[68chevelle]

Ok guys i have been reading some old posts - trying to figure out how to correctly set my timming - and all i am getting is confused. I will try to post what i've gathered from it and maybe somebody can shed some light on this and correct my wrong ideas. 1st) You need to disconect the hose going from the vacuum advance pod on the dist.to the carb and block them off. 2nd) I have heard two different theorys on how to set the initial advance - one is off the timming tab and # two is to rotate the distributer. (which is it - or is it both) - like you use the tab to get it where you want it and then rotate the dist. to readjust -maybe to make it start easier and then rotate it again to bring it back to where you want it? 3rd) I know the mechanical advance is all in the weights and how fast it comes in is due to the springs but where does the vacuum advance come into play? It's got to be there for a reason. 4th) Total timing as stated by a tech archive is the initial plus the mechanical - i can understand that ( minus the fact that i still dont know where the vacuum advance comes in) but then it says that to adjust total timming (holding the engine at 3000 rpms to 3500 rpms - depending on when all the timing is in) to rotate the distibuter - this doesnt sound right to me and maybe i am misunderstanding it but wouldnt that make the initial timming lower or higher ? I want the initail to stay the same and the "total" to be correct and to do this i would have to change the weights right? Maybe i have answered my own question here but what i think it is that i need to know is how to check the timming - to make sure it is right. I know i will need to get a balancer tape and a timming light but how do i go about it. - Once i have my intial set where i want it do i - just play with the weights until they are where i want them or do i just rotate the dist.? Thanks for letting me aimlessly ramble and for any replies that come from it.

Dan

 

[DaleM]

You're on the right track. Disconnect the vacuum line and plug it. Loosen the distributor just enough you can turn it. Set your initial timing with the engine running via the timing tab (i.e., 6 degrees, 8 degrees, etc BTDC) by turning the distributor very slowly. Initial advance w/o vacuum is to get the car started. As vacuum drops (you're on the gas), your timing will advance.

Snug down the distributor and reconnect the vacuum line. With the vacuum line attached, it should advance your timing a bit more...if it doesn't the vacuum unit in the dist is probably bad assuming you have a good ported vacuum connection on the carb.

Run the engine up to the rpm you want total advance at (say 3000 or 3500) and see if you've got all the advance you want (such as 36 degrees, 38 degrees, etc.) If you don't get all the advance you expect, you'll need to (1) give it more initial advance (w/o vacuum line) or (2) have the distributor weights/springs recurved with a kit from say Mr. Gasket or take it to a good distributor shop and have it degreed on a machine.



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Dale McIntosh
TC Gold #92
ACES #1709
www.dalesplace.com
Restore it or customize it, but drive it and enjoy it!

 

[Unclepennybags]

For an explanation of vacuum advance, think of the vacuum advance as a "load sensor" for the ignition system. As the load on the engine increases, the amount of timing that it requires is decreased. At light loads the engine can tolerate more spark advance, so the vacuum advance is added to the initial + whatever mechanical advance you have. The vacuum advance system is what allows you to run this extra spark advance at light loads. As the load increases, the vacuum advance provides less and less advance. So, you can think of the vacuum advance as a "vacuum un-advance" in that it un-advances the timing as the load increases.


Hope this helps.

MIke

 

[onovakind67]

The limits of the advance curve in your distributor are defined by the limiting pin underneath the weight plate. If it is a GM distributor, each 0.010" the pin moves changes the distributor advance by 1°. If you limit the pin to 0.100" movement by adjusting the slot it rides in to be 0.100" larger, your distributor will provide 10° of mechanical advance. Since 1° at the distributor equals 2° at the crank, this would provide you with 20° mechanical advance. If you set your total timing to 34°, your initial timing will be 14°. In order to change the range of mechanical advance, you need to change the size of the limiting slot. Vacuum advance will add another 10-20° at the crank, and you can buy vacuum advance cans with limiters in them. Here's a link to an interesting article on timing for modified engines.
http://members.aol.com/DVAndrews/timing.htm

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67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction

 

[Fine69]

<BLOCKQUOTE>quote:</font><HR>Originally posted by 68chevelle:

I have heard two different theorys on how to set the initial advance - one is off the timming tab and # two is to rotate the distributer. (which is it - or is it both) - like you use the tab to get it where you want it and then rotate the dist. to readjust -maybe to make it start easier and then rotate it again to bring it back to where you want it?

Dan<HR></BLOCKQUOTE>

Turning the distributor will change the advance setting. It will change both inital and total by the same amount. You use the timing tab and balancer marks along with the timing light to guage the changes...

As the others have stated the distributor has a set amount of advance built into it. This can be changed physically. You need a better understanding of the basics before getting into that though.

To help understand let's say your distributor has 20deg's of advance built into it. If you set the inital to 0 (zero) the total advance will be 20deg's btdc(before top dead center), if you set the inital to 10deg's btdc then the total will be 30deg's btdc...

Make sure you understand how to use a dial back timing light or how to read a degreed balancer and check your total timing and inital with the vacuum advance disconnected and plugged. Subtract the inital from the total to get the amount built into the distributor. Get comfortable with the basics before yanking the distributor and changing weights and springs....

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...Dennis
Topless '69

[This message has been edited by Fine69 (edited 04-24-2001).]

 

[cSchu]

This brings up a question for me....
I had a vacuum advance distributor on my '70 (377). I recently replaced it with a Mallory breakerless distributor, and it doesn't have vacuum advance. Of course, right after I swapped it out, it ran like a dog, so I checked to make sure I had installed it right. Then I started playing with the timing. I have plugged the vacuum ports on the carb since they are really useless now, but it still seems sluggish.
Any ideas?

 

[68chevelle]

Ok guys thanks for clearing everything up for me! I hope you dont mind me asking more questions because i have a couple of more. These relate to the performance of the engine through timming now rather that how to set it. 1) As the duration of the cam increases so should the amount of initial timming - correct? So with a stock cam is 10 degrees going to be too much? How do i know what the engine likes best. Also when i go to a bigger cam this spring how much do i need to add? Is there any equations and what factors play into this? Also since the vacuum advance is used during light loads can i use this as a way to offset extra timming - much the same as a wider lobe seperation on a cam makes it idle smoother. How do you get the best performance out of the timming without going into denotation? How much total advance should i run? and when should it all be in (3000 rpms vs. 3500 rpms) I can provide my specs if needed. Thanks again for all your help I really appreciate it!

Dan

 

[onovakind67]

If you've read Dave Andrews article you'll have come across the table equating cam timing to initial timing. If you haven't read it, you need to do this first.

Engine spark timing isn't like other engine commodities, where more is better. You actually want to use as little advance as possible, and with the more advanced combustion chamber shapes, maximum advance is decreasing. We're using 30-32 degrees on a 327 long-distance road racing motor, any more and the power starts to fall off.

Vacuum advance doesn't compensate for anything in the distributor, it is a method of advancing the spark timing to compensate for the loss of volumetric efficiency under part throttle conditions.

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67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction

 

[68chevelle]

onovakind67 thanks for the reply and the link. I now have the Dave Andrews article in my personal bookmark and have found it to be very helpfull. I appreciate everyones help!
Hey while im at it I have another question for you. On a chevy Hei, isnt the limit of the weight travel determined by the center plate/weight, that the actual rotating weights rotate around? thanks again

Dan


[This message has been edited by 68chevelle (edited 04-24-2001).]

 

[onovakind67]

On the HEI distributors, the inner posts that hold the springs extend downward into 2 slots which limit the rotation of the advance mechanism. Interestingly enough, the circle scribed by these posts has a circumference of 3.6", or .010" per degree of rotation. This makes it easy to adjust your HEI for limited mechanical advance, for every .010" larger the slot is than the pin you will have 1° of rotation of your distributor, or 2° of advance at the crank.
For example: You want 12° initial advance with 36° total mechanical advance. This would mean 24° advance at the crank, or 12° at the distributor. Take your distributor apart and use JB weld to close up one of the slots and drill it out so the slot is .120 larger than the pin, allowing the pin to move 12° total. When you put the distributor back in to set your timing, set the initial at 12°, and the total should be 36°, or set the total to 36° and the initial will be 12°.

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67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction