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Old 08-11-2012, 05:06 PM
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Initial Timing?

Can someone explain Initial Timing, and Ignition Timing for me? i think i have a grasp on Mechanical & Vacuum advance.
Does it come from the Cam?
My Set up is on a sbc 350, 30 over. 186 double hump heads with 2.02/160 ported and polished, rpm performer manifold, Lunati VooDoo 268/276 cam, Holley 650 dp, 1 5/8 headers, New HEI dist, 350 turbo tran, 3.73 gears, B&M holeshot convertor. street truck that will be run hard... but not on the track. driven 30 miles to work (HWY) a couple of times a week.
Thanks!

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Old 08-11-2012, 06:14 PM
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Give this a shot, it's a good read:

http://www.corvette-restoration.com/.../Timing101.pdf
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Old 08-11-2012, 06:23 PM
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You can find the general info on ignition timing and the whys on the wiki's on this site.

Your 268 cam will like between 18 and 24deg initial base timing.
from that you want to develop a curve that maxes out at around 34 to 36deg
at max advance. So adjust the length of the mech curve accordingly.
the curve should not start advance off the base until 1000 to 1200 rpm
and should advance smoothly to a peak at 2800 to 3400rpm.
the rate has to be found by trial and driving.
The fastest most agressive rate it not nessessarily the best.
But a good dose of initial (18 to 24deg) is warranted with that cam.
a smooth curve is that returns to base at idle is critical.

You must set the limit of mechanical advance with a
limit bushing or other method of limiting the adv travel.
More initial requires a shorter curve.
You want a 10 deg to 18deg curve length.
I suggest 10 to 14deg. (allows 20 to 24deg initial base timing.)

vacuum advance is separate. Generally 10 to 15deg max vac adv.
(at highest manifold vacuum, (deceleration)
The rate has to be found by drive testing.
It is important to limit the max possible. vac advance
10-15deg.

Use ported or full manifold vacuum. your choice.
For your set up I usually use ported vacuum. (auto trans).
try em both so you can learn the effect of both.
Make sure the vac advance does not vary wildly at idle
between neutral and in gear. Make sure it is not all pegged in at idle
. Its primary function is adding timing when under part thhrottle
cruise, light engine load.

Get a power valve that will stay closed lean at idle, in gear. (4.5 to 6.5") for the 268 cam.

If you got the holeshot 3000 or 3600 converter it should really haul the mail.
get some sticky tires.
when setting the base initial timing using a timing light slow the idle speed down to ensure you are on the base of the curve. Set it with the vac adv disconnected.
again these cams like a healthy dose of initial timing.

Last edited by F-BIRD'88; 08-11-2012 at 06:30 PM.
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Old 08-11-2012, 06:33 PM
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You can upgrade that 650 to 750hp and gain down leg boosters
adjustable air bleeds and more flow and power to good effect
using a holley or proform HP center body kit.
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Old 08-11-2012, 11:49 PM
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Quote:
Originally Posted by 60GR8Chevy View Post
Can someone explain Initial Timing, and Ignition Timing for me? i think i have a grasp on Mechanical & Vacuum advance.
Does it come from the Cam?
My Set up is on a sbc 350, 30 over. 186 double hump heads with 2.02/160 ported and polished, rpm performer manifold, Lunati VooDoo 268/276 cam, Holley 650 dp, 1 5/8 headers, New HEI dist, 350 turbo tran, 3.73 gears, B&M holeshot convertor. street truck that will be run hard... but not on the track. driven 30 miles to work (HWY) a couple of times a week.
Thanks!
Here you go.
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Old 08-12-2012, 10:47 AM
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Thanks to you all for the advive and two great reads... I'll have to read them a few more times and really look at them. One question keeps popping into my mind, does the 4 degrees of advance ground into the cam come into play while setting the initial timing? do you factor in that and set it, at say, 14 to 20dgr rather than the, 18 to 24dgr?
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Old 08-12-2012, 11:08 AM
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NO.... thats a separate thing. You are over thinking this.
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Old 08-14-2012, 08:16 AM
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F Bird.... lol you are SO right, seems to be my nature! can you suggest a good Harmonic Balancer? one that is marked for this 350?
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Old 08-14-2012, 10:45 AM
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Also... I have a new HEI dist, do they sell Mech advance curve kits? I assume they are different weights and different tensions of springs, where according to my set up should I begin my testing with these kits or parts?
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Old 08-14-2012, 12:37 PM
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If you have a stock HEI then this kit should help you along:

Crane Cams 99600-1 - Crane Vacuum Advance Kits - Overview - SummitRacing.com

It will not provide a way to limit the mechanical advance. If you inspect the HEI, you may find that either bushings or another form of limiter method can (sometimes a simple set screw) be used to limit the travel of the rotor when the centrifical force acts upon it.

The kit does include the limit for the vacuum advance, also springs for the rate at which the mechanical advance comes in vs engine RPM. It also has an adjustable vacuum adv canister which can be used to fine tune the rate of vacuum advance vs engine vacuum.

Usually you shoot for a mechanical advance limiter that holds the total amount of mechanical advance to about 14-16 deg advance. But it all depends on the initial. Remeber the total between initial + mech should not exceed 36 Deg as a ball park. So if you set your initial at 18 then you can set your mech limit at 16 for a total of 34, if you set your initial at 22 then you need a mech limit of approx 12-14 deg.

Once you get that all straight then work on the vacuum advance limit and rate. Limit the vacuum advance at approx 10-14 deg max, this is critical, most cans add about 20-22 deg at a mere 8-10" wg vacuum, too much. the crane adjustable canister will allow you to tweak the amount of vacuum advance added vs engine vacuum IT DOES NOT LIMIT THE VACUUM ADVANCE, use the limiter plate included in the kit.

The absolute total between initial + mech + vacuum adv should not be over 50 Deg BTDC advance

The one word you used to describe getting it right is "testing",,,have fun
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Old 08-14-2012, 12:43 PM
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I know the other guys have already stated some of what I put down, I am feeling chatty today,,,
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Old 08-14-2012, 03:51 PM
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Ok... I think i'm getting it down little by little. I understand setting the intial timing with the timing light between 18-24 degrees.
How do you check the mech curve? with timing light? And, I see how a set screw can limit the mech advance.
The new dist has a canister that can be adjusted, with allen wrench, to adjust vacuum advance.
Having trouble understanding still how to set mech advance limit?
Testing the springs is easy...
The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.
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Old 08-14-2012, 07:17 PM
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Quote:
Originally Posted by 60GR8Chevy View Post
Ok... I think i'm getting it down little by little. I understand setting the intial timing with the timing light between 18-24 degrees.
How do you check the mech curve? with timing light? And, I see how a set screw can limit the mech advance.
The new dist has a canister that can be adjusted, with allen wrench, to adjust vacuum advance.
Having trouble understanding still how to set mech advance limit?
Testing the springs is easy...
The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.
What distributor do you have? A stock GM HEI will use a different way to limit the mechanical advance than a MSD distributor, for example.

Unless you have a dial back timing light, the easiest way to set up the advance curve is to buy or make a timing tape or use a damper w/the outer ring indexed w/the degrees on it already.

Check the initial timing w/the vacuum advance disconnected and plugged. Note the reading. Then accelerate the engine until the mechanical advance quits rising- the amount you now see- minus the initial timing- is the amount of mechanical advance. In most cases you want the RPM where the mechanical advance quits advancing to be around 3000 rpm or less. This is set by changing the springs under the rotor that are connected to the weights of the mechanical advance.

The amount of vacuum advance needed is usually around 10-12 degrees. It can be set to come in/drop out by using the allen wrench adjustment. The idea here is to allow for as early (higher vacuum) of vacuum advance added as the engine will take w/o causing pinging on acceleration. It's a balancing deal and takes trial and error to get set right. The main thing is to not allow the engine to detonate (ping) under a load.
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Old 08-15-2012, 12:46 PM
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Good points cobalt, not sure if our OP is aware that the kit limiter plate has nothing to do with mechanical advance?

Quote:
Originally Posted by 60GR8Chevy View Post
The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.
The plate that comes with the kit is the vacuum advance limiter plate not the mech adv limiter. To install vacuum adv limiter plate on a HEI:

It goes in behind the canister actuation rod that is connected to the pickup ring on the distributor. Essentially it physically limits the travel of the rod which in turn limits the amount of vacuum adv timing added. Typically if you set it in there with the second last notch making contact with the actuation rod you will end up with approx 12 deg maximum vacuum advance added at full rod travel but it can vary with different distributors.

Note Installing the vacuum limiter plate or changing which notch is engaged will change the initial timing so that needs to be adjusted after installation or changing position of the plate. While changing the initial timing will not effect the vacuum advance characteristics we are talking about once they are set in.

Something else to consider,,,Each time you change which notch is engaged on the limiter plate you may need to re adjust the canister adjustment screw because the plate will shift the point at which the canister rod starts to move due to engine vac wg" applied to the canister diaphragm. Installing the plate on the back side of the actuator moves the range of the canister up. So if you think you have the can adjustment screw set were you want it and then add the plate better check your adjustment on the can again.

Once installed the limiter plate controls the maximum amount of vacuum advance added, the set screw in the can adjusts when the vacuum advance is added compared to engine vac.

I have never been able to adjust the crane canister for more than 12 in wg" as the top end of the range of the can, that is to say no matter how many times you turn the screw in the canister, the rod will always be at full travel once the vacuum level applied to the diaphragm is @ or more than 12 wg". This means that the timing added by the vacuum advance will be all in at engine vacuum levels at or above 12-13 wg". Do not turn the canister screw more than 9 or 10 turns out from all the way in, the screw will disengage from its internal threads and you have just pissed yourself off. It can be re-engaged with some effort.

A good place to start is 7 turns out (CCW) from all in. This will give a start point of about 6 wg" and an end point of approx 11 wg" with the plate in place. The approx 5 wg" range will remain the same and does not change much, the screw only shifts the zero (starting point).

Accell makes a can that is supposed to be able to limit the vacuum advance and also shift the range around without the plate limiter. I do not personally put faith in this canister because i have not tested it, one day I would like to bench test it, cut one open and see how this is done or if the claims are valid. The crane limiter plate works and the adjustable can does just that.

Put the plate in first then move to the initial timing setting, then work on the mechanical advance limit and curve as explained in other posts, then go back to the vacuum advance set screw tweaking. A useful method for me has been to get a vacuum gage with a long enough hose to reach the cab. Drive the truck and record what the engine vacuum level is at highway speed under light throttle load. If you are at say 12 "wg crusiing along at 70, try and get the vacuum advance canister rod to be fully engaged at 10 or 11 "wg using the screw. This can be done on the bench with a vacuum pump or just by lung power. The idea here is to have the vacuum advance start to drop out as soon as you give it more throttle on the highway but yet be fully engaged when cruising along.

This methodology and application is to provide a place to start, the proof is in the pudding.

If the engine rattles/pings no matter how you have the vacuum adv canister dialed in then you have more work to do on the initial and mechanical advance timing adjustments. Also gearing, trans shift points, type of fuel, compression ratio, air temperature, combustion chamber, piston type, A/F ratio and so on and so forth all effect the tendency of a motor to ping or not.

Have fun,,,
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Old 08-15-2012, 01:19 PM
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Good point about the vacuum advance limiter plate, Custom10. I remember a while back you did a lot of good research on this as well as the vacuum advance in general.
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