Re: highest compression on a cis e

[Dan Bubb <jdbubb@ix.netcom.com>]

This is going to be really long winded so bear with me.
Let's look at some examples:
1. Air compressor, no pressure in the tank, running very slow. As the
piston moves down the cylinder will fill with the same volume of air
that the piston displaces (since the piston is moving very slow the
cylinder has time to fill) as the piston moves up it will expele the
same volume of air as its displacement because again it's running very
slow so all the pressures are at atmospheric and the pressure that it is
pumping against is zero. In this instance the CR will make no difference
to the volume pumped.
2.Air compressor, 100psi in the tank, slow speed. Cylinder filling will
still be 100%, again since it's running very slow and the pressures have
a chance to equalize. But, actual volume pumped will be dramatically
different with CR. With a CR of 2:1 the cylinder pressure will max at 15
psi and since it's pumping against 100psi it will pump nothing into the
tank. With a 10:1 CR, max cylinder pressure will be 150psi. Air will
transfer into the tank until cylinder pressure matches tank pressure, so
it will transfer roughly 1/3 of its volume. At 20:1 CR, cylinder
pressure will be 300psi and volume transfer is roughly 2/3s.
3. Case 1 at high speed. In this instance a very low CR pump will
generate very weak suction pulses because the % volume change of the
cylinder is small. Imagine the intake valve not opening. Max cylinder
vacuum would be 15" with a 2:1 CR. The cylinder will not fill well
because the pressure difference is small. A high CR pump will generate
strong suction pulses (10:1 CR, cyl vac 27" max) with good cylinder
filling.
So, as far as air compressors are concerned CR has a big influence on
volume pumped. Despite the occasional comment that engines are just air
pumps (and in a lot of ways this is true) there are some differences.
Mainly there is a power stroke between intake and exhaust and the
exhaust stroke isn't pumping against a huge backpressure.
So, now looking at engines;
1. Slow speed. Building on the above examples, if we run the engine slow
the cylinder will have time to fill completely, so CR will make no
difference. Now, of course, the cylinder doesn't push air into a tank.
It compresses it, burns it and vents it to atmosphere. In this case,
events after the cylinder filling are irrelevant to volume flow.
2. High speed. In reference to case 3 above, a low CR engine will
generate a weak suction pulse and due to the small pressure difference
will have poor cylinder filling. A higer CR will have a stronger suction
pulse and resultant better cylinder filling. The down side is in modern
high speed engines the exhaust valve is open way past bottom dead center
(>50 deg for  a good old "G" grind) and, of course the higher
compression engine will build cylinder pressure faster on the
compression stroke forcing the inlet flow to stop sooner than it would
on a lower compression engine. So, the net difference?? Comparing a 10:1
and 12:1, the 12 will probably have slightly better cylinder filling at
higher speed because the greater inertia of the inlet gas column would
keep air flowing despite greater cylinder pressure. But, at lower speeds
cylinder filling could be worse than the 10:1 engine. Other engine
design issues such as cam timing, inlet track velocity and inlet track
restrictions will determine whether a 10:1 or 12:1 engine will have
better cylinder filling.
I'll give a little and agree that changing CR can make a difference in
volume air flow, but I believe it's a small difference based solely on
inlet tract pulse strength and nowhere near as big a difference as Shawn
asserts in his post.
Sorry for the long post.


Chris DeLong wrote:
> 
> Just to play the advocate here.... What you are saying is that a flat piston
> working in a flat combustion chamber will not suck any more air than a
> piston or combustion chamber with a huge dish in it? I think not. If there
> is no room in the combustion chamber or piston top for air then it will have
> to suck more air in to create a stoich ratio for burning fuel.
> 
> Bottom line is that higher compression will increase air flow. No way around
> it. The engine is a pump; the more "free" space you give it (I.E. lower
> compression) the less air it will flow (unless it is forced in, I.E. Turbo
> or s/c). I would have to say that Shawn is closer to reality than you when
> it comes to air flow and high compression engines.
> 
> This of course is all in my opinion and experience from working on VW's for
> quite a while for a living.
> 
> I make it my problem to do high performance shit.
> 
> >
> >Absolute Bull. Increasing the compression ratio has nothing to do with
> >mass air flow. How hard you squeeze what's in the combustion chamber has
> >nothing to do with how much air gets in in the first place. Total
> >pressure drop across the intake system, valve size, lift and duration,
> >engine displacement and intake air temperature will determine mass air
> >flow. CR may have some secondary or tertiary effect that is just as
> >likely to decrease mass flow as to increase it.  The amount of fuel
> >required also will not change, except for the possibility of running a
> >little richer to forstall the onset of detonation.
> >
> >Shawn Rader wrote:
> > >
> >Raising the compression has roughly the same effect as
> > > increasing the engines displacement - it increases the amount of air
> >your
> > > engine flows.  With more air you can burn more fuel which means more
> >power.
> > > By raising the compression from 11:1 to 12:1 your engine is flowing 9%
> >more
> > > air at any given rpm- about the same as increasing you engine
> >displacement
> > > from 2.1L to 2.3L.
> > >
> > > -shawn
> > > '81 S
> > >
> > > On Thu, 3 May 2001 08:35:05 -0400, J Miller wrote:
> > >
> > > >  i am courious what the highest compression rate that the cis e
> >injection
> > > on
> > > >  a 1.8 16 could handle,
> > > >  i know 11:1 works fine but what above 12:1
> > > >  i need a car and i am just goign to slap the 2.1L in with the stock
> >EMS
> > > and
> > > >  a few other minor mods
> > > >  Hey mr rust bucket i need an adjustable moduale
> > > >  jonas miller
> > > >  1976 2.0L 8v  scirocco   for sale
> > > >  1987 soon to be a monster 2.1L
> > > >  fzr 600 bleediun carbs
> > > >
> > > >
> > > >
> > > >  --
> > > >  Email LIST problems to: scirocco-l-probs@scirocco.org.
> > > >  To unsubscibe send "unsubscribe scirocco-l" in the message to
> > > majordomo@scirocco.org
> > > >
> > >
> > > _______________________________________________________
> > > Send a cool gift with your E-Card
> > > http://www.bluemountain.com/giftcenter/
> > >
> > > --
> > > Email LIST problems to: scirocco-l-probs@scirocco.org.
> > > To unsubscibe send "unsubscribe scirocco-l" in the message to
> >majordomo@scirocco.org
> >
> >--
> >Email LIST problems to: scirocco-l-probs@scirocco.org.
> >To unsubscibe send "unsubscribe scirocco-l" in the message to
> >majordomo@scirocco.org
> >
> 
> Chris DeLong
> ICQ#45030867
> Seattle, WA USA
> '80 16V 'Rocco ragtop
> '91 GLi 2.0 16V
> '77 Rabbit 16V
> '80 'Vert (parting out!)
> '61 17 Window type II
> '91 Vanagon
> '77 Rabbit (for sale Cheap!)
> 
> _________________________________________________________________
> Get your FREE download of MSN Explorer at http://explorer.msn.com

--
Email LIST problems to: scirocco-l-probs@scirocco.org.
To unsubscibe send "unsubscribe scirocco-l" in the message to majordomo@scirocco.org