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Moonduck · 12-07-2003, 12:11 PM

Is it loss of backpressure that causes the performance loss or loss of scavenging? If backpressure were beneficial, top-fuel dragsters would run mufflers and reducers. As it is, they run open, tuned pipes because less backpressure means better flow-through in the overall engine.

Scavenging is what occurs when an exhaust pulse travels down the exhaust pipes. The pulse is shot out of the cylinder during the exhaust cycle at a fairly high velocity. As it exits, it creates a low pressure area behind it, a vacuum if you will. In a tuned system designed for optimal scavenging, the pulses will be timed and pipes constructed so that a given pulse will pull, or "scavenge" the pulse behind it.

Using the example of motorcycle engines and their exhausts, take a look at 4 cylinder inlines and the various forms of performance (and street) exhaust patterns that you see (I'll be referring specifically to Honda engines such as the one in my CB750). You tend to have 4-into-1, 4-into-2, and 4-into-4.

4-into-4, frequently called drag pipes, have no scavenging built into them whatsoever. They are usually open, and short to enhance low-range rpm (pipe length usually being what is modified in the process by which pipes are "tuned"). These pipes are designed for optimal flow as the intake side of the engine is also tuned for optimal flow in these sorts of applications. Drag pipes can be streetable, but are usually loud and not the most efficient application over the broad-range of rpm needed on the street. Reducers are sometimes put into these pipes, but usually to reduce certain undesirable performance traits (such as pipes not running in the proper rpm, but looking great at the length they're at), and to reduce the serious noise produced by short, open pipes.

4-into-2 are commonly seen on street 4's. Collected exhausts such as this are usually designed for scavenging or for economy of manufacture. The idea is that these pairs fire at precisely opposite points in the engine cycles, 180 degrees from each other, and the pulses travel in such a timed fashion as to help pull the next pulse from the cylinder more quickly. The low-pressure area behind the preceding pulse is what allows this. The issue with 4-into-2 (and 4-into-1 as well) is runner length. For optimal performance you want your various exhausts pipes to be equal in length, this means that each cylinder will perform similarly at any given RPM range. If your collected system has 4 different run lengths, the you will have to tune your timing and intake side differently on each cylinder to compensate for the differences in performance at a given RPM. The desire for equal runner length is what inspires some of the spaghetti designs that you see in exhausts on high-end engines.

4-into-1, when properly designed, is the best for scavenging as you have each pulse being pulled out by the pulse before it. On the truly well-done designs, the pipes collect in a squarish output with the pipes stacked in pairs. The firing order will rotates along this block in one direction causing each pulse to tpull the pulse following it around the pipe stack. (Not really able to describe this well, sorry) There is some serious flow work needed at the collector to keep unwanted turbulence down. Truly hot exhaust makers put a cone with four concave runnels corresponding with each pipe, less solid makers simply cut a line in each pipe and flatten them against each other in a plus-sign look. The cone style produces better flow, less turbulence, and enhances scavenging by removing unneeded resistance to flow. The downside of a 4-into-1 is need for better mufflers. In putting all of your flow through one muffler, you need a big can, making the muffler more expensive to build. There are also, insofar as some apllications are concerned, aesthetic issues with 4-into-1

(I used motorcycles as an example as it is usually easier to visualize the type of designs common to bike exhausts, without getting into scary automotive designs that have crossovers on the #3 and #7 cylinders in their tuned 8-into-2 set-ups. Runner lengths get to be nightmarish on those engines, but scavenging can be that important in some performance applications)

Basically, a perforation kills any chance at low-pressure wash following a pulse. The perforation kills high-pressure and low-pressure conditions by allowing outside airpressure to equalize the pulse in mid-pipe. Gas pressure eithers enters or exits through the perforation depnding on where the pulse is in its' travel. This means that the engine, designed with this scavenging in mind, loses an efficiency factor, making it run like crap. Back pressure is basically bad as it prevents maximal flow, and this is why common performance mods on any internal combustion engine usually include opening up the exhaust for better flow.

Tuned open exhausts make great sense on limited RPM engines (racing applications and constant RPM designs like various small engine applications) where noise is not a problem. Tuned collected exhausts are better on others, especially if noise is an issue. Flow is always important though, regardless of exhaust theory.

--

On topic for the thread, it's been years since I watched "Bullit", and I don't recall the "sucking" noise you mentioned. I probably just didn't notice it. "Bullit" is a fascinating movie, especially on the production side. Much as I like old Stangs, they went through something like 8 1968 Mustangs, and only ever needed the one Charger. It was that much tougher a car. The Stang also needed a number of performance mods to keep up with the Charger (they chose the 390 engine, as opposed to just dropping in a better mill, they went through a lot of rigamarole to hop it up. I would've gone one year back and gotten 67's with the 427 in 'em, rawr.) It is possible that the odd sucking you hear is from exhaust and engine mods performed on the Stang. I'll have to watch/listen to the movie again to see if I hear it.

A possible theory would be scavenging in reverse. In other words, an exhaust pulse exits the cylinder at high velocity and then the driver gets off the accelerator completely, the pulse is still moving quickly and producing vacuum/low-pressure behind it. As it exits the tailpipe, with no serious volume of pulses behind it, the temporary low-pressure area still exists and "sucks" fresh air back into the tailpipe to equalize pressure. Dunno how plausible that is, nor how different from other folks' explanations.

An excuse to watch "Bullitt" again? Aw, heck.... =)

12-07-2003, 12:11 PM	#7 (permalink)
Moonduck Junkie Location: SE USA	Is it loss of backpressure that causes the performance loss or loss of scavenging? If backpressure were beneficial, top-fuel dragsters would run mufflers and reducers. As it is, they run open, tuned pipes because less backpressure means better flow-through in the overall engine. Scavenging is what occurs when an exhaust pulse travels down the exhaust pipes. The pulse is shot out of the cylinder during the exhaust cycle at a fairly high velocity. As it exits, it creates a low pressure area behind it, a vacuum if you will. In a tuned system designed for optimal scavenging, the pulses will be timed and pipes constructed so that a given pulse will pull, or "scavenge" the pulse behind it. Using the example of motorcycle engines and their exhausts, take a look at 4 cylinder inlines and the various forms of performance (and street) exhaust patterns that you see (I'll be referring specifically to Honda engines such as the one in my CB750). You tend to have 4-into-1, 4-into-2, and 4-into-4. 4-into-4, frequently called drag pipes, have no scavenging built into them whatsoever. They are usually open, and short to enhance low-range rpm (pipe length usually being what is modified in the process by which pipes are "tuned"). These pipes are designed for optimal flow as the intake side of the engine is also tuned for optimal flow in these sorts of applications. Drag pipes can be streetable, but are usually loud and not the most efficient application over the broad-range of rpm needed on the street. Reducers are sometimes put into these pipes, but usually to reduce certain undesirable performance traits (such as pipes not running in the proper rpm, but looking great at the length they're at), and to reduce the serious noise produced by short, open pipes. 4-into-2 are commonly seen on street 4's. Collected exhausts such as this are usually designed for scavenging or for economy of manufacture. The idea is that these pairs fire at precisely opposite points in the engine cycles, 180 degrees from each other, and the pulses travel in such a timed fashion as to help pull the next pulse from the cylinder more quickly. The low-pressure area behind the preceding pulse is what allows this. The issue with 4-into-2 (and 4-into-1 as well) is runner length. For optimal performance you want your various exhausts pipes to be equal in length, this means that each cylinder will perform similarly at any given RPM range. If your collected system has 4 different run lengths, the you will have to tune your timing and intake side differently on each cylinder to compensate for the differences in performance at a given RPM. The desire for equal runner length is what inspires some of the spaghetti designs that you see in exhausts on high-end engines. 4-into-1, when properly designed, is the best for scavenging as you have each pulse being pulled out by the pulse before it. On the truly well-done designs, the pipes collect in a squarish output with the pipes stacked in pairs. The firing order will rotates along this block in one direction causing each pulse to tpull the pulse following it around the pipe stack. (Not really able to describe this well, sorry) There is some serious flow work needed at the collector to keep unwanted turbulence down. Truly hot exhaust makers put a cone with four concave runnels corresponding with each pipe, less solid makers simply cut a line in each pipe and flatten them against each other in a plus-sign look. The cone style produces better flow, less turbulence, and enhances scavenging by removing unneeded resistance to flow. The downside of a 4-into-1 is need for better mufflers. In putting all of your flow through one muffler, you need a big can, making the muffler more expensive to build. There are also, insofar as some apllications are concerned, aesthetic issues with 4-into-1 (I used motorcycles as an example as it is usually easier to visualize the type of designs common to bike exhausts, without getting into scary automotive designs that have crossovers on the #3 and #7 cylinders in their tuned 8-into-2 set-ups. Runner lengths get to be nightmarish on those engines, but scavenging can be that important in some performance applications) Basically, a perforation kills any chance at low-pressure wash following a pulse. The perforation kills high-pressure and low-pressure conditions by allowing outside airpressure to equalize the pulse in mid-pipe. Gas pressure eithers enters or exits through the perforation depnding on where the pulse is in its' travel. This means that the engine, designed with this scavenging in mind, loses an efficiency factor, making it run like crap. Back pressure is basically bad as it prevents maximal flow, and this is why common performance mods on any internal combustion engine usually include opening up the exhaust for better flow. Tuned open exhausts make great sense on limited RPM engines (racing applications and constant RPM designs like various small engine applications) where noise is not a problem. Tuned collected exhausts are better on others, especially if noise is an issue. Flow is always important though, regardless of exhaust theory. -- On topic for the thread, it's been years since I watched "Bullit", and I don't recall the "sucking" noise you mentioned. I probably just didn't notice it. "Bullit" is a fascinating movie, especially on the production side. Much as I like old Stangs, they went through something like 8 1968 Mustangs, and only ever needed the one Charger. It was that much tougher a car. The Stang also needed a number of performance mods to keep up with the Charger (they chose the 390 engine, as opposed to just dropping in a better mill, they went through a lot of rigamarole to hop it up. I would've gone one year back and gotten 67's with the 427 in 'em, rawr.) It is possible that the odd sucking you hear is from exhaust and engine mods performed on the Stang. I'll have to watch/listen to the movie again to see if I hear it. A possible theory would be scavenging in reverse. In other words, an exhaust pulse exits the cylinder at high velocity and then the driver gets off the accelerator completely, the pulse is still moving quickly and producing vacuum/low-pressure behind it. As it exits the tailpipe, with no serious volume of pulses behind it, the temporary low-pressure area still exists and "sucks" fresh air back into the tailpipe to equalize pressure. Dunno how plausible that is, nor how different from other folks' explanations. An excuse to watch "Bullitt" again? Aw, heck.... =)