Thread: Reed function 101...........
09-03-2007, 04:47 PM #1
Reed function 101...........
This is a good article for those who want to understand what a reed does and the performance gains they offer....
Two-stroke engines do not have intake valves like the four stroke engines in your car. Therefore, many two-stroke engines use reed valves to control the intake of the air/fuel mixture. Selecting a set of reeds for an engine is important for optimizing the performance of that engine.
Fitting inside the crankcase and below the carburetor, a reed valve is typically a wedge shaped device with openings on either side that are covered by reeds made of steel, fiberglass or carbon fiber. The reeds control the inlet of air/fuel by acting as a one way flapper valve. When vacuum in the crankcase is sufficient, the reeds flex open allowing the air/fuel mixture to rush into the crankcase. Then the reeds slam shut trapping the air/fuel mixture in the crankcase. This process is then repeated for each cycle of the engine.
Because the reeds control the amount of air/fuel mixture entering the engine, choosing the best reeds is important for getting the most horsepower from the engine. Too much or too little air/fuel entering the engine during each cycle will cause a loss of performance.
Many tuners and racers talk about changing the thickness of the reeds to adjust its performance. For stock and lower RPM motors generally a thin and flexible reed is chosen and for modified and higher RPM motors generally a thicker and stiffer reed is chosen. This is because at higher RPM, more air flows through the engine so a stiffer reed is required to control that airflow. Try blowing on a piece of paper versus a piece of sheet metal. However a thicker reed is not necessarily a stiffer reed. In general thicker is stiffer, but more on that later. An engine cannot sense how thick a reed is. An engine knows only two things about a reed: how much air will flow past it and how quickly it responds to the engine.
The quantity of air that will flow past a reed is primarily determined by how big it is, which is determined by the size of the reed cage. The second most important factor is how flexible the reed is. A flexible reed will flow more air than a stiffer reed at lower RPMs increasing low and midrange acceleration. But at higher RPMs, this flexible reed will flutter thereby no longer controlling the airflow so a stiffer reed will be required. A stiffer reed however will flow less air at lower RPM thereby hurting low and midrange acceleration. Multiple stage reeds have been produced to overcome this tradeoff by combining a stiff reed with openings in it and a flexible reed on top of the stiff reed to cover these openings. Notice that we have said nothing about the thickness of the reed. Most tuners talk about thicker reeds when they are actually referring to stiffer reeds. For most metal and fiberglass reeds this is generally true; thicker reeds are stiffer. But with the newer carbon fiber reeds, stiffness can be increased without increasing the thickness. Why would we want to do that? This leads to the next concern for the engine builder: how quickly the reed responds.
The faster the reed responds i.e. opens when it is time to open, the more air/fuel mixture that can enter the crankcase before the reed closes and stops the flow of air/fuel. The response of a reed is primarily determined by how much the reed weighs. Basic physics says anything lighter will accelerate faster than something heavier. Now your basic reed weighs only a few grams so it seems the weight would not matter. However an engine at full throttle running 7000 RPM requires a reed to open and close 7000 times per minute. This is quite a significant amount of accelerating and decelerating in a short amount of time so a few grams can make a huge difference.
When choosing a reed for a particular engine we are therefore looking for the proper stiffness to operate at both low and high RPM and we are also looking for the quickest response we can find. That is one reason most engine builders switched from steel reeds in motocross engines to lighter weight fiberglass reeds back in the 1970's.
Since carbon fiber weighs less than fiberglass or steel it is naturally an obvious choice for reeds. The rider will notice this first off as quicker throttle response. A further benefit is that carbon fiber reeds can be made stiffer yet not any thicker. If we have a reed than is thinner but still has the stiffness that we desire, we have further decreased the weight of the reed leading to more responsiveness and therefore more power.
In this situation a carbon fiber reed has a great advantage. With a highly modified engine running at higher than normal RPM requiring a stiffer reed, a thin and stiff carbon fiber reed will increase acceleration, peak power and allow the engine to perform at its true potential. The same is true for stock to limited motors but to a lesser degree.
A further consideration in reed choice is what engineers call the fatigue properties of the material. As any material is repeatedly stressed and unstressed (like bending a paper clip back and forth) it loses some of its strength. After a large number of cycles of stressing and unstressing, steel loses 70% of its original strength, fiberglass 45% and carbon fiber only 30%. A material that loses less of its strength will have more consistent performance as a reed. Many fiberglass reeds tend to get soft after a month or so of riding and begin to lift up off the reed cage. In motorcycle racing, tuners usually change fiberglass reeds every moto to ensure consistent performance.
Another factor in choosing a reed is its potential to damage the engine in event of a breakage. Very rarely will any reed last forever. A some point in the life of the engine it is very likely that a reed or part of a reed will break off due to a backfire, overreving or just simply a wearing out of the reed material. When this happens the reed will get sucked into the engine through the cylinder and hopefully out the exhaust pipe. A carbon fiber or fiberglass reed will normally break up into little pieces without doing much damage but steel reeds have been know to score cylinder walls, break piston rings and even destroy main crankshaft bearings.
Ideally however we would like to have a reed that is unlikely to break or at the minimum have a long life. It is very difficult to determine which reed material last the longest because of the wide range of quality available from the different manufacturers. High quality fiberglass reed or OEM steel reeds typically have a sufficiently long life for recreational riders but some racers have had to change them every one to four races to prevent failure during a race. High quality carbon fiber reeds have been known to last a minimum of four races and up to a full year including practice time. Lesser quality reeds however have been known to fail after one month or as quickly as fifteen minutes.
The wide range of quality is due to a number of factors. The biggest factor is the quality of the raw carbon fibers, fiberglass and the resins used to mold the reeds. For example thinner, finer grades of carbon fiber out perform thicker grades but cost three times as much and more of it is required to achieve the same thickness. A smooth edge on the reed is also critical so that no stress concentrations are formed where cracks can start.
One factor concerning reed quality has some controversy: curvature of the reed. Stock reeds but few aftermarket reeds are made with a slight curve to help the tips of the reeds seal on the reed cage. Most aftermarket reeds however are made from flat sheets to save cost. This leads to the reeds lifting up from the reed cage, especially fiberglass reeds due to their softening over time. Many tuners fear that the reeds are then not sealing properly. But reeds never really have a chance to seal. When an engine is running at 6000 RPM, the reeds are slamming shut on the reed cage 6000 times per minute. That equals 100 times per SECOND! The reeds are opening and slamming shut so fast, they never really have a chance to sit there and seal. If the reeds are sitting open 1/16" or more there can be some slight loss of bottom end acceleration and/or a leaning of the low speed air/fuel mixture. Rejetting or flipping the reed over can help alleviate the problem, but the best solution is to replace the reeds with a new set.
As we have discussed reed technology is more complicated than it appears at first. Reed material, stiffness and manufacturing techniques all need to be considered in an effort to find reeds that will make the most power and have a long life.
09-03-2007, 08:32 PM #2
How Reeds work function Reeds How-to
This is in an excellent post K-dawg !!!
If I may add some visual aids to this post !!
Here is another link too !!
09-03-2007, 09:02 PM #3
09-03-2007, 09:07 PM #4
Broken cracked chipped reed symptoms
also, getting a good look and understanding on how a two stroke works can really illustrate how damaging a backfire can be in a 2-stroke. the reeds see the direct expansion/hit of the combustion during the explosion. reed valves must seal the crankcase pressure or the mixture will be pushed back through the intake (carbs) and then "spitting" fuel outward.
- Reeds are a one way check valve,in a nut shell.
- backfires can split/chip/crack a reed. the symtpoms can be and will vary as the degree of failure.
- Split or chipped reeds will cause a running issue at higher speeds, not too much a lower speeds.
- Broken petals will cause low speed missing and no running at higher speeds.
- Weak reeds will make a motor run great at low speed but limit the rpms the motor will turn.
- A damaged reed will not cause a backfire, it can do what is known as a lean pop through the intake.
09-03-2007, 09:09 PM #5
09-03-2007, 09:17 PM #6
Great posts. I don't even own a Yami, but I enjoyed the 411.
09-03-2007, 10:01 PM #7
- Join Date
- Oct 2006
fantastic post woody, everyday i am here i learn a little more thanks to all of you.
11-30-2007, 01:23 AM #8
Jmo. It does give gains to have a reed design to flow more mixture as how your post demonstrates nicely. But I would say that the most important factor for building high rpms is how well the reeds seal on the downstroke of the piston. This is where primary case pressure is built which is a determining factor on how fast the fuel can be pressurized and pressed on top of the piston for the intake stroke.
If you took some reeds that are slightly worn and do a pressure test of the motor without the reed cage being blocked off. The motor would not hold pressure very long if at all. Now put a fresh set of reeds on and do the same test. You will find that new reeds hold pressure better.
Because case pressure is the key factor in fueling the piston at higher rpms.....the more pressure you build the more mixture you put ontop of the piston before detonation. The most important function of the reeds are their sealing properties. More power can be saved here than almost anywere. This is the reason that seadoo could build more hp out of less cc. Because of the rotary valve which seals totaly except for the intake stroke.
I would also say that less case pressure is better for bottom end and hole shot. This is why some experience more pull on the bottom but less ontop.
11-30-2007, 04:19 PM #9
12-01-2007, 11:36 AM #10
I have used superglue to seal the reeds to the cages just around the rear edges to completely seal the back halves and help provide rigidity to the reed petel.
WARNING, If you use this method great care must be taken when it comes time to remove the petels: so you dont damage the cages. Although with a little patience and the right solvent this can be done easily.
Last edited by K447; 07-21-2012 at 05:43 AM.
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