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Sub Piston Induction. (SPI)

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Old 06-17-2010, 07:25 PM
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Default Sub Piston Induction. (SPI)

Hi,
I noticed on the Jett Engineering web site in the FAQ section its stated that almost all of their engines have SPI and although this does not benefit power it does make the final design more compact, lighter and stronger.

Just wondering why this may be the case?

Thanks.
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Old 06-28-2010, 05:44 PM
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Default RE: Sub Piston Induction. (SPI)


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Hi,
I noticed on the Jett Engineering web site in the FAQ section its stated that almost all of their engines have SPI and although this does not benefit power it does make the final design more compact, lighter and stronger.

Just wondering why this may be the case?

Thanks.
Ihonestly did not expect an answer here as sub piston induction seems to be more of a black art than anything else.

Oh well.

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Old 06-30-2010, 08:33 AM
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Default RE: Sub Piston Induction. (SPI)

Sorry for the delay.... had a bit of time away from RCU... catching up now...

SPI is not really a black art. Many engines use it. Simply helps with fuel transfer and utilizes the energy of the moving system a bit better. There is also a benifit of having the piston a bit lighter too, and less mass makes it all run smoother.

Bob
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Old 07-02-2010, 12:20 AM
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Default RE: Sub Piston Induction. (SPI)

Hi Bob and thanks for the reply.

I honestly don't know of many modern engines that use SPI due to the fact that induction systems work so much better than years gone by and so I was some what surprised that your successful range does!

And I can well see that the reciprocating mass would be minimised by having the lighest piston possible and this would in turn make the engine smoother and easier to balance.

I was under the impression that the use of any enclosed exhaust system would greatly effect how SPI in that area would work, by this I mean that if indeed the low pressure area below the piston can induct then it can only breathe in what is available to it and that surely would be a spent charge.

This is what puzzles me and why most engine constructors who have to use mufflers or exhaust pipes steer clear of it like the plague. They state that they rely on the intake port entirely for that function and that SPIis big 'no no' because it dilutes the incoming fuel/ air charge in the worst possible way and somewhat takes away the functionality of the carburettor.

But obviously it works for you guys!
I am left thinking that either there is no sub piston inducting going on at all or the little that would be apparent at high revolutions within the milliseconds of such a small port dwell time would be inconsequential, exhaust fumes or not.

Cheers.
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