Running an open downpipe?

[NateS]

So the other day I bottomed out in my friends driveway. I hit the cat just right to pull it off the downpipe. Didn't break anything just pulled the car right off. The car is ALOT louder and the turbo whines soo much. I'm in the process of putting a straight through pipe in place of the cat (might as well). Anyway, I know N/A cars need back pressure. I'm assuming forced induction cars would too however, I see tons of srt4s and what not cruising with open down pipes too. I also noticed there is an 02 sensor after the cat. The car seems to run fine other than the backfires, rumbles, and having to have the window down all the time. This is not a permanant setup but, i was curious as to what damage I am doing to it driving it like this if any.

[steviek]

nah your fine. the rear o2 does basically jack squat thats what you take out and repace your wideband o2 with. There is a circuit you can build if you want to remove the o2 all together.

[SilvrEclipse]

The rear o2 just lets the ECU know that the cat is working. Has no control over fuel or timing. Also a turbo car does not need to have any "back pressure", the turbine in the exhaust is enough.

[Thor06]

Yup, silvr is right, the turbine will do enough but I should point out that you do no need backpressure... you need chasing. Back pressure is a side effect of tubing small enough to maximize chasing, and no matter what people say, back pressure is not a good thing. If you want a pretty interesting read you can find the article about it on tuners.

[gthompson97]

By chasing I'm assuming you're talking about exhaust velocity?

And yes, you'll be fine running an open downpipe.

[Thor06]

Not exactly, though the two are directly related. When a gas molecule moves, a void follows it. The other gas molecules move in to fill this void. In our case, the exhaust gas moving out of the combustion chamber creates the void behind it effectively sucking the molecule behind it. See? This effect is called chasing. Higher exhaust velocity means bigger void and better chasing, whereas lower velocity means less void and not as good of chasing. Smaller piping means higher velocities but less flow and less flow means higher back pressure. Bigger piping means lower velocities but more flow and less backpressure. Since the exhuaust gas is moving very fast at high RPM's chasing is pretty much negated and its mainly the low/midrange thats affected by chasing or lack thereof. Its a balancing act see, we need to find a happy medium in pipe size so that chasing is maximized at lower RPMs for better midrange, but backpressure is minimized for better power at high RPMs. This is why you cant just slap a 3" exhaust on a NT... it has a lot less backpressure to aide in that top end power, but less chasing so the low/midrange torque is going to suffer. Does this make sense? Sorry if it got a bit scatter brained, I was watching a pretty good episode of CSI while doing this.

[vanilla gorilla]

I run an open downpipe. Apex'i N1 downpipe. No cat or cat back. I've got a custom made turn down bolted to the end of the downpipe though, just to direct the exhaust downward.

[gthompson97]

C'mon Dusty, how dumb do ya think I am!? More flow = more velocity = more "chasing". "Chasing" would be dependent on the exhaust velocity. The more velocity that an exhaust system has, the more exhaust gas it will "suck" out of the cylinder because of the pressure differences. So with less velocity you get less pressure differences as well, thus leaving some unburnt "hot" gas in the cylinder, increasing cylinder temps and also creating a somewhat rich mixture, resulting in less power/torque. Now if we really want to get technical, we can go into the theory about exhaust gas temperature changes as they flow from the cylinder through the system, but we won't go there today. Mmkk pumpkin?

[Thor06]

Originally Posted by gthompson97

C'mon Dusty, how dumb do ya think I am!? More flow = more velocity = more "chasing". "Chasing" would be dependent on the exhaust velocity. The more velocity that an exhaust system has, the more exhaust gas it will "suck" out of the cylinder because of the pressure differences. So with less velocity you get less pressure differences as well, thus leaving some unburnt "hot" gas in the cylinder, increasing cylinder temps and also creating a somewhat rich mixture, resulting in less power/torque. Now if we really want to get technical, we can go into the theory about exhaust gas temperature changes as they flow from the cylinder through the system, but we won't go there today. Mmkk pumpkin?

Well, you asked, so I told.

[xavier3jr]

there is alot of turbo people around here that use a "Cut-Out". its a Y pipe pritty much one side runs the reg exhaust the other comes off the side right after the Dp and has a screwd on cap over that end that you can put on and remove anytime. put it on for daily driveing take it off for strip or street raceing. pritty effective way i think, that way you also dont get in truble for have a exhaust thats too loud when just driveing.

[kjewer1]

Not entirely relevant, but the RWD does pretty well with this this much backpressure. I don't think the turbine provides much backpressure either.

Well that was a blatant case of a little showing off, I'm a big enough man to admit that, but it still proves the point. Most people with DSMs are in for the HP, not the low RPM driveability and economy.

[Black99GST]

Originally Posted by kjewer1

Not entirely relevant, but the RWD does pretty well with this this much backpressure. I don't think the turbine provides much backpressure either.

Well that was a blatant case of a little showing off, I'm a big enough man to admit that, but it still proves the point. Most people with DSMs are in for the HP, not the low RPM driveability and economy.

ok keven, we get it! yours is bigger!

[Thor06]

!!

[NateS]

Lol, I clicked the link expecting to see some of measurement and some numbers. Jealous...

[kjewer1]

That big turbo on the left can be yours today for 600 bucks. You know you want it just for your coffee table...