Adjusting timng with Vacuum Advance

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Wally426ci

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I have a stroked 360 408-412-416...... anyway, After alot of reading, I am seeing the definition of different timing refferances. Initial: being set at idle speed with plugged vacuum, Total: being set at 2500+ rpm's, and cintrifugal being the difference between the 2.

BUT, my question is, how can i be sure i am getting the most out of my Vacuum advance? when can i measure that and what am i looking for.

And, what does anyone think of the argument of using Ported vacuum as opposed to Manifold vacuum.......

Thanks!

:read2:
 
Manifold vacuum is pretty much constant or fluxuates much less than venturi vacuum. There is a resivoir in the manifold vacuum circuit on some vehicles too, which would delay the responce.
 
Walley, the MP engines book as a nice read on what to do.
What carb do you have?
Ported vacuum is what your looking for.

As I read what you wrote, it looks good, save the morning coffee not fully down the hatch........

As lon as the distrib has all the timing in by 30 something degrees, your good. I say 30 something because stock heads require more, aluminum w/ a high quench area, or alot of good quench area needs less. Somewhere around 32 or less.
 
Thanks guys. I was hearing between 32-36 Total degrees...

Im running a 750 Edelbrock. cast iron heads.

I also read that you could change intitial timing while keeping the same total, since that is due to cintrifugal.......
 
Yes, the intail is where you set the distributor and lock it down. Try plugging the vaccum source on the carb for the distributor, set intal timing @ 5*'s advanced, with a fully warmed up engine and a RPM gauge, rev it up to 2600, set the distributor @ 32 BTDC, lock it down.

Then hold the RPM's between 2000 and 4000 and find the total mechanical advance. Limit the total advance to 52*'s. Hook vacuum back up.

This is general and should get you in the ball park, very close. Tune for highest vacuum while your at it.
 
Your initial is going to be a function of the cam you are using. The more duration and over lap the more initial you will need. I have a cam with 223/230 @ 0.050 lift and it likes 17 degrees BTDC. You will need to limit the mechanical (centrifugal) advance when you get the initial this far from a stock setting. If you have an after market or MP distributor there will be an adjustment for this but you need to keep the total initial + mechanical around 34-36 with a sb with open chamber heads.

Vacuum advance only operates at steady state light load cruising. It's purpose is to improve efficiency (i.e., gas mileage).

You need to know what your manifold vacuum is when driving at a steady speed on a flat road. To get the vacuum advance set up you really need a vacuum source like a Mighty-Vac pump.

Using the vacuum source determine at what vacuum level the vacuum advance is fully in. If it is higher or lower than what you measured for the engine there is a small allen screw inside the port of the vacuum advance can that you can adjust the spring preload on the can. Don't recall which way is which to turn, but adjust so that your vacuum advance is all in when at your manifold vacuum level.

Now drive the car as long as you don't have any throttle tip in detonation with the vacuum advance connected you are good to go. If you do you will need to back off the adjustment until it doesn't.
 
Cool. im wondering though, if i set initial and then change it at full revs, how can i keep initial the same? wont turning the distributor twice change the previous setting?
 
You will need to mechanically adjust the distributor internally. The Mopar Performance distributors have a set screw that locks a stop. Using the tuning kit that is available you take your intial subtract from the 34-36 total and the result is where you want to set the mechincal stop. From the kit you pick the spacer that is the closest to what you want and put in the advance slot and adjust the stop to the spacer. MAllory distributors work the same (the MP uses a Mallory mechanism). Accel distributors have graduations that you adst the stop to and MSD use different size bushings.

If you have a stock distributor then you will need to take it apart and weld or epoxy the slots and through trial and error file them to the correct length.

There are spring kits that allow you to adjust the rate at which the mechanical timing comes in. Most kits will have charts that you pick the curve you want and from that it tells you what springs to use.

Lots of folks will just set the total timing a 2500+ rpm and call it a day. This leaves a lot of tuning and improved driveability on the table and is a very innaccurate way to set timing, it works OK for a race only motor but not for a street driven vehicle.

You need to find the initial the engine wants and you do that with a vacuum gauge by increasing the timing until you find max mainfold vacuum. Then adjust the mecahnical advance mechanism to give you what the heads want. Open chamber small block mopar heads like 34-36, more modern heads like magnums and aftermarket ones will like a little less. This is something that can only be dialed in with a dyno or many drag strip runs. The vacuum is then set up as I previously mentioned.
 
Im curious too!! I just took it out for a spin. I had 34 total, but also reading around 36 initial. then i hooked up the advance and had 20 degrees. It wouldnt run when i lowered it anymore. I think i need to get a vacuum gauge.
 
'64 Cuda said:
Where do you get the tuning kit for the MP distributor? Do you have a part number?
Click to expand...

The MP distributor uses a Mallory advance mechanism. The Mallory p/n for the kit is #29014. Summit sells a kit under there own name and is #850535. My wife put my MP catalog away for safe keeping and I can't find it but MP has it listed under a mopar p/n too. The MP kit is just a repackaged Mallory kit so it's easier to get it under the Mallory name from Summit or Jegs.
 
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