what this boils down to is nomenclature. the correct name for things. a spade is a spade, a shovel is a shovel and a plow is something else altogether. they can all still be used to move dirt. or snow. or something else altogether.
a saxaphone is tuned, a kazoo is not. they can both be used to play music, yes? but one works better in certain cases than the other?
4 strokes appreciate low back pressure, it lets the high pressure exhaust gas escape. so a trumpet pipe or anything that causes a low pressure zone in the header pipe, is beneficial.
4 strokes also have a thing called VALVE OVERLAP.
the momentum of the exhaust gas helps suck fresh charge from the intake, for a brief period near TDC. "crossflow" heads suffer from the same thing that affects two strokes at times... fresh air/fuel is sucked from the cylinder straight out the exhaust valve. so do flat heads but theyre just another kettle of fish.
the overlap makes for bad idling. it also makes for serious top end power/ rpm by increasing volumetric ratios.
so, sorry, completely INCORRECT. both valves are open at the same time on a 4 stroke. the exhaust doesnt close until AFTER TDC, and the intake is open BEFORE TDC.
at the same time, they will not appreciate a "stuffing","plugging", or "reversion" wave trying to stuff anything drawn from the exhaust port back into the cylinder. because at this point the inlet IS open, and any increase in pressure in the cylinder at this point prevents new mixture coming in, possibly even pushing it back out through the carb!
a plain TRUMPET or MEGAPHONE pipe WORKS on a 4stroke. on a two stroke, they suck, literally. they draw the charge up from the crankcase, sure. cus they suck
but then they dont produce any stuffing wave. the cylinder pressure is lower after the ex-port closes than it would be using just a plain straight pipe! they drink petrol cus most of it goes out the exhaust...enough so it will light up and burn!
and the effect gets stronger as you go faster, because then you have the exact opposite to a Ram Air intake... a void created by the venturi effect/bernolli effect of the vehicle passing through the air that sucks exhaust gas out from the pipe.
the biggest factor in a tuned pipe is the reversion wave INCREASING the pressure in the cylinder than would normally be achieved with a normal exhaust. this is how they "supercharge". by getting more into the cylinder than it would normally hold! more bang for the buck...
combined with the suction/scavenging wave, this adds up to make tuned pipes the most important factor in performance.
do some research before shooting ya trap
yep, they run turbos with big fat hi flow pipes because they try to get the largest pressure differential across the turbine.
so you get 1 point for that
10psi at the port and 1 at the exhaust makes for 9psi to extract power from.
10psi at the port and 8 at the exhaust makes for 2 psi to extract energy from.
less pressure in pipe + more pressure at port = more extracted energy to spin turbine, more work, more compression, more boost, more pressure at port, more energy, more compression, more boost ad nauseum....
idiots that run big exhausts without turbos thinking theyve just added 20KW by sheer pipe diameter alone dont know about velocity and momentum of gas flow. bigger is not always better.
it isnt "backpressure".
its a small enough diameter pipe to increase velocity of exhaust gasses. the faster they travel, the more momentum they have. the more "suction" they create at the port. the more gas removed from the cylinder, and, once the intake opens, the more fresh mix drawn into the cylinder.
go too far and fluid friction slows the flow down again, so pressure rises, exhaust dont flow, and pop. stalled it. oops
