Reed Block Mod

[millsey]

Can anyone tell me how the shortend reed block mod was done and some pictures if pos cheers

Edited September 18, 2007 by millsey

[baldilocks]

I had a TY done years ago, you need some proper engineering kit though and to be honest I dont think it made that much difference as I remember. You need to machine some of the fins off the back off the barrel to get in and I think it was shortened by 15mm but you had better check. Try and see if you can calculate the space between the end of the reeds and the back of the cylinder. What you need to do is take that space out so that the reeds are as close to the cylinder as possible. To be honest I 'd get the barrel ported by birkett if I was you and I think you will notice much more of a difference.

[rory]

I herd some where an 89 CR125 reeds do the trick ? Any body agree ?

[jse]

I herd some where an 89 CR125 reeds do the trick ? Any body agree ?

I installed an 87' CR125 reedcage, with Boyesen dual-stage reeds, in my 85' TY350 a couple of decades ago after I first saw the 8-petal, steel reed setup that was stock (a lot of the 350 components are based on the big-bore MX engine of that year). The CR cage is a 6-petal and has a "stuffer", or an insert that's a formed piece of plastic, that maintains the charge shape and increases velocity over the jet tower at low slide openings. It increased throttle response and slow speed smoothness quite a bit.

Later I cast/machined a one-piece TY350 manifold/reedcage that was similiar to the later Boyesen RAD Valve and it worked even better than the CR mod. I'll post photos if you want.

Jon

[masnoc]

Hi we can do a 275cc conversion which runs standard set-up and reed cage, that will give a big gain in torque and bottom end power. (Just changing the inlet tract is pointless.)

We are about to launch hole range of parts for the Ty 250/350 mono including pattern Exhaust system, with much improved power characteristics.

pete

[nzralphy]

Just a thought and you are likely to know this, so sorry if i'm out of line.

With 4 or 2 strokes.... to get the best benefit you should match the inlet tract, to the cam profile or 2t porting, to the exhaust system. Not doing so will only give some of the potential benefits you are looking for in your mods. However, as rough rule of thumb - reducing the length of the reed block will move the tuned length (the power band) up the rev range.

[peterb]

Hi Ralph,

I'm not so sure about reducing the reed block (exit to cylinder/crank case) length shifts the power band up the rev range, I reckon the closer the reed block to the barrel or crank case provides improved power through the rev range, particulaly at low end revs. With a reed inlet into the crankcase, for improved 2T primary compression, the crank volume must be kept to a minimum to increase primary compression and therefore efficiency. A shorter distance between the exit of the reed v/v block and the crank case will decrease primary volume and therefore increase efficiency.

Bye, PeterB.

[jse]

Hi Ralph,

I'm not so sure about reducing the reed block (exit to cylinder/crank case) length shifts the power band up the rev range, I reckon the closer the reed block to the barrel or crank case provides improved power through the rev range, particulaly at low end revs. With a reed inlet into the crankcase, for improved 2T primary compression, the crank volume must be kept to a minimum to increase primary compression and therefore efficiency. A shorter distance between the exit of the reed v/v block and the crank case will decrease primary volume and therefore increase efficiency.

Bye, PeterB.

I think Peter's general impressions tend to follow my prior experience. But, I think that Ralph is also correct, but that the reedblock/distance theory tends to be more valid in the area of cylinder reed engines (where you deal with a longer intake tract and resonance issues need to be considered). and is less valid in dealing with case reed engines, which most of the new 2T Trials engines are.

Primary compression ratios in 2T Trials engines are closer to Supercross MX engines (than, say, outdoor MX engines), where a high PCR is needed for instant mid/upper-mid output for corner-to-corner performance. The Primary compression ratio has an effect on port fuel/air mix velocity, and therefore the scavenging of the cylinder, at the cost of a slight decrease in volume (a two-stroke's crankcase compression of the incoming charge is, in effect, it's own "supercharger", unlike a 4T, which relies on ambient air pressure and fuel/air mass velocity).

There are, as in any competition engine, trade-offs, and the high PCR in a SX 2T engine make it unsuitable for regular outdoor MX as the power tends to flatten off fairly quickly due to "pumping losses" as it takes energy to compress the incoming charge in the crankcase (in a Top/Fuel Dragster's engine it's estimated that it takes between 800-1200 HP just to run the supercharger).

Jon