A Back-Pressure Discussion

[suprf1y]

Originally Posted by 2LZ

A little back pressure is quite useful to help your power curve.
.

No it isn't.
It's useful in covering up for what is an otherwise incorrectly designed exhaust for the application.

[2LZ]

Originally Posted by suprf1y

No it isn't.
It's useful in covering up for what is an otherwise incorrectly designed exhaust for the application.

I beg to differ........ 8)

[suprf1y]

Beg all you want.
You're still wrong.

[2LZ]

Originally Posted by suprf1y

Beg all you want.
You're still wrong.

Sounds like you been talking to my X wife! 8O

[Weldangrind]

Originally Posted by 2LZ

Sounds like you been talking to my X wife! 8O

You've got a good attitude 2LZ!

I can appreciate where it seems like a certain amount of back pressure is beneficial, because it seems like that if the rest of the system is not optimized for a free-flowing exhaust.

I gotta agree with suprf1y on this; an optimized induction system does not benefit from exhaust restriction. As an extreme example, Top Fuelers and Funny Cars don't use anything other than straight pipe that is tuned to the application: wide open throttle for 1/4 mile.

A regular carburated passenger car or motorcycle that has had the muffler removed (with no other changes) will not run as well as it did with the muffler, because the system is not optimized for the additional airflow.

Modern fuel injected vehicles are another matter. Due to advances like mass airflow sensors, computer controlled vehicles can adjust on the fly and increase pulse width (injector "on" time) to add more fuel to suit the application.

[2LZ]

Originally Posted by Weldangrind

You've got a good attitude 2LZ!

I can appreciate where it seems like a certain amount of back pressure is beneficial, because it seems like that if the rest of the system is not optimized for a free-flowing exhaust.

I gotta agree with suprf1y on this; an optimized induction system does not benefit from exhaust restriction. As an extreme example, Top Fuelers and Funny Cars don't use anything other than straight pipe that is tuned to the application: wide open throttle for 1/4 mile.

A regular carburated passenger car or motorcycle that has had the muffler removed (with no other changes) will not run as well as it did with the muffler, because the system is not optimized for the additional airflow.

Modern fuel injected vehicles are another matter. Due to advances like mass airflow sensors, computer controlled vehicles can adjust on the fly and increase pulse width (injector "on" time) to add more fuel to suit the application.

Thanks W. Life's to short to get all worked up....especially on the internet....and besides, if we all agreed on everything, the world would be a truly boring place.

[2LZ]

Originally Posted by suprf1y

Beg all you want.
You're still wrong.

O.k.....I give, you are correct. Zero back pressure is best.

In a ‘perfect world’ or the new, modern world of computerized engine controls and sequential port fuel injection adjusting the entire vehicles manners ‘on the fly’, yes, you are correct that zero back pressure is best.
With the ability to constantly adjust injector pulse width and a continuously changing timing curve, all babysat by sensors, actuators and ECM’s, regardless of altitude and temperature…..zero backpressure will optimize your overall performance across the power curve.
Thanks to modern science, the new vehicle is capable of maintaining a 14.7/1 ratio, richening and leaning it as you accelerate and decelerate and maximizing burn and power by that and timing adjustments due to demand, MAF sensors, MAP and BAR sensors, knock sensors, etc……..all while also keeping a zero back pressure, ‘tuned’ exhaust system silent for the modern, quiet running vehicle.

Now….let’s get to the ‘real world’ of what we are dealing with here, fixed timing curves and antiquated carburetion with preset jetting.
Of course, opening up the stock exhaust system to be totally free flowing will create more top end power. It will also lean the heck out of it, especially if you open up your breather. Hence….the need to richen the jetting or you eventually cook exhaust valves, piston domes and your bank account.
Unfortunately, this freeing up of the intake and exit breathing also eliminates an inherent, planned ‘design flaw’ of the older engines and that’s that a little back pressure was actually designed into these old systems to in a sense, double charge the air/fuel mixture by reversing the flow a fraction of a second in the carburetor bore when demand is increased….creating the side effect of midrange torque and also, cooler running exhaust valves, seats and piston domes.

But yes, you are correct…..in today’s world of modern vehicles….or specifically designed racing vehicles, zero back pressure is, by far, best.

[katoranger]

Modern systems can adjust, but we need to focus on a vintage dirt bike with old technology.

How do we get the results of peak power and torque where it is most useful?

[knothead]

There's no such thing as zero back pressure. Aint gonna happen! Even gas has mass and it takes energy to set in motion. Add to that the drag as the gas moves along past the exhaust valve(s), throught the head and down the pipe... All that will add up to backpressure.

Think of the gas column as a big spring that stretches and compresses with each exhaust pulse (which is how it works). According to the timing that spring will either work for or against you. A "tuned system" is made so that the ex. valves open when the "spring" is stretched and can pull the waste gasses straight out of the cylinder. Back pressure is important in that it can be used to tune the timing of the pulses where they will be effective in the lower RPMs where most street engines are used. A race engine usually needs it's power on the top end, the bottom end dosen't matter so we go with a WFO exhaust with little backpressure.

[FastDoc]

Originally Posted by knothead

There's no such thing as zero back pressure. Aint gonna happen! Even gas has mass and it takes energy to set in motion. Add to that the drag as the gas moves along past the exhaust valve(s), throught the head and down the pipe... All that will add up to backpressure.

Think of the gas column as a big spring that stretches and compresses with each exhaust pulse (which is how it works). According to the timing that spring will either work for or against you. A "tuned system" is made so that the ex. valves open when the "spring" is stretched and can pull the waste gasses straight out of the cylinder. Back pressure is important in that it can be used to tune the timing of the pulses where they will be effective in the lower RPMs where most street engines are used. A race engine usually needs it's power on the top end, the bottom end dosen't matter so we go with a WFO exhaust with little backpressure.

The spring analogy is very thoughtful and accurate. Well explained.

Other than that I have nothing to contribute, I've never engineered an exhaust system.

[Weldangrind]

Originally Posted by katoranger

Modern systems can adjust, but we need to focus on a vintage dirt bike with old technology.

How do we get the results of peak power and torque where it is most useful?

The low tech method is to start with a new plug, do a wide open run, cut the power and read the plug. That'll give you an idea of how close the air / fuel ratio is for maximum horsepower.

I've considered welding in an O2 sensor bung in the exhaust pipe and experimenting with a wide-band O2 sensor and a voltmeter. Of course, being a cheap China Rider, I'd need to get an O2 sensor for free.

[Weldangrind]

Originally Posted by FastDoc

The spring analogy is very thoughtful and accurate. Well explained.

Other than that I have nothing to contribute, I've never engineered an exhaust system.

Copy that. I also appreciate 2LZ's explanation of flow reversal. It sounds a lot like long duration dual pattern cams that are designed to hang the exhaust valve open. The benefit at low rpm is reversion where some of the exhaust gas travels backwards to dilute the incoming air / fuel mixture which tolerates more timing or higher compression. The benefit at high rpm is that the exiting exhaust gas helps to pull fresh air / fuel into the cylinder, creating a supercharging effect.

This thread has been very informative for me, although I still don't know how to properly optimize the exhaust system in the bike we're building. Trial and error, I suppose.

[waynev]

Originally Posted by "Weldangrind

The low tech method is to start with a new plug, do a wide open run, cut the power and read the plug. That'll give you an idea of how close the air / fuel ratio is for maximum horsepower.

I've considered welding in an O2 sensor bung in the exhaust pipe and experimenting with a wide-band O2 sensor and a voltmeter. Of course, being a cheap China Rider, I'd need to get an O2 sensor for free.

Just use an EGT sensor/readout, this will tell you if your rich or lean.

[FastDoc]

Originally Posted by waynev

Just use an EGT sensor/readout, this will tell you if your rich or lean.

That's the way we set our mixture in airplanes where we can vary altitude 15,000 in one flight.

[Weldangrind]

Originally Posted by waynev

Just use an EGT sensor/readout, this will tell you if your rich or lean.

Please tell me more (it's relative to this thread, and I'd love to learn about it). There was an O2 guage on sale at Princess, but there were no instructions with it; it was meant to be installed in the dash. There was no O2 sensor included.