Team Chevelle banner
Cancel

Timming - I'm lost

1638 Views 9 Replies 6 Participants Last post by  onovakind67, 
6
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
See less See more
Save
Like
Status
Not open for further replies.
1 - 3 of 10 Posts
O
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

------------------
67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction
See less See more
Save
Like
O
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.

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

------------------
67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction
See less See more
Save
Like
1 - 3 of 10 Posts
Status
Not open for further replies.
Team Chevelle
A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
Bench Racing Performance Engine Body Shop Chevelle Tech
Top