Title: RE: webers
Adam,
I've
been having a bit of a think about my reply to you on this one and it's
going to get a bit long and technical. Vacuum advance is very beneficial to
a petrol engine if the dizzy has been set up to use it in conjunction with
mechanical advance but you can set a dizzy up that doesn't require vac advance
to deliver the correct advance for both economical cruising and power. DCOE's
are a problem to get them to deliver a metered advance signal without pulsing
present. DCOE's don't even come with a metered vac outlet as they originally
weren't supposed to be driven on the street. A few ppl I know have tried
rigging up various systems, most of which don't work as required or nearly as
well as a well setup mechanical only system.
What
you say in your last post is true but you don't have to use vac advance to
achieve the correct timing for a given rpm and load, it's just economical and
easily achieve on the family car by using the vac and mechanical method. Most
good EFI systems don't use vac advance units these days but that's another
story, we're talking carbies. Also, there are a few variations on vac advance
units and how they achieve the desired results eg some pollution systems
actually use two vac signals, one metered and one not and they work against each
other. I'll stick to the simple 70's model as I think it's a little easier to
demonstrate how we get away with just centrifugal advance with DCOE carbies.
I'll demonstrate this technique with an example.
Take a
stock Hitachi on a L18, the vac advance unit doesn't get any vac at idle as the
vac inlet on the carbie is blocked until the throttle is opened a bit past idle.
Initial timing is 10 btdc, vac advance will start if 210mm of vac is present and
will supply up to 16 degrees at 410mm of vac and one would hope
progressively. Now on the same engine centrifugal advance starts around 800 rpm
and will supply up to 24 degrees of advance at 3800 rpm so at around 2800 rpm on
cruise both systems should be supplying around the same amount of advance above
static and from then on in the rev range the mechanical all that is left,
i.e. 8 degrees, also if you accelerate from 2800 rpm no vac is supplied
from the manifold as it will drop below 210mm instantly the throttle is opened.
Some good quality performance dizzies like Mallory, MSD etc are capable of
adding the vac advance to the mechanical advance even if you're cruising at 80
mph. These are far more sophisticated than your stocker and were developed for
pollution lean burn engines as they can supply up to 50 degrees advance on
cruise, These use ported vac up 1000 rpm and switch to manifold vac above that.
The re-mapped DCOE dizzy can't compete with this system.
Now
you know what advance you want from the mechanical advance at various points in
the rev range, the data you have is a bit like an EMS map. For a single DCOE you
would retain the initial advance of 10 degrees and the centrifugal advance stop
is set at 24 degrees which is about right for a L engine (total of 34 degrees
and if you want higher you can adjust the initial setting a couple of degrees)
Now the experts use a machine (I can't remember what it's called) that will
tell them what the advance is at a given rpm, usually in 200 rpm increments. To
adjust the advance to what is required, they fit different weights or springs
and usually a combination of both.
and I
still can't find what that meter is called.
Anyway, I hope that this gives you a bit of an understanding of what
timing advance tuning maps are about.
regards
Terry
Oh OK, I was just thinking
this.
When the motor is
working in part load mode, ie. 1/2 throttle, there are still exhaust gasses
present in the combustion chambers. This causes the combustion process
to be slowed. To counteract that you ignite the mixture earlier so power
occurs at the same time in the stroke as when at WOT. This gives better
power at partial loads. I thought that a vacuum advance would be
benificial to all petrol engines, including multi-throttle type
setup.
-----Original Message-----
From: Terry
Rudd [SMTP:[EMAIL PROTECTED]]
Sent: 13/02/2002
17:48
To: [EMAIL PROTECTED]
Subject: RE: webers
DCOE Webers are designed that way -
they may get slightly better economy on off throttle with a make shift vac
advance but I doubt it. Most ppl who have played with vac advance mods with
DCOE webers haven't achieved what they set out to. You have to check out the
science behind a side draft carbie that has a standard butterfly metering
system. DCOE/DHLA carbies are totally different to any other. Another point
is they can be very economical the way they are when setup correctly and
driven by your mother, but economy wasn't the major concern at the design
phase.
regards
Terry
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