Queston about Sub Piston Induction

I was wondering about the application of sub piston induction. I know that some had it and some didn’t so what was the thinking? If I understand it – the sub piston induction would allow additional air into the case thereby boosting the pressure when the piston came down?? More performance?

Inquiring minds want to know.

Thanks

True enough. But if you use a muffler, hot exhaust gasses are forced into the crancase, causing loss of power and overheating.

George Aldrich told me of comparing the same engine, running with a muffler, with and without subpiston induction, and finding no difference in performance. It is generally said that subpiston induction is not good with a muffler.

A little off topic, but I have been running a Tower 40, with muffler, in my OTS Easy. Engine starts readily and runs to suit me. However, the head and rear of the cooling fins are covered with dark castor crud. Also, there is oil on the wing leading edge (profile mount) after each flight. Finally took the engine apart to see what gives. As far as I can figure, there is exhaust coming up between the cylinder and the crankcase and escaping between the top of the case and the lip on the liner. No evidence of a leak from inside the cylinder. Very strange.

>George Aldrich told me of comparing the same engine, running
>with a muffler, with and without subpiston induction, and
>finding no difference in performance. It is generally said
>that subpiston induction is not good with a muffler.(snip)
>Jim

Interesting on this. You can usually drop the liner on an FP engine about 7 to 10 thousandths beyond "normal" before getting to sub piston induction. I dropped one a little beyond this once and truthfully could not tell any difference once everything else was set properly. It ran fine with a muffler. I recall Lew Woolard did a little experimenting with dropping "too far" with a Fox and found it still worked fine as well. Perhaps it is not as bad as previoiusly thought by some.

Just remember, there is a difference between a muffler and a pipe. Randy has mentioned to me several times that he has received engines back where the pipe was set too short causing the shock wave to come back too soon with the result of burning cylinders and cases. And this is without sub piston induction. A good muffler will have minimal back pressure and negligible effects on small amounts of sub piston induction.

I had the same experience with an Iron-piston FP 35. It ran just fine with about .003" of SPI. I was using one of Leonards tonque mufflers.

Sub-piston induction isn't used much these days, except for the Cox TD engines, where the bottom of the piston uncouvers the exhaust port for about .005". Early spark ignition engines like Super Cyke, Orwick, Anderson Spitfire, used this by holes in the the crankcase and sleeve. The idea was to neutralize any residual vacuum left in the crankcase when the piston was near the top. Others have said that this residual vacuum was due to the venturi being too small for the crankcase volume.

I once had a similar conversation with a friend in England, and so I taped the SPI holes in the crankcase of an Orwick 64 and ran it both ways, using the same fuel and prop. I could find no difference, but with a spark motor, it is difficult to duplicate the exact spark timer setting (and maybe I should have tried to start it with the spark timer fully advanced--but I didn't).

LAST EDITED ON Apr-02-04 AT 04:39 PM (CDT)
 
All of my Super Tiger 'G' series in the .65 abc RISE CL Speed, all expose a thumbnail width of space at the bottom. Some of the other version ST's as well. William

Way back when, wasn't there an engine review article of the G21/40
which suggested that, because of the sub-piston induction designed
into that engine, a muffler would rob it of some power?

I seem to remember something of that nature, though it is entirely
possible that was only my imagination at play.

The man that makes the very nice Super Cyclone reproductions.
States that the sub-port inductions holes are worth 300rpm over the
motors that don't have them. His engines are run on ignition and are
used in SAM events.

Ron B.
F4Fguy

On an open face engine,such as a Cox or many of the old time sparkers,or on a side port engine where intake timing is compromised,sub-piston induction probably acts as supplemental intake porting,and could easily result in added performance.Modern rotary valve timing isn't limited by the piston travel and consequently incoming air is limited only by the pumping efficiency of the engine.With a short pipe the wave returning too soon with sub piston induction would in all probability,kill the engine or at best,seriously degrade engine performance due to dilution of the case mixture with exhaust gasses.On the other hand,the probable effect of S.P.I.with a muffler would be to suck mixture from the crankcase* due to the inertia in the exhaust gasses causing some degree of power loss.Unless we're talking very large time-areas,there's simply not time for the back pressure to have much effect.All in all I'd expect very little detectable effect under normal operating conditions.

*this could also occur with a mal-adjusted pipe.