Sub induction

grampa jack wrote:Thank you for the link! For my particular needs, I believe the non-spi approach will be just fine; I may have to muffle the engine.

grampa jack wrote:I did a search on the RCG site for "sub induction" & got nothing; I may need to brush up on my search skills.

nitroairplane wrote:
It increases performance when there is no muffler but with a mufflit it does more bad than good.

Surfer_kris wrote:

nitroairplane wrote:
It increases performance when there is no muffler but with a mufflit it does more bad than good.

Have you actually tested that?

I think the SPI will help on the reed engines at high rpm, with or without the muffler.

grampa jack wrote:I did a search on the RCG site for "sub induction" & got nothing; I may need to brush up on my search skills.

PlaidHatter wrote:I hope it's ok to bring this thread back from the dead. (Still a noob here)
I just learned about SPI, (did I say noob already?) and after learning about it's performance value, my question is, can you have TOO much SPI?
Like if you were to take this particularly modified cylinder:
(Damn, I forgot I can't post a link yet cus I'm still noobin groovin. But basically a modified cylinder with a lowered exhaust port)
Which has a lowered exhaust port, creating SPI, and then also shorten a piston slightly to give the bottom maximum gap, further increasing SPI during TDC and WOT RPM.  
Forgive me if I'm repeating something that has already been gone over. (Hi. Noob.)

Thanks in advance.

RknRusty wrote:
How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next charge. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is dimished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6' is needed to get to useful RPMs for SPI do have its effect.

**Obviously, the bonus air pressure added to the existing air from the venturi leans out the mixture, so that the needle valve must be opened more(richer) to let in extra fuel to balance the mixture.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the A/F ratio, now you have a higher charge in the crankcase than if there had been no sub piston induction.

Rusty Knowlton
www.coxengineforum.com

PlaidHatter wrote:Looks like you've got a lot of well researched info there, I definitely gotta get to reading. Thanks for the reply!