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1.8L head on 2.0L block
I've been thinking about this alittle more. We all know what that means!
I was thinking about the conditions in the cylinder during the intake and
exhaust and how that is related to flow.
The power an engine produces is largely related to how much air it burns.
>From that is subtracted the various inefficiencies. Pumping losses,
inefficient combustion, heat losses.....
If your intake is restrictive then you get less air and proportionally less
power. So, 7% less air pretty much starts you at 7% less power. The probable
most important secondary effect on the intake side is intake velocity. The
same amount of air at a lower velocity will generally produce less
turbulence in the combustion chamber, slower burn and less power.
Flow thru the exhaust is not a power producer. It's a power loss. The
exhaust valve starts to open well before the piston reaches BDC on the power
stroke while the cylinder pressure is still very high. So, alot of the
exhaust gets a huge boost out the port due to the really high pressures
(compared to intake pressures or the average pressure level in the exhaust
system).
Once this slug of exhaust is out the piston still has to do work to force
the rest of the exhaust into the pipe, but the general exhaust system
pressure isn't that high (on the order of a couple psi even for an
inefficient system) so the power lost pushing the remaining exhaust out is
not huge. Now, obviously, the lower the exhaust port flow the more power is
lost pushing the exhaust out and you also will get more charge dilution with
high exhaust back pressures that will have an effect on the amount of power
produced by the incoming charge.
The main point is; power is directly related to intake flow, it is
secondarily related to exhaust flow. i.e. the cylinder pressures producing
power (in the area of 1000 psi) are directly related to intake flow and the
cylinder pressures loosing power (<10psi) are directly related to exhaust
flow.
Having said all that it seems like intake flow is the most important (and as
Dave points out, it's not just the port. it's the entire intake tract) and
exhaust flow of secondary, although not insignificant, importance.
On the topic of intake velocity; I'm not sure the 2.0 head necessarily has
better velocity despite the lower flow. I haven't seen an actual 2.0 head,
only pictures, so I could be wrong, but it seems the primary restriction to
flow is the center divider between ports is a big chunky lump on the 2.0 and
is more streamlined on the 1.8. So, it could be lower flow without the
benefit of higher velocity.
Anyway, having now shot my mouth off again in favor of the 1.8 head, I still
would like to see dynos comparing the two heads.
Dan
----- Original Message -----
From: L F <rocco16v@netzero.net>
To: Dave Ewing <MK1Scirocco16v@attbi.com>; <scirocco-l@scirocco.org>
Sent: Saturday, June 28, 2003 7:34 AM
Subject: Re: 1.8L head on 2.0L block
Dave,
You are right; the exhaust is positively expelled, whereas the intake only
relies on vacuum to fill the cylinder (NA engines).
However, the intake isn't more "restrictive" per se, it just doesn't have
the irresistable force in action that the exhaust has. This is why the
intake valve(s) is almost always larger than the exhaust valve(s)....the
intake needs all the help it can get.
You stopped short on one sentence; the exhaust has to exit the tailpipe
into the atmosphere....not just into a pipe. (that's why low restriction
mufflers/cats, mandrel-bent large diameter tubing, etc. are important)
It's one reason to try to put the end of the tailpipe in a low-pressure area
of the vehicle rather than a high-pressure area; helps scavenging.
Good disscussion.
Larry
sandiego16v
----- Original Message -----
From: Dave Ewing
To: scirocco-l@scirocco.org
Sent: Saturday, June 28, 2003 1:10 AM
Subject: Re: 1.8L head on 2.0L block
Along with Chris's statement, wouldn't you think that the pressure that
the
piston creates when forcing the exhaust out of the cylinder is greater
than
the vacuum that is created via the throttle body? As far as exhaust is
concerned (atleast the type of exhaust that most of us are running, fairly
free flowing) I would have to say that the intake is more restrictive than
exhaust. I realize you can increase intake flow by adding a cam or bigger
TB or whatever but it is limited to some extent or another, the exhaust on
the other hand only has to exit the head into a pipe. It doesn't have to
pass through the air filter, intake boot, TB, intake, etc. I don't know
if
this is relevant but something that makes sense to me.
One other point to consider is that I would rather have the exhaust flow a
little better than the intake (whether the intake ports are hogged out or
not, either way) so that the heat is leaving the motor more efficiently.
16v motors run hotter due to their increased compression and the higher
rpms
needed to make useable torque.
I understand the importance of velocity, speed, etc. but when you consider
that you could make a smaller port flow better than a larger port then
this
would be an argument against the 1.8 head considering that the 2.0 heads
are
newer and more technology has gone into the port design. I don't know the
specifics but wouldn't you agree that VW wants to consistantly improve
their
motors especially with tighter emmissions standards?
Actually, all of this is rather irrelevant as most power hungry listers
have
already ported and polished their 1.8 or 2.0 heads so stock standards
don't
really apply but I, again, would much rather have a newer head on my car
that hasn't seen as many miles or as many kids beating it to death. Since
the 1.8 heads came on the scirocco/jetta/golf and the 2.0 heads only came
on
the jetta/golf/passat, I'd think that the 1.8 heads on the sciroccos have
taken the most beating.
Dave
----- Original Message -----
From: "Chris DeLong" <green536@hotmail.com>
To: <amalventano@sc.rr.com>; <jdbubb@ix.netcom.com>;
<scirocco-l@scirocco.org>
Sent: Friday, June 27, 2003 3:52 PM
Subject: Re: 1.8L head on 2.0L block
> Ok so then you are saying that increased air velocity due to port size
and
> shape DOES NOT flow more air? Hmm, I would think that INCREASED air
velocity
> would yeild more flow due to the forced induction characteristics that
you
> mentioned below.
>
>
>
> Chris DeLong
> Fine Tuning
> 206.367.5503
> www.finetuningperformance.com
> Seattle, WA USA
>
>
>
>
>
> >From: "Allyn" <amalventano@sc.rr.com>
> >To: "Chris DeLong" <green536@hotmail.com>,
> ><jdbubb@ix.netcom.com>,<scirocco-l@scirocco.org>
> >Subject: Re: 1.8L head on 2.0L block
> >Date: Fri, 27 Jun 2003 14:05:54 -0400
> >
> > > More airflow=better efficiency=more power.
> >
> >umm
> >depends on runner diameter/taper / rpm
> >rpm change = different intake air pulse size/speed, therefore some
> >configurations make more power at low rpm, where some others make more
> >power
> >at higher rpm.
> >just boring the crap out of intake doesnt give you more airflow in all
> >situations. narrower intake passages cause faster air velocity while
> >filling
> >the cylinder, and that very momentum can actually cause a forced
induction
> >effect, as it squeezes that much more air in the cylinder just before
the
> >intake valve closes. this is how some engines can have a volumetric
> >efficiency approaching (and possibly exceeding) a value of 1. the
narrower
> >intake is not perfect though, as it begins to restrict airflow at
higher
> >rpm.
> >
> >so... from an intake perspective, a stock 1.8 head is meant to flow
most
> >efficiently at a higher rpm than a stock 2.0 head is meant to.
> >Al
> >
> >
> >
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