[Diy_efi] Venturi effect crankcase breather

[Perry Harrington]

On Thu, Jul 10, 2003 at 06:16:10PM +1200, Matt Porritt wrote:
> To put it simply for you:
> 'A' (ANY non force fed engine) 'sucks' on its inlet stroke right?
> If into the inlet tract there is 2 paths to atmosphere (ie simple Y pipe)
> then when the engine 'sucks' on its inlet stroke they will both have 'suck'
> on them right?

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

At idle, the butterfly is the primary restriction.  If your crankcase breather
is plumbed into the plenum of an engine, the registering of vacuum depends 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 atmoshperic
pressure if there is a restriction in the inlet to it.  Many of these engines
built in the last 20 years do not have much restriction, even less in newer
engines.

> 
> 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.
> 

Only if there is a) restriction to the airbox b) butterfly before plenum

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?  Because
then all of the world's airspace becomes your plenum.  The plenum is largly
there for intake tuning on the bottom end, to smooth out the otherwise rough
intake pulses.

> 
> >Measured at the plenum?  If it's properly designed, it
> > should barely register.
> 
> At idle it will register quite high actually

Application specific.  None of my bike engines will register less than atmospheric
pressure in the plenum at idle.

> 
> > Are you suggesting that car
> > engines have a completely different set of dynamics
> > from car engines?
> 
> No I'm not suggesting car engines have completely different sets of dynamics
> from car engines...????

Obviously car vs bike.  Yes, they do seemingly have different laws.  Only when
you get into formula 1 car engines, do they overlap.

I don't suppose you've seen Honda produce a 1200cc Civic engine with 200BHP?

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

F1 engines regularly make between 750 and 900HP in the 3 liter range, turning slightly
higher RPMs than production bike engines.  The specific output of the F1 engine is
higher than well tuned 130HP 600cc bike engine.

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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