Reed vs Rotary - What would YOU choose？

Reeds vs Rotaries, something that's long been discussed in historic karting. What's the difference? What's better? Is either better? Let's have a chat, let's find out. We've got a rotary valve engine. You can tell it's a rotary valve because the inlet is on the side. Saying that, not all rotary valve engines have the inlet on the side. Reed valve engine on the left here. Inlet for a reed is always at the front on a kart engine. What's the difference between the two and how do they perform differently? The rotary valve engine, as the name describes, there's a valve within here that rotates on the crankshaft. That opens and closes and that is what controls the inlet of the fuel on the engine. On the reed valve engine, you've got a pair of reeds within this section of the engine. Think of it like on a wind instrument like a clarinet. The reed, as the air moves through, the reed opens and closes. What does the reed actually look like and what does the rotary valve actually look like? I've got a rotary valve here, so this is what it looks like on the inside. Obviously, this one's damaged. It shouldn't have this score in here. It should be completely flat all the way across. This is inside. This rotates with the engine, opening and closing. As you can see, while I'm rotating this, you can see it would cover the inlet and then open the inlet again. This is an important tuning part of a rotary engine. You can change how much of it is open or how much of it is closed. This affects where in the rev range the engine is making its power. As a general rule of thumb, the more closed it is, so the bigger this section of the disc is, it will have more lower RPM power. The more open it is, so the more of this section of the disc, in which case this will be cut back on both sides, would give you more high RPM power. What does it look like inside a reed valve engine? Inside the reed engine, you would take this piece off, come off like this, and then inside you'd have the reed block and then you have a stuffer. Now, the reed block, these are the reeds. You've got one on each side sitting in a pyramid. They're actually facing that way in the engine, but as the air is passing through, these reeds are opening and closing. It's like a variable intake timing, but there is still tuning that can be done on it. With the reeds, as they're flexing, they're doing in time with the engine pulses. The thickness of these reeds can make quite a big difference to the performance. In a similar way to how adjusting the valve on a rotary valve engine adjusts where the power is within the rev range, the more flexible these are, the better they're going to work at low RPM, but in turn, you lose some of the high RPM performance. The stiffer these are, then you're going to lose some of the low RPM performance because it requires more energy, more pulses within the engine to be able to force these open. They will move a little bit slower and you'll lose a little bit of low RPM performance because when these are very soft or very thin, when you get to very high RPM, there's not enough resistance for the airflow. Rather than opening and closing in time with engine pulses, they actually start to resonate, don't fully close, don't necessarily fully open. Think of it like on a four-stroke engine, you've got the valve that is going up and down against the spring. If it's trying to do that too fast, the spring starts to act like a slinky. Some of you are old enough to know what a slinky is. A child's toy used to spring them down the stairs. If it starts to act like a slinky, it's out of time with the engine pulses and you lose performance. A stiffer set of reed petals helps with high RPM performance, but like everything, it's a compromise. You gain in the high RPM, but you lose some in the lower RPM, so it's finding that balance. Now, if you want to look at the rotary valve in a bit more detail, as we said previously, inlet is here, so the carburettor is bolted onto here. You can see the disc as I rotate the engine. You can see the disc is rotating with the crankshaft. To access this, firstly, we need to take off the engine sprocket because that stops us taking off the rotary valve cover, so we'll do that quickly. Now we have the engine sprocket off, we can take the bolt out of the rotary valve cover and gain access. Now we can slide the rotary valve cover off, being careful not to damage the gasket. Here we see the rotary valve. If I rotate the engine, you can see the valve rotating. You can see as it covers and opens the inlet, so we'll pop that off. Now, on this particular engine, this is a Rotax DSE. It mounts on a hexagon drive that's here. As mentioned earlier, these valves can come in different opening and closing versions, so if we compare it to another valve, you can see here the alternative valve that we have here is a lot more closed. Where this goes in relation to the crank position is very important. Get it wrong, the engine simply won't run or will perform very badly. Now, there is one very easy way as a rule of thumb to do this if you're at the track. That is to move the crankshaft until the piston is at top dead centre. So we can do that very simply by looking through the inlet hole and you can see the conrod through here. Get the conrod so it is completely straight up and straight down. Get your valve onto the hex, so it wants to be at, if you imagine this as the clock face, you want it at about 350, so 10 to 4. Do that as a general rule of thumb, that gets it back in the right place. It depends on the mounting. Some engines have a spline mount, in which case you can be one spline out and it makes a big difference. If you're doing that type of mounting, you need to be a lot more accurate with your measuring to find top dead centre. You probably need to use a degree measuring tool on the crankshaft to get the opening and closing positions as you want them. One thing that is very important with reed valve engines is the condition of the reeds themselves. They can start to break away on the corners or they start to chip along the edges. They can actually break all the way across. Now the main reason this happens is from either ingesting something through the air box that goes through the carburettor into the engine and it's just an impact with this. The other way is often through over choking of an engine. So occasionally we need to replace the reeds on this. Certainly we need to check them regularly. Now the easiest way to check them is to remove it from the engine, look through from this side. Now when we're checking them we're going to be looking for gaps down in here, seeing if there's a way that air can pass through here when it's not supposed to. The easiest way to do this is to hold it up to the light. You'll see very easily if something is not right, you'll see daylight through the edge. Now these ones are still in good condition. I'm pushing this open with my finger so you can see, but it gives a good example of what you would see. Now we've spoken about the reed block and the reed petals, let's talk about the reed stuffer. Now the purpose of the reed stuffer is firstly to mount your carburettor onto here, onto the two studs. The other purpose it serves is on the back side of it there is what is effectively the part that is actually the stuffer. Now this fills up the volume within the reed block itself, can fit together as such. The reason we might want to fill up some of the volume within the reed block is firstly to help direct the airflow and secondly with a two-stroke engine, because all of the air that enters through the inlet goes into the crankcase, you have two types of compression within a two-stroke engine. One is the compression above the piston but you also get some compression below the piston. Now varying the volume within the crankcase is one way of varying the engine performance. So on to fitting these back together, we have a gasket that goes on the reed block, we have a gasket that goes on the stuffer, pop that onto there, then the block and the stuffer go together. Once that's lined up you can then insert it onto your engine and put in the four bolts. So now you know a bit more about the two 100cc engine types, the reed valve engine and rotary valve engine, which one would you choose?