How an expansion chamber works. How an Expansion Chamber Worksby Michael Forrest. Two strokes can function. RPM for a faster burn and more power. An expansion. chamber does just that. But at what part of the powerband it does it is.
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That is because the pressure wave (created when the exhaust. When the wave enters the diffuser cone the sudden. When the ongoing pressure wave in the pipe hits the baffle. I used to think that. I studied professor Blair's. For more detailed info on.
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I have made an Excel file for analyzing the dimensions of an expansion. RPM of the engine. The wave is first created when the exhaust blasts into the. Graham Bell wrote that . My Excel file, sheet 2, calculates the percentage area. It would be very.
Thankfully for you I have. As a beginning temperature I measured it halfway down the. Blair. wrote that the average temps for GP is 6. MX is 6. 00, enduro 5.
It changes with jetting, load. RPM. For designing you should be precise and. RPM in the calculations. For precise pipe calculations you can. Don't take any reading till the engine is at maximum.
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EGT. But you can guess at the. RPM. I think you could. RPM is at least 1. RPM (which incurs the highest temperature). If you entered. 6.
If the perfect pipe for your engine needs a 4. I use automotive. But one run is all you need for each test length. You can. also use sheet aluminum cut out from a cooking pan rolled into a tube. Factors. affecting temperature- -> more compression gives less temperature. Header. Diameter. This is the pipe that connects the cylinder to the diffuser cone.
Most pipe builders just keep the same stock header diameter. My expansion chamber calculator lets you do whatever you want to but gives recommendations for header and belly diameter based on my own theories. What I don't like about his advice on headers is that it essentially disassociates its size from the size of. I consider because all of the exhaust typically exits before the exhaust port is half way open). This topic of header size is important because both Graham Bell and Gordon Jennings give belly diameter recommendations that are 2. So they keyed off the header. And too. wide doesn't assist in pulling up intake charge through the.
What type of powerband you want should influence your choice of header. Larger diameter headers will present a greater. Following the exhaust pulse will be a negative pressure. So a smaller. than normal header diameter will allow more post- exhaust suction which. The good aspect of its influence is that.
This can be important when the transfer duration or blowdown. RPM. The bad aspect is that the suction. This causes some loss of intake. To know what is happening you need to see.
It depends a lot on how close the transfers are to the. My idea on how to correctly size the header diameter is that it should be done by a formula using engine size, compression ratio, and open port area at the end of the exhaust pulse exiting the cylinder.
And I recommend a maximum belly diameter as 2. The formula I use agrees with the experiment done by Gordon Jennings with different header diameters for a DT2. That is to say. that they get larger with more distance from the cylinder. This. theoretically aids exhaust gas flow at high RPM but reduces the. The correct distance depends mostly on the peak RPM. Shortening it allows the engine to rev higher. For a. open classed motocrosser reving to 7,0.
RPM one inch of. length affects the powerband by approximately 1. RPM, and for an. engine reving to 1. RPM one inch changes it by 3. RPM. Its length should be so that the beginning of the diffuser wave returns.
The. Diffuser (expanding/diffusing) Cone. As long as the primary pressure wave is traveling along the diffuser it. From around 2. 50. RPM to peak RPM this vacuum wave returns.
Extra fuel/air contributes to a. Also less dilution of. This is what causes an expansion chamber. Also the transfers roof angle comes into play as the. You can tell by. looking at the pattern the intake flow leaves on top of the piston.
The advantage of a multi- coned diffuser. Since the main factor.
I designed, although the belly. A 2 or 3 stage diffuser causes the.
Click. here to see how different diffuser designs result in different. Click here to read more about diffuser cone angles. Maximum Cross Sectional Area of. Diffuser. According to Graham Bell the maximum diameter at the end of the diffuser (at the belly) shouldn. This is equal to an area 6. My recommendation after much study of different engine/pipe/dyno results is to use 6. Belly. The. belly exists to keep the diffuser and baffle waves from overlapping.
It is the pressure changes in the exhaust wave going thru the pipe that cause the secondary waves to return to the cylinder. The more pressure change, the stronger the return wave is. So although there are pressure changes all the way up to 1.
If the belly is too wide then that causes the end of the diffuser and the beginning of the baffle to be . That is why some people think that having a wide belly widens the powerband (at the expense of peak power). Well, it does but only because it's causing a longer effective belly section at the expense of the diffuser and baffle lengths. Look at the graph below to see how the end of the diffuser wave and the beginning of the baffle wave are extended weak areas of wave strength which looks almost the same as if the belly were much too long (much longer than the actual 9. The belly width was 1.
With most bikes there is limited space which sets the maximum. It is always parallel- walled. When designing a pipe it is always best to make variations to it and note the results at the power graph. Below are two graphs showing the difference in pipe powerbands for 2 belly lengths with a single cone baffle. You can see how a longer belly has peak power 5.
RPM earlier in the powerband and a more linear rise in power. The purple graphs are the pipe power graphs. The. purpose of the baffle is to create a return wave so that at high.
RPM the return wave arrives back at the cylinder before the exhaust. Without it the. piston will push intake charge out the exhaust port as it rises. Doing both things. RPM. Unfortunately, at all engine RPM.
RPM, the return wave returns when the transfers are. But if you want an engine with explosive power on.
The baffle. should be designed to match the bikes RPM range of the needed powerband. A longer baffle. due to a lesser angle will allow a slightly wider pipe powerband, but. RPM is just below. RPM range of the powerband. Tuned length. The perfect location for the baffle, in distance from the piston, needs. RPM of the desired powerband.
The. header, diffuser, and belly set its length from the piston. But if the. baffle uses many cones of differing angles then using a. Angle. For a single angle baffle cone the steeper the angle the stronger the. Lesser angles cause less peak power but. Making a 2. or 3 cone baffle with lesser angles gives. Most racing bikes.
My expansion chamber calculator. ECcalc is useful for calculating the baffle return wave length and. Here's some graphs showing the changes of pipe power from different baffle angles. Take note of the last graph which is from a baffle with 3 cones of increasing angle. This is best for enduro bikes because its peak happens 7.
RPM earlier than the 1. All that really matters is the return waves. I will speak in regards to them. If. the diffuser wave peak strength is too weak in relation to the baffle. I had a pipe do that on my 1.
It ruined my riding experience until I redesigned. I wouldn't recommend allowing that to happen. If your ride has. ECcalc. recommends on sheet 2) by increasing the stinger diameter. Stinger/Silencer.
The stinger acts as a bleed off valve for the pipe. It controls the. amount of average pressure in the pipe.
More pipe pressure adds to the. So too narrow or too. The pipe. pressure caused by the stinger increases with RPM so that the highest. RPM which is why it hurts the diffuser wave the most.
RPM. I never believed that the length of a tube would have much affect on. I blew into two tubes of the. Then I looked for a good flow. We can use these input. GPM=cc above exhaust port (or 7.
RPM), length of. stinger in feet (mm/3. P. viscosity for exhaust, no couplings, results in a calculation of psi. Experimenting with stinger. I concluded that a good range. Excel file. does a good job of calculating diameters and lengths to approximate. Although the formula in my Excel file now calculates the. For more on stinger.
For example my Suzuki 1. Excessive exhaust restriction. If the stinger originates at the side of the belly, and not the end of. Although my. computer mic doesn't capture the sharpness of the exhaust note exiting. Gordon Jennings wrote that a belly stinger adds a little bit of mid range power without losing any high RPM power. His belly stinger was the stinger pipe stuck up into the pipe so that its beginning was at the same distance from the piston that the beginning of the baffle was.
Just be sure to weld a little support connecting the inner stinger to the belly so that it won't break its weld from being moved up/down by you hitting bumps at speed. Click. here to see a belly stinger . I cut off the previous stinger and had it welded to the belly.
Then the end of the baffle cone was welded shut. Click. here to see the end product. Notice that the. - 5psi diffuser wave is reduced to 0 psi by the 5 psi back pressure: My experience tells me that if the intake of an engine is restricted with too small a carburetor or too small a reed valve then it's best to have a big diameter stinger for no back pressure.
But if the carb and reed are good then a little back pressure can give you a little more horsepower. How I analyze a reed valve is by using my fingernails to hold open the reeds and measure the opening height and width (which multiplied together gives the area). The trick is in estimating how much pulling force happens at the reeds as the piston rises to create a vacuum.