Subject: Re: Advance curves
Date: Tuesday, Jul 14 1992 16:08:12
From: John De Armond
>The easy answer is to spend $200 on a recurved distributor. But I
>was wondering what steps, even on a theoretical level, I should take
>if I wanted to recurve the distributor myself. Do I have to make
>the weights heavier or lighter to move the advance curve around?
>Or should I be asking my dyno man?
>(I guess I'm just feeling spunky because I did a mild port-match
>on this motor when I had the head off and it made an improvement
>in the car's performance, so now I'm wondering what else I can do
>with a pair of dikes and a Dremel... :-)
Excellent article. The proper way to develop an advance curve is to
run the motor under controlled conditions and generate a map of
advance vs manifold pressure and RPM. An 8 X 8 point map is fine
(8 rpm points, 8 manifold pressure points.) If you're doing a mechanical
distributor, you then develop a curve from this map and alter the springs
and/or weights to match your experimental curve. Weight weight (hehe)
in combination with spring preload determines the onset of advance while
the spring rate vs weight weight determines the slope of the advance
curve. It is obviously better to play only with the springs.
The proper way to develop this curve is to put the car on a dyno, set
an RPM, set a manifold vacuum with the throttle and vary the timing
until the engine makes the best torque without knocking. map each
RPM at your chosen manifold pressures and then proceed to the next
RPM point. A very easy way to vary the timing is with MSD's manual
advance/retard box. let's you dial advance/retard in with a simple
If you can't get to a chassis dyno, a good substitute is a long steep
hill. Hooking up a trailer, preferably with trailer brakes helps
even more. Simply ride the brakes and/or the trailer brakes
to hold the RPM steady and proceed. This will take an order of
magnitude longer than making a dyno run but it will work. I've done it
many times before. You have the advantage too of actual road air
Curving the distributor is the next trick. I'll be publishing an article
in the first or second Performance Engineering on a distributor curving
machine you can build for practically nothing assuming you have
a timing light, a tach and and an electric drill. If you want an
advance copy, contact me in email (give me some slack, guys. If you're
not gonna build the machine tomorrow, wait for the magazine. I only
have 24 hrs in a day :-) You can either buy springs or make your
own. Piano wire made for the purpose of winding custom springs is
available from most machine shop supply houses. Ziegler's in Atlanta
is an example - or at least they used to.
It is trivial to wind springs. Get some drill rod about 20% smaller
than the desired ID of the spring. Make a handle from thick strap
iron so that the drill can be inserted into a hole drilled in
the strap and a setscrew tightened. Drill another hole with a setscrew
in the strap to clamp the end of the wire in. Get two pieces of wood,
preferably hickory or oak, clamp them together and drill a hole one size
smaller than the drill rod so that when the wood is separated, a half
hole, actaully a trough, is on each one.
Insert a piece of piano wire in the handle and insert the drill rod
in the hole drilled in the wood. Lube the rod and clamp the whole
assembly in a vice. The handle should be up practically flush against
the wood with the wire sticking out. Start turning the handle. The
wire will wind around the drill rod mandrel and the process will jack
the rod out of the wood. The resistance keeps the coil nice and tight.
Wind about 3 extra turns per inch of spring to make up for the
unwinding when you release tension. CAREFULLY release tension, unclamp
the wire and bend the ends as needed.
My 23rd Revised Edition Machinery's Handbook on page 331 and following
has everything you'd want to know about calculating spring rate and
wire sizes. This book should be on every hotrodder's shelf. My book
cost $47. Not bad for 2500 pages of concentrated mechanical engineering
Another technique I've found quite useful when I need a precise spring
rate is to make one a bit over strong and acid etch it down to spec.
("Gasp!" the motorheads say, "he's talkin' CHEMISTRY!" Yup. That I am.)
This is also trivial to do for non-stainless steel wire. A simple
glass jar filled with battery acid (NEW, not drained from your nearest
boat anchor), a carbon rod from a dry cell as the cathode (- terminal)
and the spring as anode (+ terminal) driven by two dry cells in series
(2-3 volts is optimum.) does the trick. Vary the voltage to get the
current such that the surface stays bright. A dull surface generally
means too much current After the etching is done, wash the spring in
a boiling solution of baking soda to remove all acidic residue and
optionally bead blast the surface of the spring to hammer down any
microscopic iron stalagmites that sometimes form.
(boy did I get wound up!)
From: John De Armond
X-Source: The Hotrod Mailing list
Date: Jul 1992
Subject: Re: Advance curves
>Do you get more performance out of advancing the timing?
>I have a 67 mustang and keep the advance at 5-6 degrees (like the manual says)
>I could advance it more, but what kind of gain can I expect?
>Also, what problems are there related to advancing? (efficiency? extra
Assuming you're not detonation-limited, there will still be an optimum
advance setting where the engine will make the best torque. More
advance than optimum means more energy is being expended compressing
the combusting mix than is gained by the earlier combustion and/or
the pressure peak occures BTDC which fights the crank rotation.
Typical symptoms of non-detonation-limited advance include lower torque,
overheating (because much more energy is absorbed by the pistons and
cylinders) and in extreme cases, seized pistons characterized by
melting over the whole body of the skirt. This failure mode
can sometimes masquerade as oil failure.
Perfect example. Rutger's SAE car. We used a Kaw Ninja engine, pretty
much stock except for a very high zoot injection and intake manifold
system. We used the Electromotive engine management system which also
does direct ignition and has programmable timing. The specified fuel
was turbo-blue which has an octane rating of something like 115. Obviously
an engine designed to run on pump gas is not going to take advantage of that
octane. During a dyno run I experimented with timing from the keyboard
of the laptop. Torque continued increasing as I jacked the timing up
until I reached >> 55 << degrees BTDC! It slacked slightly after that
and the radiator temperature rose notably. Incandescent headers were also
Think about this for a moment. More than 1/4 of the compression stroke is
spent compressing a burning mix! Narry a knock was heard in this process.
Yes, we'll take advantage of some of that octane next year!
Date: Fri Dec 4 11:11:47 1992
Subject: vacuum for advance
When I bought my 71 it had the vacuum for the spark advance taken off
the intake balance tube between the two manifolds. I believe that
originally the vacuum was taken off a port on the front carb. Is this
It occurred to me that the vacuum at these two places is not going to
be the same if the port on the carb measures vacuum at the venturi
rather than manifold vacuum. Is the Z's advance solinoid driven by
venturi (ported) vacuum?
The car seemed to run pretty well before, so I never gave it much
thought until I put the new motor in last week.
- )V(ark)< z-club #2 71 with hot new L28!
[You should be using "ported vacuum" for the advance. This vacuum
is zero at idle, builds off idle and is gone again by half throttle.
It is achieved by bringing the vacuum line to a small port drilled
right above the throttle butterfly. As the butterfly opens, the low
pressure area around the tip passes over the port and supplies the
vacuum. As the throttle opens more, the general vacuum in the manifold
drops and the ported vacuum goes away. Ported vacuum is available off
one of the carb ports on the forward port. JGD]
From: emory!teal.csn.org!dhaile (David Haile)
X-Source: The Hotrod Mailing list
Date: Apr 1994
Subject: Re: ignition timing
In article <email@example.com>, The Hotrod List <firstname.lastname@example.org> wrote:
>The best way to find the max amount to advance the timing is to hook up
>a vacuum guage to a vacuum port on the intake. Then increase the timing
>to you get to the highest amount of vacuum you can get. Then try to
>start the motor. If the motor has a hard time turning over, then back
>off on the advance untill the starter will crank effeciently. By the
>way, don't forget that the motor must be at operating temperature.
First, I would like to preface my comments with the statement that I'm
not much of a mechanic and really don't often know what I'm doing.
However, I have tried the suggested timing technique and have concluded
that it isn't worth a hill of beans for my car. I have a 1968 Buick
Electra convertible with a 10.25:1 430 in it, exactly stock, but
rebuilt. When I tried the vacuum method, I found that my timing was
around 25-30 degrees BTDC at idle. The car seemed to run sort of OK,
no bad pinging and all that, though it was sluggish off the line.
My mileage and power didn't seem to be affected. This engine's stock
timing at idle is around 4 degrees. I took it to what I consider to
be an excellent mechanic and he adjusted it to 12 degrees. It drives
easier now because of the improved off-the-line acceleration. I do
suspect that my distributor isn't working exactly like it should,
even though it was rebuilt. I had a Jacob's Electronic ignition on
the car for a while, but I returned it because it didn't live up to
its claims. Sure made the car idle smoother, but no other detectable
By the way, I only get 9-10 mpg in town, 13 highway. Everything,
everything, everything under the hood has been rebuilt. I know a
guy with the same car and he gets 16-17 mpg on the highway. What
can I do to improve mileage? I've tried a different ignition system,
timing adjustments of all kinds, carb adjustments. Nothing seems
to help. I haven't gone into the carb yet to put in smaller jets,
but will try that soon.
/ David W. Haile - Ft. Collins, Colorado - email@example.com /
[The ported vacuum plumbed to the vacuum advance is designed to apply
extra advance to the engine during light load, medium speed engine
operation. This works because the mixture is dilluted with exhaust gas
and the load is light; both conditions lead to slow flame propagation, thus
the engine can stand more advance. The ported vacuum is achieved by
locating a small vacuum port such that it will be exposed to manifold
vacuum as the throttle first cracks. As the throttle opens further,
the vacuum in the area is reduced and the signal goes away. Ported
vacuum does not exactly match what the engine needs but like all mechanical
advance schemes, is an approximation that works OK.
A very generalized description on how to set up the timing is to set
the static timing to the lesser of either that point where the car will start
readily without kicking back against the starter or a couple of
degrees retarded from that which gives the fastest idle. Retarding the
timing a little gives better idle torque which makes taking off against
the clutch a bit easier. Then set the mechanical advance so that there
is no pinging at wide open throttle and mid to high RPM. Finally, set
the vacuum advance so that the engine doesn't ping while under light load
and while there is ported vacuum present (use a gauge).
The amount of mechanical advance is set variously by the length of the
holes in the rotor or weight stops, depending on the distributor. The
amount of vacuum advance is frequently set by a potted setscrew in the
end of the advance diaphram (typical of jap cars) or by bendable tabs
on the point/reluctor plate. Or sometimes, not at all with fixed stops,
in which case, you must be innovative. Doing it this way will give
you both the best power AND the best cruising economy.