HeadSmess
Well-Known Member
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- Joined
- May 17, 2010
- Messages
- 3,048
raising compression can only do so much for a two banger. you start getting more "pumping" losses than useful power. im not saying not to shave the head, but theres only so far you can go on a 2stroke before it becomes detrimental. now, the "squish band", thats slightly more important than sheer "high compression".
the aim is to get fuel/air mix into the cylinder, and exhaust out, with the minimum of turbulence and intermixing, with the maximum volumetric efficiency. polished/rounded corners and edges, removal of casting flash, matching ports, etc, all these little things add up to make the airflow through the engine smooth. smooth flow makes for more power with less losses.
volumetric efficiency is a hard subject for some to comprehend.
say you have a cylinder of 1 litre capacity. open to the atmosphere, that will be one litre of air. quite simple. thats the rated volume.
now pump the cylinder at a certain rate. if the top was missing, it would fill to its rated capacity, and expel the same amount, 1 litre. minus the small amount of air thats compressed/rarefied on each stroke due to the momentum of the air.
start closing the top off, like a valve or port would. as the piston descends, at first theres a lower pressure in the cylinder. the air has to "think a bit" before it starts flowing. as the chamber fills, more and more air rushes in through the (restrictive) port. it doesnt just "magically and instantaneously" fill up with air! the air speeds up, it gains momentum. inertia. it wants to keep flowing into the cylinder, even once the piston has started coming back up! etc etc.
as the gas is expelled, once again, momentum and inertia come into play. as the piston starts descending, the air/gas still wants to keep flowing OUT of the cylinder! with well designed headers, intakes and valve times, you can get more air into a cylinder than it would otherwise hold! though volumetric efficiency usually sits around 85 to 90%.(ie, a 1 litre cylinder can only be filled up with 850 to 900 cc of air/fuel) its also RPM dependant. one setup might be useless at 3000rpm but boy, does it work well at 5500!
superchargers raise VE by stuffing more air into a cylinder than would normally be possible.
hence the valve overlaps and (to someone that doesnt eat breathe and
engines) general valve timings always seem a little bit "illogical" a standard 4stroke will open the inlet 10-15 degrees BEFORE TDC. the exhausts still open! the exhaust closes 10-15 after TDC. then inlet closes at say, 190-200 after BDC. compress, boom, and what? the exhaust opens at roughly 90-110 after TDC! so the valves open before the piston even gets to the bottom of the stroke! etc etc. its that momentum the air gains that makes it work!
less overlap, and open closing times closer to the TDC and BDC marks makes for slow revs, torque, no hi speeds. large overlaps and highly advanced and retarded valve times make for bad idles, with heaps of power up top.
so, how does this apply to a twostroke? much the same way. inertia is important. but so are "pressure waves". i swear i read somewhere that (ported or valved) motors have a limiting speed of approx 30krpm. at this point, despite the ports being wide open, the air simply can get moving in time to enter it before its closed!
its a lot like "you cant get a projectile to launch faster than the SPEED OF SOUND IN THE PROPELLANT" when dealing with pneumatic rifles. the gas CANNOT expand faster than the speed of sound within itself! (compressed air has a higher mach speed than plain air, so yes, a projectile can be launched faster than the speed of sound in a standard atmosphere)
the pressure waves can only propogate through the gas at a certain speed, can only push on the projectile and rear of barrel at a certain rate, etc.
back to the question.
low/mid speed torque.... fairly large ports, stocks are way too small. a really long intake manifold, the bigger carb wont do much as the air flows through its venturi slower than it would on a stock carb, making tuning erratic, and tending to bog on WOT at low speeds. and of course, one of my weasely pipes! (which in my experience override any other mods)
iunno. motor tuning seems like a black art at first, but it isnt. you just need to be willing to spread your knowledge around. its like playing a musical instrument. it can sound like, or can sound amazing. all about harmonising some physics, some metallurgy, some experience pushing a broom even, or watching smoke in the draught from under the window... then applying everything you see, smell and touch!
the aim is to get fuel/air mix into the cylinder, and exhaust out, with the minimum of turbulence and intermixing, with the maximum volumetric efficiency. polished/rounded corners and edges, removal of casting flash, matching ports, etc, all these little things add up to make the airflow through the engine smooth. smooth flow makes for more power with less losses.
volumetric efficiency is a hard subject for some to comprehend.
say you have a cylinder of 1 litre capacity. open to the atmosphere, that will be one litre of air. quite simple. thats the rated volume.
now pump the cylinder at a certain rate. if the top was missing, it would fill to its rated capacity, and expel the same amount, 1 litre. minus the small amount of air thats compressed/rarefied on each stroke due to the momentum of the air.
start closing the top off, like a valve or port would. as the piston descends, at first theres a lower pressure in the cylinder. the air has to "think a bit" before it starts flowing. as the chamber fills, more and more air rushes in through the (restrictive) port. it doesnt just "magically and instantaneously" fill up with air! the air speeds up, it gains momentum. inertia. it wants to keep flowing into the cylinder, even once the piston has started coming back up! etc etc.
as the gas is expelled, once again, momentum and inertia come into play. as the piston starts descending, the air/gas still wants to keep flowing OUT of the cylinder! with well designed headers, intakes and valve times, you can get more air into a cylinder than it would otherwise hold! though volumetric efficiency usually sits around 85 to 90%.(ie, a 1 litre cylinder can only be filled up with 850 to 900 cc of air/fuel) its also RPM dependant. one setup might be useless at 3000rpm but boy, does it work well at 5500!
superchargers raise VE by stuffing more air into a cylinder than would normally be possible.
hence the valve overlaps and (to someone that doesnt eat breathe and
engines) general valve timings always seem a little bit "illogical" a standard 4stroke will open the inlet 10-15 degrees BEFORE TDC. the exhausts still open! the exhaust closes 10-15 after TDC. then inlet closes at say, 190-200 after BDC. compress, boom, and what? the exhaust opens at roughly 90-110 after TDC! so the valves open before the piston even gets to the bottom of the stroke! etc etc. its that momentum the air gains that makes it work!less overlap, and open closing times closer to the TDC and BDC marks makes for slow revs, torque, no hi speeds. large overlaps and highly advanced and retarded valve times make for bad idles, with heaps of power up top.
so, how does this apply to a twostroke? much the same way. inertia is important. but so are "pressure waves". i swear i read somewhere that (ported or valved) motors have a limiting speed of approx 30krpm. at this point, despite the ports being wide open, the air simply can get moving in time to enter it before its closed!
its a lot like "you cant get a projectile to launch faster than the SPEED OF SOUND IN THE PROPELLANT" when dealing with pneumatic rifles. the gas CANNOT expand faster than the speed of sound within itself! (compressed air has a higher mach speed than plain air, so yes, a projectile can be launched faster than the speed of sound in a standard atmosphere)
the pressure waves can only propogate through the gas at a certain speed, can only push on the projectile and rear of barrel at a certain rate, etc.
back to the question.
low/mid speed torque.... fairly large ports, stocks are way too small. a really long intake manifold, the bigger carb wont do much as the air flows through its venturi slower than it would on a stock carb, making tuning erratic, and tending to bog on WOT at low speeds. and of course, one of my weasely pipes! (which in my experience override any other mods)
iunno. motor tuning seems like a black art at first, but it isnt. you just need to be willing to spread your knowledge around. its like playing a musical instrument. it can sound like
, or can sound amazing. all about harmonising some physics, some metallurgy, some experience pushing a broom even, or watching smoke in the draught from under the window... then applying everything you see, smell and touch!
