Cascade Crew

[aaron_sK]

A few years back when I was a n00b on these forums, Paul (Twilightoptics) and I got into a little debate over whether improving exhaust flow would help or hurt gas mileage.

I always thought freeing up the exhaust would increase mileage, he said it would result in a drop. Today I did a little bit of calculating on the S10's mileage, and it is down about 1-2mpg after installing the Hedman's, the new Y-pipe, and Magnaflow.

I've also noticed that it doesn't like nearly as much timing, so there must have been enough backpressure to cause some sort of self-EGR.

You were right, dude!

[QwkTrip]

There is no such thing as back pressure.

You overbuilt the exhaust system. The optimum was somewhere between stock and where you are now.

[QwkTrip]

Deja vu.

[Twilightoptics]

What John said.

:O)

You might seem an improvement if what you had was less than what the engine needed, and what you have now is what it needs.

If you go too big, you lose velocity and thus some milage from the low rpm loss.

:O)

[QwkTrip]

I want to know why did you spend all that money on performance parts for an S10?

[aaron_sK]

Pfft. $170 for painted Hedmans isn't a lot of money.

Plus it does a nice pegleg burnout now.

[QwkTrip]

I agree. $170 is a good value.

You should have a custom license plate, "PEG LEG"

[85 T/A WS6]

[aaron_sK]

[DBL_TKE]

[QwkTrip]

That's just a marketing blurb from a company website. We all know what engineers think of marketers, right?

I've done graduate level compressible flow and fluid dynamics. Back pressure is a slang term and a misconception that leads people to incorrect conclusions because it implies that pressure is directional. Pressure isn't directional. A pressure differential causes air movement in the exhaust. And then because of the wildly complex nature of air flow and combustion there are other phenomenon that also cause pressure differential in the exhaust system.

You're going to find all kinds of differing opinions on what is the best exhaust system because the science of air flow and combustion is still developing. Compressible air flow is a baby concept in the world of science and wasn't even developed until the supersonic flight age. We're talking about a science that is only about 60 years old!

[85 T/A WS6]

Slang or not, its widely used by many in the auto industry.

It can be measured (helps to detect a bad cat for example)

It is what makes a 2 stroke engine work.

[QwkTrip]

Those people were in the first rounds of layoff, I'm sure. They probably work for Supertrapp now

You can't measure back pressure. What would be the units of measure? You can measure pressure, though.

So here's a parallel example to the plugged catalytic converter. I'm a little aware of diesel particulate filters (DPF) and the development happening at my company. When they needed to know if the DPF was plugged they had to use two pressure sensors to measure the pressure drop across the DPF honey comb. That's 2 sensors that cost money. There wasn't a section in the catalog called, back pressure sensors. Believe me, if they could save money by using one sensor they would have done it!

[85 T/A WS6]

Putting "back" in front of "pressure" just helps you define where its at in a system. In turbo charging, back pressure is good on engine port but bad on the turbine outlet. This is the only automotive area that really applies to compressible flow.

Here it is, the test kit!

http://www.sjdiscounttools.com/waebpt01.html

It is here, we have it now.

Some Ford Powerstokers use a exhaust back pressure valve

That silly Flowmaster company talks about back pressure here.

http://www.flowmastermufflers.com/backpressure.html

And here is Magnaflow giving lip service to the same thing

http://www.magnaflow.com/05news/magazine/05sportcpg02.asp

The campaign to eliminate the term back pressure will fail.





Long live the Leaf Blower!

[QwkTrip]

[Dewey316]

[QwkTrip]

To add to that, air is a compressible fluid so any air flow problem is governed by compressible flow.

I designed turbine blades in college. There was no mention of back pressure and the word can't be found in my text books (just checked). I'll bet money on McGraw Hill over Supertrapps and Magnaflow any day. Most people used to believe the world was flat, too. Popular opinion usually overcomes truth.

[85 T/A WS6]

Turbines rotate because of enthalpy difference, which inlcudes pressure, but also includes temperature and velocity. Good turbo design maintains enthalpy by maintaing outlet velocity, (engine port to turbine inlet). The statement I made specifically applies to turbos, not NA engines.

By the way, a turbine would spin, with no pressure difference because of the flow of exhaust gases. This would require a large, inefficient turbine design. How does a water wheel, running in a stream turn? No pressure difference there, just water flow.

Compressible flow doesn't apply much in typical NA engine exhaust systems. Yes, exhaust gas is compressible, but the flow velocitys are so low it doesn't matter. There isn't much mystery in design of automotive exhaust systems, just mostly hype as vendors try to convince people that they have a new idea.

Not trying to bust chops here, I just wish you guys would take off the lab coats for once lol.

[Schultzy89GTA]

[QwkTrip]

I did get rained on pretty hard today and had to go home and chance my clothes. You better hope I didn't forget anything in the rush!