Thanks Terry,
I will have a read and digest it a bit.
-----Original Message-----
From: Terry Rudd [SMTP:[EMAIL PROTECTED]]
Sent: 20/02/2002 23:53
To: [EMAIL PROTECTED]
Subject: 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.
�
I just logged into Google to see if I could find out what the dizzy tuning meter is called. The first article that came up was this on that basically explains what i've been trying to do - worth a read <http://www.chevytalk.com/tech/ignition/Timing_Advance.html>
�
Here's another article that shows how to use the map method (all my 30 yo secrets are out there now) <http://www.boyleworks.com/ta400/psp/distcurve.html>
�
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
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Rowe, Adam
Sent: Thursday, 14 February 2002 11:07 AM
To: [EMAIL PROTECTED]
Subject: RE: webers
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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