Vacuum Advance - Converting to Full Manifold Vacuum

[jeff68]

I have a 68 L30 327 4-speed car.  It originally had the AIR system on it, but the entire system was removed shortly after delivery.  It has the original distributor and the vacuum advance is hooked up to ported vacuum.  The car has always seemed to run well.  Original static timing spec for this car is 0 degrees (IIRC).  So at idle the timing is at TDC (0 initial + 0 vacuum adv + 0 mech advance)  

Based on my research, I want to change the vacuum advance to connect to full manifold vacuum, and play with the springs on the mechanical (centrifugal) advance to start advancing just over idle and be "all in" by around 2800 rpm.  Research also indicates that I want my initial static timing at about 10 degrees, with max mechanical advance limited to about 20 degrees (rough numbers).  My issue is that my mechanical advance plate allows for 34 degrees of crankshaft advance (17 degrees distributor advance).  I measured this on the mechanism, and it corresponds to the stamping of "734" on the part, the last 2 numerals indicating total crank advance.  The original bushing is in place on the mech advance limiting pin, and the mechanism operates smoothly.  With 34 degrees of mechanical advance, I will still need to keep my initial static timing at about 0 degrees so that my high rpm low load  cruising timing isn't excessive ( cruise timing = 0 initial + 16 vacuum advance + 34 mech advance = 50 degrees).

My plan is to:
1)  Install a vacuum advance canister that will provide 16 degrees crank advance at idle vacuum conditions (connected to full manifold vacuum).
2)  Change the mechanical advance springs to start advancing just over idle and be "all in" by around 2800-3000 rpm.
3)  Keep initial static timing at 0 degrees.
4)  Keep existing mechanical advance limit arrangement at 34 degrees (I really don't want to modify the limiting slot).

Is this a good plan?  I know my timing curve will still not be ideal, but this should be a step in the right direction, correct?  Any other comments or suggestions?  This isn't a race car, but I'd like it to perform well.

Thanks,
Jeff

[Everett#2390]

Reads like a good plan.
I would start out where you are at now and record the running condition of all driving methods, plus MPG.
Then make a change and record the results and compare with previous.
You can always go back to where you where or where the Camaro ran the best.

[JohnZ]

My plan is to:
1)  Install a vacuum advance canister that will provide 16 degrees crank advance at idle vacuum conditions (connected to full manifold vacuum).
2)  Change the mechanical advance springs to start advancing just over idle and be "all in" by around 2800-3000 rpm.
3)  Keep initial static timing at 0 degrees.
4)  Keep existing mechanical advance limit arrangement at 34 degrees (I really don't want to modify the limiting slot).

Is this a good plan?  I know my timing curve will still not be ideal, but this should be a step in the right direction, correct?  Any other comments or suggestions?  This isn't a race car, but I'd like it to perform well.

Thanks,
Jeff

I think you'll feel an improvement with your plan, but you'll see even more if you set your initial at 10* and use a larger limit pin bushing to limit centrifugal to 24*; that increase in initial will make a VERY noticeable difference.

[jeff68]

^^Thanks.  That's what I was thinking, but it looks like I would need a limit pin bushing with an offset center hole to accomplish this.  I've never seen one of these.  Do you know if they are available?  I was thinking about trying to make one.  Maybe I'll give it a shot.

[jeff68]

Well, I decided to make a new limit pin bushing to limit centrifugal advance to 24 crank degrees.  I had 2 brass bushings that came with a Mr. Gasket and Moroso advance spring kit.  I sacrificed a brass fitting I had 'in stock', and soldered a piece of it to one of the bushings.  Doing some math using the original pin bushing OD, the slot size, and the original advance, I determined the bushing width required to get me 24 degrees maximum.  I carefully shaped the new bushing, then installed it.  The bushing went about 1/2 way onto the pin, then I needed to carefully press it in place the rest of the way with a bench vice.  I used a little Loctite retaining compound on it just to be sure it wasn't going anywhere.

The bushing was too big at first (better than too small I guess).  So, I carefully filed it until I got 12 degrees movement at the centrifugal advance mechanism (24 crank degrees).  This was a real PITA to file in place, but I didn't want to ruin the bushing by trying to remove it.  Hopefully this exercise was worth the effort.

Making the bushing:


Bushing installed:


Putting things back together:


I decided to try the Mr. Gasket silver & black spring combo as a first shot.:

[TODD]

Jeff good job on that eccentric bushing, there is a ring groove on there that you could snap ring it on. Now when that vacuum can is hooked to full manifold vacuum keep ensure you don't have too much total advance at cruise (pinging) if you do you'll need to limit your vac can with a mechanical stop. I have a sun distributor tester so it makes the whole set-up a lot easier.

[jeff68]

^^Thanks Todd.  I don't think I'll need to install a clip on the pin - the new bushing fit very snug, and the original rubber/plastic bushing was on there for 44 years without a clip.

My VAC should produce 16 degrees crank advance at 14" vacuum.  With an initial of 10 degrees, and 24 degrees mechanical advance, I should have a total advance of 10 + 24 + 16 = 50 degrees at higher-rpm cruise.  My total idle timing should be 10 initial + 16 vac = 26 degrees, and my maximum WOT should be 10 initial + 24 mechanical = 34 degrees.  All these numbers seem to be very good based on my research.  I don't have access to a distributor machine, so I'll have to set up the distributor on the car & see how it runs (and adjust as necessary).

[TODD]

Those numbers look good, just keep an ear open at highway cruise. My 302 would exhibit a slight ping accelerating at cruise (highway speeds 70 mph). So I put a mechanical limiter on the vac can to keep it a max 10 degrees. Hopefully you won't have that issue.

[jeff68]

Update:
Here's what I ended up with:
9 degrees initial static timing
Idle timing with VAC connected = 9 + 16 = 25 degrees.
23 degrees of centrifugal advance
Centrifugal advance starts at about 900 rpm, and is all-in (9 + 23 = 32 degrees) at about 2500 rpm.  This is a little earlier than I wanted it to be all-in by, but the car doesn't seem to ping or knock under any conditions.  Should I mess around with even more springs to bump this up to 2800-3000 rpm??

The car runs great with the timing set-up as above.  However, there is one weird issue.  I have the idle set at 700 rpm per the manual, but I like the idle a little lower - around 500 rpm.  If I try to go below 700 rpm I get a loud whistle from the carb.  I contacted Cliff's High Performance, who rebuilt the carb.  He said that with my idle timing at about 25 degrees, I have too much idle vacuum (I measured it at 20") and that's what is causing the noise.  He says that to get my idle down, I'm closing the primaries too much and the airflow is whistling past them.  He suggested that I run ported vacuum to reduce the advance at idle, thus decreasing vacuum.

So, I hooked the VAC back up to ported vacuum and I get a nice idle at 500 rpm, but I haven't driven the car this way.  So, if I keep the VAC connected to ported vacuum, what are the downsides of having the idle timing at only 9 degrees instead of 25 degrees?  Should I give it a try?  

Under all other driving conditions, the distributor advance system should function the same whether the VAC is connected to ported or full manifold vacuum, correct?

[TODD]

Tough to get an old mechanical distributor to do what you want, but not impossible.
The car will run best on full manifold vacuum for sure, so sounds like you have two issues.
One is to limit that vacuum advance at idle with a limiter plate like Summit sales Crane Vacuum Timing Limiter Plates 99619-1.
You could dial in the limiter to 10~12 degrees at idle.
Once that is worked out you asked about all in advance rpm, and that is dependent on what the motor likes.
Some people suggest lug the motor at rpm to see if it pings, but how is the seat of the pants feel during acceleration?
If its not pinging I would leave that alone. There are very few spring types to select from, and I haven't found one that is just right.
But I haven't tried to make springs yet and that is the only way I know of to dial in the curve.

Todd

[JohnZ]

If I try to go below 700 rpm I get a loud whistle from the carb.  I contacted Cliff's High Performance, who rebuilt the carb.  He said that with my idle timing at about 25 degrees, I have too much idle vacuum (I measured it at 20") and that's what is causing the noise.  He says that to get my idle down, I'm closing the primaries too much and the airflow is whistling past them.  He suggested that I run ported vacuum to reduce the advance at idle, thus decreasing vacuum.

Another guy who hasn't a clue about how the vacuum and centrifugal advance systems work together - all he knows is the ported vacuum setup he sees in the shop manual.

Leave it on manifold vacuum, set your idle above the "whistle" point, and drive it. The centrifugal advance system is completely independent of the vacuum advance, and doesn't care how vacuum is set up - it works the same way regardless of manifold-vs.-ported vacuum - it only responds to engine rpm.

[jeff68]

... but how is the seat of the pants feel during acceleration?
If its not pinging I would leave that alone.

The car drives great and pulls harder than I ever remember before changing the timing curve.  I have not noticed any pinging, and I'm running 93 octane fuel.

 Should be relatively easy to make a home-made limiter plate.
So, keep everything the same, just limit my vacuum advance to 10-12 degrees?

[jeff68]

Leave it on manifold vacuum, set your idle above the "whistle" point, and drive it.

Well, there is one other piece to the puzzle that I haven't told you (sorry for holding back).  With the stock return spring, the carb doesn't always like to return to idle - it stops just short of the idle speed stop screw.  If I blip the throttle or pull the gas pedal up with my toe it goes  back to a nice idle.  It only does this with the engine running, and when the engine isn't at full operating temperature (but not running on the choke).

Could too much vacuum at idle be trying to keep the throttle plates open?  I hate to throw a stiffer return spring on it because I'm worried about premature wear on the throttle shaft bores.

[Steve68]

Leave it on manifold vacuum, set your idle above the "whistle" point, and drive it.

Well, there is one other piece to the puzzle that I haven't told you (sorry for holding back).  With the stock return spring, the carb doesn't always like to return to idle - it stops just short of the idle speed stop screw.  If I blip the throttle or pull the gas pedal up with my toe it goes  back to a nice idle.  It only does this with the engine running, and when the engine isn't at full operating temperature (but not running on the choke).

Could too much vacuum at idle be trying to keep the throttle plates open?  I hate to throw a stiffer return spring on it because I'm worried about premature wear on the throttle shaft bores.

Jeff, a couple things come to mind on the throttle return issue.  Plates may be sticking on carbon build up inside the carb throat.  Does the car have a dashpot?  I think you mentioned you have A/C on the car so if the AC is on the dashpot holds the throttle open more.

Steve

[jeff68]

Steve - No dashpot.  Carb was just rebuilt, so it's clean.  The shaft bores were bushed during the rebuild, so maybe it needs a little time to settle in.  It's just strange that it only does it with the engine running.  It will be just above the stop when idling, then I shut the engine off and it closes against the idle stop.