62 Impala vacuum advance.

A friend and I have restored his 62 Impala. We found the original
engine and carb. Long story short, the carb doesnt have a ported
vacuum source for the timing advance. the manual I have shows it
hooked up to manifold vacuum. If any one has this info ,it would be
most appriciated. It is a rochester 4 barrel, and the engine size is
327cid. Thanx in advance.

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It may not make a difference for the vacuum question, but is it the 250hp or
the 300hp version of the 327cid they used that year? I had one of the '62
300hp models and made some modifications but am scratching my head on the
vacuum source. Both versions used vacuum advance, but the 340hp version
used in the corvette that year was straight centrifugal advance if I recall
correctly.

"homi" > wrote in message
news:1_604443_1c655bb2155ebf8adeda99d538554fd2@aut oforumz.com...
>A friend and I have restored his 62 Impala. We found the original
> engine and carb. Long story short, the carb doesnt have a ported
> vacuum source for the timing advance. the manual I have shows it
> hooked up to manifold vacuum. If any one has this info ,it would be
> most appriciated. It is a rochester 4 barrel, and the engine size is
> 327cid. Thanx in advance.
>
> --
> Posted using the http://www.autoforumz.com interface, at author's request
> Articles individually checked for conformance to usenet standards
> Topic URL:
> http://www.autoforumz.com/Antique-Ca...ict124140.html
> Visit Topic URL to contact author (reg. req'd). Report abuse:
> http://www.autoforumz.com/eform.php?p=604443

AFAIK they were hooked to manifold vacuum on FI Corvettes as well, but
that doesn't make sense since the point of vacuum advance is to advance
the timing at low RPM and high manifold pressure (open throttle), not
low.

Dunno. Academic-I'd put on a ported carb and an electronic
distributor. Points suck for driving.

I,m rebuilding a car with vacuum advance but I do not rember WHY I need
It, can you go through the basics...someone.?

wrote:
> AFAIK they were hooked to manifold vacuum on FI Corvettes as well, but
> that doesn't make sense since the point of vacuum advance is to advance
> the timing at low RPM and high manifold pressure (open throttle), not
> low.
>
> Dunno. Academic-I'd put on a ported carb and an electronic
> distributor. Points suck for driving.

wrote:
> I,m rebuilding a car with vacuum advance but I do not rember WHY I need
> It, can you go through the basics...someone.?

Sure. The general idea is that the fuel charge in a cylinder takes a fixed
amount of time to burn. You would like the burn to start just before the
cylinder reached top dead center on the compression stroke, really burn well
just after the piston passes TDC, and end about the time it reaches bottom dead
center on the power stroke. You *don't* want it to start too early, because the
pressure of the expanding gasses will push backwards against the piston if it is
still rising on the compression stroke when the burn peaks. Too late, and most
of the power is wasted out the exhaust system.

The problem is that the burn takes a fixed amount of time, but the time it takes
the piston to go from TDC to BDC depends on the rpm; the faster the engine is
turning, the shorter the time span. What you want is for the spark plug to fire
pretty late (close to TDC) if the engine is turning slowly and fire pretty early
(perhaps 30 degrees before TDC) if the engine is turning rapidly. This will get
the peak burn right after TDC regardless of the engine speed. The vacuum advance
uses intake manifold vacuum to change the spark plug timing to accomplish this.
It doesn't do a perfect job, but it's pretty good.

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

God explanation, George, but its the *centrifugal* advance that adjusts
for RPM. Same logic, but the vacuum advance adjusts timing for the
speed at which the mixture burns. A thin mixture (high vacuum) burns
slowly, so the timing must be advanced, a dense mixture (low vacuum)
burns more rapidly.

The combination of vacuum and centrifugal adjustments help to optimize
the timing for all the varied types of engine operation. At high vacuum,
high RPM operation, like decelerating down an offramp, the timing may be
36-40 degrees before top dead center (BTDC) due to the combination of
advance techniques. At low vacuum, low RPM, like leaving a stop light
hurriedly, the timing may only be 2 degrees BTDC.

However I have never seen a satisfactory explanation as to why the
vacuum signal should be ported or from the manifold. It would seem that
at idle, where there is high vacuum, the timing should be advanced.
But most late model vehicles (50's - 80's) seem to use ported vacuum
which effectively disables the vacuum advance until the throttle is
slightly above idle.

If you look inside the carburetor, you can see the "port" that gives the
signal its name. It sits just above the throttle plate at idle, so that
the vacuum is on the other side of the plate. If the throttle is opened
slightly, the edge of the throttle plate passes above the port, exposing
it to the vacuum in the manifold.

regards,
Leon

George Patterson wrote:
> wrote:
>
>> I,m rebuilding a car with vacuum advance but I do not rember WHY I need
>> It, can you go through the basics...someone.?
>
>
> Sure. The general idea is that the fuel charge in a cylinder takes a
> fixed amount of time to burn. You would like the burn to start just
> before the cylinder reached top dead center on the compression stroke,
> really burn well just after the piston passes TDC, and end about the
> time it reaches bottom dead center on the power stroke. You *don't* want
> it to start too early, because the pressure of the expanding gasses will
> push backwards against the piston if it is still rising on the
> compression stroke when the burn peaks. Too late, and most of the power
> is wasted out the exhaust system.
>
> The problem is that the burn takes a fixed amount of time, but the time
> it takes the piston to go from TDC to BDC depends on the rpm; the faster
> the engine is turning, the shorter the time span. What you want is for
> the spark plug to fire pretty late (close to TDC) if the engine is
> turning slowly and fire pretty early (perhaps 30 degrees before TDC) if
> the engine is turning rapidly. This will get the peak burn right after
> TDC regardless of the engine speed. The vacuum advance uses intake
> manifold vacuum to change the spark plug timing to accomplish this. It
> doesn't do a perfect job, but it's pretty good.
>
> George Patterson
> Why do men's hearts beat faster, knees get weak, throats become dry,
> and they think irrationally when a woman wears leather clothing?
> Because she smells like a new truck.

Lots of misconceptions and half truths so far. Yes the engine wants
certain amount of advance in different situations and RPM. Cylinder
pressure is as much a consideration as rpm. Fist starting. just enough
advance to start but not enough to kick back on starter. Now the
mechanical advance will advance some on start up and continue to advance
till (depending on manufactures spec) 2500 to as high as 4000 but 2200
to 2500 seems a norm.
The reason for ported vacuum comes from a proper explanation of what
vacuum advance is meant to do. vacuum advance is mainly a gas mileage
enhancer (yes back in the day gas mileage was a factor) Now the reason
for ported vacuum and not manifold is that at idle and light throttle
manifold vacuum is high at idle you would not want the vacuum advance
pulled on full. next at part throttle and full throttle manifold vacuum
drops dramatically or to 0 at WOT so if you were counting on the vacuum
advance to give you advance for power when you needed it, it would not
be there. Now at highway speed at light throttle a car can take much
more advance because the cylinder pressures are down. This advance (46
to 52 deg total) increases gas mileage at cruise but if you were open
throttle and increase cylinder pressures it would detonate (ping) like
crazy so you want the advance to retard. The ported vacuum is created
within the carburetor itself by the speed of the air moving through the
carb and past the port inside. that speed is faster at lower throttle
openings than wide open and of course shut! The ported vacuum is 0 or
near 0 at idle increases at light throttle and helps the advance curve
under light acceleration when Cylinder pressures are increased but still
lower. At wot it is lower by design.
Two ways to tune a car (almost three)
1. is to set it up as close to factory specs but that can somewhat
skewed by today's gasoline's! you may get ping at light throttle thus
turning some advance out of the vacuum advance with a adjustable vacuum
advance may be needed with today's gas it may not like the 46 to 52
cruise advance. If the car is fine with advance disconnected but pings
in some driving conditions with it on you may/will have to reduce the
amount and rate of vacuum advance by adjusting it with a 3/32 allen
wrench (mopar anyway) in through the advance canister port turning
counter clockwise (reducing amount of vacuum advance and rate) till a
result you want is attained (elimination of ping).
2. is maximum performance with vacuum plugged and forgotten as the max
power and performance enhancement is the goal not the little added gas
mileage that can be achieved. In this case. (most likely when mods are
done and acceleration rules) you plug the vacuum advance install
lighter advance springs so the manual advance comes on much faster for
more performance gain. Then you set the total to your engines max total
advance for performance each engine is different can be from 30 to 40
total . Total is the total mechanical and initial advance. it can be
checked by increasing RPM until the timing quits advancing and that is
total . Now properly applied timing take or a dial back timing light are
required for this test. Now say you engine say's best results for total
performance will be at 35 degree (likely in the park for a small block
chev) you set total to that and drive. if it pings reduce till it's gone
and be happy. if it is fine go one with your life and new found power.
3. is to tune for a combination of total performance and gas mileage
enhancement. Usually not a overly successful endeavor but as with method
two unhook and plug vacuum advance. tune to total performance and drive
assuring no ping. Now re attach vacuum advance to ported vacuum on carb
and drive. if it pings vacuum advance must be reduced till pinging
disappears which may be down to 4 to 5 deg of vacuum advance down from
15 to 20 hardly worth the effort????
Now to do this if your vacuum canister is not adjustable it must be
replaced with one that is. they are available for most common
applications. I suggest like others. convert to a electronic ignition
with a nice adjustable (vacuum that is) new distributor. You old one
likely has worn bushings anyways!!!!
Dodgem

Took me years to understand and I'm not sure I have it all but I have a
good successful grasp!

Leon Corley wrote:
> God explanation, George, but its the *centrifugal* advance that adjusts
> for RPM. Same logic, but the vacuum advance adjusts timing for the
> speed at which the mixture burns. A thin mixture (high vacuum) burns
> slowly, so the timing must be advanced, a dense mixture (low vacuum)
> burns more rapidly.
>
> The combination of vacuum and centrifugal adjustments help to optimize
> the timing for all the varied types of engine operation. At high vacuum,
> high RPM operation, like decelerating down an offramp, the timing may be
> 36-40 degrees before top dead center (BTDC) due to the combination of
> advance techniques. At low vacuum, low RPM, like leaving a stop light
> hurriedly, the timing may only be 2 degrees BTDC.
>
> However I have never seen a satisfactory explanation as to why the
> vacuum signal should be ported or from the manifold. It would seem that
> at idle, where there is high vacuum, the timing should be advanced. But
> most late model vehicles (50's - 80's) seem to use ported vacuum which
> effectively disables the vacuum advance until the throttle is slightly
> above idle.
>
> If you look inside the carburetor, you can see the "port" that gives the
> signal its name. It sits just above the throttle plate at idle, so that
> the vacuum is on the other side of the plate. If the throttle is opened
> slightly, the edge of the throttle plate passes above the port, exposing
> it to the vacuum in the manifold.
>
> regards,
> Leon
>
> George Patterson wrote:
>
>> wrote:
>>
>>> I,m rebuilding a car with vacuum advance but I do not rember WHY I need
>>> It, can you go through the basics...someone.?
>>
>>
>>
>> Sure. The general idea is that the fuel charge in a cylinder takes a
>> fixed amount of time to burn. You would like the burn to start just
>> before the cylinder reached top dead center on the compression stroke,
>> really burn well just after the piston passes TDC, and end about the
>> time it reaches bottom dead center on the power stroke. You *don't*
>> want it to start too early, because the pressure of the expanding
>> gasses will push backwards against the piston if it is still rising on
>> the compression stroke when the burn peaks. Too late, and most of the
>> power is wasted out the exhaust system.
>>
>> The problem is that the burn takes a fixed amount of time, but the
>> time it takes the piston to go from TDC to BDC depends on the rpm; the
>> faster the engine is turning, the shorter the time span. What you want
>> is for the spark plug to fire pretty late (close to TDC) if the engine
>> is turning slowly and fire pretty early (perhaps 30 degrees before
>> TDC) if the engine is turning rapidly. This will get the peak burn
>> right after TDC regardless of the engine speed. The vacuum advance
>> uses intake manifold vacuum to change the spark plug timing to
>> accomplish this. It doesn't do a perfect job, but it's pretty good.
>>
>> George Patterson
>> Why do men's hearts beat faster, knees get weak, throats become dry,
>> and they think irrationally when a woman wears leather clothing?
>> Because she smells like a new truck.
>
>

when you set the timing with vac hose plugged you are setting it in the
retarded position which is where you want it to be when you are under power.
when you hook up the vac hose it advances the timing . until you step on the
gas then it goes back to the retarded position.

--
>Remember, it's not,
>"How high are you?"
>It's "Hi, how are you?"
little mike
> wrote in message
ups.com...
> I,m rebuilding a car with vacuum advance but I do not rember WHY I need
> It, can you go through the basics...someone.?
>
> wrote:
>> AFAIK they were hooked to manifold vacuum on FI Corvettes as well, but
>> that doesn't make sense since the point of vacuum advance is to advance
>> the timing at low RPM and high manifold pressure (open throttle), not
>> low.
>>
>> Dunno. Academic-I'd put on a ported carb and an electronic
>> distributor. Points suck for driving.
>