It could be because you put the rest of the exhaust on.... added more restriction to the exhaust track. Thats just my opion cause i take exhaust off when i wanna got race down 75 and run just downpipe.

Maybe your butt dyno needs calibration. I think most of the power loss you are experiencing is the vibrations and noise from your old open down pipe.

The gas mileage increase may be from not getting on it as hard with the new exhaust. Kind of hard to tell gas mileage short term though. Check it over a span on 3-4 tanks of gas.

Correct me if I'm wrong, but to my knowledge, your increased fuel mileage is due to the back pressure of the exhaust which increases torque/power at lower RPMs when not in boost. You should notice that you don't have to give it as much throttle during normal driving conditions. However, this same back pressure can effectively "choke" your exhaust stream throughout higher RPMs at WOT. You want a large pressure gradient across the turbine wheel...

As others have said, "your butt dyno may need recalibration". Noisy, sadly as it may be, sometimes seems faster. I have also seen the turbo-to-manifold gaskets blow due to the increased stress of a free hanging down pipe on the bolts. It's actually happened to me. This pre-turbo exhaust leak causes extremely slugglish performance and could be what you are experiencing; give it a check.

Just Noticed this was 9 months old. What was I doing; whoops!

backpressure exhaust benefits from no such thing after the turbine all the turbine wants is the least pressure behind it that the key to making more power.

MidShipCivic said:

backpressure exhaust benefits from no such thing after the turbine all the turbine wants is the least pressure behind it that the key to making more power.

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I understand this, that is why I stated that at WOT you should reduce back pressure as much as possible in order to cause the greatest possible pressure gradient across the turbine wheel.

However, I mistakenly confused back pressure and scavenging characteristics. While cruising (out of boost; not trying to boost; relying purely on motor), reduced scavenging (what I was calling back pressure) causes sluggish performance and slightly less gas mileage. Have you ever driven with an open downpipe? It loses noticeable bottom-end under normal conditions such as starting off from a traffic light but feels great up-top.

With open down pipe you were probably boost creeping to 20 psi. Now with a real exhaust on the car you're probably at stock boost level.

_1466_ said:

I understand this, that is why I stated that at WOT you should reduce back pressure as much as possible in order to cause the greatest possible pressure gradient across the turbine wheel. However, while cruising (out of boost; not trying to boost; relying purely on motor), no back pressure causes sluggish performance and slightly less gas mileage. Have you ever driven with an open downpipe? It loses noticeable bottom-end under normal conditions such as starting off from a traffic light but feels great up-top.

Can a wiseman please chime-in on this. Maybe I'm smoking crack

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On a turbo motor, this theory need not apply. As long as the turbine housing is properly sized, it will create more than enough back pressure.

On N/A, completely different. A proper manifold on an N/A car makes a huge difference, as well as the proper sized exhaust.

_1466_ said:

I understand this, that is why I stated that at WOT you should reduce back pressure as much as possible in order to cause the greatest possible pressure gradient across the turbine wheel. However, while cruising (out of boost; not trying to boost; relying purely on motor), no back pressure causes sluggish performance and slightly less gas mileage. Have you ever driven with an open downpipe? It loses noticeable bottom-end under normal conditions such as starting off from a traffic light but feels great up-top.

Can a wiseman please chime-in on this. Maybe I'm smoking crack

Click to expand...

Let's stop using the word back pressure I don't know who ever gave you that word it shouldn't exist for your understanding. Do you have any empirical data to show your car is using more fuel with no tubing after the turbine?

You're talking about velocities that work be aiding in extraction at low to mid variations at lower throttle angles you are not seeing a back pressure issue.

Some exhaust systems exist where variable geometry is used to optimize horsepower.

standard235 said:

On a turbo motor, this theory need not apply. As long as the turbine housing is properly sized, it will create more than enough back pressure.

On N/A, completely different. A proper manifold on an N/A car makes a huge difference, as well as the proper sized exhaust.

Click to expand...

On an NA car the exhaust manifold isn't pressurized however proper sizing of tubing and tubing geometry still apply before a turbocharger.

My understanding of basic exhaust theories has increased 10 fold over the past hour and I must say my initial responses were incredibly uneducated.

MidShipCivic said:

...You're talking about velocities that work be aiding in extraction at low to mid variations at lower throttle angles you are not seeing a back pressure issue.

Some exhaust systems exist where variable geometry is used to optimize horsepower.

On an NA car the exhaust manifold isn't pressurized however proper sizing of tubing and tubing geometry still apply before a turbocharger.

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1.) Yes.
2.) Yes there are; many Ferrari and Lambo models utilize them.
3.) Yes. Turns out several significant principles change when a turbo is added. Smaller manifold tubing yields higher velocities and quicker spool but may choke high-end performance. An application specific balance must be found and it most often yields the greatest power or area under a dyno curve.

MidShipCivic, thank you for the insight! I'm finally released from the back pressure B.S. I'll keep on reading