MarkF@Cartelligent

I own a '98 1.8T with a 1 bar Wett chip.

I have just installed a 3" exhaust from the flex pipe back and have been told that that large an exhaust will reduce my low end torque becuase the exhaust is too free flowing.

Can someone explain how freeing up the turbo to spin up more freely affects my low end torque?

Or is it all a load free spinning *******!!!!!

Thanks,

Mark

DustysA4

make of it what you will...coming from a company trying to sell a free flowing exhaust.<ul><li><a href="http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm">http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm</a</li></ul>

fusilier

This isn't easy to explain so I'll do my best without putting my foot in it.

The turbo is an exhaust restriction which means an increase in exhaust pressure. The exhaust pipe is also an exhaust restriction. So the two together determine the pressure and air velocity in the exhaust manifold. Thus At low RPMS, the exhaust manifold pressure is higher than the intake pressure (no boost) Means the turbo spins up with more umph down low where it wouldn't be spinning at all with a free flowing exhaust. Keep in mind a certain amount of exhaust pressure and velocity is required to spin the turbo. This difference in pressures in a round about way equates to low end torque(through the turbocharger). At higher rpms as the exhaust pressure comes closer to the intake pressure (less torque, more HP). You can see that the less restrictive the exhaust the more air can be pulled through the system. Overall a more restrictive exhaust on a turbo is a bad thing, cause althought the turbo might yield more low end torque since the exhaust manifold sees highter pressure sooner, the engine works harder to push the exhaust gases out as velocity and pressure builds in the exhaust. Remember this all happens at low RPMs where the total flow through the exhaust is still less than the maximum. it will feel more bogged than a free flowing exhaust and will have less high end horsepower.

The gain in torque is quite small for normal exhaust systems, but generally a small gain in low end torque with an associated larger decrease in high end HP means a more restrictive turbo back exhaust.

Rallye cars work the same way with an intake restrictor (something I took off my car) This decreases the pressure on the intake side thus increasing the air velocity. The pressure differential works the same as back-pressure. Creating more low-end torque with a loss in high end HP.


This of course is with all exhaust manifolds of a design that are sized correctly for the turbine.

fusilier

thats accurate for non-turbo engines

Mike O.

...the pressure differntial has nothing to do with turbine vs. compressor differentials. It has everything to do with the pre and post turbine differential.

Back pressure helps turbo engines when they are operating in a vacuum below the point at which boost is developed, just as occurs in a NA engine.

Mike O.

fusilier

Its both. The pressure gradiant in the exhaust is obviously steeper with a high back pressure exhaust. And like you said, the pressure differential across the turbine is greater which means the turbo has more power. If both didn't hold true, then having a high back pressure would be good all the time all the way up the rev. band and torque and HP would be constant, But the amount of torque is indeed a good representation of the pressure differential available between the intake manifold and the exhaust manifold. Thus torque goes down and HP goes up as RPM increases. This is directly related to the fact that the intake manifold pressure approaches the exhaust manifold pressure at high RPMs.

I'll walk it through. You start with a high back-pressure exhaust manifold pressure to get the turbo going. We know that the pressure at the exhaust tip is 0. The pressure at the manifold is some number. So if its high back pressure then the gradient is steeper and the pressure seen at the entry of the turbine is greater than that at the exit. This is the whole point of the thread. So the turbo starts spinning sooner in the rev-band thus building turbo pressure sooner. Intake manifold pressure quickly rises beyond exhaust pressure reaching the point of max boost and max engine torque. From this point on, the engine revs increase and total airflow increases. But with a restrictive exhaust the max flow is less than a free flowing exhaust so the whole system reaches equilibrium (intake=exhaust) sooner. Thus less total HP is created. We can agree on that.

So atmospheric conditions and turbo intake restrictors can indeed do the same thing as a high back pressure exhaust(more low end torque, less high end HP). Again this is why the same effect can be accomplished by changing things on the intake side. intake restrictors create high velocity air entering the turbo housing which allows the turbo to spin up faster for all rev bands were the restrictor isn't limiting max flow. WRC cars do both restrictive exhaust and turbo restrictors to get massive low end torque while limiting to 300hp at high revs. Very high boost levels at low RPMs. Its not effecient and the engines get very hot with all the restriction, but it works within the rules. So they are indeed related. Watch WRC the cars rarely rev above 6K. They stay between 4-5K.

BTW, F1 cars are complete opposites. Normal aspirated very very high flow. Relatively low pressure differentials between intake and exhaust so they have low flat torque but massive high end HP.

4kq

backpressure is always the enemy of power, but it is unavoidable, so engineers make the best of it.

The only reason why you would possibly want backpressure is because of a specific cam timing, to reduce the amount of airflow passing directly from the intake valve to the exhaust. But that is a kludge.

A turbo produces backpressure because it is a exhaust restriction. Engineers have two options on a turbo (and non-turbo for that matter). 1. Small restrictive turbo (high torque, low relative HP [Audi]); or; 2. large efficient turbo (moderate torque, high HP [Pooorche])

You want an engine that pressurises quickly at low rpm vs. one that is most efficient at high rpm. Use a staw analogy very crudely., thin straw high velocity/low volume, fat straw low velocity/high volume.

Kevin K2

did specific tests on 2L saab turbo with hi-flo muffler. initial boost response at lower rpms was much quicker with oem cat, vs midpipe. compared several times, same day. the measured backpressure at turbo exit was zippo at this power range. this would not likely show up on a dyno run. my conclusion was that it was all about turbine exit flow dynamics, not simple backpressure variable.

dyno example ... dyno runs on perky tt rx7 showed big gains in upper rpm quadrant with mid-pipe vs hi-flo cat, but both were exactly the same up to about 6k rpm vs 8k redline.

fusiler ..

wrc rally cars use 34 mm restrictor, by fia law. as it is a restriction to flow, the compressor is still pulling the air into the wheel .... never seen the output of a centrifical fan increase with choked intake? inlet velocity does help on
turbine side, where exhaust flow drives the turbine.

wrc engine makes 270+hp at 3500 rpm. I suspect a negative pressure at the inducer face will occur quickly with a 1.34" restrictor, increasing the pressure ratio for a given manifold boost, and decreasing response.

citroen for 2003 wrc?

fusilier

Kevin I agree with your findings. You got more initial boost at lower Rpms with the cat than you did with the midpipe.

I'm not sure about the reason why you found similar back pressure readings with both installs.
A cat would respresent a blockage in the exhaust which to me would produce turbulent flow. turbulent flow would create more back-pressure than laminar flow. Its also very possible that your saab OEM cat wasn't very restrictive. Nevertheless you got more low-end boost with the cat than without.

I have run my rally car with and without the cat and have produced similar results to yours. Easy to break all 4 tires loose with the cat in there. Harder without it. More low end torque and pull with the cat, but clearly less high end power. I have a very restrictive cat used for registration. It takes my 80mm straight pipe exhaust down to approx 2.5" so its a dramatic example.

I would expect the same with your dyno runs on the RX7. More high end power without the cat.

all things being equal a high flow system will rev faster (more response) etc.. and the differences between a 2.5" and a 3" exhaust will be hard to tell on a dyno with a small sub 2L engine and low stock boost. Very minimal. I'd always go with a a high flow setup with good manifolds. Just because the overall engine responsiveness is better and less heat is generated.

As to the FIA restrictor thing, The output doesn't increase with a choked intake, in fact there is a clear bust in max HP.

Here's how I think the restrictor produces more torque(WRC notables have indicated that it does so I don't argue this point). There are some websites that state, the restrictor only limits max flow. For all other lesser total flow rates it just increases the air velocity since the area is smaller and adds a bit of turbo lag. faster air hitting the compressor=good thing. Think of the airflow going into the compressor as volts and amps. Analogy (Would you rather have 1 amp at 10V or 5amps at 2V. Either way its 10watts. So what if the max flow without the restrictor is 30W and 25W with the restrictor. It makes no difference if the engine is limited to 25W.)

I've heard an argument that since the restrictor is a venturi(meaning it narrows then widens back out), there isn't any increase in velocity at the compressor. Instead the compressor initially spins in a pocket of reduced pressure created by the restricted airflow. This creates faster spool up time once the negative pressure in the housing starts to overcome the resistance of the restrictor it pulls air like a normal turbo but it got a headstart, so to speak. To me, this makes more sense.

WRC engines are limited to 300hp but there is no torque limitation. So the designers use the restrictor to their advantage to create insanely high low end torque curves but nearly flat 300hp dynos.! Weird stuff, but once the flow is "on" so to speak at low to mid RPMs, the torque is higher than a high flow system. They can manipulate the rules to create a legal engine that produces tons of torque at low RPMS while intentionally limiting HP at high rpms. 9/10th of what they do is NOT the restrictor, its proper turbo and engine mapping, but the restrictor does help this strategy.

Aside from WRC, i know of no other cars that use this turbo restriction priciple to build torque rather than HP, and I'm certainly not recommending a restrictive exhaust to anyone.

BTW,
Here is a picture of the 40mm restrictor that I have removed from my turbo housing. I have less low end torque but more overall HP which benefits track driving. Notice that the exit of the restrictor is tapered to a razor sharp knife edge that is exactly the size of the intake on the RS2 housing... The neck is exactly 40mm. The intake of the restrictor is machined to the diameter of the hose.

<img src="http://pictureposter.audiworld.com/38899/restrictor_0003.jpg" border="0">

Mike O.

...that is to say, in vaccuum mode, backpressure helps develop torque. Backpressure on NA engines helps with low end torque because of dynamics of cylider filling and valve timing. Anyway, more energy in vaccuum mode helps the turbine spool up and helps the transition to the + side of the heat pump equation.

Think of the heat generated during the vaccuum operating range as the starter for the heat pump that the turbine is.

Mike O.