One thing I should have added is that we can still get good petrol over here in England and this has a bit to do with it as well. Low octane gas will limit the amount of timing you can put in the motor, Shell have started selling 100 octane at some of their station and super unleaded is still available in a few garages as well.

J.

 

Ok Bob, from what I'm reading here you are limiting your mechanical advance with spring choice. That's not right.

Your mechanical advance (the springs and weights) is a set amount of advance. It doens't change. Ok here's an example. I'm assuming your running a HEI? Anyway with a HEI the mechanical advance is set to advance 20 degrees period thru out the curve.

With your vacum disconnected, no matter how high it takes to rev it to make it stop advancing, it will only advance 20* . And it should advance the entire 20* by 2800 RPM with no vacum. So no matter where your initial timing is set, it will only advance 20*. 16 initial + 20 mech = 36 total. 18 initial + 20 mec = 38 total, see what I'm getting at.

Without physically modifying the advance slot on the HEI (welding it up some) your stuck with this extra 20* total advance that is added to the initial.

Aftermarket dist.'s are different. Some use swappable bushings to reduce or expand this 20* without welding.

Centrifical force it put on the weights when the dist spins. The faster it spins...the more centrifical weight is added. The springs only affect how much weight can be applied before they open fully and hit the stops (amount of total advance... 20* in our previous example) at a given RPM.

The springs just limit how fast this 20* comes in. This is called the curve. Lighter springs might let the weights hit the stops at 2800. Heavier springs may make it have to spin to 3500 before the weights open fully and hit the stops. You following me so far?

That's why you can't limit the total advance with springs. You won't get a true reading if they aren't opening fully and hitting the stops, because the faster you rev it the more it will advance untill it hits the stops.

To figure out your total advance do this. Put the heaviest springs in and take a reading at idle. Then take the spring off completly and take another reading. The difference between these 2 numbers will be the total mech advance your dist is capable of.

Now... no matter where your initial is set... you want all of this in and the weights hitting the stops by 2800 rpm's with 36-38 total advance. Play around with different springs untill you get it.

Then you can hook the vacum advance back up and it will give it around 50* total at cruising vacum or it should. That's why they make adjustable vacum advances.

Just email me if you need anymore help setting it.

 

"To figure out your total advance do this. Put the heaviest springs in and take a reading at idle. Then take the spring off completly and take another reading. The difference between these 2 numbers will be the total mech advance your dist is capable of."

So what your saying EDDIE82, is that all distributors may Not be capable of giving a total mech advance of 20. Right? The reason I point this out is that I read on one of the HotRod forums that the emission distributors limited advance to much lower than 20 and are not capable of producing the 36 degrees in by 3000rpm without setting initial timing to say 26 degrees. This is the problem I have with my 1979 HEI. Initial timing of 16 plus total centrifugal of 10 degrees, without advance vacuum attached. Can't get more centrifugal out of it.

 

To the best of my knowledge...and I'm probably wrong... but all the HEI's before the computer controlled ones came out it 81 had the mech advance total set at 20*. But whether I'm right or wrong you can check it yourself with my example. It should be very close to 20 + or - a couple of degrees.

Check to make sure your weights are opening completly and smoothly, sometimes they can stick and this will affect the total advance.

 

JHL81 thks for the article...
EDDIEJ...thks ok heres what i have....all in by 2500 rpm with no change for higher RPMs...
no weights in distributor/centrifugal advance of 26*
gold springs (lighteset) centrifugal advance of 16*
silver springs (med) centrigugal advance of 13*
black springs (heaviest) need to find my notes

so it appears to me that my springs are in fact limiting the centrifugal advance both in terms of curve and total advance.... i thought that both the bushings and the springs controlled this?

 

Bob was that a misprint "no weights " or did you mean no springs?

The weight are what causes the dist to advance

 

oops i meant no springs....this is what i think is going on..i have just read and reread a bunch of articles on this....

1. HEI distributors give false readings on centrifugal advance when measured with no springs...so that original measurement i took is useless..

2. lighter springs/heavy weights make the centfigural advance come in much earlier skewing the initial base reading...

so even though i carefully measured stuff its relatively pretty meaningless.

so i am going to do this...

A:with no vac canister connected and with my medium springs, set my timing at 2800 rpm so i have full centrifugal advance to 36*...let the base timing fall it it may, (knowing that the centrifugal advance is skewing it higher)

B: hook up vacuum cannister and measure timing at idle to make sure i have no more than 30-32*

how does that sound?

 

The springs only control the rate of advance, not the amount.

I assume your talking about the bushings that the weights use. The bushing only allow the weights to move on the posts without wearing out the weights themselves...unless you have aftermarket weights that have an elongated slot with different bushings controlling the stop point... but it still couldn't allow the stop point or total mech advance to move much before the weights hit the cap.

 

Yes thats going in the right direction.

But on your #1 point if you bring it up to 2000 and hold it steady for a 30 seconds or so to make sure the weights are against the stops and not bouncing on and off them, Then take a reading and it should be true.

Make sure it's not advancing past 36* no matter how high you rev it and you will be set

And you don't hold the idle at 2800 rpms and set the timing unless you know that it has stopped advancing at 2800.

Rev it at a steady rate till it stops advancing any further and set it at 36 no matter what the rpms are. It is desirable to have it all in by 2800 rpms. That's what the different springs do.

 

thanks.... i do have aftermarket weights

 

I should have wrote that like this. The springs are supposed to just control the rate of advance, not the amount.

With heavy enough springs you can make it to where the weights will never open fully enough to hit the stops.

 

and conversly really light springs open stuff up really soon and can come into play even at idle...

 

Exactly

 

good info have a few to do

 

i think i just got it...... i want heavy enough springs that they do not come open at idle so i get a real 16-18* base inital timing

i want these springs to be loose enough that by 2500RPM i have full centrifugal advance

and i want a combined initial timing of base timing + vacuum advance to be in the 30-32* range...


now with this sudden insight (I never said i was a genious just persistent) i think my medium springs may be opening up at idle which is making my base idle just a little higher then it should be...so i'm going to add a heavy spring in there and see how it effects my base timing and timing at 2500 rpm and adjust accordingly...stay tuned for tonights results

 

Does the base timing really matter? I'm set up for 36* @ 2500 with no vacuum advance. I set the timing with an adjustable timing light to give me the 36*. I don't care where the base timing is, as long as the car starts ok. Been running like this for the last few years without problems. Balancer has not shifted, so, I knew I was accurate.

An adjustable timing light is really needed if you want to know what is really happening.

 

well thats a good question that i'm not qualified to answer...but it seems that the issue is if the vacuum cannister is giving you say 15* and the base timining is giving you 20* because the weights are opening early then you really have about 35* of initial timing which is out side of the 30-32* range of base timing and vacuum advance. this 30-32* is suppose to be a really good starting point....now your car does not pull vacuum at cranking ....so what does this all mean..... I'm not sure, and if your car is working good i guess you are ok...

 

Here's something for the "ported vs. manifold" vacuum advance hook up debate. I prefer ported and this is why - If you start with 15-18 degree initial timing that should be plenty unless you have a huge cam or something, I see no benifit to more timing than that at idle, so why add more timing with a vacuum can at idle hooked up to the manifold (although I can see a reason for this if your stock distributor travels 30 degrees and you need to set the initial at 6 to reach 36 at full RPM)? But that is not my concern. My concern is that if you ARE using the manifold vacuum can to get your timing to 16 or 18 at idle then what happens when you do a hard throttle launch? Your vacuum goes down as you open the throttle suddenly and the timing is reduced probably at a greater rate then the centrifigul timing advance as you increase RPM. You are effectevely retarding your timing, if but just for a few seconds, at the very time that you need it to increase.

That being said, the total effect on the vacuum can and timing during these few seconds may be immaterial but still I have a feeling this may be a bad thing for performance.

 

BOB, what cr. do you have and what octain fuel you using ??

 

Bob,

Forget the vacum advance to start out with, timing is set with it disconnected. Glen's suggestion is a good one provided you know you are getting the full mechanical advance of 20* in the HEI. You just bring the engine up to the desired RPM and with your adjustable timing light set it accordingly to what you want, say 36*.

Then check the timing at idle (with the vacum disconnected) take that figure away from the total it showed at say 2,800 - 3,000 rpm and you should find out how much mechanical advance you have vs initial timing. If for example you read 16* at idle and it maxes out at 36* at 3,000 rpm then you know you have the full 20* in the HEI.

The vacum advance can is basically an emmision/economy thing the car would run fine with out it. If you want to run a lot of initial timing you should consider getting one of the ajustable vacum cans as the standard HEI ones give to much advance when you run with a lot of initial timing. When you are cruising at a steady 50 or 60mph the vacum advance comes into play, if your motor sounds like someone is in there working with a hammer and chisel you need to back down the initial timing or as I said get an ajustable vacum can so that loose some total advance.

Have Fun

J.