Exhaust Not Worth It

[RZT5]

I think its actually something in metric that works out to somewhere between 2.25 and 2.5

[Fishey]

Been saying this for 10 years now and no one listens. If you want to go fast run 2.5 all the way back. If you want to be a paper racer 3" or gtfo......

Sure, if the car is n/a otherwise I disagree.

[Q-Ship]

Here's an article that I found interesting on the subject; apparently it's correspondence with a Garret engineer.

http://tiny.cc/9vuij

[Q-Ship]

-sorry, haven't figured out how to edit yet. In my understanding, a higher back pressure is desirable between the exhaust port and the turbo, but after the turbo, the less restriction the better. Admittedly, it's all theory for me as this 850 is my first turbo experience besides Knight Rider.

[Dammit]

Thanks very much for posting that article, very interesting.

[TRACStar]

Che's arguement seems to be more about the fact that the O2 is not expecting a 3" diameter. Therefore the O2 will read the exhaust as being lean due to the increased volume that the pipe has. It makes perfect sense in theroy but how much of a difference it actually has I have no clue.

Flow wise for power it was always my understanding that on a turbo car, since the turbo supplies all the backpressure the engine could ever hope for the best exhaust would be a straight pipe exiting the side of the fender. Would you lose some bottom end? Probably, but the increase in the mid range and up top would more than make up for it.

[AverageVolvo]

I have experimented with several setups. For me 3in Dp and cat 2.5 catback is ideal(with methanol inj). Good spool, moderate Egt's, and no drone or obnoxious exhaust noises. Just sexy turbo spool

3in does yield lower egt's but most would not even notice the difference unless your tracking the car.

[troyhyde]

George, the most restrictive part of the stock exhaust system (on your '99) is the crush bend after the suitcase muffler, second biggest restriction is probably the muffler itself.

On your car, you already have a good angled turbo housing and downpipe. Considering the LPT internals I wouldn't do more than replace the cat-back on the car. The IPD/TME offering is of very high quality and is quiet. I think you have hit the point of diminishing returns on your set-up right there.

To do more would be a waste of your time, effort, comfort (noise), and pocketbook in my humble opinion.

You could put on a larger downpipe which would free up a few more horsepower. The cost, labor, and possible danger to the engine would probably negate any minor benefits.

I've got a '95 850 with the stock restrictive conical turbo. With a TME cat-back the car is pretty darned quick. I've got other cars with angled turbo housings, large exhaust all the way to exhaust tip. Honestly, they all seem to produce about the same amount of power.

[Ellsworth]

Che's arguement seems to be more about the fact that the O2 is not expecting a 3" diameter. Therefore the O2 will read the exhaust as being lean due to the increased volume that the pipe has. It makes perfect sense in theroy but how much of a difference it actually has I have no clue.

Flow wise for power it was always my understanding that on a turbo car, since the turbo supplies all the backpressure the engine could ever hope for the best exhaust would be a straight pipe exiting the side of the fender. Would you lose some bottom end? Probably, but the increase in the mid range and up top would more than make up for it.

I'll probably be down rated for this but, The A/F ratio does not change because the tube the exhaust gas flows through is bigger. If it did where would the extra air or fuel come from? It's common sense here. The A/F ratio changes when the parameters of the ecu are exceeded, because flow through the system as a whole is increased and that's where tuning comes in. Turbos work on a pressure differential and back pressure IS the enemy, unless you don't have a way to tune and are trying to not boost too early for fear of bending rods, as is overly regurgitated here, you don't have any reason other than noise to run a smaller than optimal exhaust. Would you loose bottom end with a straight pipe exiting the fender? No, quite the opposite you will spool faster and have more power everywhere, assuming the straight pipe, or tube as it is commonly referred to in the industry was of sufficient diameter to not cause any back pressure.

[Che'_Moderator]

I'll probably be down rated for this but, The A/F ratio does not change because the tube the exhaust gas flows through is bigger. If it did where would the extra air or fuel come from? It's common sense here.

So ummmm I suppose changing the diameter of the tube the MAF sits in has no effect either?

[flaco]

So ummmm I suppose changing the diameter of the tube the MAF sits in has no effect either?

MAF = O2 sensor = FAIL!

I remember narrowbands having trouble at WOT when gas speed/massflow was high. Dunno about widebands. A bigger pipe would slow down the exhaust speed.

[Che'_Moderator]

MAF = O2 sensor = FAIL!

I remember narrowbands having trouble at WOT when gas speed/massflow was high. Dunno about widebands. A bigger pipe would slow down the exhaust speed.

Monkey = Doormat = Epic

I can post random meaningless equations also.

[troyhyde]

A larger exhaust pipe will decrease the amount of oxygen the O2 sensor sees in its pathway, reduced by both volume and speed. It will read this as higher voltage, a signal the car is running richer since it appears more oxygen has been burned in the combustion process - less measured oxygen implies more burned (more burned implies excessive fueling). The ECU will reduce fuel to compensate for what it sees as a rich condition.

Bigger exhaust with no changes to the ECU will make the car run lean!

If you DECREASE the diameter of the MAF enclosure the same percentage as you increase the exhaust pipe, you should theoretically get the right mixture in the cylinder... You are now tricking both sensors.

[Ellsworth]

So ummmm I suppose changing the diameter of the tube the MAF sits in has no effect either?

I would not make that claim, I thought we were discussing post turbo exhaust and it's effect on a device that measures the ratio of air to fuel, not the mass of air passing through.

[Spanky]

I'll probably be down rated for this but, The A/F ratio does not change because the tube the exhaust gas flows through is bigger. If it did where would the extra air or fuel come from? It's common sense here. The A/F ratio changes when the parameters of the ecu are exceeded, because flow through the system as a whole is increased and that's where tuning comes in. Turbos work on a pressure differential and back pressure IS the enemy, unless you don't have a way to tune and are trying to not boost too early for fear of bending rods, as is overly regurgitated here, you don't have any reason other than noise to run a smaller than optimal exhaust. Would you loose bottom end with a straight pipe exiting the fender? No, quite the opposite you will spool faster and have more power everywhere, assuming the straight pipe, or tube as it is commonly referred to in the industry was of sufficient diameter to not cause any back pressure.

The gas is diffusing to a certain volume, as this volume reaches infinity your losing accuracy with respect voltage change displayed by the O2 sensor. As the diameter of the DP goes to 0 the back pressure goes up and you loose power. I believe Chuck is referring to the "ideal" diameter for 95% of people on this board who are trying to maximize the capabilities of the stock engine.

To get the best accuracy you would need to know what temperature the O2 sensor/wideband is calibrated at to read Lambda. Obviously if your measurements are off, your actual AFR ratio will also be. If your using a DP thats too big your going to vary your accuracy for Lambda at a given engine RPM/boost level. If the ideal operating temperature is close to EGT's seen just prior to the turbine wheel you could easily plumb in a collector (say 1/2" line) that follows the DP which would be used for Lambda readings only (much like a external WG plumb), or move the sensor upstream on a larger diameter DP. My guess would be that the ideal temperature is much less then temperatures just prior to the turbine wheel do to the distance of the 02 sensor on a stock car.

If you DECREASE the diameter of the MAF enclosure the same percentage as you increase the exhaust pipe, you should theoretically get the right mixture in the cylinder... You are now tricking both sensors.

Wanna sell me a bag of whatever your smoking?