I am willing to accept that many of you know more about this than I do, but
on the 69 the retard connects to the manifold. The later ones go to the
carb, but not the 69.
Jim Altman jaltman@altlaw.com Illigitimi non Carborundum
http://www.altlaw.com/metro/jaltman.html 69-TR6#CC28754L W4UCK
-----Original Message-----
From: owner-triumphs@autox.team.net
[mailto:owner-triumphs@autox.team.net]On Behalf Of Tomislav Marincic
Sent: Friday, October 09, 1998 9:50 AM
To: Triumph list
Subject: Advance/retard
Jim,
"Moreover, retard is connected to
manifold vacuum, which declines as engine speed increases, as opposed to
the
advance line which connects to the carburetor and increases vacuum as speed
increases."
I disagree. The advance line connects to the carb after the
butterfly, and
measures essentially the same relative vacuum as the manifold line. Later
TR6's had the retard line hooked to a carb connection too, to eliminate
the faulty idle-vacuum switch on TR250's and 1969 TR6's. The later retard
line was ported, meaning it primarily sensed vacuum only at idle. The
advance
line is also ported, so it doesn't sense vacuum until the engine is above
idle.
That's why one is connected to the top of the carb and the other to the
bottom, they need to be on different sides of the carb butterfly.
Manifold vacuum is not directly related to engine speed
unless the car is at constant speed on level ground. It is directly related
to the load on the engine, which is a function of engine speed and
throttle butterfly position. If the designers wanted more advance
based just on engine speed, they would have simply added more
mechanical advance.
Vacuum advance is an economy feature, not power.
An engine at lower load can tolerate more advance, which will result in
greater
economy, so the VA system does just that. Picture climbing a long
hill in a full truck in high gear with cheap gas. You push the gas pedal
further down, increasing load and decreasing manifold "vacuum"
and then the engine starts to ping. (not detonate) You've just reached the
point where your engine cannot tolerate the amount of advance it has.
On all TR6/250 engines, by whatever means, the vacuum
retard was intended to work primarily at idle. The vacuum advance,
if present, was intended to work at above-idle speeds where the
engine was operated at part-throttle conditions. At full throttle,
neither system would be doing anything, regardless of engine speed.
Best Regards,
Tom Marincic
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