I wrote a very extensive post on this a long time ago based on David Vizards findings along with a whole slew of testing. I just can't type that much anymore, hopefully we can find it...lol

-Shannon

 

Zero_to_69, if you take Basic Fluid(air is a fluid ) Mechanics and then Compressible Flow/Intermediate fluid mechanics you'll understand about 80 percent of the article and probably the theory.

I honestly would not understand a thing these guys are talking about if I hadn't take the above two courses. Gotta start from conservation of Mass, Momentum and Energy

If you're into learning how to super-tune an engine I'd suggest taking combustion chemistry classes, materials/mechanical engineering dealing specifically with failure, but most important, fluid mechanics type courses.


Fluid mechanics is still not very well understood. I.E. OldSStroker's example of how header design is very unchanged over 100 years. Naviar-Stokes equation (the basis for cutting edge CFD code) is 100 years old as well (I think).

Sorry for the long post folks

 

English is not my first language so I apologize up front for any difficulties you might have reading me.

I agree with Oldstroker in that it is always harder unlearning something than it is learning it in the first place. Without a strong foundation we should refrain from postulating too much, otherwise our head might explode.

Many of the pre WWII papers on this subject were founded on the gas-slug "Kadency" theories. These were based on sound wave models and were later found to be inaccurate as finite amplitude waves have their own governing laws. Much of the good stuff came during WWII and after.

On sound waves vs finite amplitude waves:
If I were to say, "Good Morning" to you in a room with twenty other people conversing. you would understand me from five meters or twenty (if I spoke loudly enough). The sound waves travel without interaction or distortion (with other sound waves). Yet, if it were physically possible to speak in finite amplitude waves. You would not understand a word I say (you would lose an ear drum too). The waves would interact and mutate as they traveled. Just an example of the complexity involved in predicting this type of wave motion. Again, we can be thankful of software that employs these mathematical models for us.

One thing I want to say in regards to pipe ends, transitions and so on was already stated but maybe missed:
Any change of flow area along the path will result in opposite sign wave "reflection". The extent of area change is directly proportional to that wave's amplitude. It doesn't matter if the change is as mild as a diameter change (like a step tube header) or an open pipe that discharges to the atmosphere (your example of a header without a collector).

Oldstroker goes on to mention the importance of the pipe shape, path, length and size. I would suspect this is because he knows that these things are much more important than an in depth understanding of wave interaction. The goal being to tune a pulse (of given order) so as to arrive during the cam overlap period throughout a predetermined rpm range. That's where all this study is headed in the first place, at least as exhaust designers see it. The timing of these pulses is dependent on the pipe length with the amplitude and duration dependent on the pipe size and collector design. The pipe size is a counterbalance of pumping work versus mean gas speed. There's an optimal design per given application in there somewhere. Perhaps one with adjustable pipe lengths/areas.

Best of luck.

 

You said it eloquently!

Thanks for the information.

 

I have to agree with OldSStroker. That book has got to be oneof the best books out there. I up and bought it and now understand a lot more that I thought I had a graps on.

Just like the intake, the exhaust is tuned for a certain rpm. You want the exhaust pulse to come into the cylinder and leave right as overlap happens to increase scavaging. Also another notable thing that they mention is that if you increase the size of the exhaust primary by 50%, it will slow that exhaust pulse to the collector so it will refect to the cylinder at a later time. That way you do not have the pulse come into the cylinder and fight the intake pulse coming into the cylinder.

 

Melkor, if English isn't your first language I'll be a monkey's uncle...you write better than I do!

I wont even begin to understand the Finite Amplitude waves vs. sound waves until I grab the book.

With repsect to low pressure in the exhaust and pipe diameter;
can I conclude that diameter affects velocity of the exhaust gas,
more than length of the pipe?

For example, the volume of gas that it takes to "fill" a 1 5/8 inch
primary header tube at 4500 RPM for a 350 CID motor creates
enough velocity to increase Volumetric Efficiency though scavenging.

If a 2 inch primary header was installed on the same motor yielding
the exact volume of gas at 4500 RPM , the velocity in the header
runner would decrease and cause higher pressure at the exhaust
valve.

Even though the pipe is larger (which some may think will reduce back pressure)...
going too big will result in poorer performance.

Correct?

I can also begin to comprehend the timing of the pulse back to
the cylinder during overlap. When both valves are open, the
intake runner and exhaust runer become one long path.

If the pressure in the exhaust runner is lower than the pressure
in the intake runner, more air/fuel will be moved into the chamber.

The reflected pulse (if in proper phase) can create a 'standing wave';
or a wave of significant amplitude to further induce the air.

The length of the pipe has more to do with the timing of the
reflection and the resulting phase.

Correct?

Unfortunately, I'm not able to take courses at the moment. As much as I'd like to, time and $$$ don't permit.

I promise to stop asking any more questions until I buy the book. (hopefully tomorrow)

 

Just for the record, my first language is Japanese. I moved to California in 1995 but starting learning english a year before. I don't think Americans realize how difficult a language english is to master but I take much pride in knowing it well enough that people can't tell. Living in a country as great as this one I can say that it has been a labor of love. So thank you for the compliments.

And don't worry about your comprehension of wave dynamics. Unless you like to do hyperbolic partial differential equations for fun, a basic understanding of the concepts is all you need. After the basics, it just boils down to the math and I don't see where the math will help you.

I encourage you to read up on Bernoulli. You will find many of your answers in the basics of Bernoulli's work. Flow through a pipe in its most basic form is about differences in pressure. You have a pressure at one end (p1) and a pressure at the other (p2). There is most likely a difference between p1 and p2 and in some situations like choked flow, there may not. The area of the pipe is a variable in calculating velocity. The length of the pipe may play a significant role in the head loss but it depends on the type of flow (laminar or turbulent), the viscosity of the fluid and a host of other variables. The number of ingredients you add to the equation depends on the accuracy of answer you wish to achieve. This can be a daunting task when thermodynamics and other factors are thrown in the mix.

Short answer is yes. Velocity is more dependent on flow area than the pipe length in the flow of exhaust gasses. But now we have a generalized statement that may offend some academics and that is why I do not like to generalize too much.

The pipe should be sized in a way that achieves a high enough mean gas velocity so as to provide a balance of scavenging efficiency vs pumping losses. If the pipe is too large and the mean velocity too low, the pipe will tend to revert exhaust gasses and spoil the intake charge. An engine like this would be very inefficient at anything but the highest rpm and then performance may still suffer at the lower rpm threshhold the engine must operate in. In other words, this would be of more detriment to some types of engines than others.

Your on the right track with the pipe length and reflected pulse. In there is an optimal intake valve closure point. You need software and enough knowledge of the variables at work to determine this though. The more you understand, the better off you are. Not only that but you begin to realize just how much the integration of technology is going to do for engine performance. Variable valve timing, displacement on demand and the like.

Not very detailed in my explanation but there is not enough space for that kind of explanation. Besides that, your book will be of much greater help than I can.

Been much fun,
Yasutsuna Mitsuyo

 

"He who shows his power" is a good forum name, Yasutsuna. I am very impressed with your contributions.
Stick around here.

Reversion: if the exhaust pipe is larger than the exhaust port opening in the head you can often get some anti-reversion effects without harming the outward flow. On a SBC, putting that mismatch on the lower or "short side" can be beneficial.

There is some empirical evidence that primary pipe diameter relates to rpm where torque may peak regardless of flow. For example: Formula 1 engines with about 90 hp per cylinder have primary pipes which appear to be in the 60 mm range, while 100+ per cylinder engines with half the rpm have about 55 mm primaries. Both of these engines operate only in the upper 1/3 of their rev range.

Exhaust gasses are always turbulent flow, I believe.

There is some software being used for exhaust system design, I believe. Sometimes it predicts well, sometimes not. I'm not familiar with it, but I'm fairly sure it is being used.

Packaging of the best theoretically or experimentally derived header in the racecar is a daunting challenge. There is some software that does this very well. My guess is that some of the folks who use it don't advertise that they do because they don't want the competition to know. Formula cars are probably much more difficult to package than Nextel Cup cars.

 

OMG, you have the best quote out of all of these. I’ve seen some things, and said some things, probably have said one or two myself (and hopefully corrected them) and have seen ones so ridiculous that I never said a thing. Most times what I see has to do with camshafts and the "follow the heard" practice seems tied into the more complex matters. I.E. camshafts, backpressure and pulse tuning.


One of the reasons I get so much pain watching first time racers either autocross or “race” around a road course is because they are going out there without proper schooling under them. By proper I mean a top name race school training not some guy yelling at you as you try to make it around the track. I never raced a day in my life before I headed off to school and the things I had to be broken of never ceased to amaze me, now compound that with years of bad habits and I would be farther behind. Usually the guys who do this are the ones who always end up doing it the school of hard knocks way and are spiteful of those who did it the other way. (The Seven Dealy sins are just that) It’s not just cars it applies too, any skill is usually advanced much faster along if it is educated in the proper environment to start. Just what I have seen in my short time on the planet.

Yep that’s the big one right there! That’s why a stepped header or a tri-Y or even a open exhaust collector all effect power. The scary thing is to look at a F-Body Y-Pipe, it doesn’t just end at the header collector folks it goes the whole way back until the tuning effects are so small they are not seen.


I got to say that is a great way to look at it. I have to admit that my understanding of engines is not always based on understanding the physics of it, but understanding that when you change a parameter you will get X result if you go one way and Y result if you go another. Now it is important to understand a good amount of aspects of every component of an engine system. (Notice not just an engine, the whole system from air cleaner to exhaust pipe) Material differences, component specs, production processes and the design of individual parts are all good areas to know and also good areas to use to your advantage. The big thing is that you have to know enough to talk to the people who specialize in different parts such as camshaft companies or connecting rod companies but not always know more than they do. I know there are certain things that I look for in different components and loaded questions that I will ask to someone about their products. The good thing is when you get the right answers to your questions you know you are talking to the right people. One reason why I recommend and use Oliver and Comp Cams among others. Understanding the physics of engines is important, I think understanding the theories and visualizing the concepts is more important than doing the math to prove that it works. (Hey I know some guys need everything proved to them) I’ve seen people who can do the math, but don’t get the theory or concept. That scares me personally. Once you get the concept then you can take the theory and use it to explain to yourself or others why such a reaction comes from a certain change in an engines power output. That in the end is what we are ultimately after.


Yep and it goes back to the whole premise that a motor is a system. A big diameter pipe is going to need less camshaft or it’s going to cause more bad problems down low, but still give you tuning help up high. Then again the volume of waste going thru the pipe has a lot to do with the diameter needed. Same principal applies to cylinder heads, two pipes/ports the one flowing less volume (CFM) will have less velocity even if they are the same size. Usually on the exhaust side it means you are making more HP so you need more diameter for the same job.

Reversion, that is one of my key words I love to hear! Most of it’s benefits come from other areas of the motor but it’s a great way to improve power without any ill side effects.

Bret

 

Bret and Jon, as far as im concerned, after your posts i now believe you two make up the entity known as god.
Mr. Mitsuyo - i had never figured out how bernoullis law quite figured into differential equations...but now i do. and for me that is a good thing (im one of those weird people that understand the math before the concept...the math leads me to the concept)
but i have a stupid question that is slightly related to this topic: why is it that when i disconnected my exhaust right after the cat when i had a stock exhaust (manifolds + stock 95 y-pipe + cat) it seemed to slow the car down. was it just the lower end and therefore the seat of the pants feeling missing? or is it because of wave tuning (wouldnt think so because i think the cat would destroy all that) or maybe something more sinister.
rich

 

IMHO(and someone else who came up with this origninal idea but I can't remember whom) that the car seeming to slow down is all an illusion caused by the change in volume(dB) of the exhaust. You associate the louder exhaust with higher performance and higher rpms. If you watch your tack carefully you'll notice that your really only at 2300 rpms when you punch it and your car sounds like NASCAR versus your old exhaust which only began to scream at 4500.

Of course with the stock LT1's incredibly flat torque curve you should feel the same surge over a wide range of RPMs as long as your comparing within the same gear.

The loudness factor is how I rationalize the phenomenon of people believing they lost low end. Another possiblity is that the increased airflow is just enough to throw the computer out of its optimum range until it 'learns' by logging some more data. Usually the dyno doesn't agree with the SOTP meter on this subject and therefore i don't think that it affects the computer that drastically.

-brent

 

That all sounds (pun intended) about right. Wouldn't a chassis dyno test of that be fun?

It kinda goes along with my theory that I can drive my car harder without pi$$ing the neighbors and cops off if it isn't too loud. They think it's going slower than it really is. That's one of the reasons I have Corsa "touring" rather than "Indy" exhaust on my C5.

 

Hey guys,

I've been trying to source the book, "Scientific Design of Intake and Exhaust Systems".

The search comes up with this link:
http://www.chapters.indigo.ca/item.a...D3F03204640630

It does not specify which edition. The lady at the store cannot
find any specifics for 3rd Edition either. Is it safe to say the above
link points to the latest and greatest version?

Do I have your blessing to place the order?

 

Here's a link to Barnes & Noble with a picture of the cover.

http://search.barnesandnoble.com/boo...7X&cds2Pid=946

That's third edition, and maybe cheaper, too, but there can't be old editions still in print. Besides the major new stuff is emissions related, I believe.

Pax tecum!

 

Thanks for the photo link Bret / Jon.

I ordered the book yesterday from that site and should be in my
hands by next weekend.

For those trying to source the same book from other sites, it's
temporarily not available.