belgiumbarry

100 would be 2 stroke. 50 here i suppose ..

GregBBRD

Thanks, corrected.

ptuomov

I like this explanation, where you can just multiply (or divide, depending on the context) the numbers by three:

http://www.epi-eng.com/piston_engine...technology.htm

So the step placed 20cm from the valve reflects back a suction wave that arrives at the exhaust valve 22 about crank degrees after EVO at 6000 rpm and 11 crank degrees after EVO at 3000 rpm. This happens to be about right, as it takes less crank degrees for the cylinder to evacuate to non-critical pressure at 3000 rpm than at 6000 rpm. In fact the time in ms is about the same as long as the torque is about the same, with some provision to the exhaust cam profile.

The explanation/calculation is not perfect, just a back of the envelope calculation. It's imperfect because it doesn't take into account the wave traveling fast when it goes out in the direction of gas flow speed and slow when the wave moves upstream against the valve flow. This is not a huge issue early in the exhaust cycle right after EVO because although the exhaust valve seat sees sonic flow the pipe itself has much larger area and therefore doesn't see anything like that sort of gas flow velocity.

GregBBRD

Tuomo:

Quote:
For purposes of approximation, assume that the mean temperature of the exhaust gas in the primary up near the head is 1500°F (815 °C). The speed-of-sound-in-air equation (close enough for approximations, according to Professor Blair) produces a sonic velocity of 661 m/s (2168 feet per second). At 18,000 RPM, (300 RPS) one crankshaft rotation takes 3.33 milliseconds (ms) or 3333 microseconds (μs). Therefore one degree of crank rotation takes 9.26 μs (3333 ÷ 360). If the first step in the primary is 200 mm from the back of the exhaust valves, then using the calculated speed of sound as an approximation of the propagation speed of the finite pressure wave, the 400 mm round trip from the valve to the step and back takes about 600 microseconds, or 65 degrees of crankshaft travel.Assume that, in an 18,000 RPM engine, the establishment of enough exhaust valve opening to allow meaningful flow would occur in the neighborhood of 100° after TDC. Therefore, it is clear that this first reflection is timed to arrive back at the valves even before the piston reaches BDC. For what purpose? Recalling that during blowdown, there is sufficient pressure ratio in the cylinder to establish choked (sonic) flow through the exhaust valve orifice, then it would certainly be advantageous to maintain that gas velocity for as long as possible.

A noted engineer in the world of Formula-One confirmed that this is exactly the reason for the one-or-more large-magnitude steps in the primary: to place a negative pressure at the back of the exhaust valve timed so as to extend the duration of the critical pressure ratio.
http://www.epi-eng.com/piston_engine...technology.htm

Sorry, but to me, the conclusion, above, is pure gibberish.

Why in the world would anyone want to have the returning pulse (moving back towards the exhaust valve) arrive just as the valve opens? That pulse would "push against" the exhaust trying to leave the cylinder, right? Wrong direction, to create a scavenge effect!
And what, at the change in the primary dimension pipe size (getting bigger) creates a returning pulse? Well, nothing....that's not why it is there!
Truthfully, any reverse pulse which reaches the port/exhaust valve has to be detrimental towards scavenging exhaust...that pulse is in the incorrect direction! (All anti-reversion chambers (popular right now, but not necessary) installed in the primary pipes are to keep that returning pulse from getting near the exhaust port!)

The question I asked: "What does the airflow through the primaries look like, in a cross sectional picture?" is the important part of exhaust design, to me.

Here's my theory:

Air flowing through the primary pipe is not only laminar, but the returning pulse (which originates at the collector) is actually flowing backwards along the walls of the primary pipe. (I do not believe that any "returning wave" is stronger than the air flow through the center of the primary pipe (the out flowing sonic (for awhile) gas will always be flowing towards the collector, through the center of the primary pipe.....nothing is going to be able to travel upstream, in this area of the primary pipe....there's far too many pulses going towards the collector for a returning pulse to "swim" upsteam.

Just intuitively thinking about the outflowing gasses exiting the cylinder head, they are cooling as they travel down the primary pipe (reducing the velocity.) It seems logical (to me) that a smaller "stepped" pipe downstream from the exhaust valve would keep the velocity up, yet this is not what people do...they install a bigger stepped pipe. Completely counter to what one would think on initial examination, right?
Why?
My theory is that as the reverse pulse travels up the primary pipe (along the walls), the change to a smaller pipe (the "step") turns the laminar flow (which is along the walls of the pipe) back into the main flow....reversing its direction. Note that this can also be done, right at the exit from the port, if there is a small "step" in the exhaust port. (Part of the reason that "D" ports work so well is that they "block" the returning pulse from getting to the exhaust valve, on the "short wall" of the port.)

There's one other thing to consider....laminar airflow tumbles....and the smoother the surface, the bigger the "tumbling" cross section is (intake airflow 101....never polish a port, as the flow will almost always drop, significantly.)
There's almost nothing smoother than the inside of a tube (stainless or mild steel) exhaust pipe! You can figure out how to "fix" that, right?

We can talk about collectors, which is where the reverse pulse originates from, later. Design of the collector is very, very important.

ptuomov

a lot to think and discuss here, will try to get back to you after a weekend of dance recitals!

Strosek Ultra

My lack in the English language permits me to get deeply involved in this discussion. However I believe Mr. Brown is wrong. There are two main phenomenon occurring in the exhaust, gas particle flow and pressure wave propagation. The proper handling of the pressure waves in the exhaust can help us create low pressure (vacuum wave) at the exhaust valve during overlap which will help removing as many gas particles as possible during the exhaust stroke. I have never heard of anyone stepping the primary pipe from large (at the exhaust port) to small at the collector, on the contrary.
Åke

This is what Burn´s say about the matter with simple words easy to understand:
Quote: In order to explain the effect of exhaust tuning on performance, let’s take a quick look at the 4-stroke engine cycle. The first step in the 4-stroke process is the intake stroke. With the intake valve open, the piston travels down the cylinder pulling a fresh air and fuel mixture into the cylinder (intake stroke). When the piston nears bottom dead center, the intake valve closes and the cylinder travels up the cylinder compressing the air/fuel charge (compression stroke). With the piston at the top of the stroke, the spark plug fires and ignites the compressed mixture causing essentially a closed explosion. The pressure of the ignited fuel pushes the piston down the cylinder transferring power to the piston, rod and finally the crankshaft (power stroke). After bottom dead center, the exhaust valve opens and the piston is pushed up the cylinder forcing the exhaust gases out the exhaust port and manifold (exhaust stroke).
As the exhaust valve opens, the relatively high cylinder pressure (70 – 90 psi), initiates exhaust blowdown and a large pressure wave travels down the exhaust pipe. As the valve continues to open, the exhaust gases begin flowing through the valve seat. The exhaust gases flow at an average speed of over 350 ft/sec, while the pressure wave travels at the speed of sound of around 1,700 ft/sec.

As one can see, there are two main phenomenon occurring in the exhaust, gas particle flow and pressure wave propagation. The objective of the exhaust is to remove as many gas particles as possible during the exhaust stroke. The proper handling of the pressure waves in the exhaust can help us to this end, and even help us “supercharge” the engine.


As the exhaust pressure wave arrives at the end of the exhaust pipe, part of the wave is reflected back towards the cylinder as a negative pressure (or vacuum) wave. This negative wave, if timed properly to arrive at the cylinder during the overlap period can help scavenge the residual exhaust gases in the cylinder and also can initiate the flow of intake charge into the cylinder. Since the pressure waves travel at near the speed of sound, the timing of the negative wave can be controlled by the primary pipe length for a particular rpm.


The strength of the wave reflection is based on the area change compared to the area of the originating pipe. A large area change such as the end of a pipe will produce a strong reflection, whereas a smaller area change, as occurs in a collector, will produce a less-strong wave. A 2-1 collector will have a smaller area change than a 4-1 collector producing a weaker pressure wave. Also, a merge collector will have a smaller area change than a standard formed collector producing a weaker wave.


So, the trick to proper exhaust tuning is to tune an exhaust system is produce a negative wave of the proper strength timed to occur at cylinder overlap. Various exhaust designs have evolved over the years from theory, but the majority are still being built from ‘cut & try’ experimenting. Only lately have computer programs like the Burns X-design or high end engine simulation programs been able to help in this process. Practical tools like adjustable length primary pipes and our B-TEC and DynoSYS adjustable collectors allow quicker design changes on the dyno or in the car. When considering a header design, the following points need to be considered:

1) Header primary pipe diameter (also whether constant size or stepped pipes).
2) Primary pipe overall length.
3) Collector package including the number of pipes per collector and the outlet sizing.
4) Megaphone/tailpipe packag

SwayBar

Greg, that returning pulse is a low-pressure one which creates almost a vacuum which helps evacuate the cylinder of high-pressure exhaust gases.

When the pulse is timed correctly, it arrives and there is a vacuum behind the exhaust valve right before it opens. When it opens, the low-pressure sucks the exhaust gas out, which amongst other things, reduces pumping losses.

GregBBRD

Well, that's certainly what some theorize, no doubt about it.

I also believe there is a returning "wave". I said that.
I just don't believe it is helpful, in the vast majority of cases (these cases are for engines that run in very narrow rpm ranges...and what people "think" is happening, is not....different discussion, for later.)

There's one obvious problem with the "returning wave".....how does it suddenly become a source of vacuum if it arrives once the valve is opening?
Magic wave reversal?

I believe that the proper header design, for the vast majority of engines, will "block" that "return", not allowing it to reach the port.

SwayBar

The principle also apply to the inlet tract.

These pulses travel at the speed of sound which is pretty much a constant. Since we know how fast sound travels, we can then determine the appropriate lengths of tubing required for both intake/exhaust for a target RPM.

Knowing/understanding these principles would be beneficial for both intake and exhaust design.

For example, an exhaust slug is expelled from a cylinder into its header-pipe, travels down its fixed-length of pipe, and hits the collector. At that moment, a reverse-pulse is sent back up thru the other primaries connected to the collector where it travels back up the length of the pipes connected to the collector, to the exhaust valves. The hope is that a low-pressure wave arrives just when another exhaust valve opens at a target RPM.

This of course is all firing-order and RPM dependant, and taken into consideration during design.

GregBBRD

Yeah....
Step back from the Internet and just think about what you just wrote.

A returning "wave" from the intake system, timed to arrive when the intake valve opens is moving in the correct direction....towards the cylinder. When "inertial supercharging" occurs, this will cram the cylinder with more that 100% of the available volume.
A returning "wave" in the exhaust system, is moving back towards the exhaust valve (the incorrect direction)....and if timed to arrive at the time the exhaust valve opens, pushes exhaust back into the cylinder.

Go back and read what the guy said that Tuomo posted....complete gibberish. There's no possible way to have a returning "wave" magically turn into a vacuum when the exhaust valve is opening!
You have to be careful about trusting what every idiot happens to say and be able to dismiss what is complete bullsh!t.

On the other hand, if the "returning wave" is timed to arrive just before the exhaust valve opens, the wave can bounce off of the closed valve and rebound back down the exhaust system....creating a vacuum. "Inertial vacuum."

There's a few very important things to remember about this:
1. This occurs in a very narrow rpm range.
2. Outside of that very narrow rpm range, the returning wave can arrive at the wrong time. (Like when the exhaust valve is opening/open.)
3. This will only work on a 4 cylinder engine (or a flat plane V8 and a handful of V6 cylinder engines).
4. Any irregular one bank cylinder firing (like on a 928 engine) will have the "wave pulses" arriving at all kinds of crazy times, changing those intervals as the rpms change. (A 928 engine fires #7, skips one firing cylinder, fires #6, #5 fires immediately right after #6, skips one firing cylinder, fires #8, then skips two cylinders firing, and fires #7 again.) Note that the same irregular firing occurs on the other side of the engine. Also note that there is no possible way to "correct" for this with pipe length....it must be blocked, or negated by some other means!

ptuomov

In my opinion, the main objective in header design is to produce a low-pressure condition in the exhaust port and thus the cylinder during the valve overlap IVO-EVC period. The piston is not pulling during the overlap and usually any intake tuning is relatively weak at IVO, so the responsibility falls on the exhaust pipe produce this low pressure wave.

For a single cylinder engine, the way to do this at a specific rpm is to have a straight pipe of the tuned length and correct diameter. In a single cylinder engine, one wants that relatively small diameter straight pipe out of the port for some inches, say 12" from the valve, and then a step up in size to a larger pipe and adjust the pipe length to tune the exhaust. The narrow (port-sized) pipe off the head needs to be as straight as possible. This tunes for a narrow rpm range (and some multiples). Incredibly, in this case I've actually done that experiment with a single cylinder four-stroke engine over 30 years ago! One can widen the tuned rpm range and reduce the rarefaction wave magnitude making some fraction of the rear section of the pipe conical as in expanding the cross-section. Expanding cone or megaphone in the end spreads the suction wave over a longer time and distance.

Then there are multi-cylinder engines that combine exhaust pipes. The worst thing that a multi-cylinder engine header can do is letting the initial high-pressure wave that starts from EVO enter other cylinders' exhaust valves during their overlap. This is that is called "exhaust blowdown interference". A simple solution is just leave the pipes separate like in "zoomies". During the world wars, people developing piston aircraft were experimenting with exhaust routing options and to their surprise they learned that combining the pipes could make 0-5% more peak power at the crankshaft (ignoring exhaust thrust effects) than zoomies and of course even more peak torque. This is due to the suction effect of the collector. The problem that comes with the collector is that unless the primary lengths are right (or, rather, not horribly wrong), we're risking the exhaust blowdown interference. What we need is the pipe lengths to be long enough, such that at low rpms the 180-degree apart firing cylinders have long enough pipe distance from one cylinder to another and that at high rpms the 90-degree apart firing cylinders are separated by enough pipe as well. Because the pulses are irregular in a cross-plane V8, the exhaust primary pipe lengths and those lengths being equal across cylinders aren't that critical provided that that the pipes are long enough to avoid 180-degree and 90-degree pulse interference. The pipe after the collector is useful for tuning below the peak power rpm (but doesn't matter that much above the peak power rpm as the exhaust gas is flowing so fast thru a correctly sized ID, i.e., not too large) collector.

ptuomov

The wave can be either high pressure wave or low pressure (rarefaction) wave. This is independent of the direction that the wave moves. What are you talking about?

When a high pressure wave moves forward and enters a reduced cross-sectional area, the constriction sends back a high pressure wave splitting the original wave’s energy. In the extreme, it could be a closed pipe end and the whole wave would be reflected back. Similarly, a low pressure wave entering a smaller cross section will reflect back a low pressure wave while the original wave continues forward with reduced energy.

Increases in the cross-sectional area send back a wave of opposing sign. A high pressure wave entering an increase in cross section sends back a low pressure wave. The most extreme case of this is an open pipe end. When a high pressure wave exits the pipe, it sends back a strong low pressure wave.

I don't think the physics works the way you describe below.

FredR

As to what is going on inside the exhaust system might best be described as being akin to sailing the likes of a Hobie 16- hit it right on the crest of a wave whilst running [wind behind that is] and off you go, hit it wrong and you slow right up- in racing those things the difference between winning and losing was invariably to do with catching the surf just right and that in effect is what the exhaust system needs to emulate. Rightly or wrongly I have been under the impression that the cross plane crank and its uneven firing order make optimising the headers difficult to say the least and that most of the potential benefit is obtained by fitting the X pipe configuration which surely works and even they can be optimised if what I have read is correct..

For years we saw the MSDS headers advertised only problem being that I never personally saw anyone demonstrate on a dyno that they actually made any additional power of any note. As I am aware the "Devek headers" were specifically intended for stroker motors albeit they seemed to do something positive on a stock 5 litre motor like the GT that Jim C purchased from Louie [now GB modified]. I have no idea what GB's header do for a stock motor or whether he offers soemthing for stock 5 litre motors specifically.

I used to write to Tom when he was developing his Medusa system - incredible piece of plumbing but never saw any specific dyno verified benefit posted about them before he sadly departed this world..That GB is still "developing" his variant also tells its own tale but he is doing what I alluded to earlier in that he is using his "feel" to fine tune the system as a $100 software programme is not going to be able to help him in any significant way. Formula 1 spend a fortune fine tuning these things and try to guard their designs robustly. Rightly or wrongly I get the impression that given the constraints they were working to the Porsche engineers did not do too badly with their manifold and exhaust system design in general. Mine is modified as per a Louie formulation but it would not meet environmental requirements of that I am sure- I would be probably be arrested in Germany and thrown in jail in Switzerland. Even the local police challenge my exhaust at annual inspection but gave me some lattitude that US Vee 8 owners do not get due to the "Prestige factor" - that and they know US cars still have stock silencers available!

ptuomov

Some comments: during the turbo exhaust manifold design and the turboback exhaust design, we used two pieces of software (in addition to the CAD software) that were very helpful. About $100 for PipeMax to get the pipes ballpark right and About $400 for EngMod4T (https://vannik.co.za/EngMod4T.htm) to see how the waves move inside each simulated exhaust configuration. Both were predictive and useful.

If things upstream of the crossover are sized right, the crossover doesn’t do much to peak power or power above the peak power rpm. It can help with lower rpm torque and it also takes down the noise level by a decibel or two.

Whet are those Ott prescribed exhaust modifications?

FredR

Proves my point about the limitations and software cost and whereas doubtless a useful tool, Pipemax is very limited as to what can be achieved with it.

Louie recommended the Devek headers that I do not have- just the stock headers- the front box [a sort of H pipe?] is removed and replaced with his X pipe. The central resonator is removed and fitted with a Bullet muffler and the rear box is removed with the RMB fitted. The bullett muffler is quite a decent size- a fair bit bigger than the GTS resonator boxes. Louie ran this system on a number of cars he worked on or so I understand. One of these days I might drop the kit behind the X pipe and try the GTS resonator and rear muffler to see if it changes the "feel" at all.