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M's Factory
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Discussion Starter #1
First of all let me clear something up, as I had a chat with a friend a few nights ago and it seemed a little confusing to them this principle.

A turbocharged motor works best when it can get air in and out of the combustion chamber as quickly as possible. Plainly put, your engine needs the maximum volume of air in as fast as possible, and to blow it out equally as quickly.

There is no discussing it, the bigger the exhaust on your turbocharged car, the better.

You can argue all you like about exhaust velocities etc, but basic physics will show that once the exhaust leaves the turbocharger, it doesn't matter how fast it goes as long as there is no extra pressure exerted on the turbocharger, and it's volume that can facilitate that.

On a naturally aspirated engine I agree, that there needs to be, enough pressure in the system to heat the pipe and increase flow but if you own a blown car then back pressure is the enemy.

The Turbocharger is our source of minimal back pressure, but that's the back pressure we need... the difference in pressure before vs. after the turbocharger is what makes the turbo spin so fast... It makes our boost! more back pressure will make your performance suffer.

A turbocharger has a relatively low inertia when compared to other rotational parts in your car and therefore it is most drastically affected by things impeding it from spinning. Things such as bearing quality (or type) oil quality, housing shape, impeller and compressor shape intake restrictions and, most importantly, exhaust back pressure.

All of these factors will all affect the efficiency of your Turbocharger and exhaust back pressure is the biggest culprit of performance degradation on an engine. This is why it is recommended that the exhaust be your first point of modification of a performance turbocharged motor.

Generally with stock exhaust systems, you will feel an initial push from the turbocharger boost. After a certain amount of time, the engine will reach a RPM and airflow rate greater than that of the exhaust system. What happens shortly after this is that the exhaust gases start building up and pressurising in front of the restriction, which is usually your catalytic converter which leads to…

· Reduction in Impeller Speed
· Degradation in Scavenging
· Heat

I cant think of much else at the moment. I will think some more on, the three things above and put something up this afternoon.
 

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M's Factory
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Discussion Starter #3
Cool glad it came out ok.

Let me finish it.
 

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M's Factory
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Discussion Starter #4
Impeller Speed

The turbocharger works best in an air-in/air-out as fast as possible situation. The reduction in impeller speed is a direct result of the speed of the exhaust gases pushing the impeller.

This a result of the exhaust stroke of the motor having to push against not only the gases in the chamber to make them exit, but the mounting pressure in the exhaust system.

As the gases move slower and slower past the exhaust wheel of the turbo, the turbo slows down resulting in a decrease in boost.

This is one of the reasons that power rolls off at high RPMs.

Reduce the back pressure and you have not only greater power throughout the power curve, but the power curve now extends further in the rev range.


Degradation In Scavenging

Scavenging is important in all motors, but especially important in performance turbocharged motors where combustion mixture burn is of the utmost importance ( not wanting it to run lean or rich ).

Scavenging happens when both the intake AND the exhaust valve(s) are open for a split second after the exhaust stroke.

What exactly takes place during scavenging is that the pressurised air in the intake blows through the combustion chamber and clears out all unburned gases.

If there is a severe amount of exhaust back pressure, the expelled exhaust gases will actually be pushed back into the combustion chamber! - Potentially OH ****.

This heats up the intake charge further and can lead to detonation, leading us nicely into the last subject…


Heat

Heat is also a byproduct of exhaust back pressure Back pressure heats up the turbo tremendously, which also heats up your intake air temperature which ultimately means, less power.

Thats it, im all out, cleared up , packed and ready to leave. Im off to lunch as I need a pint now Ive done this.
 

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M's Factory
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Discussion Starter #7
Av it ! Mr Nova Boy Puffa Jacket , Baseball Cap Kid Popey. YIP.
 

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You are a world of knowledge Mr Sarky!! :)

Is there a limit then when the size of the exhaust starts to be detrimental? I know its probably different for most cars, but say on a Skyline?
 

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Its Bitchin' :D
 

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M's Factory
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Discussion Starter #10
only if you dont like speed humps, for a turbine engine its all about how quickly you can remove the air.

Not that i have heard of an experiment to find out if a too large exhaust would be unsuitable to the applicaiton.
 

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Its just a new way of smuggling refugees in to the country, they make reasonably good sound insulation too

Paul
 

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Gets Blown by Twins :D
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Would it be worth de-catting a standard car or a slightly tuned car?

I ask because lots of owners said "Yes get rid of the cat" whilst others in the trade (Tim at SVS being one) say "All it does is make it louder". I was thinking of doing this to mine (I'd only like 350bhp nothing more) and, whilst I can see the point behind reducing the amount of restrictions in the exhaust in big bhp cars (400+), if it's not going to make much difference to my car other than noise (as per Tim) then I don't see the point, but if it is worthwhile (as per most others) then I don't mind spending £80 on it.

You can see how confused I can get, anyone care to help a poor & befuddled young man!!
 

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M's Factory
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Discussion Starter #13
You car will feel smoother , but on a turbo car, it is a yes.
 

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Theory runs that for ooomph at lower RPM keep the cat, and for OOmph at higher RPM, lose it. Fits in with Sarky's psots above quite nicely.

I noticed a difference losing the cat I must say. Esp once boost comes on. Less responsive below 3000, but defo more responsive above.

Got a HUGE folder of notes from my 'Engines' theory classes years ago, around 50% of which is Piston engine specific, with a lot of stuff on turbos/superchargers. Must look it out and see if I can add to this damn fine thread.
 

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Gets Blown by Twins :D
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So then if you de-cat your car then you lose a bit of response below around 3000rpm (because the turbo hasn't come on boost and the car needs the back pressure - like an non-turbo car does); BUT; at higher rpm you need it to be as free flowing as possible. OK, I'm convinced :D
 

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Skip said:
Just do it anyway, everyone else has!!! :)
Moi, bow down to peer pressure!!!...................................................Oh OK then :D
 

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Ian H....

As far as my brain goes, thats how it works in terms of basics.

Obviously thermal dynamics, fluid dynamics and everything else going on inside your engine (think of all those bernoullis going on in there!) is too bloody complicated, but the gist of it amounts to that being how it goes.
 

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G-SPORT DRAG GTS
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the greater the differential of pressure across a turbo the higher it's specific output/spool up will be. On an engine designed for maximum output you cannot have an exhaust system that is too big when turbochargers are present, backpressure only exists on N/A production cars to assist with the reduction of charge flowing thru the head on overlap, Even on N/a race cars back pressure is not desireable and the limiting factors there is using gass pulses to create flow scavenging which increases gass flow thru the head.
Enough rambling.....
 

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and when you start running frighteningly high revs backpressure becomes VERY undesirable, as it all gets way too complicated.

I believe.

Would be cool to design a tailpipe with a section which narrows at the end, like a jet exhaust. Try and get the gases to come out supersonic. :D :crazy:

Pity its virtually impossible on a piston engine.
 
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