Date: Fri, 5 Feb 1999 10:50:51 -0800
Reply-To: Doug Jones <duge4wd@EMAIL.MSN.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Doug Jones <duge4wd@EMAIL.MSN.COM>
Subject: Re: extractor exhaust?
>Mac wrote:
>I think I understand basically the concept of an extractor exhaust,
>but how do they go about "extracting?"
I'll take a shot at this. The first thing you should notice about an
"extraction" exhaust is that the primary pipes (those coming from the head
flanges to the collector) should be of equal length. The individual pipes
may appear snake-like and excessively long; but, there is a reason for this.
This ensures the exhaust charge from each cylinder travels the same
distance. This equal distance equates to equal timing of
these exhausts charges passing through the collector, thus the charges from
each cylinder pass through the collector in sequence...one right after the
other.
As one exhaust charge passes through the collector, it goes from exerting a
positive pressure in it's primary tube, and to exerting a negative pressure
on the other primaries which feed into the collector. This associated
negative
pressure "extracts" exhaust gases from the affected primaries. This helps
remove exhaust gases quickly from the primaries and from the combustion
chamber. This is extremely important when large valves are used, and there
is allot of valve lift and duration which increases the intake/exhaust
overlap. So
the extraction caused by each charge passing the collector and 'pulling' on
the other exhaust charges helps remove spent gases from the cylinders and
helps maintain sufficient intake charge velocity and fuel atomization.
In the case of large valves, long valve duration and lift, high intake
charge velocity helps fuel aerosolized (which equals more fuel surface area
and better, more complete burn), poor evacuation of exhaust gases would
contaminate the high velocity mixture and slow it down, causing the fuel to
drop 'out of solution.'
This is why most race engines (especially F1) tend to have a convoluted
snake-like exhaust (especially when space is at a premium), b/c they run
large valves, and hi-lift/duration cams; They also make a great deal of
their power at higher rpms which helps maintain the intake charge
velocity==>better burn, more power.
>Does anyone have enough
>experience with these things to know whether or not I would get a
>noticeable advantage on an air-cooled with one of these?
Yes, you should see instant results. Low restriction exhaust systems are one
on the quickest bolt-on ways to increase power. I've put several on
different autos. Corvettes, Jeeps, and aircooled VWs. You may have some fit
problems with VW extractor exhausts, nothing major I think. I've never put
one on a aircooled T4 engine; but, the fit on most aftermarket T1 systems
leaves something to be desired. Just requires a little "persuasion" ( read
that as a big hammer for some subtle bending). Maybe a torch in the
extreme cases.
>What kind of noise level do these things put out? Is it going to
>"muffle" at all or will it sound like those annoying (imho) glasspacks
>that everyone around here seems to have on their trucks?
My experience is that they tend to be louder b/c the mufflers used with
these headers tend to be less restrictive; This helps maintain the
"extraction" and reduces backpressure. You could try single or dual quiet
pack type mufflers. They are a little deep and throaty but not offensive. I
don't have much experience with what low restriction mufflers are available
for Vanagons.
I hope I helped somewhat, especially since this post was kinda long. I
apologize
for the length to those not interested in this subject.
BTW, where's Berea College? Just curious, up until two days ago I was a
post-bac student at Univ of MD; I've had to take a leave of absence for
medical reasons. I can't really miss classes already, can I?
Doug Jones
No Vanagon, No Syncro; but, I hopefully
see something in my future. I'm just tied up at the
moment. The money is burning a hole in my pocket.
I'll keep you posted.
