Відео

The gas is a mass or air molecules, the wave is energy moving through the air mass. The wave raises or lowers the pressure of the gas only at the location of the wave. The same as a wave in the ocean causes a rise pr trough in the water level where the wave is. So the sonic wave doesn't push the gas anywhere, if the face of the wave arrives at the port just before it closes, which it does if timed right by the tuned length of the pipe. Then the leading edge of the wave peak will block fresh fuel and air gas mixture from spilling out of the cylinder into the port as the port closes thus trapping more of it in the cylinder resulting in more power.

Another example Image a gate at the edge of a harbor that allows or stops water from moving out into a channel of water and there is a boat drifting with the current that is about to drift through the gate into the channel and a wave comes back up the channel against the current. The wave arrives at the gate just as the boat is about to go through it, at that moment the front of the wave cause the forward speed of the boat to slow or stall until the peak of the wave has passed it but before the wave has passed by the boat the gate suddenly closes trapping the boat in the harbor which, if the wave had not arrived and slowed the boat momentarily, the boat would have passed through the gate just before it shut into the channel. The harbor is the cylinder and the boat is the fresh intake charge that is about the spill out into the channel which is the exhaust.

That is an excellent answer. Thank you@

See link to software in description section below video Download at birdcagesoft.com

Port timing is fairly specific to the RPM of an engine, as in port time area. The compression is a the main game with power, but unlike a 4 stroke increases in CR will only provide gains in the power curve or "band" of the engine. Ignition timing is basically a setting that follows other conditions, how much advance you need is dictated by density. So improvements of porting or compression that yield a higher density in the compressed charge will burn faster and hence require less ignition advance. Squish itself has no effect on power, its purpose is to allow higher compression ratio with detonation than could be used otherwise. So if you raise compression ratio to get more power you need to optimise the squish band to ensure you don't have any detonation at the higher compression. To the question of raising the barrel, more port timing will allow you to rev the engine a bit higher which will results in most power but there is a point with a reduction in compression will effect the lower end of the power curve so it will take longer to get to the higher RPM, hence the need for some trial of various base gaskets to get the optimal output for the particular track you are3 running at. Higher revs or more over rev for a longer track is good, but better power end for a shorter tracks is better.

@@AuMechanic Thanks heaps Dave. That info is invaluable

In short you need to get some starting figures by using a port time area calculator software. Not aware of any free ones around though.

@@AuMechanic Thanks! any program that does not cost a fortune?

@@erikjohansson9039 Try torqsoft www.torqsoft.xyz/disc-valve-inlet-time-area-two.html Not sure of prices but they have a disc intake time area calculator

You could do....but The trench left in the piston top would mean that area doesn't come close enough to the squish region. Although in an engine like this the CR is pretty low so it wouldn't matter much. OTH there is probably not much to be gained by doing it anyway. It just highlights the fact that this engine basically has the wrong cylinder on it for the stroke among other flaws with it.

Was thinking of putting alumi-rod /filler for that, but true @@AuMechanic :)

The mid section diameter is more a construct of the cone angles which dictate if the pipe favours peak or wider RPM range. In short the steeper the cone angle to more it favours peak power. Whereas a shallow angle will be better over a wider RPM range. In addition to that multiple angle cones with favour wider RPM range then single straight angle cones. So if you look at a pipe that has straight cones with a sheepish angle to them it will favour peak power. And converseley it will have a longer mid section to make up the tuned length. No part of the pipe equation takes into account the pipe volume or engine volume.

@@AuMechanic Interesting…Crazy how much math goes into these things! Thanks again for info!

Hi The scope I used back then was a very old CRT scope so no invert or other features. Also the signal is taken from the primary side of the coil however due to mutual inductance of ignition coils you can still see the secondary winding behavior through the primary with some variation to the pattern, as opposed to taking it directly from the secondary where you can measure actual voltages.

Any questions feel free to ask.

No to convert milliseconds into seconds divide by 1000 because there are 1000 milliseconds in each second.

That exhaust is port time in milliseconds. 1 minute = 60,000 milliseconds What the equation is doing is finding out what division of 360 degree is the port timing. So if its 180 degree then it 1/2 or 0.5 of 360 degrees. So the equation is 0.5 multiplied by the time the port is open at during one crank revolution in milliseconds based on the RPM. So if its 10,000 RPM then the equation is 60,000 / 10,000 = 6 So the port that is open for 180 degree each revolution is open for 6 milliseconds when the engine is spinning at 10,000 RPM And based on the speed of sound inside the pipe at the given temperature which is "Velocity" at metres per second. So we convert metres per second Velocity multiplied by 1000 equals millimetres per second. Velocity in millimetres per second X Exhaust port time in milliseconds. So that is converted to seconds by dividing it by 1000. Then we divide it by 2 allowing for the fact the wave goes to the end of pipe and back to the port and we only want to know how long it takes in one direction to find out the tuned length of the pipe.

If you are struggling with the equations it might be easier to just download the free software and use it to get tuned length. See link in description section under video or in later videos.

Need to do a rebuild on my 372XP that the piston cooked in a few weeks back

Same here, specially when they take flight across the workshop.

Do you mean valve "guide" tool ?

Yes anything over about 11 you would use 650C as an estimated value. You would need to use an EGT gauge for an accurate reading. Keep in mind that is the peak temp. Under the peak RPM or lower gears that value drops quite a bit but we work with peak temp in top gear. For racing purposes that provides for peak RPM at the end of straights in top with some room for a bit of over rev. Also be aware that the RPM value is the maximum engine redline RPM value not the peak power RPM.

@AuMechanic OK thank you dave. . . Will be putting degree wheel on today to find the exhaust port timing for the rd400f. . . My guess its around 190 to 200. Could be wrong. Thanks again

@AuMechanic Hey there. I've read several comments on that particular topic in just 2vids of this playlist. You wrote this in your comment above : "...the RPM value is the maximum engine redline RPM." So, in my case, the Redline goes from 10k to 12k on the tachometer of this '93 aprilia rs50. Should I use 12K, as I understand from your above comment, as my input value for RPM ? I've made all my sims until now with a value of 10K (thinking "redLINE"). I'll run some more estimations using 12000RPM to see the differences. I'm working on a mod for the stock exhaust pipe. And I do because of your work, both teachings and freeware. Thanks a lot for your contribution to the community, cheers !

Thank you.

It is See the link in the description section below the video Later videos cover some info on using the software.

For most applications the port increases in area from the piston outward to the pipe. The header may also expand over its length or can just be straight in the case the pipe is for peak power.

Most 2t pistons have a wire clip so the groove as a round profile. If you fit a C clip that has a square profile that could be risky as it does seat on the rounded groove. And using any clip that has ears is problematic as the video covers.

So long as you put in the right port window diameter info that will be right. See more recent video that shows how to measure port area and put it into the software.

@@AuMechanic so for port window diameter i should put in the area?

@@samdehaan2395 Use tools menu to go to converters window to convert area in to diameter then use that.

Can you explain that in a bit more detail ?

@@AuMechanic Okay, so. my moped has a 600mm pipe that comes all the way from the cylinder to the expansion chamber. The inner diameter is the same as what the outer exhaust diameter is. But i want to know how can i implement that 600mm pipe that's between the cylinder and expansion chamber in your calculation software? The expansion chamber on the old moped is only 400mm long but combined it's 1000-1200mm.

@@AuMechanic Also can you explain why your exhaust calculator doesn't take cylinder displacement into account?

@@benjaminnurmentaus4232 Cylinder displacement has no bearing on the design of the pipe. Diameters are based on flow which is determined by port time area, and lengths are determined by tuned length that is a factor of maximum RPM and port timing.

@@AuMechanic Okay. But can you explained the long lenght of pipe in between the cylinder and expansion Chamber and how it effects the back pressure made by the expansion Chamber?

In short, no.

The long answer as to why it wouldn't help is that an expansion chambers relies in a pressurised intake such as a 2 stroke has, the crank case creating a compression of the intake mixture that is then fed to the cylinder via transfers. A rotary or any normally aspirated engine lacks that. It would work with a super charger and has been used but as the super charger pressure can be dialed up it negates the need for an expansion chamber that would only work over a limited RPM range anyway.

@@AuMechanic do you know of any rotary exhaust tunning science, like matching extractor lengths to engine port sizing?

@@shaunessex3095 Never built an exhaust for a rotary only 4t and 2t piston engines so I don't have the experience to give advice on it. A general guideline to matching ex port to header is aim to have a the same or slightly large header than port, but never smaller header. And always use mandrel bent sections for bends to maintain the cross sectional area of the pipes.

I've always been in the understanding rotary exhaust requires a sertain amount of back pressure but not many people mention it though, rotaries are essentially 4 cycle 2stroke simular animals ,but the gas tem

Might be an idea to replace it before it breaks and bits of it go into the top of the engine.

See the description section below the video for the link to it.

There is a link to it in the description section under the video.

Thank you Dave, hey question for you , I put a 2014 rotax 800 etec in my bushplane and the factory pipe is the wrong shape . Can I just take measurements from the factory pipe and use those values with different bends to achieve the fitment ? If the L and D values remain the same does the shape or routing matter?

Also what is your recommended material to use ? Lighter is better for my application

@@davepalmer8925 0.5 to 1 mm The thinner it is the easier it is the bend cones but that harder it is to weld.

@@davepalmer8925 So long as the L and D dimensions are right it doesn't matter what shape or how many bends you put in it.

Do a search for metal timing degree wheels.

Is the plate in the intake or exhaust ?

@@AuMechanic in the exhaust

​@@T-crisp Ok I would say no change. If you for example reduced to diameters of the pipe including stinger to suit the restrictor hole size rather than based off ex port area you may cause too much back pressure and overheat the piston.

Not really Expansion chambers don't operate on resonance like a pulse jet or the like does. They shape of them reacts to a single compression wave as the exhaust port opens to produce the effect. Resonance (as in Helmholtz) applies to highly specific frequencies and harmonics of them where the expansion chamber exhaust operates over a wide range of frequencies over the range of engine RPM.

Yeah it chews it up alright. But unfortunately the clip is steel and the piston is ali and steel wins. The other issue is the clip is perpendicular to the ex port in the piston pin hole and more in line with transfers so to make it out of the exhaust it would first have to go down the transfer port without the piston chopping it, and then pop back out the transfer port once the piston is at BDC and then go out the exhaust and that's pretty long odds.

A butterfly choke isnt the only means of enriching the mixture for cold starts, many carbs use other means.

The software is available now, if you see newer videos it shows where to download it. Or see description section for link.