[Diy_efi] Venturi effect crankcase breather

[Jack Vines]

Greetings, Adam,

Can you help me better understand what specific factors involved in pulling
a vacuum in the crankcase cause the dramatic drop in engine life and which
parts wear faster and why?  Are we talking ring wear, bearing wear, piston
skirt wear?

thnx, jv.

-----Original Message-----
From: Adam Wade [mailto:espresso_doppio at yahoo.com]
Sent: Wednesday, July 09, 2003 4:44 PM
To: List for general do-it-yourself EFI talk
Subject: Re: [Diy_efi] Venturi effect crankcase breather


--- arnie <arnie_ at charter.net> wrote:

>> 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:

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 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.

> 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. 
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.

> 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.

> 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?

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.

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

__________________________________
Do you Yahoo!?
SBC Yahoo! DSL - Now only $29.95 per month!
http://sbc.yahoo.com

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi


The information transmitted is intended only for the person or entity 
to which it is addressed, and may contain confidential and/or 
privileged material. Any review, retransmission, dissemination or 
other use of, or taking of any action in reliance upon, this 
information by persons or entities other than the intended recipient is 
prohibited. If you receive this in error, please contact the sender, 
and delete the material from any computer. 



_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi