Hi guys!
I've seen quite a few threads here regarding various exhaust setups including using automotive mufflers etc. There's discussion about the sound, of one being better than another, and also comparison of performace of one over another. I'm including some rather lengthy info here for those interested in getting the most out of their engine's exhaust.
For maximum performance a properly tuned exhaust is a matter of tubing diameter, muffler's inside diameter, muffler packing/silencing method and very important: the total tuned length of entire exhaust system. Large diameter exhaust is great for a top end screamer, just like big fat valves and hogging out ports. For torque we want to slow the motions down, let the engine take a little longer to chew its food. Top end horsepower wants fast flow: big valves, large and short distance intake runner or intake tube, highest compression, fattest bore, largest carb to feed it, with high lift/long duration cam with lots of overlap, plus short fat exhaust. This all adds up to gobbling more fuel, exploding it as quickly and completely as possible then dumping out the leftovers as fast as possible. This lets the heat pump factor, which is all an internal combustion engine really is, work itself so that incoming charge helps push out the outgoing and outgoing helps suck in the intake charge: FLOW, FLOW, FLOW!
Opposed to that, a torque monster wants a whole different scenario. There's three different ways to build: under-square, square, or over-square. This refers to the bore/stroke ratio, where bore diameter is divided by stroke length. Under square engines have a longer stroke length than bore diameter; square are equal like 60mm bore x 60mm stroke; and over square is the typical larger bore/shorter stroke engine. Simply because bore becomes larger in over-square short stroke engines, there's more room in the head for larger valves, with the converse being true of under square: smaller valves. Over-square also says the piston has a shorter distance to travel, so a quicker trip getting to higher rpm faster. Because of that everything should be sized to faster motions of incoming and outgoing air columns in over-square engines.
For a Torque build an under square, long stroke engine is first preferred. From the top you want a tuned length intake tube with a smaller, good flowing and correctly atomizing carb, breathing into smaller valves to slow down the moving column of fuel/air. This is pulled in by a longer stroke, to take a deeper breath of intake charge, that bore/stroke ratio favors the stroke side. High lift valves are good but the camshaft duration, as length of time valves are physically open during stroke, are lower numbers, being open for less of the full rotation of cam. The cam profile's 'overlap' is the time that both the intake and exhaust valves are open, acting like a tunnel, pulling air/fuel in and then shoving out exhaust columns simultaneosly. Ever heard of a 'Tunnel Ram'? Overlap of valve events in a long stroke under square engine, rotating at slower rpms, means you're dumping out the fuel/air mix you just worked so hard to cram in!
The Torquer's slower motions mean we want a longer intake runner to let carb more completely convert raw liquid fuel/gaseous air into an air-atomized fuel-gas mixture. The smaller the droplets of fuel the greater total surface area of fuel available to explosion, so more complete combustion. A high rpm A/A Top Fuel blown Hemi dragster is milliseconds away from hydrostatic lock, that it is actually pushing an almost solid column of raw fuel through intake tract and cylinders in its hugely oversquare engine. Instead, we want to take maximum advantage of every molecule of atomized fuel in mixture. Long narrow intake, through smallish valves, with valves open for intake and then closing for combustion, filling combustion chamber at maybe 7.5:1 to 8.8:1 compression, then spark or compression explodes mixture. Flame front travels leisurely and pushes down piston with full power of fuel applied, exhaust then opens and clears cylinder just in time, before next intake opens again. We need to scavenge that outgoing charge, because last event's gases are burned and useless, taking up cylinder volume that should be a new fuel charge.
Outgoing exhaust pulse wave travels at the speed of sound, roughly 750 feet per second. By using a 'tuned length' exhaust according to speed of that column we can literally pull it all out. The outgoing is under explosion pressure, but when it reaches the end of pipe and nothing's in pipe behind it, the external atmospheric air pressure and surface area is suddenly higher than empty pipe's internal psi, so a reversion wave at 1/2 atmospheric pressure instantly travels back up empty pipe toward valve again. Once it gets to closed valve it reverses yet again, like a bounce, and becomes a negative pressure wave traveling out compared to next pressurized exhaust event. So pipe itself and last event's lower pressure wave literally, physically PULLS the next exhaust event out into a pipe with a standing vacuum! That is, it does this only if the tuned length is precisely right for that action. Otherwise the reversion wave coming back up pipe fights against the next outgoing column of exhaust gases and must reverse direction in flow, which also slows outgoing, hindering the whole process.
We can see that piston speed, stroke length and bore are critical to this, that bore/stroke ratio play the first major role in events. Also diameters and lengths of the tubes coming into and going out from engine have to be precisely correct to time everything together for maximum efficiency. A tuned length intake and exhaust is called that because it's balanced according to the speed of sound: harmonically tuned! A long ram intake is called that because it physically pushes or rams mixture through to engine if all is in balance.
Repeating some, each exhaust pulse is a high pressure wave traveling from exhaust valve face to exhaust exit tip at the speed of sound. When this wave reaches tip it exits into atmospheric pressure. Then a secondary wave at 1/2 of atmospheric pressure rushes back into exhaust and travels up pipe toward valve face, where it reverses direction if valve is closed. For a properly tuned exhaust you want this lower pressure wave to arrive at valve face just before it opens, so next outgoing exhaust wave only has to work half as hard to exit on next pulse into this low pressure condition. Because it's at 1/2 atmospheric pressure it actually draws the new high pressure wave of next exhaust valve opening into the pipe, acting as an extractor exhaust.
Where you have 4 into 1 exhaust this wave motion becomes more complicated in timing of pulses but it still occurs. A 4 into 4 system [also a thumper] is much easier to tune because we're only dealing with one exhaust valve and the same pipe. The trick is to adjust the total length of the entire exhaust pipe so that the low pressure wave arrives at the precise moment before valve opens and reverses, giving a low pressure path for exhaust to exit through.
In addition engine rpm has a major effect as it changes timing sequence of pressure waves. A tuned length that's correct for 5,000 rpm is going to be incorrect for 7,000 rpm. What you want is a happy medium as much as possible, with the total exhaust pipe length tuned to the rpm range where you most often need or want maximum power, and/or to the rpm range of your particular engine. This is where an adjustable exhaust tip is handy, to dial in or 'fine tune' the length to suit your engine's needs and your mode of riding. You want shorter for high speed and high rpm, and longer for your lower rpm torquer. The adjustment of the position of muffler/s on exhaust pipe according to overall length will also adjust extraction effect. In addition it can change exhaust note by adjusting amount of time the pulse is in the muffler's silencing area.
It's also possible to modify your exhaust with a baffle, as a washer with holes drilled in it, that is installed inside muffler to cause a 'stumble' in the exhaust pulses. This 'stumble' occurs in both directions of wave flow and is only for changing sound, as it works against maximum performance of extraction effect. The baffle washer with holes in it may be welded under the the head of a long bolt, with bolt head and washer on cylinder head side and bolt's threads toward exhaust exit tip. With a cross piece with a hole in it welded near exit, and a nut on bolt threads at cylinder side, bolt passing through hole, another nut on exit end on other side of cross piece, the baffle can be adjusted back and forth along length of threads inside of tube to vary its effect.
I have some figures available for optimum tuned length according to desired rpm. One set of numbers is for maximum horsepower engines according to Norton-Vincent Engineer's research. The other is for torquer engines, from Chrysler Corporation Engineering, that total tuned length per rpm is diferent between them. The longer the exhaust the lower the effective rpm range.
