A discussion on common terms of gas dynamics in internal combustion engines

Or why can't I just bolt a huge 2in1 manifold onto my engine and expect it to make infinite torque. (This has been taken out of an actual discussion and is a slightly simplified view on the things going on in the intake and exhaust side of a 4 stroke engine. This is a recurring topic in the V-twin world.)

Imagine that your engine isn't breathing air, but jelly just to make it easier to imagine what's going on.

Scavenging: When the exhaust-jelly leaves your engine it will create a bit of a low-pressure area behind it (imagine a plastic water/soda bottle) and this scavenging will help you fill your cylinders with fresh gas/jelly. That's why your valves have overlap, i.e. both the intake and exhaust valve are open at the same time for a very short time period. Scavenging is (mostly) dependent on the exhaust design. If you sit down and think about it, you will probably be able to work out, why a 2in1 exhaust will work better than two separate exhaust pipes (which aren't connected in this example) in terms of scavenging.

Intake resonance: Now let's go back to our jelly-filled cylinder, if the intake charge (air-fuel-mix) rushes into the cylinder head (doesn't matter if there's a carb or a 2in1 manifold at this stage) a good amount will enter the combustion chamber until the inlet valve closes. Now what happens with the rest of the intake charge? It bounces off the inlet valve and flows backwards towards the carb/manifold. (Ever noticed that your bike/car makes some weird noises, when you open the throttle and don't have the air-cleaner on? Yep, you just heard the intake resonance.)
Now as our little ball of jelly travels back the wrong way through the intake, the inlet valve opens again and fresh gas comes the other way. They both crash into each other, mix and then flow towards the inlet-valve/combustion chamber again. The amount of mix is determined on the one hand by the restriction on the inlet outside of the head(airfilter, carb) and on the other side intake-port, valve-size as they determine the mount that can physically go through the port. The tricky thing is, if you time these collisions right, you can actually achieve to get more fuel into the engine than you normally would. E.g. imagine the fuel molecules bounce into each other very close next to the inlet valve and you have a strong vacuum from the scavenging-effect in your exhaust and all of a sudden at a certain rpm you suck in more fuel than you could, if there weren't any resonances happening in your intake and exhaust.

Intake runner length: Is the distance between your intake valve and the slide of your carb. And pretty much simply determines the maximum time it takes for the intake charge to bounce back and force or better it can take.

Any intake setup can (and should) then be tuned for a certain frequency, i.e. a certain rpm where this works best for your engine. Because all of the above would be way too simple the way it is, said intake resonance happens not only once for a given rpm, but multiple times. Out of which the 2nd and 3rd wave contain more energy and thus provide a stronger charge. (Otherwise we would simply bolt our carbs directly to the inlet valves as that would make the shortest possible runner length and ensure that the inlet charge bounces off the valve and carb-slide and then back into the engine.)

You can probably guess by now, where this is going... the longer the intake runner length the more time there is, which is especially important at low rpm, the shorter the intake runner the length the better at a certain higher rpm. (Remember we're not hunting for the first reflection, but for the 3rd.)

Plenum volume: The amount of air-fuel-mix available in the area from the (closed) carb-slide to the closed intake valve. This can fulfill multiple purposes: a) it can be imagined as a sort of storage (we already noticed above that under ideal situations we can suck in more air-fuel-mix than can otherwise be provided in one go) and as a dampener (e.g. for unwanted) resonances at certain rpms. If we get back to our jelly example, if you have a lot of jelly waiting in a plenum, it's pretty easy to imagine how this will slow down any way bouncing off the intake valve. The amount of energy consumed over time (and this being the result of distance divided by speed) basically determines the strength of the intake signal. (Which is what the resonance is called in professional terms.) Again a case of think this through and should explain, why you can't create an infinitely large plenum on an engine or infinitely long runner lengths or at least it won't do you any good. But what it will explain is, why a longer will at some point flow less than a shorter one.

As such we can conclude, plenum volume makes sense up to a certain limit. Above that it won't do much harm, but also no extra good. Also it should become apparent, that there isn't no such simple wonder-solution to all induction problems. As changing the intake runner length isn't a simple "make it longer, make more torque" thing, but there's a lot more to it. Hence why you can't say that a 2in1-manifold will automatically make more torque, because if the intake runner length and plenum are too large you will basically kill all the potential gains.