Engine and Transmission Tech

It's getting deep in here. :)

Jim B. on the subject.

Quote:

The shorty headers we run do not scavenge like a long tube header. There simply isn't enough primary length for velocity*mass tuning. The length of the exhaust pulse and the gap between pulses are such that the pulse completely exits the header well before the next valve opening, causing turbulence in the collector and reversed flow back up the tube. At the same time, when the OEMs can get 650 hp out of that LT4 package obviously they know something about tuning an exhaust with a log type manifold (or shorty header which is very close to the same thing). I believe this is because they have decades of experience with reversion tuning, which can improve extraction with a log manifold.
Until very recently there was no way this type of exhaust system was ever going to approach the results of a long tube header system. That seems to be changing and I credit computer modeling with the improvement. As it now stands, the OEMs are eking every little advantage they can get out of it which is why you see the shorty tri-Y system on that engine. Note the outlet size, it looks small for 650 hp doesn't it? But I don't think for a second you could bolt on just any exhaust system and get that level of output. I'm very certain in fact that the requirements are quite specific. They probably demand a specific header pipe and a specific type of muffler at the very least, and if you look closely with a mind to reversion tuning I think you will find a specific volume between the muffler and the ports. For this type system I'm pretty well convinced that reversion tuning is the key. I do not know yet what this critical volume is, but eventually we will find out.

On our cars though we almost certainly destroy any chance of reversion tuning with inadequate volume in the header tube, and then to make it worse, in many cases we use a glass pack muffler, which works by absorbtion rather than reflection and therefore produces no reversion waves that could be used for tuning in the first place. Now a glass pack is great with long tube headers, where reversion is a very minor part of exhaust tuning, being vastly overpowered by the velocity*mass aspect. Further, any reversion pulse is well divided between the 4 ports by up to 6 ft of primary pipe. But for reversion tuning you need a hard reflection of the exhaust pulse energy, just the same as in a 2-stroke scavenger pipe. Canister type mufflers naturally provide this reflection, but glass packs do not. However there is a fine balance between the power of the reflected pulse and muffler restriction, and this is an area where we could probably learn much from expansion chamber-stinger design.

But the bottom line is that we put a shorty header on the engine, eliminating the potential benefits of mass*velocity tuning, then a short header tube eliminating also the potential benefits of reversion tuning, and we are left wondering why our performance on the dyno is dismal.

I believe there is a way out, but it requires us to think a little bit. Reversion tuning relies on volume. You are taking an exhaust pulse, sending it down a tube, reflecting it off the end back up the tube (yes this can happen while other pulses are coming down the opposite direction) so that it hits the valve seat as a low pressure pulse just as the valve opens. For a V8 there are two of these pulses per engine rotation per bank, or twice the number as in a 2-stroke engine with a scavenger pipe, and they impinge on all 4 valves simultaneously, which is why a log manifold or shorty header works in this mode. The suction can be transferred to the opening valve. Timing is not as critical as you might think as the pulse can cover about a 3000 rpm band, but the low pressure pulse does have to get to the valve when it is open, not closed. That is where the volume comes in.

We could experiment with this. I think that a properly designed chamber substituted for the muffler in the typical location could be constructed with a suitable volume and exit configuration to generate this reversion pulse in phase with the valves at midrange engine speeds. We would just need to know the right volume first, which we should be able to approximate by inspection of modern vehicles. Further complicating the matter is that pulses on one bank are not equally spaced, but we should take our major cues from the work of the OEMs who currently are obvious masters in this department.

Anyway, my suspicion is that with our short pipe between the shorty headers and the muffler we have inadvertently timed the pulses to give our engines a pressure wave when the valves open rather than a low pressure and this accounts for the less than stellar dyno results these configurations tend towards. Thus far our remedy has been brute force and more cubic inches but I think that has to evolve.

[www.mgexp.com]

More.

Quote:

Reason I asked is that I'm gathering data related to the use of shorty headers vs long tube headers and my theory is that with shorty headers and log manifolds, reversion tuning theory analysis will promote greater efficiency which would benefit most of our cars. I was curious to see if your build supports that theory or opposes it. Interestingly enough it supports it. I note that the canister type mufflers (the only component in your system which could generate a reversion pulse) is located as far from the exhaust ports as is possible, making the volume of the reversion chamber as large as is practical for this size car. That combined with the tube size gets it into the range I think is needed to put the reversion pulse in phase with the exhaust valves. Note that the glass packs do not create reversion pulses but act by attenuation. Do your canister type mufflers have an internal baffle, or are they straight-through?

Exhaust Scavenging and Energy Waves

Inertial scavenging and wave scavenging are different phenomena but both impact exhaust system efficiency and affect one another. Scavenging is simply gas extraction. These two scavenging effects are directly influenced by pipe diameter, length, shape and the thermal properties of the pipe material (stainless, mild steel, thermal coatings, etc.). When the exhaust valve opens, two things immediately happen. An energy wave, or pulse, is created from the rapidly expanding combustion gases. The wave enters the exhaust pipe traveling outward at a nominal speed of 1,300 - 1,700 feet per second (this speed varies depending on engine design, modifications, etc., and is therefore stated as a "nominal" velocity). This wave is pure energy, similar to a shock wave from an explosion. Simultaneous with the energy wave, the spent combustion gases also enter the exhaust pipe and travel outward more slowly at 150 - 300 feet per second nominal (maximum power is usually made with gas velocities between 240 and 300 feet per second). Since the energy wave is moving about 5 times faster than the exhaust gases, it will get where it is going faster than the gases. When the outbound energy wave encounters a lower pressure area such as a second or larger diameter section of pipe, the muffler or the ambient atmosphere, a reversion wave (a reversed or mirrored wave) is reflected back toward the exhaust valve without significant loss of velocity.

The reversion wave moves back toward the exhaust valve on a collision course with the exiting gases whereupon they pass through one another, with some energy loss and turbulence, and continue in their respective directions. What happens when that reversion wave arrives at the exhaust valve depends on whether the valve is still open or closed. This is a critical moment in the exhaust cycle because the reversion wave can be beneficial or detrimental to exhaust flow, depending upon its arrival time at the exhaust valve. If the exhaust valve is closed when the reversion wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates its energy in this back and forth motion. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve.

A wave is comprised of two alternating and opposing pressures. In one part of the wave cycle, the gas molecules are compressed. In the other part of the wave, the gas molecules are rarefied. Therefore, each wave contains a compression area (node) of higher pressure and a rarefaction area (anti-node) of lower pressure. An exhaust pipe of the proper length (for a specific RPM range) will place the wave’s anti-node at the exhaust valve at the proper time for it’s lower pressure to help fill the combustion chamber with fresh incoming charge and to extract spent gases from the chamber. This is wave scavenging or "wave tuning".

From these cyclical engine events, one can deduce that the beneficial part of a rapidly traveling reversion wave can only be present at an exhaust port during portions of the powerband since it's relative arrival time changes with RPM. This makes it difficult to tune an exhaust system to take advantage of reversion waves which is why there are various anti-reversion devices designed to improve performance. These anti-reversion devices are designed to weaken and disrupt the detrimental reversion waves (when the wave's higher-pressure node impedes scavenging and intake draw-through). Specifically designed performance baffles can be extremely effective, as well as heads with D shaped ports. Unlike reversion waves that have no mass, exhaust gases do have mass. Since they are in motion, they also have inertia (or "momentum") as they travel outward at their comparatively slow velocity of 150 - 300 feet per second. When the gases move outward as a gas column through the exhaust pipe, a decreasing pressure area is created in the pipe behind them. It may help to think of this lower pressure area as a partial vacuum and one can visualize the vacuous lower pressure "pulling" residual exhaust gases from the combustion chamber and exhaust port. It can also help pull fresh air/fuel charge into the combustion chamber. This is inertial scavenging and it has a major effect upon engine power at low-to-mid range RPM.

There are other factors that further complicate the behavior of exhaust gases. Wave harmonics, wave amplification and wave cancellation effects also play into the scheme of exhaust events. The interaction of all these variables is so abstractly complex that it is difficult to fully grasp. There does not appear to be any absolute formula that will produce the perfect exhaust design. Even super-computer designed exhaust systems must undergo dyno, track, and street testing to determine the necessary configuration for the desired results. Last but not least, the correct choices and combinations of carburetor, air cleaner, cam shaft, ignition, and exhaust used in the proper relationship to each other for the intended riding application will always produce the finest quality results. Most important of all, is to do your research prior to purchasing the combination of products and equipment best suited to your individual style of riding.

Original reference lost.

[www.zggtr.org]



Edited 1 time(s). Last edit at 01/15/2017 11:12AM by MGBV8.

All that is very interesting, but waay to complicated for my li'l BV8, so I think I'll just study this link a bit more.

[www.chevydiy.com]

Good references Carl. I was missing the point about the speed of the pressure wave. What I didn't see though was what happens to that wave when you do one of two things: either 1) divide it between 4 ports using a long tube header or 2) run it into a log manifold or shorty header.

This is in a nutshell the premise of the entire discussion. That being that reversion tuning holds only small potential for a long tube header system, yet has 4 times the potential benefit for a log or shortie system like most of us run.

Also I had a reference from a guy who was actively involved in developing 2-stroke scavenger exhausts that they were effective over about a 3000 rpm range. That is pretty significant.

Jim

In the link to the Chevy website, that article talked about porting exhaust manifolds. I like that idea. 10-15 ft/lbs of added torque.

That article was written by David Vizard and he seems to be right most of the time. But I don't always agree with him, he's just as human as the rest of us and has the same tendencies to overlook things. For instance, in his description of his reversion damping chamber (forgot what he called it) he is writing about it giving a small increase in power when used with a long tube header. Well, if it works that way then he is using it to eliminate an out of phase reversion pulse, as opposed to making use of an in-phase pulse. Another small detail, if the reversion pulse is actually created by the opening of the tube into a much larger chamber as he describes, then wouldn't that reflection be immune from cancellation effects created from the front wall of that chamber? It seems much more likely to me that in this case the stronger reversion pulse is generated at the rear of the chamber before being reflected back by the front wall, creating cancellation within the chamber itself. At any rate, a pulse reflected by the open air end of a tube is going to be much weaker than one generated off a reflecting surface.

David, I think is approaching reversion from the perspective of a standard performance configuration, which is to say, long tube headers, and despite his inclusion of information about iron manifold porting, does not go on to consider the effects of reversion tuning of the entire exhaust system on a manifold or shortie header system. As such his advice is of only limited benefit within our group.

Now if someone wanted to run the numbers, ( I may or may not get around to it) I would imagine you would find that the more rapid travel of a pressure wave front means that for a reversion wave to arrive in phase at the valves it has to travel several feet down the pipe before being reflected back, and the length of that pipe will determine at what RPM the synchronization is optimal. Knowing that you should be able to tune your exhaust for your desired peak and band. Of course, the nature and firing order of a V8 engine will broaden that band and attenuate that peak somewhat, because the cylinders of one bank are not evenly spaced in time.

Also, you could choose a muffler determined by the amount of reflected energy desired. I agree with David that the flow characteristics and the pressure wave characteristics must be considered separately, but I'm not convinced he has done that in his muffler analysis. Let's go back to what is undeniably the state of the art in pressure wave design practice, the 2-cycle scavenger pipe. What you have here is a megaphone into a large chamber, acting as a transition with no reflections. That spreads and dissipated the pressure wave. Then it is followed by some form of reflecting back wall. Here there has been a great deal of variation but the more recent trend has been towards a smaller flat wall rather than the earlier "funnel" configuration, This makes perfect sense as a flat wall perpendicular to the flow is a more effective reflector. Then the wave, traveling back up the megaphone is compressed and condensed into a reversion pulse approaching the strength of the initial pressure pulse. Finally the stinger is fitted to carry off the gas flow, usually with a significantly smaller diameter than the inlet, and perhaps opening into a part of the chamber less affected by the exhaust pulses. Even a weed eater exhaust is made this way, despite it's boxy appearance. (largely minus the long megaphone section)

Now we can understand the zero reflection of a megaphone easily enough. Reflection off a flat surface also makes sense. So how do you get reversion off a tube terminating in open air? Seems a reasonable question doesn't it? Does it bounce off the entire worldwide reservoir of air at atmospheric pressure? Does it somehow bounce off the square cut end of the tube? However it does it (and we are assured by Science that it does) there is no way that this reversion pulse is ever going to be anywhere near as energetic as one reflecting off a solid obstruction and contained by a tube or chamber. So my point is that it is very easy to misunderstand and misinterpret what is going on in a muffler. Or a a catalytic converter, although by design we can see that they seem to be intended to be largely invisible to the exhaust stream, flow and pressure pulses alike. Still, if we do no more than theorize that a flat wall make a good reflector it is usually fairly easy to see where the reversion pulse will come from. Which can lead to a reasonably accurate measurement of the length of the reversion tube or chamber, which can then be used to get in the ballpark of what is needed to be in phase at the desired target engine speed. Which should then prove quite useful in a shortie header equipped car. We are beginning to see some evidence that a location close to the rear bumper could be most favorable, and a shorter system would favor higher rpm use. A hybrid muffler could be even more effective if it were constructed with a cone at the entry and a flat wall at the exit.

Jim



Edited 1 time(s). Last edit at 01/16/2017 06:05AM by BlownMGB-V8.

Equal-length headers, whether long-tube or short-tube would cause the resonant frequencies to be the same for all cylinders. That would optimize at a particular RPM range but that might produce a spike in power in a more narrow range. I had noticed that the intake runners on a Ford 5.0 were not equal length and the header tubes were likewise not equal length, and that the front cylinders 1, 4 and 2, 5 were similarly longer for both the intake runners and the exhaust header tubes. I had wondered if that was deliberate, an attempt to tune resonance for different cylinders at slightly different RPM ranges to flatten and spread the peak torque.

This spreading of resonant frequencies might also require a modification of the design in the exhaust system. I'll think about the firing order of the engine to see where the different cylinders with their different resonances occur and see if there are any other related factors, such as the front cylinders, being at the front end of the crankshaft, might benefit from being tuned at a lower or higher RPM, and the wind-up of the crankshaft during power strokes and similar.

That was a long sentence.

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Well, this danged html coding takes out the spaces but if you write the column as two columns, one even numbers and one odd, and retain the 1 line per number you can see the pattern.

Grouping the cylinders by banks you see that the interval between cylinders per bank varies, complicating the issue somewhat. Yet considering that the operating principal of long tube headers is that a second pulse from a single valve enters the tube before the first has left it, and that these primaries are almost never longer than 40 inches, it seems apparent that the reversion pulse should be timed so that it's anti-node should impinge the originating valve on the next engine cycle. The issue is that the node of an adjacent cylinder in the firing order (such as 4 or 7) could interfere, and indeed can be expected to at certain engine speeds.

Now considering that the the rate of travel of the energy pulse is some 5 times the speed of the slug of exhaust gases moving down the tube, a simple calculation based on known dimensions for header primaries would put the distance between valve and reversion wall or reflector at something in the range of about 10 and 16 ft. Which puts the rear wall of a canister muffler back somewhere near or beyond the rear bumper of an MGB. Yet this simple calculation differs from that of the exhaust gasses because unlike the mass*acceleration theory where we draw a vacuum behind the slug until the next one exits, here we want to open the valve during the anti-node, not at peak pulse pressure, and the anti-node is somewhere around the middle of that 10-16 ft length. Yet again, for maximum effect we want it to supplement the next returning reflection so that the scavenging effect continues to build. So, at the target speed we want the exhaust opening just before the next node or high pressure area of the pulse. This would be as you approach redline. (Ever wonder why that old 2-stroke never needed a rev limiter? This could have something to do with it, perhaps the anti-node had not reached enough intensity to scavenge yet.)

Next, the rpm limitations tell us that a shorter distance favors a higher target speed or redline because the valve is going to open sooner. So what do we have at this point? Well, a header pipe length of not much less than 5 ft under any conditions including race engines, of not more than say 12 ft to stay in the anti-node even with a low redline, and a reflection free tube all the way back to the reflector, after which the remainder of the system is of lesser concern and can be smaller diameter tube. Interesting how similar that is to old school muscle car exhausts isn't it? Note that the engine speed does not affect the distance calculations as the wave propagation speed is not rpm dependent. It does however affect the distance between nodes, and therefore both the length of the anti-node and the interval until the next node. This explains the need to tune the pipe to the required response band. By plotting time vs distance you can get a clearer picture of what would work better for a given engine speed.

Jim