Backpressure

I go to a local cruise in and there are two guys there who are generaly full of shit. The one claims that the K24 is a V6 and he is going to put JDM K20 heads on it. And that he he has it at home and it makes 350HP. And that stock JDM Integra Type-Rs make over 300WHP. The other says that the entire under body of his Cavalier is titanium, and that he has a motor in for it that is a 550HP ecotec that when he had that motor in it, it runs mid 9's with a full interior. But one day they said they knew some "dumbass kid" (I think they are the dumbasses) who had a WRX and he put such a big exhaust on it that there was no back pressure and the turbo would never spool. Is that true, that is slightly more believeable than that other stuff. I thought that if you put massive exhaust on a N/A car you can lose power, but a turbo can will make more power, even if there is just a down pipe that blows the exhuast outside the car. Unfortuneatly he sold his turbo ecotec so I can't see it, but he is putting a C6 corvette LS2 and tranny in it and converting it to rear wheel drive. The car show starts again in a few weeks and it is every week, I'll post some pics of their cars, you laugh too.

[sv650s]

k24 is not a v6 you should bitchslap him for that, i think it's possible to fit k20 heads on a k24 since it's basically a bored out version of the k20. the 550hp cavalier that runs 9's is bullshit especially if he's just got a big turbo on it cos that's a whole lot of lag for dragracing so he'll probably run even worse than a stock cavalier (lower compression) till the turbo is done spooling. About the backpressure thing i'm not really sure but i think the bigger the exhaust gets the more the backpressure it gets and isn't the turbo never spooling a bad thing? some people would say you should get out of that cruise but i'll say stay in there to fuck with them a little bit

[SaabJohan]

No backpressure is the way to go.

The best exhaustsystem of a turboengine is a conical diffuser.

For a turboengine, a turbocharger can provide the fastest response and the highest boost pressures with the lowest expansion ratio over it (pressure after/pressure before).

You can use a K20 head on a K24 block, I just posted that because they are funny. O go to the cruise in because it is mostly old school muscle cars driven by old school guys. There are a few cool tuners and then these losers. Plus I can't pass up an opportunity to ride with my nieghbor in his 680HP all motor 1969 corvette

[beef_bourito]

you never want backpressure, it slows exhaust gases down. but going with a bigger pipe with less backpressure will sometimes decrease velocity, depending on how big you go. it's a fine balance between backpressure and velocity that you want to achieve. pre-turbo you want high velocity, so you don't go puting a 4" exhaust on an sti. after the turbo, you want the lowest backpressure you can get, so it's not uncommon to see some turbodiesels with 4" exhaust.

[zx2guy]

Originally Posted by beef_bourito

you never want backpressure, it slows exhaust gases down. but going with a bigger pipe with less backpressure will sometimes decrease velocity, depending on how big you go. it's a fine balance between backpressure and velocity that you want to achieve. pre-turbo you want high velocity, so you don't go puting a 4" exhaust on an sti. after the turbo, you want the lowest backpressure you can get, so it's not uncommon to see some turbodiesels with 4" exhaust.

i wouldnt totally agree. we were talking about this a while back in the zx2 forum (thats ford for you non ford people) i started the thread labeled backpressure and someone gave a rather enlightening site to go to that fully explains backpressure... ill try to find it.

edit: found it. http://www.flowmastermufflers.com/backpressure.html . but i agree with the velocity part. but the site points out that for some parts to operate at max effieciency, you need backpressure. and having the perfect amount will also help with your overlap in your cam (if it sucks out too fast you lose a/f mixture, to much and you contaminate.

[TerminalVelocity]

ok, does anyone know how a turbo works?

Not talking mechanically, im talking scientifically.

The exaust gases are hot...very hot...they go through the housing with the turbine. They cant spin the wheel fast enough to compress shit...BUT! The heat transfers from the exaust into the turbine spining it.

If you have too large of an exaust the gases dont flow fast enough and the heat dosent transfer well...less power

too small of an exaust eather forces the gas out soo fast it dosent heat up the turbine enough....or you get backpressure and destroy the turbine.

[zx2guy]

this is quickly becoming one of those "you need a little bit of both" threads. yes the turbo has to get up to temp, like the motor. scientifically the hot expanded air (which is harder to compress then when taking the origional amt of air prior to heating in the same volume of space< basically you heat the air in an air tank, the pressure goes up) this is where some of the boost comes from. now im no pro in turbos... by far... but i know enough to be dangerous. and anyone who needs a refresher can go to howitworks.com.

im not arguing with terminalvelocity.... however i would like to know more on the science part of it.... mechanically i have it.... but you can never find anyone with the balls to say for sure about some of this stuff. because really alot of it is still refered to as theory.

[SaabJohan]

Originally Posted by zx2guy

this is quickly becoming one of those "you need a little bit of both" threads. yes the turbo has to get up to temp, like the motor. scientifically the hot expanded air (which is harder to compress then when taking the origional amt of air prior to heating in the same volume of space< basically you heat the air in an air tank, the pressure goes up) this is where some of the boost comes from. now im no pro in turbos... by far... but i know enough to be dangerous. and anyone who needs a refresher can go to howitworks.com.

im not arguing with terminalvelocity.... however i would like to know more on the science part of it.... mechanically i have it.... but you can never find anyone with the balls to say for sure about some of this stuff. because really alot of it is still refered to as theory.

You NEVER needs backpressure. It's as simple as that. For exhaust scavenging you need exhaust velocity not pressure. To maximize efficiency you want to decrease the pumping losses as much as you can, this also includes the reduction of exhaust backpressure as that is partly responsible of the pumping losses during the exhaust phase.

Turbocharger turbines work by expanding exhaust gas over the turbine, hence converting thermal energy into kinetic energy. The expansion ratio depends on the pressure difference over the turbine and it's today common (on small engines) to take advantage of the high pressure during blowdown by minimizing the exhaust manifold volume. This will also help to reduce pumping losses. However, a turbine that operates with varying expansion ratio will suffer from a low efficiency so for more static use (like ship diesels) a more constant turbine inlet pressure is used. A stable turbine inlet pressure can be achieved by making the exhaust manifold volume larger.

The desciption how a turbine works given by TerminalVelocity wasn't that correctly I'm afraid. Turbocharger turbines usually have a reaction rate of about 50%. This means that 50% of the heat is konverted into kinetic energy (gas velocity) in the turbine nozzle, the rest is expanded directly in the turbine itself.

[534BC]

Exhaust driving the turbine is done by flow (cfm) the temp affects slightly, but is really just along for the ride. Whatever it is it is. for temps.

Even though it is not a positive displacement, at the flow rates from engine it is quite similar to sizing the compressor side.

[nissanfanatic]

For the simple reason that maximum airflow provides maximum horsepower, wouldn't it be rather foolish to believe that somehow pressure in the exhaust, the opposite of boost(pressure in the intake), is going to promote airflow/horsepower?

[nissanfanatic]

Exhaust driving the turbine is done by flow (cfm) the temp affects slightly, but is really just along for the ride. Whatever it is it is. for temps.

and hot air ballons don't work...

[534BC]

Originally Posted by nissanfanatic

For the simple reason that maximum airflow provides maximum horsepower, wouldn't it be rather foolish to believe that somehow pressure in the exhaust, the opposite of boost(pressure in the intake), is going to promote airflow/horsepower?

The pressure differential across the engine will promote hp/airflow on any given engine. The size and rpm will determine the cfm at any given differential, when boosting increasing the intake at a faster rate than the exhaust is increased will result in mor power from the same amount of boost.

[534BC]

There's nothing magical that happens at the point of pressure loss at the turbine other than the normal loss of temp as determined by the temp/gas/press laws (whoever those guys were??? boyle newton ,,,)

[534BC]

Terminalvelocity is talking about sizing issue where as a turbine that is too large will not make any boost and one that is too small will be overspeeded. It is size we are talking about, not temperature. I am not saying temp has nothing to do withit, but the temps remain pretty constant for like engines and is determined by mixture and such.

For instance there will be no size difference when sizing turbine for pryro temps of 1100-1400. It is what ever it is.

Air balloons run on lp gas right??