Distributor timing advance

Distributor timing advance

Joined: September 13th, 2007, 7:49 pm

February 19th, 2012, 11:54 pm #1

OK guys I have started to work out my timing problems. I will verify TDC and the marks on my balancer tommorrow.I have ordered a new timing light, with all the bells and whistles so that I will be accurate when I set my timing.
I got out my install info on the Mallory Unilite distributor. The factory preset mechanical advance is 24deg. at 3,000-3,200 rpm. I have the degree key so that I can change the advance in 2 deg. increments, starting at 18 deg. all the way to 28 degrees. Where should I start? Am I right saying that if I use the 24 deg. setting I would need 12 deg. at the crank to get a total of 36 deg. at 3,000-3200 rpm?
What should I shoot for? This is a vacum advance system.
I also ran a compression check all cylinder running from 205 psi to 225 psi does that sound right for a closed chamber flat top motor?
Thanks! Eddie
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Joined: May 20th, 2005, 4:48 pm

February 20th, 2012, 12:09 am #2

With 24 degrees of (mechanical) advance, starting with 12 degrees initial will put you at 36 degrees total.

Most of my stuff likes 32-34 degrees total, but you can try different setups. With the vac advance, you'll get much more than that.

I can't remember your exact combination, but if I remember right, you have a pretty steep rearend gear. You may want to experiment with a little quicker curve...maybe with all of the mechanical advance being in by about 2700-2800 or so.

As for the fancy timing lights, make sure that if you bought one that's a dial-back light, that you have the dial set to zero if you're not used to using one.



Brent Lykins
B2 Motorsports, LLC






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Joined: October 1st, 2004, 8:05 pm

February 20th, 2012, 12:47 am #3

16 initial + 20 centrifugal (in by 2800 rpm)

Gotta use ported vacuum so the vacuum advance doesn't add to the ignition advance at idle

your other alternative is:

6 initial + 30 centrifugal (in by about 3600 rpm)

This curve allows you to use manifold vacuum, thereby adding 10 degrees to the ignition advance at idle.

In this example I assume your vacuum advance adds 10 degrees maximum.

-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
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Joined: February 28th, 2008, 4:15 pm

February 20th, 2012, 5:01 am #4

I would use full manifold vaccum. I set the advance curve at 20*-24* and the initial at 12*-16* and then the vaccum on top of that. Then tune the carb on these setting. With this timing I get a sharp running motor. Your motor needs alot of timing for the lean run condition at idle.
Andy
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Joined: December 16th, 2003, 1:59 am

February 20th, 2012, 5:55 am #5

but the point is that the amount of timing the vacuum pulls has to be limited significantly from what is normally available out of a vac can that's set for ported vac

i've seen 60* + from a wide open vac can

there'll be phantoms, there'll be fires on the road... and the white man dancing
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Joined: October 1st, 2004, 8:05 pm

February 20th, 2012, 11:16 am #6

I tried to make things simple ...

Disclaimer: These ignition settings are generalizations and do not apply to every motor.

For the 351C with iron 4V heads Ford suggested a centrifugal advance curve of 17° to 22° (nominal 20°} that starts at 1200 rpm and is all in by only 2000 rpm (OHO manual of 1972) or 2800 rpm (Muscle Parts catalog supplement no. 2 of 1970). The initial advance was then adjusted to provide 36° to 40° total advance (14° to 23° initial advance).

Motors equipped with alloy heads featuring high swirl combustion chambers usually call for 28° to 32° total ignition advance. Motors equipped with quench combustion chamber iron heads and dished pistons will also call for a little less ignition advance, whereas motors equipped with pistons having pop-up domes will call for a little extra ignition advance.

I consider Fords OHO centrifugal advance curve too fast for street motors. I prefer a centrifugal advance curve that starts 200 to 400 rpm above the idle rpm speed and swings 10° every 800 to 1000 rpm, dependent on gearing. To determine how many degrees of centrifugal advance is needed subtract the degrees of initial advance from the degrees of total advance. Total advance is best determined on a chassis dyno, I shall describe how to optimize initial timing advance below.

To optimize the initial ignition advance setting:

(A) Connect a manifold vacuum gauge, an accurate tachometer or an exhaust gas temperature (EGT) gauge to the motor. Leave the vacuum advnace hose connected if the motor has vacuum advance. Loosen the distributor clamp bolt if the motor is equipped with a distributor, start the motor and allow it to warm up to full operating temperature.

(B) While the motor is running at idle speed in neutral (manual transmission) or in drive with an assistant firmly pressing on the brake pedal (automatic transmission), and assuming the initial advance is in a retarded setting; begin slowly turning the distributor clockwise to advance the initial advance setting, or increase the initial advance setting of your distributorless ignition controller using a potentiometer or a lap top computer. As you do the idle speed shall increase, the intake manifold vacuum shall increase and the EGT shall decrease.

(C) Continue advancing the initial advance until you reach the setting where idle speed no longer increases, manifold vacuum no longer increases or EGT no longer decreases. The setting where the readings first stabilize is the optimum initial advance setting.

(D) Tighten the distributor clamp bolt if the motor is equipped with a distributor, and reset the motors idle speed, you are done.

(E) After optimizing the initial advance setting the amount of centrifugal advance will require adjustment to attain the proper total advance; this is very easy to do with an distributorless ignition.

High compression motors often have trouble hot starting with the ignition set at optimum initial advance. If your motor will not hot start after setting the initial advance as described above you have two recommended choices to resolve the problem:

(A) Install a new or more powerful starter and/or a battery with more cranking amperage
(B) Install an ignition module that retards the ignition during starting

And if all else fails, there's a third alternative. This solution is detrimental to the engines low speed performance, which is why it is not a "recommended" solution.

(C) Set the ignition for less initial advance and compensate by increasing the centrifugal advance.

Street motors should utilize vacuum advance for optimum efficiency during part throttle (high intake manifold vacuum) operation. Vacuum advance will more than improve fuel economy, it will prevent burning up the exhaust system during partial throttle (high manifold vacuum) cruising. The general consensus is to limit vacuum advance to about 10° to 12°.

I do not wish to enter into a ported vacuum verses manifold vacuum debate. Truth is either method can be used, each has its advantages.

Ported vacuum sets a higher limit to how low the advance will be retarded under low rpm/low manifold vacuum conditions, it allows a race motor type ignition calibration while taking advantage of vacuum advance for cruising (high manifold vacuum, part throttle) conditions. It is the method I prefer and the method recommended by aftermarket ignition manufacturers such as MSD. This method results in peppier throttle response. It should never cause a problem if a car is geared low enough and if a motor is in a good state of tune. However, the manifold vacuum method is useful when a high compression motor has trouble hot starting using the ported vacuum method, because the static/initial advance is significantly retarded. It is also useful for high geared or heavy vehicle applications in which the motor "pings" with the ignition tuned for ported vacuum.

For example:

Assuming a motor's optimum total advance is 36°, its optimum initial advance is 16°, its vacuum advance is limited to 10°, and it idles at 1000 rpm.

Ported vacuum:
16° initial advance + 20° centrifugal advance = 36° total
centrifugal advance curve starts at 1200 rpm, ends at 2800 rpm (10° per 800 rpm)
advance at idle = 16° (16° initial setting, no vacuum advance)
Partial throttle advance gain above idle is a combination of centrifugal advance plus vacuum advance
The vacuum mechanism always supplements total advance

Manifold Vacuum:
6° initial advance + 30° centrifugal advance = 36° total
centrifugal advance curve starts at 1200 rpm, ends at 3600 rpm (10° per 800 rpm)
advance at idle = 16° (6° initial plus 10° vacuum advance)
Partial throttle advance gain above idle is solely due to centrifugal advance (manifold vacuum decreases above idle)
The vacuum mechanism can either supplement or diminish total advance under various engine speeds and loads

Ignition advance can actually take a "dip" at throttle tip-in using the manifold vacuum mehtod, which is why ported vacuum provides peppier throttle response. As motor speed increases the centrifugal mechanism increases advance while decreasing manifold vacuum results in the vacuum mechanism decreasing advance, therefore the advance mechanisms buck one another using the manifold vacuum method, resulting in less advance during part throttle cruising compared to the ported vacuum method.




-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
Last edited by gpence on February 20th, 2012, 11:50 am, edited 1 time in total.
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Joined: May 20th, 2005, 4:48 pm

February 20th, 2012, 11:42 am #7



Brent Lykins
B2 Motorsports, LLC






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Joined: September 13th, 2007, 7:49 pm

February 20th, 2012, 8:07 pm #8

16 initial + 20 centrifugal (in by 2800 rpm)

Gotta use ported vacuum so the vacuum advance doesn't add to the ignition advance at idle

your other alternative is:

6 initial + 30 centrifugal (in by about 3600 rpm)

This curve allows you to use manifold vacuum, thereby adding 10 degrees to the ignition advance at idle.

In this example I assume your vacuum advance adds 10 degrees maximum.

-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
Thanks "Old Man"! That is so simple even a dumby like me could do it! I do use the ported (top) port on the Holley. I will take the distributor back to the 20 deg. as a base line. To get the advance to come in earlier I will have to go to lighter springs in the distributor, right? The 16deg. initial sounds "normal" to me on the Cleveland.
One more question Gearge, about setting valves. I am going to re-set all of my valve train while I am at it. I have guide plates, Crane gold roller rockers, with a hydrolic cam. I have mine set at just till all the slack is taken up (puch rod to lifter to rocker) and then I have gone !/4 turn more (per my machine shop). I have seen everything from 1/2 turn to one full turn. What do you guys recomend?
Bret I did buy the back lit,w tach,dwell,advance reading timing light. I have thrown out my old plain cheepo!
Thanks again for all the info guys! You are the best! Eddie
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Joined: May 20th, 2005, 4:48 pm

February 20th, 2012, 9:00 pm #9

On setting the valves, here's the procedure I follow:

Set the intake valve when the exhaust valve is starting to open. This insures that the intake lifter is on the heel of the lobe. As you say, take out all lash....sneak up on it and look for axial play, not necessarily how easy the pushrod will twist between your fingers. Once you get to zero lash, I would add 1/2 turn. Set the exhaust valve when the intake valve is starting to close.

The amount of preload basically amounts to the rocker stud thread pitch, as well as how much preload the lifter is designed for. However, a 1/2 turn is pretty safe in most cases.

Brent Lykins
B2 Motorsports, LLC






Last edited by blykins on February 20th, 2012, 9:22 pm, edited 1 time in total.
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Joined: October 1st, 2004, 8:05 pm

February 21st, 2012, 4:13 am #10

Thanks "Old Man"! That is so simple even a dumby like me could do it! I do use the ported (top) port on the Holley. I will take the distributor back to the 20 deg. as a base line. To get the advance to come in earlier I will have to go to lighter springs in the distributor, right? The 16deg. initial sounds "normal" to me on the Cleveland.
One more question Gearge, about setting valves. I am going to re-set all of my valve train while I am at it. I have guide plates, Crane gold roller rockers, with a hydrolic cam. I have mine set at just till all the slack is taken up (puch rod to lifter to rocker) and then I have gone !/4 turn more (per my machine shop). I have seen everything from 1/2 turn to one full turn. What do you guys recomend?
Bret I did buy the back lit,w tach,dwell,advance reading timing light. I have thrown out my old plain cheepo!
Thanks again for all the info guys! You are the best! Eddie
Assuming your rocker studs have 7/16 x 20 (national fine) threads on the adjuster end, each full turn of the adjuster nut = 0.050".

Therefore a quarter turn = 0.0125". With iron heads on an iron block, adjusted at room temperature, 0.012" will keep your motor's valve train clearances out of trouble (gap free) from freezing to over-heating. A half turn as Brent suggests provides a big margin of safety.

Now the philosophy part ...

Remember anti-pump-up lifters? they had a heavy duty clip of various kinds holding the lifters together so they wouldn't come apart if the lifters pumped up. I eventually stopped using them, and stopped worrying about pre-load and pump up. Although I didn't stress over pre-load, I also avoided excessive amounts of it. I think the range we're talking about, 0.012" - 0.025" is cool.

I was taught the idea is to engineer the valve train so as to avoid pump up, not plan for it ... because pump up only happens when valve train parts lose contact with one another creating clearances that aren't supposed to be there. So instead of anti-pump-up lifters, I used anti-pump-up valve train. In other words ... stiff push rods, light weight valves, light weight spring retainers, and adequately stiff springs.

Its very reassuring to know that you can rev the balls off a motor and not worry about anything bad happening because you planned for it in advance. Its called good engineering. I think the expense of light weight valves is more important than all sorts of parts people spend their hard earned money on. Among the first hot rod/race parts Ford sold over the counter for Cleveland cylinder heads (Boss 302) were titanium intake valves and light weight exhaust valves.

I don't mean to infer you need to change anything now or spend any additional money, just store that philosophy away for future use; and hope the data is retrievable.

end of philosophizing ... back to drinking ... cheers

-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
Last edited by gpence on February 21st, 2012, 5:00 am, edited 1 time in total.
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