Yamaha pipes

[alvin l nunley]

AL, since I have some of your pipes, what length would you suggest for stock appearing Yamaha/gas and Stock appearing Reed/alky. Clutch slip. Thank You

what pipes do you have? I've made quite a few over the years.
Clutch slip on the KT is about 10,300. That number is pretty close for almost any pipe. There are a couple of exceptions. Of course the direct drive pipe and the Horstman HPV.
Read/alky would depend on the exhaust port opening in degrees. The KT exhaust opens at about 92° ATDC, so if you're exhaust port is higher than that, set it shorter, and lower, set it
longer. A change of one quarter inch is significant.
The reed valve clutch stalls a little lower, the rotary valve a little higher. I've never tested any of those engines with alky, and of course any change in the port timing would change all this information. As a general rule of thumb, a higher exhaust port would tend to raise the RPM where the engine reaches peak horsepower, and in that case, you would want to try a shorter header.
Comments compliments criticisms and suggestions always welcome.

 

[Don K]

I had a person PM me for a better description of wet line tuning. Here is my reply. Maybe it will help others.



Expansion chambers are designed to develop resonant harmonics that allow a fuel charge to be drawn through the exhaust into the header and flex and then jammed back into the open exhaust port before closing. The effect of this is to pack the cylinder with a much denser fuel charge than would be achieved with the 14.3lbs/sq" atmospheric pressure can provide. It is like a simple super charger.

When the exhaust port closes some of the fuel charge is left outside of the cylinder inside the header. This forms a barrier. The cool fuel air charge and the hot exhaust. This forms a visible line in the exhaust header. Shortening and lengthening the flex moves this wet line. Shorten flex and the wet line moves closer to the beginning of the header and aluminum of the exhaust port. The aluminum will get too hot and distort if the wet line is too close. Lengthen the flex to far and you leave fuel air charge outside the cylinder and loose power. I always tuned the flex for a wet line about 3/4" inside the header. This worked very well for us and we never seized pistons. Once you determine flex length for a given pipe just leave it. The guys that try to tune flex at the track for given atmosphere conditions are the dudes that seize pistons.

Google up how expansion chambers work. There is good information on the net.

Don K

 

[arc100]

I had a person PM me for a better description of wet line tuning. Here is my reply. Maybe it will help others.



Expansion chambers are designed to develop resonant harmonics that allow a fuel charge to be drawn through the exhaust into the header and flex and then jammed back into the open exhaust port before closing. The effect of this is to pack the cylinder with a much denser fuel charge than would be achieved with the 14.3lbs/sq" atmospheric pressure can provide. It is like a simple super charger.

When the exhaust port closes some of the fuel charge is left outside of the cylinder inside the header. This forms a barrier. The cool fuel air charge and the hot exhaust. This forms a visible line in the exhaust header. Shortening and lengthening the flex moves this wet line. Shorten flex and the wet line moves closer to the beginning of the header and aluminum of the exhaust port. The aluminum will get too hot and distort if the wet line is too close. Lengthen the flex to far and you leave fuel air charge outside the cylinder and loose power. I always tuned the flex for a wet line about 3/4" inside the header. This worked very well for us and we never seized pistons. Once you determine flex length for a given pipe just leave it. The guys that try to tune flex at the track for given atmosphere conditions are the dudes that seize pistons.

Google up how expansion chambers work. There is good information on the net.

Don K

Thanks Don, I will try this in the future. I assume you need perfectly clean header and exhaust before you start or is it easier to see if there is a bit of black carbon stain so the liquid can retain itself in the cavities of a thin carbon layer?

Does it show itself the whole way down the header or is it only visible 1" or 2" from the header flange at the cylinder?

What i mean is if the pipe is way too long will it show the wet line say 3" down the header?

 

[arc100]

Also do you have to kill the engine at full throttle or just drive it back to pits as per normal? Or is this something only done on the dyno?

 

[Don K]

arc100,

Clean your header with a steel brush. We used one that fit in the drill.

The line is very distinct unless you are too rich and everything is wet.

We always started with the recommended flex length which put you in the ballpark. We were never 3" down the header or into the flex.

I always taught the kids to tune the low speed on the track and to close the low speed to kill the motor.

DK