Barney Gaylord wrote:
>
> On Tue, 08 Apr 1997 10:05:36 +0000 Robert Allen <boballen@sky.net>
> writes:
>
> >By golly, you're sure right. MGs use manifold vacuum for their
> distributor and the carbs are CD -- constant depression (or continuosly
> drips) so there will always be vacuum on each side of the throttle plate.
> Curious. That would imply that vacuum advance contributes to total
> advance even under periods of moederate acceleration. From a traditonal
> carb perspective, that is pretty strange.
> > ..... [snip] .....
> >Also, there is the perverse "vacuum retard" on emission MGs (that I have
> zero experience with) so I'm curious where that vacuum source is. I
> understood that the "retard" feature was supposed to be active only at or
> near idle so I wonder where the vacuum source sees idle. Or I could be
> wrong again.
>
> MGA and early MGB have ported vacuum from the rear carb and vacuum
> advance distributors. Later MGB with smog stuff use manifold vacuum and
> vacuum retard distributors.
>
> Ported vacuum is low at idle, comes on strong on acceleration at low
> speeds, then increases slightly with engine speed. Manifold vacuum is
> high at idle, drops dramatically under hard acceleration, climbs again at
> higher RPMs, and hits the highest when you close the throttle. Ported
> vacuum with the vac-advance dizzy and manifold vacuum with the vac-retard
> dizzy have roughly the same effect. Spark advances when you hit the gas.
> As I recall, the primary reason for using the manifold vacuum retard
> setup is to retard the timing dramatically on overrun to prevent
> backfire.
>
> Vacuum spark timing modulation is usually limited to less than 10 degrees
> advance and maxes out before 2500 RPM. Mechanical spark timing advance
> is nearly linear, starts just above idle, and maxes out around 3600 rpm.
> Total spark advance should be limited to no more than 32 degrees on a
> stock MG 4-cylinder engine.
>
> A hot cam may tolerate up to 36 degrees advance for a racing setup, but
> often a radical cam cannot take much spark advance at low speeds. So, if
> you install a wild cam in your street machine without changing the dizzy,
> you may have to back off the idle timing to zero (TDC) or even a few
> degrees ATDC.
>
> Now I know someone is going to tell me I'm in way over my head here, but
> hey it's free.
>
> Barney Gaylord
> 1958 MGA
good discussion, but - in all the dsirbutors we've checked ove r the
years (30 + fyi) we find that most distribs have 25 - 30 degrees and
have all advance in normally by 2700 - 3500 rpm. the cam shaft has an
effect on distrib. timing but of more concern is the compression. a
stock engine can stand far more timing than a high copmpression engine.
stock being up to 9.0 : 1 as you approach the higher numbers, the
timing needs to be less. stock can go up into the low 40 degree range
while 12.5 compression needs to be around 30 degrees. no matter what,
chekc the distributor on a machine to see what the total advance is and
then set initial timing accordingly. on our current race engines, the
technology allows for adjustable advance curves with a retard at the top
end to overcome the tendency for the spark to fire "too far down the
cylinder" as the piston is coming up to TDC. thanks, ted
--
Ted Schumacher
TS Imported Automotive
404 Basinger Rd.
Pandora, Ohio, USA 45877
Ph. 800/543-6648 (sorry, USA only)
Ph. 419/384-3022 - tech./general information
FAX 419/384-3272 - 24 hrs
New-Used-Rebuilt-NOS-Performance British car parts.
We also have used sports cars for sale.
Always 200 - 300 cars for parts in our British-only
salvage yard.
For the finest in British car apparel, accessories and
regalia, be sure to check with our MotoringXtras store.
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Check it out!!!
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