Difference between Late and Early model Cylinders....

[ja-moo]

Tygrant,
The way to think about reeds is that they allow you to raise the state of tune without all of the harmful side effects.

I would disagree a "bit", as anytime you can keep the complete intake charge in the crankcase, give the motor maximum intake duration and also stop the reversion which results in rich mixtures, it does increase performance and efficiency.

Just fitting reeds won't make much difference to most motors in terms of power. On some it will improve the bottom end. If the reeds are small they will tend to restrict flow and top end performance, and too large and they barely open, so that's not a lot of use either.

I will have to disagree a "bit" here too. Of course the degree of port timing has an effect. On the H2's a few of my customers have found on totally stock bikes, there was a noticable increase in "rideabilility" and low rpm torque. One customer loved to take lone rides and actually went from the stock 15/47 gearing to 16/40, that's a huge jump and not something he could of done without reeds.

Also the reed design has a lot to do with "total" flow. Just about all manufactureres went with a "it works, so copy it" attitude with the "V" block. Which is really not a great design, as anytime you have obstacles in a flow path it causes flow problems. And "V" blocks have that sealing bar right in the middle of the flow pattern.

So, it's not really "only" the size of the reed, but the design also.

As long as the motor can pull in a full charge, (in the appropriate time) the size of the reed area can vary greatly.

So if you intend to tune a motor especially if you want more intake duration from shorter piston or from dropping the floor of the intake port, reeds help to avoid a big loss of tractability.

Reeds do not open instantly, but take a finite time to open and close. Stiff reeds are even harder to open and are better for race motors because they don't flutter as much as less stiff reeds.

Sometimes, smallish reeds that open very wide can flow better than big reeds that don't open as easily. Reed block angle also effects the way a reed flows.

On a TZ race bike reeds cost power but make teh bike easier to ride. A TZ700 makes about 90HP with reeds and with a pair of TZ350 cylinders with identical porting it makes about 115, but it's almost unrideable.

That's part of the "V" design (and one teh) and the use of mismatched reed area. There should not be as 25 hp loss with reed valves. With the proper sized reeds, maybe around a 5hp loss is achievable.

It's not the difference in top end that makes the difference, it's the way it comes on the pipe - the sudden transition from inefficient to efficient at lower revs.

But in a high performance two stroke that's rideable, reeds are a must.

It all comes down to porting and intended use.