[Diy_efi] Venturi effect crankcase breather

[Matt Porritt]

On 10/7/03 6:40 PM Perry Harrington wrote

> Well, if you wanna get technical...  An engine does not "suck", the atmos=
hpere
> pushes.  You will have less than atmoshpere when the path between atmosph=
ere
> and an air space has a restriction.

Smarty pants! :)
It was intended as a 'suck, squeeze, bang, blow' example for someone that
could be... Well.. I'll leave that bit out.

>=20
> At idle, the butterfly is the primary restriction.  If your crankcase bre=
ather
> is plumbed into the plenum of an engine, the registering of vacuum depend=
s on
> where the restriction is.  Most bike engines use IRTB setups, which puts =
the
> vacuum after the plenum.  A bike plenum will ONLY see less than atmoshper=
ic
> pressure if there is a restriction in the inlet to it.  Many of these eng=
ines
> built in the last 20 years do not have much restriction, even less in new=
er
> engines.

'If' being the operative part here. (also position of throttle body in
relation to plenum)


>>=20
>> If you put the crankcase breather one of these parts its gets 'suck' on =
it
>> also!
>> Not quite magic, but a vacuum gauge easily shows there is vacuum.
>>=20
>=20
> Only if there is a) restriction to the airbox b) butterfly before plenum
>=20
> The crankcase breather evacuates because of pressure buildup within the
> engine,
> which is higher than atmospheric.  These are routed to the plenum for
> emissions
> reasons.  Every race bike I've seen hangs a K&N filter off the crankcase
> breather
> and dispenses with the plenum in favor of individual filters.  Why?  Beca=
use
> then all of the world's airspace becomes your plenum.  The plenum is larg=
ly
> there for intake tuning on the bottom end, to smooth out the otherwise ro=
ugh
> intake pulses.

Also oil vapour in the intake charge is detrimental to fuels antiknock
characteristics

>=20
>>=20
>>> Measured at the plenum?  If it's properly designed, it
>>> should barely register.
>>=20
>> At idle it will register quite high actually
>=20
> Application specific.  None of my bike engines will register less than
> atmospheric
> pressure in the plenum at idle.

Once again, dependant on when your TB is.

>=20
>>=20
>>> Are you suggesting that car
>>> engines have a completely different set of dynamics
>>> from car engines?
>>=20
>> No I'm not suggesting car engines have completely different sets of dyna=
mics
>> from car engines...????
>=20
> Obviously car vs bike.  Yes, they do seemingly have different laws.  Only=
 when
> you get into formula 1 car engines, do they overlap.
>=20
> I don't suppose you've seen Honda produce a 1200cc Civic engine with 200B=
HP?

Yes I have. That=B9s the sort of work I do daily. 1200cc Civic? They havn't
been built for over 20years! (well.. JDM anyway)

>=20
> For that matter, the engine getting 42MPG while cruising and still making
> 166BHP
> per liter?

Little bit optimistic on the gas there I think ;)


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