspection time, it will look like they are they and functioning, and I'd still be able to pass inspection.

I wonder if there would be any gains?
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#5 (permalink) Old 09-01-2004, 10:29 PM
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you only pass inspection if you pass emissions, the tester dosn't give a shit if it looks like you got them installed or not, if your car dosn't put out more 'bad chemicals' than allowable, then you will pass.
BTW: Most cats are honeycomb, not stuffed with stuff. If you take all that out, the catalyst isn't there, and wont do the job of converting the fumes into less harmless chemicals, hence, you may fail inspection.

Removing the cat wont make the sound much louder, stright pipes from a cat or headers will sound VERY loud.

If your worried about too much backpressure in your exhaust system, get some freeflow cats and a preformance muffler.

I think if you reduce the backpressure, you get less low-end torque. It may not make sense but I've seen dyno plots where it's clearly obvious. Net horsepower is higher but low end torque is lower.

Instead of a permanent solution, take a look into that solenoid activated Y pipe... you can put that Y piece infront of the cat, and when you want some instant serious preformance boost, you can flip a switch you isntall somewhere in the cab, and the alternate path Y will open up, and it will be just like you had stright pipes.


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#6 (permalink) Old 09-02-2004, 02:31 PM
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ghead hei what the F.... E36 do not need backpressure we got something call ECU that will ajust air/fuel ratio so we do not need backpressure I'll tell you who needs it maybe a 2002 bimmer witch run with carburator if you wanna find out the true about backpressure don't beleave this people go to:

http://www.uucmotorwerks.com/html_product/...etorquemyth.htm

they are the only ones that have post an article that tells the true. anyway if you have someway to work around the ammision test go straight that's the way to go, it will be loud but you can put a tip with deep sound and i'll fix it. I went from dual straight pipe with a borla tip to a singel 3" straight pipe this is call a track pipe tha sh.... is loud every revmach downshift you will hear it loud a clear. I still got a tip at the end 'cause every saturday i take it for a spin around the city.

Please free the beast inside i'll garantee you you'll car will gain power.
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#7 (permalink) Old 09-02-2004, 04:04 PM
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I just read that UUC article, and while the writer is correct on most points, he is missing one critical piece of the exhaust knowledge. while it is true that the DME makes adjustments for proper stoicometric efficiency, and so you dont have to worry about valves burning from a lean condition, what the author does not tasske into account is the misconception that exhaust gas has no mass..

Let me explain. Most people think that an exhaust systems only job is to help get as much of the spent gases out of the engine as quickly as possible. however, there is a concept inflow dynamcis known as engine scavenging. what this concept says, in a nutshell is that a column of exhaust gas (like the exhaust in your exhaust pipe) has mass and veleocity. as it moves through the exhaust pipe, it also creates a vacuum behind it. (think of a syringe sucking up liquid for an analogy) well how does this matter you ask? It matters because how much power a given engine makes is tightly tied to how efficiently it fills the cylinder with fresh combustable charge. (thats how superchargers and turbos work right? they pressurize the intake charge so more air and fuel per given unit of space is in there , that is the definition of pressure) well lets think about how that fresh charge gets into the combustion chamber... the valves are open the piston comes down and sucks in fresh charge right? Ah but notice I say valves, the exhuast valves are ALSO open for some of that time. It is called overlap. Why you may ask.. becuase the the exiting exhaust gas has mass and volume, and that column of exhuast gas in your exhaust pipe is moving at a pretty good clip, AND it is creating a vacumm behind it. well that vacuum is being filled by yup you guessed it FRESH Charge. Thus, having sufficient exhaust gas velocity to pull this vacuum at less than full throttle is pretty important for low rpm torque.

This by the way is not my theory by any stretch of the imagination. do your own research and you find it to be true as well... If things worked the way the article says they did why dont we see 4 and five inch exhaust systems or dual 3inchers of physical size is an issue?

Now I know UUC makes a good quality product, but please think about the fact that they are selling cat back systems not complete from exhaust port to tail pipe systems .... the column of gas is left intact for like half the total length of the exhaust by a cat back system, so sure a free flowing cat back system is good, becuase the cats are restrictive and you have not changed the useful lenght of the exhaust column of gases to any extent that matters.

As other posters pointed out.. look at all the jaapanes wanna be tuner carsd with massive exhuast systems that seem to drone as their motors rev out but dont seem to be accelerating very well, except at like 6000 RPMs where the exhaust output of the engine is finally sufficeint to fill that massive exhaust pipe and do some good in the filling dept..
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#8 (permalink) Old 09-02-2004, 09:08 PM
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in short: UUC was talking about a reduction in backpressure, not an elimination.


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#9 (permalink) Old 09-05-2004, 11:29 PM
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ghead ghead ghead ghead
are you

I'm going to put it out very simple we live in the era of 4 stroke engines with fuel injected motors, so:

stroke 1 : Piston goes down and sucks Air/fuel mixture into the cylinder chamber
stroke 2 : Piston goes up and compress mixture and spark plug ignites mixture
stroke 3 : Piston is push down by the explotion of the mixture.
stroke 4 : piston goes up pushin gases out of the cylinder chamber to start over again.

so 1 + 1 = 2

the less force that the piston has to make to get the gases out, the more power you going to get.


remember that the force apply by the piston to get the gases out is coming from the next piston in time to fire up so it just make sence

plus I think I personally belive in a company like uuc that sales performance parts on more that 3000 BMW dealers around the states
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#10 (permalink) Old 09-06-2004, 09:59 AM
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Quote:
Originally posted by RJ'sE36ITS@Sep 5 2004, 11:09 PM
ghead ghead ghead ghead
are you

I'm going to put it out very simple we live in the era of 4 stroke engines with fuel injected motors, so:

stroke 1 : Piston goes down and sucks Air/fuel mixture into the cylinder chamber
stroke 2 : Piston goes up and compress mixture and spark plug ignites mixture
stroke 3 : Piston is push down by the explotion of the mixture.
stroke 4 : piston goes up pushin gases out of the cylinder chamber to start over again.

so 1 + 1 = 2

the less force that the piston has to make to get the gases out, the more power you going to get.


remember that the force apply by the piston to get the gases out is coming from the next piston in time to fire up so it just make sence

plus I think I personally belive in a company like uuc that sales performance parts on more that 3000 BMW dealers around the states
but the more "fresh" or combustable air and fuel in the piston, the more force the piston will require. So by your explanation you are saying that superchargers and turbo chargers would hurt your engines performance, because it would take more force to get the pistons down. As long as there is uniformly the same amount of combustable materials in all of the pistons then the system works just fine.

Remeber that the car needs cool-cold air coming in and hot air going out. Why? Cold air is denser there by giving you MORE combustable material. And you need it hot coming out because hot air moves quicker than cooler air. If you have a larger exhaust pipe from the cat back or a larger straight pipe with not cat, and you have a stock engine you are not helping performance at all. Why? Because the exhaust gases cool down because they can dissapate more and have a larger contact area with the metal pipes. The heat of the exhaust gases bleeds out into the exhause pipe and the out under the car, thereby causeing your exhaust gases to move slower. But wait!! I have a larger pipe so there is room for more slower moving exhaust gases right? Wrong. When you first start your car there isn't a problem. But as soon as you start moving, ie push the gas pedal. The exhaust gases will enter the afore said exhaust system and spread out for the diameter of the pipe and slow down as it looses heat. Then the exhaust gases coming out of the engine will be trying to pusht the exhaust out of the pipe. This cuts down on the vaccum needed to suck in new combustable material.

Gotta run to class, will pick this up again later, I'm probably going to be late because of this post...
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#11 (permalink) Old 09-09-2004, 10:45 AM
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ok i will let it go

but force inducted motors are a totally diferent animals remenber as the pisto work a little bit more to get all gases out the turbyne will push 2 or 3 times more presure that the piston actually does.

I agree with you about the desity of the hot gases there is a old belive in the race world old guys when did not have a set of headers one for top end and another for Low end what they used to make a dent on the pipe right where the dow pipe ends so by them doing that they thought they where creating turbolence and that will created a vacum.

But all this for what I haven't seen the first car that actually the loger of the pipes cool down the gases come on

Have you seen a header after on two laps around a track "red hot" have you touch the tip of the catback of a street car after a cpouple of laps around the track real hot so the biger the better i garantee you the gases couldn't be more hot.

plus have you seen or hear back fires or flames coming out of the back if you run any car with pedal to the metal it will run ritch on mixture and not all of it will get burn so gases like you said will be more heavyer.


anyway I tell from my experience from tial and error but maybe the books have a diferen opinion.

I working in my proyect " Daily Driver" it will be a 2.5 motor with cold air intake TMS ship and straight pipes I will like to dyno this car vs. same but with catback and then i migth be ablel to prove my point.
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#12 (permalink) Old 09-09-2004, 05:28 PM
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For track use is slightly different because the "track" cars run a lot hotter than street cars do. And what I said only applys for the larger diameter pipes with stock engine internals/parts (as said in the post). Track cars are anything but stock and they have ran their cars and tested ect enough to know exactly how much air flow they need inorder to make the maximum use of thier engines. For a heavily moded car or a car with non stock F/I then yes a straight pipe could help performance. However, with a CAI and chip, I think that a larger pipe would only hurt performance if not make no change at all except in sound (if the pipe wasn't much bigger).

Also you would have to dyno the cars both with stock exhaust to find out if they both have the same power before hand or if one has lost more power than the other over time.
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