Distributor advance ... Does mechanical effect vaccum ??

[Canadian1968]

I have MP electronic ignition kit ( orange box) . Using the distributor that came with the kit . My question is if I limit the mechanical advance in the distributor with the tork screw adjusting plate. Will that effect my total timing that can be pulled with the vaccum advance hooked up ?

My problem is I only seem to have about 15* mechanical in the distributor unless I set my base at 23* I can't get my timing up to the desired 38* . If I hook up the vaccum advance it rips me off almost 35* again unless I wan to set my base timing at 0* which is not an option I will be getting way to much total advance  with the vaccum.

So if I close the slots in the distributor will that drop my total timing with the vaccum hooked up??  Spring kits only change the curve not the limit of timing correct ??

What should i do

[firefighter3931]

It sounds like you have the vac advance hooked up to manifold (full time) vacuum instead of ported vacuum. At idle the vacuum advance should not be functioning. Only under light throttle/steady cruise should the vacuum canister be activated. 

All carbs have a ported vacuum fitting for vac advance....you need to switch the vac advance hose to the correct port on your carb....or leave it disconnected and the port capped. 



Ron

[Canadian1968]

Haha no I have it hooked up right.

If I can set my timing at 10* base, hook up the vaccum no change. When I reach about 2500 RPM I am hitting around 43*
If I leave the Vaccum Disconnected and leave my timing at 10* Base, I will only reach about 25* at 2500 RPM

[BSB67]

I have MP electronic ignition kit ( orange box) . Using the distributor that came with the kit . My question is if I limit the mechanical advance in the distributor with the tork screw adjusting plate. Will that effect my total timing that can be pulled with the vaccum advance hooked up ?

My problem is I only seem to have about 15* mechanical in the distributor unless I set my base at 23* I can't get my timing up to the desired 38* . If I hook up the vaccum advance it rips me off almost 35* again unless I wan to set my base timing at 0* which is not an option I will be getting way to much total advance  with the vaccum.

So if I close the slots in the distributor will that drop my total timing with the vaccum hooked up??  Spring kits only change the curve not the limit of timing correct ??

What should i do

I'm not clear on exactly what your question is.  Start with what Ron recommends.  You should be able to adjust it to get a total mechanical advance of about 21 to 23° (crankshaft °) for the 15° initial and 38° total.  Hook up the vacuum advance to the ported vacuum and you are done.  If the car starts bucking at low speed in top gear, you might need to either slow the mechanical, or back down on the amount of vacuum advance.  There will be times cruising that the combined total timing will be in the 40s or maybe even 50s.  Its okay

[Canadian1968]

My initial question was if I adjust the mechanical advance does it ditectly effect vacuum advance at all ? Or are they completely separate systems .   
My example : base = 5 + 15 mechanical = 20
                     Hook up vaccum total timing is now 40

Now if I open up distributor and adjust to limit the mechanical advance by 5* should I get ..
                     Base= 5 + 10 mechanical = 15
                     Hook up vaccum SHOULD my total now be 5* less ??


But if BSB is saying  that my 43* is ok then. I guess I don't have a problem . I thought I was supposed to aim for 36 to 38 total timing , any more can/ will cause some problem . The engine is stock internally right now just a set of headers and 2 1/2 exhaust with Holley carb.  I have a Edelbrock rpm waiting to go on

Sorry for confusion I am just frustrated because I had the timing set where I was happy with . I was not running a vacuum line at all. I sent the distributor to a shop just have the pickup coil tested ever since then I can't seem to get it back to where it was

[firefighter3931]

Bump the timing up to 22-23* so it will have ~ 37-38* total. The ported vacuum will only kick in under light load. At full throttle the vacuum advance will not engage and you will not be pulling in any additional timing. 


Ron

[ChargerST]

It sounds like you have the vac advance hooked up to manifold (full time) vacuum instead of ported vacuum. At idle the vacuum advance should not be functioning. Only under light throttle/steady cruise should the vacuum canister be activated. 

All carbs have a ported vacuum fitting for vac advance....you need to switch the vac advance hose to the correct port on your carb....or leave it disconnected and the port capped. 

Ron

Hi Ron,
can you explain why the vacuum advance shouldn't be activated at idle? Someone posted a text about vacuum advance a few weeks back (can't find the thread but here is it on another site: http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm ) and there the author said (a GM/Chrysler engineer) to use manifold vacuum. The reason he gave was that the lean idle mixture burns slow and needs additional timing to burn completely.
what's your take on that?

[billssuperbird]

I have a Ellerbrock 750 looking at it from the front of the car witch side should the vac hose be on

[Canadian1968]

Bump the timing up to 22-23* so it will have ~ 37-38* total. The ported vacuum will only kick in under light load. At full throttle the vacuum advance will not engage and you will not be pulling in any additional timing. 


Ron

Alright. But under light load my vaccum will pull my timing up to almost 50 then. But this ok ?

[histoy]

In answer to your first question, the vacuum and centrifugal advance systems work independently of one another.  So you have 3 types of advance at work on a stock engine: initial (base) timing, centrifugal (controlled by rpm), and vacuum (controlled by manifold vacuum).   For a stock engine you set the base timing (zero or 5* btdc) for a '68 440.  You can then accelerate the engine to determine additional centrifugal advance using your timing light.  A total of 36 to 38 degrees is what most big block guys run when not running a vacuum advance.  The get up to that amount you'll probably need at least 18 degrees base timing, or higher as Ron explained.   If you had a stock engine, you would then connect the vacuum advance and it should start to kick in around 9 inches of manifold vacuum and be all in around 15 inches of vacuum.  The total vacuum advance should be around 10 -11 degrees.  On a stock engine all 3 timing settings work together, and the vacuum advance helps increase gas mileage and helps the engine run cooler.   When you eliminate the vacuum advance like you were running before, the high initial base timing covers up the fact that you've eliminated the vacuum advance.

Your other question about the combination of base, centrifugal & vacuum advance giving a total of around 50 is quite possible.  My wife's '64 327 Vette runs a base timing of 4 degrees, but with the centrifugal and vacuum advance included it can go as high as 52 degrees total.   

[BSB67]

Bump the timing up to 22-23* so it will have ~ 37-38* total. The ported vacuum will only kick in under light load. At full throttle the vacuum advance will not engage and you will not be pulling in any additional timing.  


Ron

Alright. But under light load my vaccum will pull my timing up to almost 50 then. But this ok ?

Ron, could you tell him 50° is okay, otherwise he might be stuck forever.

[firefighter3931]

Ron, could you tell him 50° is okay, otherwise he might be stuck forever.

Yes, as BSB67 stated 50* of total timing at cruise speed is fine....no worries. 

It's sometimes hard to wrap your head around that number when we all talk about 38* being the magic number but keep in mind that is under a full load/heavy throttle condition. Under light load the engine will not be so octane dependant and the extra timing can help a bit with fuel economy.

To explain further : say you're cruising along at 2200 rpm and the engine is running nice and smooth. At this point you're making roughly 18* of vacuum and the vacuum advance is engaged. Now you decide to stomp the loud pedal and the vacuum drops to zero so the vacuum advance disengages and you're back to 38* total and still in the safe zone. That is how vacuum advance works.

Generally, only mild engines that make more than 15in hg of vacuum will properly function with the vac can hooked up. If your engine begins to surge/buck....more than likely you don't have sufficient vacuum to run the vac advance and it should be disabled.

Hope that helps explain the concept 



Ron

[Canadian1968]

Thanks for the patience . I thought anytime your timing got above 40* was bad news . Your explanation makes sense to me now. Guess that why you do all your timing adjustments with the vaccum line Disconnected haha.  Your not exactly going to stand there and put it at WOT with your head hanging over trying hit the timing marks with your light !! ... well at least I'm not going to !!.

[flyinlow]

From your origonal post.

The mechanical (centrifical) advance works by advancing the reluctor star  wheel. The vacuum advance works by advancing the pick up plate.

I have to limit my vacuum advance to about 8*.  I used a small washer ,spilt it then epoxied to the back of the vacuum chamber. Crude but effective.

Try yours first, might be fine as is.

I have tried running vacuum advance at idle on Mopars and ended up going back to ported vacuum signal. GM used it Olds, Buicks and some of their other low output engines.