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421 Bird
Tribal Scout

USA
29 Posts

Posted - 04 Sep 2015 :  10:26:26 AM  Show Profile  Reply with Quote
I have a 1965 421 motor. #76 heads, Crower 60916 cam, Performer RPM intake, and Holley 750 DP carb. Compression is 10.75 and I run 108 octane fuel.

Since I a using a Pertronix distributor instead of points, should I increase the gap from 0.35? If so, what do you recommend?

Also, the distributor has adjustable vacuum advance. The factory setting is 16 degrees. Is this too much? The cam card says that the cam has 4 degrees of advance ground into the cam. Is that reflected in my timing numbers when setting the timing?

Obviously, I am new to setting timing. Thanks for the help.

Joe

2barrelpower
Sitting Bull

USA
135 Posts

Posted - 04 Sep 2015 :  11:29:49 AM  Show Profile  Reply with Quote
Gap at .035. More is hard on coil.

On cam timing, no. Four degrees of advance ground into the cam does not affect timing in a 1 to 1 sense. It will affect the characteristics of your engine, so in a roundabout way yes it will affect optimal timing, but not in the sense that you're asking.

As far as vac advance, 16 is in the ball park.

Someone will come along and link to or write out the proper procedure.

69FB, 350P, 9 to 1, Comp 260H, Stock Valve Train, Performer Intake, 1406 Carb, Hooker Headers, Points Style, Crane XRI, Crane PS20, TH350, stock stall, 3.08 gear, and 235/60/15 Firestones.
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Admin
Ye Olde Webmaster

848 Posts

Posted - 04 Sep 2015 :  11:43:26 AM  Show Profile  Reply with Quote
Hi Joe-
Your electronic distributor provides a bit more of an electrical pulse than the points, so you can comfortably increase spark plug gap to .045". Many HEIs during the 70s specified .060". IMO, that's probably too wide on a non-smooger performance engine.

When the cam manufacture states that the cam is ground with 4 degrees advance that number really has more to do with setting up the camshaft (degreeing it in). Comp Cams for example grinds most of their off the shelf cams with 4 degrees advance. Setting it up, dot to dot, usually means it really is at 106 degrees, but the LSA comes out at 110, because of the built in advance. This number has absolutely nothing to do with ignition timing and setting up initial mechanical timing on your distributor.

You need to differentiate between mechanical and vacuum assisted advance. What the Petronix people are saying IMO is that the vacuum advance unit will provide as much as 16 extra degrees of advance to the mechanical side. And that comes in during part throttle. So, if you engine pulls enough vacuum at lets say 70 miles per hour, the vacuum could very well provide as much as 16 degrees. At 70 on most street Pontiac V8s the mechanical advance should be at it's plateau, meaning it can't advance any more degrees mechanically. Typically this should be around 3000 rpm.

So, if your Petronix provides 20 degrees of mechanical advance, 16 from the initial timing, and as much as 16 degrees more from vacuum at 3000 rpm, you engine might see as much as 52 degrees advance. HOWEVER, it's highly unlikely that your engine would have a sufficient vacuum pulse at the rpm to deliver 16. It might be only 10 or 8 extra degrees of advance The only way to tell is to drive at that rpm with a vacuum gauge and line attached to manifold vacuum and read the amount of vacuum.

Your 421, IMO should need 34 degrees of mechanical advance, all in by 3000. So, if your Petronix unit delivers 20 mechanical degrees, set you initial timing at 14. This has to be set with no vacuum assist in play.

The other thing you should know is that vacuum can be attached to manifold or a ported source--usually at the carb. What this means is that at idle, let's say 800 rpm, the mechanical is set at 14. Plugging the vacuum line to a ported source at the carb will generate no additional advance until the throttle opens. In contrast, manifold vac is typically at its highest at idle. It could be anywhere from 10 to 20 of mercury depending upon the camshaft duration and overlap. For example, if the vac at idle provides 14 additional degrees of advance, your idle advance would be 14 mechanical plus (+) 14 vac providing 28 degrees of total timing advance. This is probably too much.

It is preferable to set timing with an HEI type electronic unit using a ported source.

"Dedicated to keeping the classic Pontiac engine alive."

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421 Bird
Tribal Scout

USA
29 Posts

Posted - 04 Sep 2015 :  3:10:25 PM  Show Profile  Reply with Quote
Thanks for the great information. I do have the vacuum advance connected to a ported source on the carb. When I disconnect the hose from the carb, the engine slows by about 250 rpm's. Is this normal?
The distributor has 23 degrees of mechanical advance that is all in by 4,000 rpm's. I can change the springs to have the mechanical advance all in by 3,000 rpm's, but this will provide 9 degrees of mechanical advance at 500 rpm's, and 13 degrees at 1,000 rpm's. I'm guessing there will be about 11 degrees of advance at idle. Should I then set the initial timing at 22(11 initial and 11 mechanical) giving me 34 total at 3,000 rpm's?

Thanks again,
Joe
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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 04 Sep 2015 :  11:01:33 PM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
How are you coming up with those advance numbers?

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.

Edited by - Bill Boyle on 05 Sep 2015 08:45:50 AM
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421 Bird
Tribal Scout

USA
29 Posts

Posted - 07 Sep 2015 :  2:20:39 PM  Show Profile  Reply with Quote
I disconnected the vacuum advance and plugged it. At idle, I set it at 11 degrees. Then brought the rpm's up to 4, 000, but only had 25 degrees of total advance. I then raised the idle advance to 15 degrees and brought the rpm's up to 4, 000, and read 29 degrees total. I am not sure why I am not getting the full 23 degrees of mechanical advance.
I hooked the vacuum advance up and raised rpm's to 4, 000. Total advance was 41 degrees.
The car seems to be running fine. The motor does not run hot, and there is no hesitation. I have not gone wot yet, but when I did get on it briefly, the motor ran fine. Also, starts right up when hot.
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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 07 Sep 2015 :  7:55:39 PM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
Hmmm. The weights are only providing 14 degrees of centrifugal advance (mechanical). The tension of your springs is preventing more or there is a stop in place limiting further advance. Typically most HEI type distributors allow 19-20 without modification.

The total amount of mechanical advance should terminate around 3000 rpm. With vacuum assist the advance will add to the mechanical advance. At 3000 rpm you should see a total of 34 mechanical degree (14 initial + 20 mechanical). Add another 10 with vacuum at 3000 rpm under part cruising throttle and you engine should see 44 total timing (vac and mechanical).

Based on what you've posted, I can only conclude that more tuning is needed.

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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421 Bird
Tribal Scout

USA
29 Posts

Posted - 08 Sep 2015 :  12:23:40 PM  Show Profile  Reply with Quote
Bill,

I have attached the mechanical vacuum specs for my distributor. The distributor currently has the silver springs that are factory installed. I should be all in at 4,000 rpm's with 23 degrees of mechanical advance.
I guess it's possible that I am not getting an accurate reading from the timing light. I am going to get a new light and see if I get the same readings, or not.

Joe

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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 08 Sep 2015 :  1:18:55 PM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
The pair of silver springs is your best bet. However, there needs to be a stop or a limit to further mechanical advance beyond 3000 rpm.

Then adjust your initial setting at 15 degrees to yield 34 at 3000.

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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421 Bird
Tribal Scout

USA
29 Posts

Posted - 08 Sep 2015 :  4:04:39 PM  Show Profile  Reply with Quote
I have the timing set at 15 degrees now. At 4,000 rpm's, I have a total of 29 degrees. For some reason, am not getting the full 23 degrees of mechanical advance. I took the rpm's above 4,000, but there was no additional advance. I am going to double check the numbers with another timing light. If they are correct, it seems that I would need to advance to 20 degrees to get a total of 34. Is this too much?
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cortcomp
Coyote

USA
5339 Posts

Posted - 08 Sep 2015 :  4:07:43 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Likely hard to start. You need to find out what's stopping you from getting more timing (a positive stop maybe in the dist?) and then work it/file it/twear it whatever to get MORE timing than you're getting, without changing initial.
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421 Bird
Tribal Scout

USA
29 Posts

Posted - 08 Sep 2015 :  4:33:47 PM  Show Profile  Reply with Quote
The car starts as soon as I turn the key. The engine is not running hot. The motor seems to be running fine. No hesitation and no pinging. I am going to put a different timing light on it and try to find a road where I can really get on it so I can see how the motor performs.
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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 09 Sep 2015 :  08:55:12 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
421 Bird, your chart reflects an expected curve based on spring tension. In my experience, I have found that many times the recommended springs are inaccurate. Mindful of this, re-curving a distributor, whether for a points distributor or HEI, is a test of patience and requires a variety of spring choices to dial it in.

The design of the points type distributors was to allow more mechanical advance from the weights in comparison to the later HEI electronic units. Tuning an engine with points required different, usually lower, initial settings at idle (drive). Points distributors typically ran off of manifold vacuum getting increase advance as soon as the vacuum line was attached. HEIs in contrast typically worked off of a ported vacuum source. When the vacuum line was attached to an HEI there was no assist at idle. HEIs required more initial anywhere between 14-18 degrees. The carb had to be adjusted differently to support these different ignition nuances.

Re-curving either unit is tricky. On the car it's more difficult; off the car on a dedicated distributor machine, the re-curving process is a bit easier and much faster.

A performance curve takes many variables into consideration. The stock settings on a points distributor or HEI are a compromise, meaning some performance is hidden. Modifying the stock distributor for performance considers the engine, transmission, gearing, and type of use with an eye towards better acceleration and gaining more overall power. Economy is a second thought.

Because of the physics involved with a 4 cycle gasoline combustion engine, the curve should be all in around 3000 rpm. That's a typical goal. Faster curves, those coming in quicker may be good for some engines but not for all. Use is important. A street car doesn't need a curve that comes in by 2000 rpm, for example. Idle quality is most often inconsistent--okay for a strip car, lousy on a car sitting at a stop light.

The amount of mechanical advance should stop. Meaning there should be no more creep as the rpm increases. Typically a stop is created to prevent the advance. So, if 20 degrees of advance has been achieved with the springs at or near 3000 rpm, a stop containing it there should be included in the re-curve.

From time to time, the weights and center piece will not allow the desired amount of advance. In that case, trimming the center piece may gain those few extra degrees. This is risky though, because OEM distributor weights etc are hard to come by. Grinding too much ruins things. This is best left for those with extra weights and related parts. Scavenge a junk yard for these critical parts to have on hand as extras. Keep the parts paired up.

Here is an excellent article written by Jim Hand. It is informative and mostly applies to HEI and specifically Jim's engine in his wagon. Please read it when you have the chance.


http://www.pontiacstreetperformance.com/psp/distcurve.html

I hope this is helpful to you.

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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421 Bird
Tribal Scout

USA
29 Posts

Posted - 10 Sep 2015 :  1:54:36 PM  Show Profile  Reply with Quote
Thanks, Bill. That's great information. I also called Pertronix to see what they thought. I was told to take the rotor off and look at the underside. He said the there is sometimes a little tab from the mold when the rotor is made. If so, trim the tab, as it may be holding the weights from moving all the way. He also told me to make sure the weights are installed correctly, as sometimes they may be installed for a clockwise rotation. I will see what I find.
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