[Diy_efi] Venturi effect crankcase breather

[arnie]

----- Original Message -----
From: "Adam Wade" <espresso_doppio at yahoo.com>
To: "List for general do-it-yourself EFI talk" <diy_efi at diy-efi.org>
Sent: Wednesday, July 09, 2003 1:22 PM
Subject: Re: [Diy_efi] Venturi effect crankcase breather


> --- arnie <arnie_ at charter.net> wrote:
>
> > More than a fairly small drop in pressure..., how
> > small of a drop in pressure?
>
> We did not discuss actual numbers, as he was not at
> liberty to do so.  He did indicate that running an
> open exhaust on a motorcycle would allow one to exceed
> that level of pressure drop.  How much it will alter
> the life of the engine is anyone's guess, and I am
> sure it depends on the materials used in a particular
> engine.
>
As a comparo, the electric motor driven vacuum sources for the automotive
aftermarket are generating 12-15 in.Hg. of vacuum.
This has shown to be an asset in competitive drag racing. :shrug:
>
> > Being closer related to the motorcycle industry, I
> > reason that you would be more familiar with this.
> > Though it is referred to as a 'compression' ring,
> > it's main function is to control oil.  A one
> > compression ring (along with conventional oil ring)
> > has been used successfully for many years
> > (decades ?) especially in the motorcycle camp.
>
> It's been used almost exclusively on two-stroke
> motors, where there is no oil control needed (when
> using crankcase induction), and friction is a huge
> enemy.  Very few four-stroke bikes use only one
> compression ring.
>
> > OEM (as well as aftermarket) has been prescribing
> > (and using) larger gapped second rings for a decade
> > that I am aware of.
>
> What constitutes a "larger gap"?
>
If one were to use .014" for a top ring gap, .020" could easily be
used for the second ring gap.
>
> > This aids in the compression sealing of the top
> > ring, by preventing pressure buildup below top
> > compression ring.
>
> Where is this pressure coming from?  If both
> compression rings have the same sealing
> characteristics, then there should never be a build-up
> of pressure between the two.
>
Why would you expect the sealing characteristics to be the same?
The top ring lives/operates in a much harsher (heat) environment,
so it's job is much tougher by comparo.  Any seal leakage past the
top ring that is sealed by a efficient sealing second ring will induce
a pressure buildup below the top ring.  Any pressure, hench the
reason a constant vacuum in the crankcase aids in sealing, below
the top ring will have an adverse affect on it's ability to effectively
seal.  By relieving the second ring of any sealing (compression)
responsibility to that of oil control alone, the top ring has fewer
negatives to overcome in it's role as a compression seal.
>
> Further, since compression pressure is what seals the top
> compression ring, and it is absent from the second ring, the
> second ring should not seal nearly as well as the top ring,
> regardless.
>
This implies the second and top ring are of the same material,
shaped the same, react the same when exposed to gas pressure,
AS WELL as react the same to wall containment.  This is not the
case to any of the above.  For example; where is chamfer
positioned on the top ring in relation to the second?  Why is
positioning on the piston critical to effective (as designed) sealing?

No, it (second ring) won't seal AS WELL.  This does not imply it
does not seal at all.  It was not stated that it would take an equal
amount of pressure from below, as above, to affect adversly, top
ring seal.  This has been verified by engine builders for circle track
competition.  No test results available.  :shrug:

GAS




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