[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 6:44 PM
Subject: Re: [Diy_efi] Venturi effect crankcase breather


> --- arnie <arnie_ at charter.net> wrote:
>
> > 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:
>
> If you don't much care about engine life, then running
> lower crankcase pressures can generate more power.  I
> was just saying that ring sealing does not improve
> past a certain point, and ring life shortens at lower
> crankcase pressures.  Since there are several things
> going on with crankcase pressure and power, it doesn't
> surprise me that there are advantages in other areas
> that "override" certain disadvantages, depending upon
> the application.
>
I'd like to know how improving the top ring seal adversly affects
engine longevity.
>
> I'd also note that the engineer in question said that
> the effect was most pronounced at higher rpms, and was
> not terribly useful at boosting power at lower rpms.
> This would be a concern for streetable engines, as you
> won't get very much benefit from running high vacuum,
> but will shorten the life of your engine.  A bad
> choice, IMHO.
>
Sure, it is gonna be more pronounced at high rpms, at WOT there
is very little vacuum generated, so improving vacuum via outside
source is designed to benefit WOT specifically.  At part throttle, the
engine generates it's own vacuum.  Don't see the concern.  Definate
benefit, and dont' see how it will adversly affect engine longevity
by having good vacuum at WOT to go along with good vacuum at
low to mid rpms.
>
> > Why would you expect the sealing characteristics to
> > be the same?
>
> In cases where the rings are made from the same
> material and have the same edge, I can see no reason
> why they would not share characteristics of behavior.
>
Those cases are in the minority.  Let's concentrate on the majority.
>
> You are correct in what I am reading between the
> lines, as in many cases there are differences between
> the top and bottom compression rings.
>
>>
> > The top ring lives/operates in a much harsher (heat)
> > environment, so it's job is much tougher by
> > comparo.
>
> Why would ambient heat change sealing characteristics?
>  True, the ring has to have different properties to
> last a long time when it is exposed to more heat than
> another ring, but I don't see where it directly
> affects sealing per se.
>
> > 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.
>
> Possibly.  Obviously, there is bound to be a "knee" in
> sealing efficiency based on pressure differential
> between the two sides of the ring.  As long as you
> stay above the "knee", you are in good shape.  Racers
> will be interested in the extra tenth of a horsepower
> at the expense of engine teardowns that happen more
> frequently, but I doubt it would be a useful trade-off
> for most of us.
>
Tenth of a hp?  :)  Expense of teardown?  Don't see how.  What
evidence is there of longevity compromise?
>
> > 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.
>
> In many cases.  Not all, though.  Generally more
> recent designs have differeing ring design for the two
> compression rings, but it was not always this way, and
> is not always this way even today.  I think we are
> basically in agreement on this point.
>
> > No, it (second ring) won't seal AS WELL.  This does
> > not imply it does not seal at all.
>
> Of course it seals, although the primary mechanism is
> spring tension rather than combustion pressure.

???  My point was it (the second ring) seals well enuf to cause
pressure to build below the top ring.
>
> > It was not stated that it would take an equal
> > amount of pressure from below, as above, to affect
> > adversly, top ring seal.
>
> What is "adverse"?  To whom?  Where on the graph does
> the point fall?  Which compromise do you wish to make?
>
Again, don't see the compromise.
>
> For my street bike, I have no interest in rebuilding
> the whole top end every 2000 miles, so I don't run
> drag racing levels of crankcase pressure.

Am I missing something.  Reduction of second ring sealing, with
larger end gaps, has been done by oem auto mfrs. for a decade, as
I noted earlier.  I'd say closer to 200,000 than 2000.



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