Steven Hill wrote:-
Should I disconnect the vacuum advance when using triple Webers, as I have
been told by one source??
What is the optimal distribution curve for the engine, given compression of
9.5 :1, triple webers, street cam, lightened flywheel, tuned exhaust. Seems
the earlier hotter TR5 engines used a 6 degree distributor (12 degrees at
the crank) and most of the later TR6 car that came to the US used a
centrifugal advance with quite a bit more advance (22 or 28 degrees at the
crank). Any guidance on this would be appreciated re: curve/springs, etc
would be appreciated. Thanks. Stephen
Dear Steven,
I hope this will be of help. Normally I would try to keep any vacuum advance
on a distributor as it will help slightly with fuel economy and part
throttle engine response but you need to have carbs that have a vacuum
take-off at the edge of the throttle plate. This cuts off manifold vacuum
when the throttle is closed and gives max vacuum at small openings.
Unfortunately Webers do not have this so you have no choice. Do not feed
straight manifold vacuum to a vacuum advance module, it will give you too
much advance at idle. Do not be confused by the fact that vacuum retard
modules do get raw manifold vacuum. This purely for emissions.
As for the advance curve, I have found that on a tuned, carburettor engine
you need in the order of 20 to 24 (engine) degrees of centrifugal advance on
top of the usual 10 degrees static. This advance curve is easy to get from
the more usual later distributors but the standard springs are not optimal.
The short, light spring can be used but the long, loose, heavy spring slows
down the rate of advance above approx 3000 rpm. It is best to replace the
heavy spring with another light one as a start, or even remove it altogether
and use just the first spring only. The maximum advance will then come in
earlier so you might get some "Pinking"(Knocking) at low speed if you are
near the edge on compression ratio or available quality of fuel. The problem
is actually eased if you have a cam with a lot of overlap because the
volumetric efficiency at low speeds is reduced, reducing the compression
pressures. My 2 litre GT6 engine uses a standard distributor with one light
spring, 10 to 1 compression, Sprint 83 Triumphtune cam, 1.75" SUs etc and
runs quite happily on UK four star leaded. I will have to see if all is
still well when we lose our leaded fuel, not because of the lead but of the
drop in octane rating. If all else fails I will have to buy Super unleaded
at an even more exorbitant price.
I don't know why Triumphtune seem to recommend the early low advance TR5
distributor, it seems to be at odds with all technical articles I have seen
over the last 30 years.
Peter French
Bedfordshire, UK.
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