>Hey GMC'ers I just bought a 74 P.D. Would like to upgrade to
>electronic ignition..Whats the best way??? Will the later model HEI
I used the HEI distributor from a 75 Olds Toronado (Cardone #1893) and
changed the vacuum advance to 10 degrees (Delco #1973577). You also need to
change the wires and plugs. Use .060 gap plugs, (not the .080 ones). The
larger gap plugs tend to burn holes in the rotor. I use AC/Delco R44SX
plugs but they are getting hard to find. When you install the distributor
you must run a heavy wire (12 ga or better) directly from the distributor
to the ignition switch. This new wire will bypass the resistance wire that
powers the old ignition system.
You will probably have to modify the air cleaner so it will clear the
larger distributor cap. I used a hammer and it worked find. Or you can get
one from a junkyard 75 olds Toronado.
It would probably be a good idea to check the timing curves after
installing the new distributor. Here is what I have been told they should
be for the GMC:
Speed No Vac With Full Vac
-----------------------------
Idle 8 18
2000 18
2500 20
3100 27
In order to check the timing more easily I painted the harmonic balancer
black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I
calculated the positions of these marks by measuring the circumference of
the harmonic balancer (20.5") and dividing by 360. This gives the length,
in inches, of one degree on the surface of the harmonic balancer. The zero
degree mark (TDC) is the reference line notched into the harmonic balancer.
When I cranked the engine so the zero degree mark is on the drivers side
(looking up from under the engine), the BTDC marks are toward passenger
side in order as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum
advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000
rpm and the vacuum advance disconnected. It is also the timing at idle with
full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100
rpm and the vacuum advance disconnected.
I used white paint to make the lines marking each of these timing positions
on the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing
light, I set the timing (at idle) to 8 degrees. If the lines on the
harmonic balancer are painted correctly the 0 degree mark (the notch in the
harmonic balancer) will line up with the 8 degree arrow on the engine and
the 8 degree mark (the first painted line on the harmonic balancer) will
line up with the 0 degree arrow on the engine. Note that the zero degree
arrow is the first point on the drivers side. The other arrows on the
engine are spaced 4 degrees apart (4, 8 and 12 degrees).
To check the centrifugal advance, I ran the engine up to 2000 rpms and
checked the timing. If the centrifugal advance is working properly the 18
degree mark on the damper (the second painted line) will be lined up with
the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms
and checked the timing. If the centrifugal advance is working properly the
27 degree mark on the damper (the third painted line) will line up with the
zero degree arrow on the engine.
The centrifugal advance on my distributor is pretty close. At 2000 rpms it
was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not
quite as advanced as I have been told it should be (27 degrees at 3100 rpm)
but I rarely run much above 2500 rpms so I live with it.
Before checking the operation of the vacuum advance be sure that the idle
timing is set to 8 degrees when the advance hose is disconnected. Then
unhook the hose that goes to the transmission modulator and connect the
vacuum advance hose to the transmission modulator port on the intake
manifold. This will apply full vacuum to the vacuum advance and should pull
in an additional 10 degrees of advance. Use the timing light to verify that
the 18 degree mark painted on the damper lines up with the 0 degree arrow
on the engine (8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should
replace it with the correct one - Delco 1973577. If you see less than 18
degrees then the advance is defective or there is a vacuum leak or blocked
hose.
The curve on this distributor (75 Toro) is pretty close (after I replaced
the vacuum advance). Here is a summary of what I measured:
Speed No Vac With Full Vac
-----------------------------
Idle 8 18
2000 18
2500 20
3100 24
There are a number of places that will rebuild and properly recurve the
distributor for you if you would like it to be exact. Bob Lamay and Duane
Simmons are two people that where recommended to me.
Hope that helps.
Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI)
>electronic ignition..Whats the best way??? Will the later model HEI
I used the HEI distributor from a 75 Olds Toronado (Cardone #1893) and
changed the vacuum advance to 10 degrees (Delco #1973577). You also need to
change the wires and plugs. Use .060 gap plugs, (not the .080 ones). The
larger gap plugs tend to burn holes in the rotor. I use AC/Delco R44SX
plugs but they are getting hard to find. When you install the distributor
you must run a heavy wire (12 ga or better) directly from the distributor
to the ignition switch. This new wire will bypass the resistance wire that
powers the old ignition system.
You will probably have to modify the air cleaner so it will clear the
larger distributor cap. I used a hammer and it worked find. Or you can get
one from a junkyard 75 olds Toronado.
It would probably be a good idea to check the timing curves after
installing the new distributor. Here is what I have been told they should
be for the GMC:
Speed No Vac With Full Vac
-----------------------------
Idle 8 18
2000 18
2500 20
3100 27
In order to check the timing more easily I painted the harmonic balancer
black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I
calculated the positions of these marks by measuring the circumference of
the harmonic balancer (20.5") and dividing by 360. This gives the length,
in inches, of one degree on the surface of the harmonic balancer. The zero
degree mark (TDC) is the reference line notched into the harmonic balancer.
When I cranked the engine so the zero degree mark is on the drivers side
(looking up from under the engine), the BTDC marks are toward passenger
side in order as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum
advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000
rpm and the vacuum advance disconnected. It is also the timing at idle with
full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100
rpm and the vacuum advance disconnected.
I used white paint to make the lines marking each of these timing positions
on the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing
light, I set the timing (at idle) to 8 degrees. If the lines on the
harmonic balancer are painted correctly the 0 degree mark (the notch in the
harmonic balancer) will line up with the 8 degree arrow on the engine and
the 8 degree mark (the first painted line on the harmonic balancer) will
line up with the 0 degree arrow on the engine. Note that the zero degree
arrow is the first point on the drivers side. The other arrows on the
engine are spaced 4 degrees apart (4, 8 and 12 degrees).
To check the centrifugal advance, I ran the engine up to 2000 rpms and
checked the timing. If the centrifugal advance is working properly the 18
degree mark on the damper (the second painted line) will be lined up with
the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms
and checked the timing. If the centrifugal advance is working properly the
27 degree mark on the damper (the third painted line) will line up with the
zero degree arrow on the engine.
The centrifugal advance on my distributor is pretty close. At 2000 rpms it
was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not
quite as advanced as I have been told it should be (27 degrees at 3100 rpm)
but I rarely run much above 2500 rpms so I live with it.
Before checking the operation of the vacuum advance be sure that the idle
timing is set to 8 degrees when the advance hose is disconnected. Then
unhook the hose that goes to the transmission modulator and connect the
vacuum advance hose to the transmission modulator port on the intake
manifold. This will apply full vacuum to the vacuum advance and should pull
in an additional 10 degrees of advance. Use the timing light to verify that
the 18 degree mark painted on the damper lines up with the 0 degree arrow
on the engine (8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should
replace it with the correct one - Delco 1973577. If you see less than 18
degrees then the advance is defective or there is a vacuum leak or blocked
hose.
The curve on this distributor (75 Toro) is pretty close (after I replaced
the vacuum advance). Here is a summary of what I measured:
Speed No Vac With Full Vac
-----------------------------
Idle 8 18
2000 18
2500 20
3100 24
There are a number of places that will rebuild and properly recurve the
distributor for you if you would like it to be exact. Bob Lamay and Duane
Simmons are two people that where recommended to me.
Hope that helps.
Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI)
