Hey it's me again. I'm in a heated debate concerning torque
production in a motor.

I'm trying to explain that backpressure can't be beneficial in an
exhaust system to make power.

I'm also trying to explain that resonance tuning (short/long pipes)
shift the torque peak according to when the motor is most efficient.

IE: Open headers drop torque at low RPM, but shift the power
band up higher.


My Points briefly are:

- Backpressure in the exhaust degrades torque production because
scavenging cannot occur effeciently when the exhaust pressure
is higher than intake manifold pressure.

- Shockwave/Pressure pulses exist in the exhaust system and
are tuned by length of the exhaust pipes.

- Short exhaust pipes resonate better at higher RPM, larger diameter
exhaust pipes improve exhaust velocity at higher RPM (when
the motor is able to produce the flow).

- Shockwaves/Pressure Pulses require a reflective surface to
bounce back upon.

IE: An open header cannot reflect a pulse back because the
wave exits the collector into the atmosphere.

The header length tunes the resonant RPM where the wave can exit at it's highest velocity (any longer would act like a silencer?)

Please note, I'm not a Physic's major, but I do grasp the concepts
well. If you can lay on it thick, I'd appreciate it...examples are nice too!

Thanks for any clarification.

 

Perhaps your grasp of this is not quite as tight as you think. No offense intended, but don't bet the farm on the statements you made.

Some of what you said is pretty close, but some of it isn't. I don't want to pick it apart, but I will suggest a book to read and study which might help you:


Scientific Design of Exhaust & Intake Systems - Third Edition
by Philip H Smith and John C. Morrison. It's published by Bentley and you can find it on line. It's under $25.

If you understand the first couple hundred pages, you'll know a lot more than most folks. You don't have to be a physics major, but that wouldn't hurt. If you were a music major (no flame intended) and understood organ-pipe theory, you would be well on your way.

My $.02.

 

Well, no flame taken.

The backpressure statement is pretty clear. I've found several
credible links to support that statement as well as dyno testing
on my car.

My biggest problem is understanding the wavelength issue.

Can anyone clarify the pressure pulse statement?

Is there reflection in an open ended tube?

I will look up that book that you recommend.

Thanks Bret.

 

 

Thanks Jon.

I'm in the process of looking up the book as we "type".

I don't expect anyone to reveal secrets about engine tuning,
but in general, how does a wave reflect without a surface to
bounce back upon?

Is the wave hitting exhaust gas at the open collector and reflecting
back?

Is it phase of the wave striking the tube at some point which
causes the reflection?

Thanks again.

 

My standard retort for anyone retreading the old saw about backpressure improving low torque is to give them the dictionary definition of backpressure:

Backpressure is a positive pressure opposing exhaust flow away from the cylinder.

I then ask them to explain how torque can increase when cylinder filling efficiency is decreased?

 

Try this:

http://www.nissanperformancemag.com/november02/nerds/

 

I read that link referring to Headers that Jon posted.

Understanding the concepts, my original questions still stand.

The article explains that a pressure pulse is created as the exhaust
gas leaves the cylinder.

This creates a low pressure area at the exhaust valve in the primary
header runner.

The confusion sets in when the article talks about pressure waves
diffusing in the collector and the prodcution of sonic waves (shock waves?) traveling the length of the remainder
of the exhaust.

I do believe that these sonic pulses are reflected back because they
are bouning off of baffles in the mufflers or the catalyst in the
catalytic converter.

The pipe organ theory is acceptable. The pipe organ has lengths
of tubes which are tuned to harmonic lengths of the sound wave...
however, I don;t see any supporting data that shows there is
a reflected sound wave in a pipe organ tube.

In electronics, an electical signal can travel down a wire and
reflect back out of phase if the load (impedence) does not match
the source.

IE: Speaker loads 8 ohm speaker on a 4 ohm output amplifier causes
reflections out of phase which cancel the orignial signal causing
less than perfect power transfer to the speaker.

My theory is...(based on my College level Physics) an open tube
acts like an antenna and radiates energy. A signal which travels
on the antenna does not reflect back, but rather radiates energy.

The length of the antenna can attenuate or boost the signal depending on where the peak of the wave leaves.

When relating that to an OPEN header...when that pulse reaches
the collector and there's nothing after the header for the pulse
to travel down, I believe it is released as energy and NOT reflected
back toward the exhaust valve...rather creates a higher pressure
at the valve reducing scavenging.

The length of the header primary tunes the efficiency of when
the pulse leaves.

IE: A pressure wave with a length of 2 feet is resonant with a
primary header length of 2 feet.

If that same scenario occured with a shorter primary of 1.8 feet,
the peak of the wave would not exit in phase causing a "block of
flow" due to the peak of pressure pulse residing in the tube waiting
for the next pulse to help push it out of the collector.

Having said that, it's just my thought and I'm not stuck on it...that's
what is coming across to me at this point.

**Note: I realize there are acoustic waves and pressure waves
occuring in the exhaust doing their own task. I may have been
vague in my original post when asking about relfections in an open
tube...when I should have said Relflections in an Open ENDED tube.

I will locate that book at our local Chapters book store to learn
more about this.

Maybe I'm blinded by just a 'piece of the pie" when it comes to my
Physic's education? Maybe I shouldn't relate electical waves to
pressure waves because they behave differently in their respective
environments.

 

What you're saying there flies in the face of better than fifty years of scientific research.

If you want to learn more about pressure waves then I'd suggest a search for governing laws of finite amplitude waves or some search similar to this. The laws governing acoustic waves while similar are still very different. We have thermodynamics, waves in superposition & resulting shockwaves. Things can get complicated in a big hurry. If you're an engineering student then you'll have to do the calcs. I'm looking forward to them about as much as I look forward to seeing the dentist. Otherwise, thank god for computers and software that can do it for you.

Only thing a person needs to grasp are the basics:
There are two types of finite amplitude waves, a positive "compression" wave (pressure ratio greater than 1) and the negative "expansion" wave (pressure ratio less than 1). Compression wave particle movement is normal to flow. Expansion wave particle flow is opposite. As a compression wave reaches an open pipe end it is reflected as an expansion wave and vise versa. The amplitude of this wave is proportianal to the extent of area change. Larger collector, stronger wave, et cetera.

In my opinion, it's not that important knowing why the waves are reflected, just understanding that scientific research has shown that they do. From that you leave with a better understanding of what's going on inside the engine.

Good luck.

 

Lucky for me I'm confused enough to think that way!

As far as being an engineering student - I'm not. It's been a goal
of mine for ages to learn the depths of engine technology and this
forum quite frankly is the closest thing to it.

I don't normally use internet forums as a source of information,
but you boys and girls (if any?) seem to know the ropes real well.
My debate is going well, I just need to learn more about what is
going on before I try to relay facts. That's one thing I'm good about - not speaking before I have some credible info.

The chain of ignorance never ceases with internet forums. Some
of the posts I read are just plain ***-backwards and people reply
with support because the member has been around for a while.
Seem more like a popularity contest than an information session at some web forums.

I would like nothing more than to walk up to Ron Capps, or John
Force with a low 4 second motor one day. That's where I want
to be!

Thanks for the links and suggestions for study. My intention is to
read up and come back here to ask even more questions!

 

the exhaust pulse travels down the primary, when it hits the collector it sends the shockwave back up the other primaries.

it is the collection point of the primaries that causes the pulses to go back up the other primaries.......having an open header doesnt change that, because the gasses havent even reached the end of the header collector yet.


i guess the only way to stop that would be individual runners like a drag car, where none of them goto a collector they just exit directly by themselves.

 

Quote:
"i guess the only way to stop that would be individual runners like a drag car, where none of them goto a collector they just exit directly by themselves."

That's what I've been trying to say all this time...only I didn't realize the shockwave reversed up the other primaries with
a collector present.

So is it safe to say, I was partially correct in thinking a reflection
can not occur with an open ended tube (like a dragster setup
with individual runners)?

 

well i was just taking a guess........i might be wrong.............the gasses leaving the primary into the open atmosphere might still cause a reflection up the tube.......one of the guys here should know the answer to that.

 

I think you are confusing yourself and maybe overthinking it, and I'm not sure you do understand the concepts.

Physics classes should have explained sound with its compressions and rarefactions. It isn't like electricity.

Remember that you are dealing two things: a flow of hot gasses, and the pressure pulses in this flow. It's critical to have pipes of the correct size and shape to handle the flow without physically restricting it with sharp bends, etc. Tuning is important, but not more so than good flow. Resonance tuning, which you are trying to understand is superimposed on the flow.

Reflection occurs whenever a pipe ends like in a collector, or when it joins another pipe, not just when it sees the atmosphere.

If you get into the referenced text you'll find "independence" and "interference" tuning. My advice is to read some before you think more about it. The danger is getting a wrong idea in your head and not being able to accept something which disagrees with it. Been there, done that!

There's are a couple good reason supercharged Top Fuel cars don't use collectors. Downforce is one.

As Melkor said, don't try to reinvent the wheel. Actually exhaust tuning goes back into the 1920s or 1930s. The first edition of Scientific Design... was published about 1962 when I was taking physics.

Wheels are still round, and exhaust tuning hasn't changed a lot. I chuckle when I see the Pro Stock and Nextel Cup exhaust systems using 4 into 2 into 1 (4-2-1) headers. Yep, it's state of the art in 2004 and also in the early 1900's! It still works, and evidently better than some 4 into 1 systems. About the only thing that has changed is the material, either stainless steel or Inconel, and merge collectors everywhere.

Think about why 4-2-1s may be better than 4 >1 headers. If they are, why don't we see lots of them for sale for high output V8 street cars or even amateur drag cars?

My $.02

 

You're right about training your brain to believe something and
then try to twist it into reality!

I've pretty much hit rock bottom on this topic at the moment.
The little bits you post help to steer me in the right direction,
but I'll need to study up and see the whole picture.

As for Physics class, grade 12 touched on basics and College
was strictly Electronics/Electrical.

I'm at fault for trying to equate the two, even though I know deep
down they react differently...obviously there is mass involved and
atmospheric conditions that can't be reproduced in a wire.

Stay tuned when Zero to 69 finishes Chapter 1 of, "Scientific Design of Exhaust & Intake Systems - Third Edition"