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From: larryhoy
To: CREICHLE
Cc: mgs
Subject: Re: Cams and Distributors
Date: Tuesday,April 08,1997 10:40PM
Chris, a while back you composed the following response (at the end of
this message) to Cory regarding pinging, timing, distributors and Weber
downdraft carbs. I have a couple of questions, I was wondering if you
could address them?
Does your information about vacuum advance (on the DVG) only effect idle
timing, and not the 3000 rpm ping?
*The 3000 rpm ping thing is mechanical advance related. You need to retard
your spark a little to tune this ping out. However, doing so may cut your
low rpm power. To avoid this you could install lighter springs that would
allow more of the advance to come in at lower rpms/less at high. Net result
is: you keep your low rpm advance but cut back a little at by 3000. If it
only happens at part throttle, you might think that it is vacuum advance
related but it is really just a symptom of the base mechanical advance being
to high at that rpm and the vacuum advance added just exposes it. You could
tune the part throttle ping out by reducing your vacuum advance (if you have
anadjustable unit) but that would only fix the symptom not the problem.
I've got the ping.
How do you choose lighter springs? As I remember the mechanical advance
springs in my distributor are two different weights now, one is very
light, one is very heavy. Wouldn't lighter springs allow too much
advance and increase ping? If I've got this right you retard timing to
eliminate ping. The problem I have is if I retard enough to reduce the
3000 rpm ping then the car runs like crap.
* The total amount of total mechanical advance will stay the same, say ~35
deg at 4500 (Stock 80 B). Meaning, the advance cam will adjust the advance
~35 deg. Lighter springs allow the cams to move more at lower rpm so there
will be more advance than there was before at the lower rpms if all else
stayed the same. Because the cams are moving more for a given rpm, the max
advance ~35 deg is reached at a lower rpm say 3500 rpm. Remember that the
advance curve is just that, a curve. On stock springs it comes in gradually
tapering off to 4500. With lighter springs it comes in quicker tapering off
at 3500. Now that you have changed the springs, you are going to retard the
static timing so that the static + mechanical advance at the lower rpm is
the same as it was before the changes (where you liked the way it ran).
We've increased the mechanical advance at lower rpms so we must decrease or
retard the static timing so it still adds up to the same amount of advance
we had before. At low rpm the car will run the same as it did before but
now the advance starts tapering off at lower rpms and there will be less
advance at 3000 than there was before. You just tuned out your 3000 rpm ping
while preserving low end performance.
*The best way to figure out the springs to use it to take the dizzy to a
distributor shop and give them the advance curve you want. They will put it
on a machine and change springs till it matches. If you would like to do it
yourself, get a good timing light, one with a dial to adjust the timing of
the light. With the vacuum advance plugged, plot your current mechanical
advance curve on a piece of graph paper at at least 3 rpms (every 500 rpm
gives a very acurate curve). At higher rpms you can use the dial on the
light to retard the light 15 deg so the mark on the pulley will still be on
the timinc cover scale, just add 15 to the scale reading. To find the curve
you want we'll use the 3000 rpm ping problem as an example. If you ping when
it is at 25 deg advance @3000 and the ping goes away when you retard the
static timing 7 deg, plot a point on the graph at 18 deg @3000. Now draw a
new curve using most of the same low rpm points but passing through the new
high rpm point. A word about curve shape: as I noted before, the smog curve
is gradually increasing in advance. A performance curve increases sharply
then tapers off. The difference in shape of the curve results in an increase
in mid range power due to increased advance in this rpm range. If you have
stripped most emissions, no cat, free flow exhaust maybe a cam etc. I would
go for a curve that brings in all the advance by 3000 - 3300 rpm. Go to a
performance shop or machine shop and get a few distributor springs. They
come in different sizes and stiffness. A common setup is to use a light
short spring that will control your advance rate for low and mid range rpm
and then use a second longer stiffer spring to taper off the advance at
higher rpms. Try swapping the smaller spring for a lighter one and see how
it looks when you plot it.
What if I eliminate the vacuum advance on the Lucas dist? Would the
mechanical advance be all I need?
*Mechanical advance compensates for engine speed. There is less time for the
fuel to start burning at 10 deg btdc at 3000 rpm than 2000 rpm so mechanical
advance moves the spark earlier as rpm increases to give it time to start
burning. In reality as rpms increase, turbidity in the combustion chamber
increases causing the fuel to burn quicker. Above ~2500 rpm this
relationship is linear meaning you do not have to compensate for increases
in rpm by adding advance. However, at low rpm it is not linear and that is
what the mechanical advace is for.
*Vacuum advance adjusts for charge density. At WOT higher charge density
means that the fuel molecules are closer together and the flame front
travels faster so less time is needed for the fuel to burn. At idle when the
throttle is closed (high vacuum) the charge density is low and the flame
front travels slower requiring more time to burn. The vacuum advance unit
senses this condition and advances the spark to compensate. It is not an on
off switch. The amount of advance is proportional to the amount of vacuum to
handle part throttle conditions. Vacuum advance is great for fuel economy
and smooths out the engine a bit.
How about the Mallory Distributor? In a previous post you write:
>I would suggest the mallory electronic ignition distributor and
performance coil. It's the same
as the dual point distributor you see in vicky brit but it has a mallory
unilite electronic ignition unit in it instead of points. Available from
Summit, it was cheaper than buying the dual point from vicky brit and is
more reliable."
I assume that if I ever put a Weber DCOE (got one in my garage somewhere)
that the Mallory distributor would work for that too.
*Not exactly, I don't know too much about DCOEs but I have heard that they
are incompatible with vacuum advance type distributors. Not sure why it
wouldn't work. I'm sure you could tap a hole in the manifold but I'm not
sure how well it would work with the distributor. There is nothing stopping
you from not connecting the vacuum advance unit and adjusting the mechanical
to compensate a bit. Most of the time, ther is some vacuum adding some
advance that you might want to adjust for. Maybe it has to do with that
whole ported/manifold vacuum connection issue seeing as the DGV does have a
port and I believe the DCOE does not. Don't know enough to say. Might I
suggest calling seven or Pierce Manifolds for more info.
*Chris Riechle
Thanks for any help.
Cheers,
Larry Hoy
1969 MGB Roadster
1987 Jaguar XJ6 Vanden Plas
1976 MGB Parting out
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