Is the amount of vacuum the same in the advance and decel ports on an SC

[lonewolf]

There are two small ports on the back of the 80-83 SC ,CIS throttle body.
One for vacuum advance and one for the decel valve.
Does anyone know if the amounts of vacuum drawn as the butterfly opens are or should be the same?

I see the questioned asked in many threads so I thought I would create a post where the answer could be quickly searched out instead of reading thread after thread trying to find it..

Thanks to all who help.

[Bob Kontak]

Originally Posted by lonewolf

One for vacuum advance and one for the decel valve.

Not the same.

Vacuum advance port is above the throttle plate. It starts sucking when the throttle is opened. It works with the mechanical advance a good way through the RPM range. I am not sure at what RPM the vacuum advance is maxed.

The decel valve is below the throttle plate. When you lift off the gas at high rpm's there is massive vacuum. The vacuum opens the decel valve diaphragm and bleeds air into the airbox buffering the sharp drop in RPMs. Lot's of folks plug the small hose with a golf tee or nail and just leave the small port on the valve open.

At idle - big vacuum below plate, not so much above. Off idle, the difference is lessened. WOT, pretty equal.

[Pazuzu]

Originally Posted by Bob Kontak

Not the same.

Vacuum advance port is above the throttle plate. It starts sucking when the throttle is opened. It works with the mechanical advance a good way through the RPM range. I am not sure at what RPM the vacuum advance is maxed.

Vacuum above the throttle plate is highest right as the throttle is tipped open, the engine is pulling hard but there is only a small gap so you get a high vacuum situation right above the butterfly. Vac advance helps at any RPM when cruising, so the butterfly is only tipped open, while the mechanical advance helps only at high RPM, but even when you have the butterfly wide open.

[Flat6pac]

The port below the butterfly is the vacuum retard for the distributor. It stays pulling vacuum until the throttle is cracked then the distributor advances to the timing light setting.
As the RPMs increase the more vacuum is caused in the throttle body causing a vacuum above the throttle butterfly. The vacuum advance begins to kick in the last few degrees of advance available in distributor..
Bruce

[Bob Kontak]

Originally Posted by Flat6pac

The port below the butterfly is the vacuum retard for the distributor.

This is incorrect. The port for the retard hose is on the rear bumper side of the TB, down low on the driver's side on the later SC's at least. That port will work fine for vacuum retard though. Just not the factory location.

Going to check my 81 to see where the small decel hose ties in. Not a big deal. Just checking to make sure I am not blowing smoke.

[Bob Kontak]

Originally Posted by Pazuzu

Vac advance helps at any RPM when cruising

Originally Posted by Flat6pac

The vacuum advance begins to kick in the last few degrees of advance available in distributor.

Three different views.

- Any RPM

- At max advance

- Stops before max advance (my thoughts)

I do know the max advance on an SC (later ones, at least) is kind of wimpy. Like 2,500 RPM. Far from screaming.

[Flat6pac]

The only vacuum available to retard the distributor at idle is below the butterfly.
Bruce

[tirwin]

Originally Posted by Bob Kontak

The decel valve is below the throttle plate. When you lift off the gas at high rpm's there is massive vacuum. The vacuum opens the decel valve diaphragm and bleeds air into the airbox buffering the sharp drop in RPMs. Lot's of folks plug the small hose with a golf tee or nail and just leave the small port on the valve open.

Pics would be a big help for those playing along at home.

I'm pretty sure on my '83 there was originally a 3-way tee on this vac line.
- airbox
- cruise control
- decel valve

I'll see if I can take some pictures later to help.

[Bob Kontak]

Originally Posted by Flat6pac

The only vacuum available to retard the distributor at idle is below the butterfly.
Bruce

Yes Sir. It's port is absolutely below the butterfly on the rear bumper side of the TB - drivers side.

[Bob Kontak]

Originally Posted by tirwin

Pics would be a big help for those playing along at home.

Here is the (orange/red) advance hose. You can see that it is just above the throttle plate. I can't get a shot of the rear side below port. Note the blue retard line running right next to it. It travels between the air sensor and TB towards the rear bumper and hooks on in the front of the TB down low. See 2nd pic.



Blue retard hose hooks TB behind air box about where white arrow points.



Here is a better pic. You can see the decel valve hose hooked in below the plate. Advance above and the retard hose snaking on through - although pic does not confirm. This is a later SC. Stole pic from Edgy01 on Rennlist via Google.