Re: highest compression on a cis e

["J Miller" <sjmill@erols.com>]

now that is an explanation
yeah baby
jonas
rocco post
76  may have a new home as of sunday
87 sittign still goign to be stock and sold
2.1L race motor for sale heheheheh


jonas
getting an 86 quantum any need a cheep car under 1K

----- Original Message -----
From: "Dan Bubb" <jdbubb@ix.netcom.com>
To: "Chris DeLong" <green536@hotmail.com>
Cc: <s_t_rader@excite.com>; <scirocco-l@scirocco.org>
Sent: Friday, May 04, 2001 1:19 PM
Subject: Re: highest compression on a cis e


> 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
>


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