paulzig wrote: ↑
Wed Apr 03, 2019 6:19 am
cgarb wrote: ↑
Wed Apr 03, 2019 2:48 am
Sorry for the side track...but why is it that they step the tubes larger as you go down the primary?
I would think its because each step up creates an anti-reversion barrier ...
Every step up or increase in diameter provides a negative pressure pulse. The small diameter start to the primary keeps each gas 'parcel' at high velocity , not allowing the gas to slow down right after the port helps the chamber scavenging process,, but keeping the pipe at this diameter for too much length then becomes a restriction, a 'small' step up gives a controlled reduction in gas velocity (losing to much velocity causes the rapidly decellerating gas become a restriction to move) and a controlled reduction in pipe restriction.
Yes the gas cools as it moves thru the pipe and in the process of cooling becomes denser and loses velocity, common sense would say reduce pipe size to keep velocity high and keep the gas moving but prior to the collector where the cylinder scavenging process of the header as a unit is not complete this does not work.
Remember as the gas cools and becomess denser it also has more molecules per sq" to become a frictonal loss against the pipe wall which creates a slower moving boundary layer of gas near the pipe wall and a faster moving area of gas nearer the center of the pipe.
So the stepped primary is a juggle of keeping velocity high off of the port for long enough to creat high scavenging potential but not for so long that it then becomes a restriction, and then stepping the pipe up enough to allow good flow and create a negative pressure wave to help the proceeding pipe but not slow the gas so fast that it becomes a slow boggy mess and is consuming so much energy from the proceding pipe/gas velocity just to keep it moving that its scavenging potential is lost/reduced.
Then there may or may not be be another step up,
Then the collector will also still function properly and enhance the benefit of the stepped pipe and create its own negative pressure pulse.. that is just the basics and there is so much more to this. Extremely interesting stuff and each engine combo will respond differently, some will really benefit from a stepped primary, some not so much.
Going on after the header pipe and collector process is complete is then another matter and this is an area where some pipe reduction to enhance gas velocity can work very well.