Lotus Elan and Lotus Elan Plus 2 Forums

It seems like there is a preference for centrifugal-advance
distributors over those with both vacuum and centrifugal advance
mechanisms. Why?

Is it because the vacuum's rubber diaphragm is prone to failure in
the engine's environment? Or just another mechanism that isn't
needed. Are vacuum systems perceived as "smog equipment" by enthusiasts?

Doesn't a vacuum-advance system act as a load sensor, adjusting the
advance for load (in addition to RPM-based adjustment of the
centrifugal advance). Isn't load sensing a good thing? Am I missing something?

Rich Boyd

How about the vacuum RETARD that '74 Europa's had., and probably Elans too.
Explain that one..



Ken

'69 Lotus Elan +2 with BDR

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From: ***@***.***lto:***@***.*** Behalf
Of Richard A. Boyd
Sent: Thursday, March 16, 2006 2:07 PM
To: ***@***.***
Subject: [LotusElan.net] vacuum advance; why not?



It seems like there is a preference for centrifugal-advance
distributors over those with both vacuum and centrifugal advance
mechanisms. Why?

Is it because the vacuum's rubber diaphragm is prone to failure in
the engine's environment? Or just another mechanism that isn't
needed. Are vacuum systems perceived as "smog equipment" by enthusiasts?

Doesn't a vacuum-advance system act as a load sensor, adjusting the
advance for load (in addition to RPM-based adjustment of the
centrifugal advance). Isn't load sensing a good thing? Am I missing
something?

Rich Boyd











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The vacuum advance is to get over that initial hurdle of accelerating
from low revs. On performance engines you don't do much low speed
lugging so it wouldn't be necessary.

I'm only guessing of course and if anyone has a definitive answer I'd be
interested to know as well.

Peter
66S2

Richard A. Boyd wrote:

Vacuum advance is used for improving fuel economy at part throttle
cruising. When cruising at part thottle the density of the cylinder
fuel air mixture charge is low and burn rate is slower than experienced
at the same engine speed and at full throttle. The vacuum advance
corrects for this slower burn and produces a more efficient engine and
thus more fuel economy.

The problem with vacuum advance on a Weber engine is in getting a
representative vacuum signal given the individual chokes and heavy
pulses in each manifold. Also most people with this sort of engine
setup are not particularly interested in fuel economy versus
performance and thus most manufacturers dont bother just as Lotus did
not either with trying to setup a vacuum advance. If you really want
good cruising fuel economy out of your Weber Elan you could set it up
with an arrangement to monitor and average the vacuum in each intake.

The non emissions Stromberg engines could have easily used vacuum
advance and as a result had improved fuel economy. I dont know if they
did or not as the manual does not detail it and I have never seen any
original ones in real life.

The emission engines Strombergs with vacuum retard where all about
meeting emissions regulations and are not worth even thinking about.

Rohan

You have the idea of it, technically with a good responsive vacuum
signal I think it helps performance and economy. One of the reasons
hot-rodders disabled this was to eliminate breaker plate wobble by
screwing/rivetting/brazing the two plates together. And with large
carbs and wild cams there wasn't enough vacuum at the low end,
adding static timing and lots of mechanical advance to overcome that
meant that at high RPM and getting your boot out of it the vacuum
signal would add too much advance, at the wrong time.
So, in performance circles vacuum advance didn't work, and it
doesn't take long for racing technology and ideas to filter down to
the street. On the other hand, street driving, even most Lotus
street driving, is mostly on part throttle and not max RPM. Two
different worlds. But, it's still a problem with Webers and large
chokes, and there's still the performance mindset and having to
figure out both the vacuum advance and mechanical advance curve so
it's a lot easier to just forget the vacuum advance.
Or at least thats my view.
Interesting if you understand whats been going on with computer
controls, they use many more signals to determine ignition mapping
and fuel injection which is why all of that "emmission control" crap
has actually improved performance far beyond what people like me
ever thought possible. Hot rodding is still alive.
Mike Henry


--- In ***@***.***, "Richard A. Boyd" <richboyd@...>
wrote: