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Engine basics => Engine set up tips => Topic started by: Gordon Tarbell on August 14, 2011, 09:00:44 PM
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Is just a wee bit of sub piston induction a non issue when using a pipe setup. I can just barely see the piston skirt rising above the sleeve opening? Just noticed it while fixturing up the engine crutch.
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Well according to Jett engines its no issue at all, I asked the exact same question over at RC Universe and their Tech guy answered it. :)
From their FAQ's -
Question:
When I put my piston at top dead center and look in the exhaust there is a gap below the skirt of the piston. I can see inside the engine. Is this a problem?
Answer:
Almost all Jetts have this gap. It is called "sub piston induction". Many manufacturers have claimed to use this to enhance power. Unfortunately, it does not make the engine run better. However, during the design of the engine a certain amount of sub piston will allow the engine to be made more compact and thus lighter and stronger. Power is not sacrificed, but a strength and weight advantage is achieved.
The sub-piston area will only induct if the normal intake system is inefficient anyway and its most commonly found on ye old time diesels where restrictor spray bars and small crank shaft passages made it hard to get a good lungful of air under racing conditions.
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OK I will label it a non-issue and move ahead.
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Gordon, I kinda think that Cox engines with their very close fitting muffler rings scared fliers off with sub piston induction and any form of exhaust collector as the proximity of bounce back wave played havoc with its tune.
I am guessing here though that any pipe derived wave form would be very out of tune when the sub piston induction actually opens.
Cheers.
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Gordon, I kinda think that Cox engines with their very close fitting muffler rings scared fliers off with sub piston induction and any form of exhaust collector as the proximity of bounce back wave played havoc with its tune.
I am guessing here though that any pipe derived wave form would be very out of tune when the sub piston induction actually opens.
Cheers.
You are right on the target! The sonic wave on a perfectly tuned pipe, will induct pressure into the open exhaust when the piston starts going up/close thereby increasing the "real" compression ratio. It is pref ferably to run a piped engine a tad rich to forecast the right mixture once it "gets on the pipe". This meaning that the engine unloads to speed. Since the timing on the sonic wave changes as rpm increase/decreases, it is a trial and error setting between needle, prop and pipe. We can theoretically get in the vicinity of the distance/length of the resonant wave, but in the end, a tach, experiments and notes will get the best results. The sub induction was meant to equalize the internal pressure with the external atmosphere, thereby eliminating any vacuum when the compression cycle starts. Essentially, it allows the engine to "catch up"...
I love to talk physics... #^ y1
H^^