I am looking at buying the above engine but notice that there is not a standard silencer that comes with the motor. Pe has suggested the MVVS 3246. Can anyone tell me what sort of performance/ rpm I could expect with the Q500 motor, 3246 silencer and an 83/4inch x 7 or 8 prop?

Many Thanks

Stuart

Stuart,


You can expect about 18,000 RPM, after the engine has freed up a bit.
Initially you may have to extend the header a bit (longer silicone coupling) and you will get a bit less.

A 9x6 MA black prop is spun at 18,300 RPM, right after break-in is completed (but this RPM is over the limit Master Airscrew set for this prop).

Thank you for your quick reply. It seems that I may have made a slight mistake. In this country the rules for Q500 state that you must use a standard silencer but is the 3246 not a mini pipe. If so what standard MVVs silencer if any could be used on this motor and what sort of performance could be expected?

Thanking you in advance for any help that is forthcoming.

Stuart

Stewart,


The 'normal' #3245 silencer is not tuned.

Since the engine will not get 'on the pipe' with it, some 2,000 RPM loss should be expected.

the 3246 is by no means a minipipe, but a Helmholtz resonator just like any side muffler or mousse can muffler. The only difference is that like the SuperTigres "super paw" it is constructed to work well with the engine, and not against it.
More power can be had with a fullblown tuned pipe like the 3247

PĂ©,


I believe Svajda said the #3246 and the #3248 were 'folded pipe' type...

Every tuned pipe is a Helmholz resonator by definition.


About the #3247, it was intended for the discontinued #3070F/R GRFT/GRRT engines.

I have someone here running it on an FAI Rossi engine.


Are you sure it can be made to boost power in the 'low' RPM range the Quickie is associated with, i.e. ???

As far as I know, the GRRT/GRFT were in the 25-30K range...

The quickie cylinder is a drop in replacement for these now abandoned engines, and yes, the quickie also works in these high rpm applications if coached to do so. A mini-tractor puller customer of mine uses these cylinders at ~30000rpm to beat the hell out of the competition.

A tuned pipe is much more than just a mere resonator. The only thing they have in common is the ability to bring a pressure wave back to the exhaust port, but a closed off pipe will also do that. Both the black MVVS and the closed off pipe will hardly do more than stop the gas outflow. A tuned pipe will reverse flow in the header pipe and thus push back the expelled fresh charge into the cylinder. That causes a raise the pressure in the cylinder after the scavenging ports are closed, acting like a supercharger or non-symmetrical exhaust port timing. A well tuned engine will have about 120% crankcase pumping efficiency. This can only be made to work with a tuned exhaust pipe.
To illustrate the similarity in principle between the 3235 and 3243 mufflers:
Czech MVVS users have experimented with 3.5cc engines, and compared the 3235 one-chamber muffler with the 3243 black muffler. The 3243 muffler had loads of power from 13000 rpm upwards, but cut off power at about 17000 rpm. That is where the humble 3235 muffle came into resonance and began gathering steam and happily guided the power curve up to 20000 rpm. Best power being had using the 3235 muffler in the high rpm realm. All as a matter of resonance volume and wall thickness (read: header length).
Due to the function of a tuned pipe with it's careful nursing of gas flow and pressure waves, at high rpm even more power would be available than with the 3235 muffler, but low rpm torque would be non-existent.