Hi All,


I'm struggling to set up a OS 91 Fx with a baffled tuned pipe. I'm not sure how long it needs to be.
THe formulae for calculating the length all require exhaust port duration in degrees and well surprise surprise that information is not given in the manual.

would be greatly appreciated yif you could assist with the exhaust port duration or length of tuned pipe required for this engine.
My header is 29cm from the centre of the piston or 27.5cm from the exhaust flange.</p>

I used to run 56 cm from exhaust flange on the engine to first baffle in the pipe for the Hatori
# 700. Using 15 % nitro and APC 15x8 prop. Static RPM around 9500. On a real hot summer day you might run 60 cm.

The OS 91FX runs better with an extra head shim and the OS # glow plug.


Hope this helps.
Henning

Sorry

OS # F glow plug

Thanks Henning, its a great help. In fact you have helped on quite a few issues since I was also in the wildeerness on prop choice. My LHS told me 15x8apc on a tuned pipe but some of my pattern colleagues say 14x8.
Your suggested set up is sort of where I was leaning toward with the OS F type of Model Technics 3d PLUG
thanks

MyOS 75 AX runs quite well on a 14x8 with a pipe on 15% fuel, so i would think the 91 would easily turn a 15 inch prop.
just my 2 cents worth
Dave jr.

it does. The engine is not even run in yet and it turns a 15x8 at 8500rpm with unlimited vertical on 10% nitro and Model echnics 3D plug.
Pipe + header lenght set to 56.5cm.

Bloody thing won't idle though. Guess it needs more running before it settles.

So far it meets the performance criteria