complete free flow exhaust, bad?

[stockmkt8r]

Are complete free flowing exhuasts bad? For example, my friend who has a gs-t disconnected his exahust from the cat and now has a huge pipe missing.

Is this bad for our turbo cars? What does it do? What are the negatives & positives?

BTW, his car is L O U D as hell... and annoying

 

[13secGST]

you have an N/T....leave the downpipe on...

 

[fstlane86]

Real quick question from a noob here about to start stock piling parts for first car.
I've been looking over the tuning guide, on here and found two exhaust sytems that I was interested in. One being the Diamond Star Turbo Back, and the other being the Extreme Technologies Full exhaust system. Both I like and just need a little more info on.

My questions are, if I get the DSS one, will I still need to get a down pipe, and will I still have to get an aftermarket o2 sensor housing. It does say Turbo-Back. Sound, not an issue with me, also, I'm looking to do a cat-elimation too. What exactly is the ET Full Exhaust Sysytem, is it Turbo-Back, is it Cat-Back, little confused here. Any help appreciated.

-Bryan

 

[AlecW81]

If backpressure helped spool-up, why would anyone use an aftermarket intake, exhaust, I&E manifolds, etc... explain Mr. Wizard.

 

[AlecW81]

From a discussion on Backpressure in the Extreme Tuner forum...

Exhaust back pressure is _always_ bad (long)
I'm writing this to explain why backpressure is bad. If you want to argue, please read up on it first, then come back. If you want to just continue on with your life, please skip this.

This has always irked me. People are always saying how engines need exhaust backpressure, or all these mysterious bad things happen. I know it's kindof just semantics, but I just want to clear it up.

I'm here to tell you that pressure in the exhaust is 100% always a bad thing, turbo or not.

I'm going to focus on a non-turbo engine, because putting a turbo into the mix makes a bit of pressure in the exhaust manifold/headers no matter what, and I just want to explain the concepts.

A long time ago, back in the caveman days of carburetors, changing anything on the engine meant adjusting the carbs, because they're designed to work for that very particular engine flow rate, ve, blah and blah. That is why when people would install larger exhausts, they might burn valves, because the car was actually running lean, because they wouldn't adjust their carbs to compensate for the exhaust. Not because of the exhaust itself.

To explain the loss of torque in the lower rpm ranges that does happen: exhausts, if well designed, use the principle of scavenging to help pull exhaust out of cylinders. How this works is, imagine you have a manifold with equal length runners, and they all meet up in a collection area. One exhaust pulse races down its runner, into the collection area, and down the exhaust. It has forward momentum, but is eventually slowed, because the valve has closed behind it, so the pulse will create a slight vacuum behind it, then the next valve opens, and it's pulse is helped slightly by that vacuum. This is scavenging. Scavenging only works at one particular frequency/temperature/volume level though. Move out of that frequency, and you have exhaust pulses that actually run into eachother, creating a restriction. The frequency all depends on the size of the exhaust, runners, and many other factors. In general, a smaller exhaust (read stock) is most efficient at a lower frequency/rpm range. Car manufacturers do this, because most people drive around at 1-3k rpms all the time, and making the exhaust efficient there increases gas mileage. Larger, aftermarket exhausts are more efficient at scavenging at a higher rpm. Thus the apparent loss of low rpm torque. The exhaust is like a musical instrument that can only play one note. Try to play another note through it, and it's not going to work as well. However, if you were to remove the exhaust system altogether, you would pretty much see gains all across the board (unless you just removed a very well constructed/designed/tuned exhaust, and then you would only lose torque where the scavenging effect was the greatest). This is because there is _no_ exhaust back pressure anymore at all. There is no scavenging effect, but you wouldn't have to worry about it. However, you would have to worry about your car burning down.

The easier the exhaust can get out, the better, no matter what. Back pressure in the exhaust is always bad, and just causes the engine to have to work harder to push the exhaust out, losing torque. Saying that the engine needs back pressure in the exhaust is like saying that you need vacuum in the intake. The intake and exhaust are very similar, and all the scavenging applies to the intake too, that's why runner length/diameter/shape on both the intake and exhaust are important.

A vehicle with a turbocharger, you can still worry about all of these things. However, the gains you get from making the turbo more efficient far outweigh the gains you get from making the exhaust more efficient. Turbos make the engine have to push the exhaust out into a manifold with lots of pressure in it, but the gains you get from the turbo allow you to say screw it. It's like how a supercharger may take 20 ft-lbs of torque to run, but you gain another 60 ft-lbs back, so you ignore the fact that the supercharger is sapping energy, and just look at the overall. Turbos work well with high manifold pressure, and low exhaust pressure, engines do not work well with high manifold pressure. You ignore one for the benefits of the other.

So, exhaust backpressure is always bad. Good exhaust design can move the efficiency range to a higher rpm, where it's more useful for racing, but decrease the low end torque, and bad exhaust design just sucks (not in a good way). It's not backpressure you need, it's a well designed exhaust suitable for your needs.

-Jesse

 

[stockmkt8r]

Awesome detailed post there! I read an article in SCC once that explained the workings of turbos and superchargers that also went into exhaust systems. Thanks for the repost

 

[cummins4953]

WOW so that explains how 5-inch mandrel-bent straight-piped stacks helped my truck so much...just when I was about to ditch them for a 2-inch baffled-muffler exhaust with press bends in them


Best Wishes, Hammer Down, Enjoy the Ride,
Will

 

[Sotarts]

Originally posted by scuRRed whiTey
hah i love it when 10 people say u dont need backpressure on Turbo cars...when thats basically what they spool up off of...

should i back this up or just say i know cause im very good friends with a mitsu mechanic

now im gonna here he doesnt know what hes talkin about...ur lieing...blah blah right?

ok well turbocharged cars work off of a back pressure to spool up...in order to now have boost creep, u need back pressure...not much but u need it

anything over a 3 inch exhaust on our cars...consider its laggin like hell at high RPMs cause of the boost creeping...

the theory of backpressure spinnin the turbo the other way...what kind of idiot are u man?...that was dumb...like bottom of the barrel dumb heh...enough of that one though


but if u got questions and someone wants to see a diagram of it ill draw it up...but untill then just understand

TURBOCAHRGED CARS NEED BACK PRESSURE TO RUN!

WTF! You can't even spell, what makes you think you make sense?

When you have sh*tloads of backpressure on the turbo car, that same backpressure prevents exhaust gases from flowing faster in larger quantities, thus preventing the turbo blades from being spun faster by those gases and obviously preventing from building more boost. (That's how boost is built, genius--faster compressor spin)

Obviously your dealer mechanic is taught with around economical methods - saving fuel. This is primarily a turbo-charged performance site. Please be relevant.

The ONLY time you need a LITTLE amount of backpressure is with N/A cars, and ONLY up to no more than 3,000 RPM, for economical reasons.

How do I know? When I took out the oxygen sensor, I had an open free-flowing hole in the oxy sensor housing = faster/more exhaust evacuation = less backpressure. Right off-the-line performance was mildly downgraded (lower combustion chamber pressures) compared wth closed hole, but when turbo starts spooling--2,000+ RPM, it pulls like 10 motherf*ckers compared with closed oxy sensor hole.

Thank you, and have a nice day.

 

[AlecW81]

He has been sufficiently flamed... let this one fade into obscurity

 

[Sotarts]

I'm really concerned about that boy. .