Back pressure

[dsm-onster]

I'm bringing this out to the public forum from PMs because I think it prbably woul be a good topic to discuss... Especially for those considering a performance turbo header beyond an EVO3 manifold

i dont know if this [guy] is still on here but i just checked my pm's from way back when i used to come on this site and found this this is after i posted on some thread:

backpressure
actaully your vary wrong my friend ... backpressue is vital in all engines, you have never taken any mechianical classes i assume or gone into any sort of VE theory. In a turbocharged engine the turbo itself is a huge restriction and provides backpressure, too much at that. We have a pressurized intake and so want to .. well i won't explain about pressure diffrentials, you wouldn't understand. A n/a engine has no turbo so the diameter of the pipe directly controls backpressure. you really should not just yell out "thats the stupiest thing ever" cause to those of us that really know engines your the only one that looks like the stupist one ever... sorry! BUT STOP ACTING LIKE A KID.

BTW stay on tuners, don't ever go to DSMtalk .. you would get mualed.

what a [guy]. i know about VE obviously he doesn understand VE. i should start another thread on it and see if he shows up.

He's retarded. Backpressure is the unwanted sideeffect of scavenging. Scavenging is where one pulse leaves behind a vacuum pocket and sucks the next pulse out. The only way to get scavenging is to have a tube or pipe that is a certain diameter to match pulse frequency. Once the rpms move into the range that the exhaust pipe diameter/length is tuned to the back pressure goes away for the most part. And, at this point, the engine VE goes up and consequently more power is generated. A 3" exhaust pipe is tuned to 7-10,000 rpms (or exhaust pulse frequency). Unfortunately, a pipe with any diameter other than infinite has restrictive surface tension on the inside walls this adds to back pressure even when the exhuast is tuned to the rpm range you are driving in.

None of this applies to a turbo engine after the turbine blade. Before the blade (exhaust mani), scavenging has been proven to give a marginally benificial increase in high horsepower applications. But you won't get much or any gain going to a turbular tuned turbo header with an EVO3. Stock exhaust manis have gotten guys into the 9s.

After the turbine blade, the pulses are chopped up to almost a constant flow so there is no way to create a scavenging affect. Therefore, one would want to reduce the backpressure to 0. A turbo works on the premise of differing pressures. If you have 3 bar in the exhaust mani (before the turbine blade) and 1 bar in the exhaust pipe then there 2 bar difference is doing the work spinning the turbine wheel. Then if you remove the 1 bar in the exhaust pipe (0 backpressure), you now have 3 bar doing the work over the turbine wheel. 50% more work!!! This is why 3 inch exhaust on a little 4 cylinder turbo engine does so well. Spool up time is DRAMATICALLY enhanced. The negative sideeffect to having a turbo is the 30-50psi of back pressure it exerts on the exhaust gasses trying to exit cylinder. But that negative effect of the backpressure is worth it once the turbo has begun to spool.

"backpressue is vital in all engines, you have never taken any mechianical classes i assume or gone into any sort of VE theory."

WTF So his class in thermodynamics and fluids displacement really showed him that pressure opposing the flow increases flow

Jester, respond on here. This will get some experts in here to pick their brains. . .

So, guys. It's been 7 years since my Thermodynamics class and materials chemistry class in college... Am I on the right path here?

 

[the_jester]

thats funny matt you edited my post. maybe everyone on here nowadays is smart enough to know how absurd that guys statement was, maybe it doesnt need to be rehashed.

 

[the_jester]

Originally Posted by silver2gst
backpressure
actaully your vary wrong my friend ... backpressue is vital in all engines, you have never taken any mechianical classes i assume or gone into any sort of VE theory

he's right i havent taken any classes, but i was born with a wrench in my hand, really, ask my mom. no, but i have read a ton on this subject and dsm-onster is on point. i geuss it just sounds better coming from someone who took Thermodynamics class and materials chemistry class in college

 

[hereitcomes]

Backpressure can be caused by numererous things...like scavenging, or overlap on timing....restrictive exhaust and so on...

Some backpressure is good - but too much is a bad thing. Think of backpressure in an easy form of - how hard the engine has to work in order to expell exhaust from the car.
Some kids freak out on this subject as if it is the most important thing next to fuel management and boost control.
lets all be friends and we all drive the same cars. Be helpful guys!

 

[underradar92]

When people say "backpressure" aren't they more referring to more of a side effect of exhaust restriction? Example: given a properly tuned header/collector, for the desired rpm range, isnt any restriction after the collector just hurting perfomance? I'm talking n/t apps. here.

 

[the_jester]

that what every performance engine building article i've ever read has said. its not back pressure that you want. its the scavaging effect dsm-onster was talking about.

 

[tstkl]

just drop all the exhaust after the turbo and be done with it.

I've had people tell me that the best flowing exhaust isn't the shortest because when this kid took off his cat back exhaust, he started having boost creep problems, so a downpipe doesn't flow better than the same downpipe with a cat back.

all you have to know is back pressure is bad, if too much back pressure is made by the exhaust, the engine will have a harder time getting the newly combusted air (new exhaust) out of the engine and that will require the engine to put a force on the gases to move it a distance (think, work=force x distance). open headers is the best flowing exhaust for an n/a car, because the engine has to apply the force on the exhaust for the shortest distance.

why do people say "some back pressure is good"?

because if you have a huge 7 inch pipe on a 2 liter inline 4, even with a turbo, the exhaust gas will fill up the pipe before leaving the pipe. this causes it to spend more time in the pipe, and while that doesn't increase the required distance traveled significantly, it does increase the force required. why? because the exhaust gas has time to cool off while its sitting in this huge pipe for a long amount of time. this causes it to become more dense, and take up less volume, increasing the time it stays in the pipe even more, allowing it to cool more, ect. this colder exhaust is denser, and since force=mass x acceleration, a great mass requires more of a force to be accelerated to the same speed. since the pipe's walls create friction, the gas is constantly slowing down, and the engine has to constantly accelerate it until it leaves the pipe. since its heavier it requires a larger force, aka a great amount of work must be done by the engine to the exhaust gases. thats work that could have been used to get your fat mass accelerating.

 

[the_jester]

exactly, i dont know why this guy jumped out there and said that mess about needing backpressure WTF maybe he was a new guy. but my MO is if i'm not sure i keep my mouth shut. or at least i try to.