HEI Distributor Vacuum Advance

[67 Burb]

On a HEI distributor, does anyone know the relationship between the amount of movement of the vacuum advance rod and the number of degrees of advance?
My vacuum advance rod moves 3 mm (about 1/8"). How many degrees of advance would I get?

[Ziegelsteinfaust]

Typical about 6-9 degrees. Some give more on the aftermarket front

[crazy longhorn]

what I have seen....mostly on aftermarket canisters. there will be a number ,stamped on the arm of the can......if you haves , say a number 11, that is figuered in cam shaft degrees.......remember that the cam runs 1/2 speed of the crank. you will see 22 degrees on the balancer on that advance! I will hit you with a generic curve.....12-14 initial/32-34 on the total, vac advance unhooked & plugged. the vac advance is more of a load sensing device, & should be hooked to manifold vacumn. You may pull 48-52 degrees of timing (lite load) @ idle.....when you open the throttle, the vac advance is gone, that engine is getting what it wants to answer your ? What I have seen in my travels.....you want.085- .090 on the travel on the vac can to get you close to where you want to be. for more info.....text back, or PM me& I will give you a number to call & talk Longhorn

[burnin oil]

Spot on advice. I usually run a mechanical and static timing at 36 degrees for old school sbc heads. Vortec and good aftermarket tend to take less. I seen some that worked well at 29 degrees. All depends on combustion efficency.

[67 Burb]

Originally Posted by crazy longhorn

what I have seen....mostly on aftermarket canisters. there will be a number ,stamped on the arm of the can......if you haves , say a number 11, that is figuered in cam shaft degrees.......remember that the cam runs 1/2 speed of the crank. you will see 22 degrees on the balancer on that advance!

I have a vacuum advance that has 30 stamped on it. If that was distributor (or camshaft) degrees, that would be 60 crank degrees. Sounds a bit much, doesn't it.

[burnin oil]

The ones I have seen stamped were farther out on the arm. I believe that is a part number. The factory 80s cars did have some crazy curves so maybe it is that much. 0-4 degrees initial with really lazy advance curves. I could see it but I don't think they made them with that much advance in the can.

[crazy longhorn]

Originally Posted by burnin oil

Spot on advice. I usually run a mechanical and static timing at 36 degrees for old school sbc heads. Vortec and good aftermarket tend to take less. I seen some that worked well at 29 degrees. All depends on combustion efficency.

Totally agree....32-36 is about right for that gen 1 small block on the total(initial+ mechanical). I have had some, depending on the camshaft/decks cuts for the Quench, that will tolorate more timing. One 355/mild cam,liked 33 degrees ......the 383 liked more! 34-35 0n the street.....but she had aluminum heads. That old 383 is in my bros truck, & is a 12 second ride down the track. 38 degrees of timing/3200 stall, curve cut loose (around 2200-2500)....no vac advance. Dont play this on the street, it will bite you in the azz! Longhorn

[68 P.O.S.]

I found these numbers around the forum.

.086 inch travel for 8 crankshaft degrees
.112 inch travel for 10 crankshaft degrees
.120 inch travel for 12 crankshaft degrees

[67 Burb]

Here's what I ended up with:

I used the vacuum advance that was in the distributor (it gives 14° of advance).

I set the initial timing at 12° BTDC.

With vac hose disconnected and plugged, at 3000 RPM, I get 30° of advance.

Connect vacuum advance to full manifold vacuum and timing at idle (700 RPM) now reads 26° and at 3000 RPM I get 42° of advance.

The engine starts well and has nice idle. On the road it has no bog or stumble and pulls well (as well as big pig of a suburban can. It ain't any race car!)

[burnin oil]

Now double check the mechanical at roughly 4k rpm and make sure nothing else comes in. If nomore advance try bumping up the initial a degree or two and retest. With 30 degrees total you are probably leaving a little power and mileage on the table.

[67 Burb]

Originally Posted by burnin oil

Now double check the mechanical at roughly 4k rpm and make sure nothing else comes in. If no more advance try bumping up the initial a degree or two and retest. With 30 degrees total you are probably leaving a little power and mileage on the table.

30° was with no vacuum advance. With the vacuum connected, I'm getting 42° total at 3000 RPM. Now, that's at a standstill. Won't I still have that 14° of vacuum advance available cruising at 100 km/h on the highway?

I don't want my drive to end like this trying to hit 4000 RPM on the highway.

[68 P.O.S.]

Here's an easy way. Take the distributor cap off, pull off the advance springs (cover them with a rag so they don't go flying and disappear). Put the cap back on, start it up, rev it up and hold it at 2k rpm (the weights will be fully advanced at that rpm), and reset the timing for 36 degrees total. Now you won't have to rev it to 4k rpm with your head under the hood. Put the springs back and you're good.

Nice pic lol

[67 Burb]

Originally Posted by 68 P.O.S.

Here's an easy way. Take the distributor cap off, pull off the advance springs (cover them with a rag so they don't go flying and disappear). Put the cap back on, start it up, rev it up and hold it at 2k rpm (the weights will be fully advanced at that rpm), and reset the timing for 36 degrees total. Now you won't have to rev it to 4k rpm with your head under the hood. Put the springs back and you're good.

Nice pic lol

Thanks. About that photo, Of course, I wasn't really getting cuffed. I saw that one of the car guys at the show 'n' shine was going to snap a pic of the RCMP officer and me talking about the Suburban, I turned and put my hands behind me and said "make like you're cuffing me" and it turned out good. Even Mounties have a sense of humour.

[burnin oil]

With vacuum advance hooked to manifold vacuum it will be available at any rpm that has sufficient vacuum to engauge the unit. In other words low throttle input. It should function at idle and at cruise on flat ground. Once the throttle is ioened then the vacuum drops which disengaged the vac advance cannister. You are now running on base timing and what ever the mechanical advance is contributing at that rpm.

[68 P.O.S.]

Originally Posted by burnin oil

With vacuum advance hooked to manifold vacuum it will be available at any rpm that has sufficient vacuum to engauge the unit. In other words low throttle input. It should function at idle and at cruise on flat ground. Once the throttle is ioened then the vacuum drops which disengaged the vac advance cannister. You are now running on base timing and what ever the mechanical advance is contributing at that rpm.

^^^ This