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RD 266

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BlueSmoked:
1976 RD250C

Not the beautiful, but I like that I can hop on and ride! Technical and safety first!

Engine rebuild in 2019
56mm R28 Cylinders
DT125 Pistons
0,35mm polini membranes
Homemade exhaust for more mid range performance. Now ready to make new ones for more top performance.
Tyga short exhaust muffler

Electric wire harness
YSS rear shocks
Racetech front springs 80+ kg, fork filled with 10W oil
Taper steering bearings

Fitted the new rear shocks and stiffer front springs combined with the steering bearings gives much more handling on high and low speed corners. Even when there are bumps on the road.

https://www.youtube.com/watch?v=oRqxFLGbKek

robrd:
Sounds good, looks good, looks like it rides good, what is there to dislike about the bike. Thumbs up from me!

GAZZATT2:
nice
can you tell more about polini membranes ?  fibre reeds ?

BlueSmoked:
Thanks guys! Bike ride today on dike roads sweet, but need some little more top end power to make it finish!

Yes fiber reeds GAZZATT2. You could get Polini membranes as sheets and cut them by your self. Easy to do and deliverable on different sizes, I saw a shop that sells 0.25 0.30 0.35 0.45 mm thickness.

Before I could consider which one I would take, and to backward test the expansion chamber ASC1.0 and ASC1.1(now with larger outlet pipe to muffler(15->26.5mm) and design new expansion chamber ASC2.0 (more top performance, little bit less mid-low range) I used software ENGMOD2T.

Pictures are from the ASC1.0 version to compare as baseline the original steel membranes. What make the difference in weight of membranes and stiffness to 1st graph the power curve. 2nd graph intake pressure curves and 3nd the exhaust pulses. All from 6500 rpm where the difference between baseline is at his largest.

All simulation data from one cylinder! So double power curves for engine!

What it makes on real street, that is not day/night difference comparing with suspension upgrades(!). Maybe dyno would measure a bit of gain.

BlueSmoked:
Last year I did a dyno test with the original mufflers and with oem membranes, running on E5 gasoline.

28.4 hp on the wheel. Efficiency of the geartrain?? I don't know.



Al simulation on ENGMOD2T are pure theoretical. Put in a lot of dimensions of cylinder ports and intake volumes etc. Could play with theoretical setups.

The software could set you a hint of needed expansion chamber on Blair design basis. For ASC2.0 expansion chamber I used now this software before an other. Now I want more top performance. ASC1.0 was designed for a okay mid-range and top performance at higher rpm's 8000. But practical the engine runs to a "wall" when driving to max 7000 rpm. Where 6000-7000 rpm is a little bit flat.

The muffler broke off the bike with version ASC1.0, so for ASC1.1 I modified it to weld the muffler connection direct to the baffler (26.5 inside diameter over 15 mm) to have more meat to weld on.

Black power line is ASC1.0 , red ASC1.1 with larger outlet pipe

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