Pontiac GTO Forum

Pontiac GTO Forum (https://www.gtoforum.com/forum-index.php)
-   -   Distributor Tuning (https://www.gtoforum.com/f170/distributor-tuning-134141/)

PontiacJim 04-13-2019 06:35 PM

Distributor Tuning
Originally posted in another discussion by cij911 and moved to begin a new discussion in solving the timing issues.

I am trying to learn from those of you with more experience.

The distributor is NEW and there are NO problems with it, just that the vacuum can it came with has too much advance 25* vs recommended 10*. Additionally it appears the distributor has more mechanical advance than ideal - 30*, which has my base timing at 6* to get a total advance of 36* coming in before 3500. The car as it sits runs well and pulls hard.

What I am trying to understand is why changing the distributor would help. Basically the recommended setup has base at 16* (vs my 6*) and a 10* vacuum can so at idle it would be 26* (vs. my setup at 31*). My understanding is if a car runs well at 31* (vs 26*) it is even better as it will run cooler at idle. At WOT, I am at 36* (just as the recommended), so really no difference there.

So basically it comes down to partial throttle and transient response (which is a major part of normal driving). My question is am I likely running too much timing? While the recommended distributor has base timing 10* more than mine, I feel that is really a meaningless (as described above). So maybe the concern is I am running too much timing mid rpm range with modest load ? That I could understand, but I am unsure if that is true as I haven't seen any signs. Basically I guess the car could see as much as 51* (full mechanical + full vacuum) vs. the recommended setup as much as 30* (full mechanical of 20* + full vacuum of 10*). So is that the issue ? And if so, what would I see / hear / or feel that would indicate it was too much? (I haven't heard pinging, car feels responsive, and stays under 190* under the hardest driving.)

PontiacJim 04-13-2019 09:11 PM

1 Attachment(s)
Let's forget about vacuum advance for the moment.

Keep in mind that as you move total advance, whether advance it or retard it, it will affect your initial timing.

You state your distributor seems to have 30 degrees of mechanical advance. My personal opinion is that your total advance should be between 32-34 degrees. Lemans guy said 36 degrees total, but I think this is too much, but can still work. He tunes distributors and is more qualified than myself, but the most important thing is not having any engine detonation/pinging.

Taking your initial timing of 6 degrees, and adding 30 distributor degrees gives you a total of 36 degrees (6 + 30 = 36). If you wanted to go with 34 degrees as your total, you would do a subtraction, 34 degrees total minus the 30 degrees distributor and get 4 degrees initial (34-30 = 4). Obviously 4 degrees is too low and probably won't work to well.

So at this point, your initial advance is dictated by your distributor advance - they are fighting each other in that if you set your initial higher where your engine will like it, lets say 10 degrees for now, you will have WAY too much total advance (10 + 30 = 40). You say your engine likes 16 degrees total. If we add this with your distributor advance, 16 + 30 = 46, you will be experiencing burned holes in your pistons. So you cannot at this time set your initial advance on anything more than 6 degrees.

Problem #1 that I see is the distributor itself. 30 degrees advance may need to be reduced by limiting the advance of the mechanical weights in the distributor - if you were to set total advance to 34 or 32 degrees.

Now for the vacuum advance. 25 degrees advance from the vacuum seems excessive. Looking at the 1968 factory Pontiac specs (attachment included) shows the maximum vacuum advance listed under each distributor. You will note you only see 10 degrees.

The vacuum source can be either a ported or unported fitting on the carb or to the manifold. Both unported and to the manifold will give you direct engine vacuum.

If you use manifold vacuum (ported fitting or direct to a manifold fitting), you would have too much initial advance. Setting initial timing at the balncer for 6 degrees and then adding 25 more degrees of vacuum advance from direct manifold vacuum gives you 31 degrees initial advance at idle (6 + 25 + 31). Too much. An unported fitting would typically provide no vacuum at idle and not add the 25 degrees, so the engine will be running at 6 degrees initial which is not your engine's preference as it wants 16 degrees initial.

Vacuum advance comes into play under light load or steady throttle cruising. It is constantly changing as does engine vacuum depending on throttle position or load on the engine. 25 degrees of vacuum advance added to your total mechanical (36 degrees here) will only happen periodically and will vary from 0-25 in a constant state of change. It gets added to whatever the mechanical advance happens to be at specific RPM's. So for example if you were puttering along at 2,300 RPM's, you have not yet reached your total mechanical advance of 36 degrees because total advance does not come into play until you reach 3,500 RPM's. You might have a total mechanical advance at 2,300 RPM's of 28 degrees. Then when you let of the gas pedal, maximum engine vacuum results and you would now take your 28 degrees mechanical and add the total 25 degrees vacuum for a total engine advance of 53 degrees (28 + 25 = 53). My opinion from my reading is that around 50 degrees total engine advance is about where you want to be. But in any case, you have 53 degrees in this example. If the engine begins to surge at light throttle, then try reducing the vacuum advance total.

Now lets wind the engine up through the gears. Once you go past 3,500 RPM's and continue on your way to 5,600 RPM's, your engine has reached maximum mechanical advance (3,500 through to 5,600 RPM's). You momentarily let off the gas to make a shift for the next gear (assuming you are not pulling a fast hard slam-bang speed shift). When you let off the gas at any time after 3,500 RPM's (maximum distributor advance), engine vacuum soars to its maximum for just an instant and your vacuum advance kicks in for that instant. What you now get is your 36 degrees mechanical advance plus the 25 degrees of vacuum advance for a total engine advance of 61 degrees (36 + 25 = 61). Yikes! Way too much. It is possible that this is where your problem is coming from.

Going with a different vacuum advance that limits the total vacuum advance to 10 degrees and which has been suggested, would be the correct way to go. 36 + 10 = 46 degrees total engine advance. A little shy of the 50 degrees, but better than 61 degrees.

Problem #2 as I see it - incorrect vacuum advance as used with your total mechanical advance of 36 degrees. 25 degrees of vacuum advance is too much.

The above is to illustrate what I see is happening.

Next, you mentioned setting your initial at 6 degrees and letting the 10 degree vacuum advance can pull your initial up to 16 degrees at idle. This should work IF you connected the vacuum line to an unported fitting on your carb or directly to your intake - both of which would be drawing off full engine vacuum. So initial would be 6 plus 10 from the vacuum advance for a total of 16 initial advance (6+10 =16) and puts you were you want to be. Some engines like more or less initial, but if 16 seems to be best for your engine, then keep it there.

If you switch to a different vacuum advance can, you will also need to know at what engine vacuum it operates. Some will provide maximum vacuum advance at 10 Hg, or 13-14 Hg, or 20Hg of engine vacuum. The vacuum can may not add all of its timing at idle. As an example, you may have a can that max's out at 12 inches of vacuum while your engine was only idling at 10 inches. So you want to check the amount of vacuum advance supplied by the can at idle and you want to match your engine vacuum with the 10 degree advance type cannister.

Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than the idle vacuum level so idle advance is solid and stable.

Another option is to install a vacuum advance stop which limits the amount of movement of the vacuum arm that pulls on the distributor plate.

My Conclusions: I would go with 34 degrees of total advance, but if 36 works and you are not getting any detonation, leave it for now.

Get a vacuum can that has 10 degrees of vacuum advance OR get a vacuum advance stop.

Keep your engine timed for 6 degrees without the vacuum can hooked up to set it and then connect your 10 degree vacuum can to direct manifold vacuum to give you the preferred 16 degrees initial at idle.

This should get your timing on track. Test it and see how the engine performs. If you still have issues, just for fun, drop the total timing from 26 degrees down to 34 and then 32 to see if this cures it. Don't worry so much about your initial timing (which will drop proportionally to 14 and then 12 degrees), you are just trying to adjust total mechanical advance to see IF it cures the problem. If not, go back up to 36 degrees. If it does, then you will have to limit the amount of mechanical advance in the distributor so you can return your initial back to its 16 degrees.

bigD 04-13-2019 09:12 PM

There have been lots of discussions on other Pontiac forums, as well as other online sites, about how to set up the timing curve in a distributor. You can find a lot of info about it with just a Google search.

The basic idea is that you can set your total combined mechanical advance where your engine runs best, (say maybe 34-36°), while also having your idle speed advance where you want it to be, (say maybe 12-15°). And, you can tailor your mechanical advance curve to attain full advance where you want it.(say maybe 3000rpm).

Then, you can buy an adjustable vac advance, so that you can have the amount of vac advance you want. Some of this adjustability can be found in some new dist. But, a factory dist can be modified to accomplish your timing needs. Weaker advance weight springs, allow the mechanical advance to come in sooner. Kits are available with 3 pairs of different strength springs. Sometimes a stock spring & one of the weaker springs is all it takes. And, there are several ways to make a positive mechanical advance stop, so that you can set your idle speed advance where you want it, without having too much total mechanical advance.

PontiacJim 04-13-2019 09:26 PM

Response transferred/added from Lemans guy
So cj if I had your distributor I would put a brass bushing in it to limit the travel and knock that 30 back to 24 or 26.

You would then set the base at 10 or 12 for 26 total. You would then add 10 degrees from a vacuum can for 46 to 48 light throttle cruise.

Once there if no pinging, you cab turn the base up 2 degrees and check if ok you can run 38 total with 22 or 24 degrees idle timing.

You say you must have 26 or more idle timing,...well really you should see it this way a very radical cam CAN take more timing at idle....

But there is not a Mandatory number....24 or 26 at idle is smooth on a hot cam,...20 degrees is smooth on most hot ones.....can you go more?

Maybe but it is a distinction without a difference usually. The total timing is the most important number,....if you want more at idle that you have to take it out of the centrifugal weights,....I have done them 18 centrifugal and 18 base =36....+10 vacuum.......

That is 28 at idle, a real hot cam can take it, but the difference in idle cooling from 24 to 28 is not really of concern. The concern is when guys have base idle at 6 or 8 or 10.......and use ported vac with no more advance at idle.....even at 16 degrees idle timing like you have you will be ok.....you can run it like that....

There is no absolute number, some engines even can take 40 degrees, but you have to be really tuned in, like racers and engine builders like PJ and BigD....they know what their car needs.....by feel and experience..

So don’t get hung up on it must be 26 base etc,..the difference of a few degrees will not likely even be noticeable, as long at the total is right and no knocking

cij911 04-14-2019 08:28 AM

Thanks guys!

A few points of clarification:

(1) PJ - I did not say that my car likes 16* of initial, rather that is what I was told it should be around by my friend and seems to sync with LemansGuy's advice (albeit slightly lower).

(2) BigD - I have the car set for 36* total and the car runs strong. My questions were a bit more advanced and were focused more on the transient response and moderate load conditions (not wide open throttle or idle). My belief, even though I don't hear or see any issues, is that I am running too much timing because of my vacuum can advance (25*) and the amount of mechanical advance (30*) my distributor has.

So to recap my current setup:

Total timing = 36* (comes in by ~3400 rpm with Mr. Gasket black springs)

Initial timing = 6*

Idle timing = 31*

Vacuum advance = 25*

Mechanical advance = 30*

So if I understand this correctly, under light loads or engine braking, I could be seeing timing in the ~61* range (6+25+30) which seems way too high and is caused by two things the WRONG vacuum can and too much mechanical advance. All of the great engine builders appear to want timing to be more like 10 - 16 base, 10* vacuum, and 20 * mechanical, which at most would be ~46* at moderate loads.

I have a 10* vacuum can and will pull the distributor and swap out and log the affects of AFR and performance. I will report back.

Thanks again for your help.

Lemans guy 04-14-2019 10:07 AM

Right PJ, Right BigD and tight cj...... first thing ditch the 25 degree vac can.....

Put on one with 10

Also if you a curve kit about $15 bucks, the weights and center piece will usually drop that centrifugal total down into the 20’s.

Sometimes that can work without a bushing or a stop.

61 degrees way too much,..cause misfires. In the 60’s all gasoline was formulated different, then a 52 to 54 degrees was perfect for light throttle cruising. 36 total and a 16 degree vac can could get you there. Today’s gas is reformulated very different and so the timing at cruise must be knocked back for efficient running to 46 to 48 degrees.....so a 10 degree vac can does it,...otherwise. Misfires, backfires and as PJ said when the throttle slam shut you don’t want to add way too much timing.

You are close and your friend is right as 26 at idle 16 and 10 vac would be a good spot.....but 24, 22 or even 20 is ok....

16......6 base and 10 vac is the low side of OK...think of the range at idle of 16 to 28 as a sweet spot.....always getting 10 off full manifold vac

You can do the subtraction to see where that puts the centrifugal. But you have to get the centrifugal right first.....then set the base

PJ is right that some run good or best at 34 or 32 total....depends on your combo....I set the dist for 36 to start and then adjust up or down a couple degrees depending on what it likes. Mainly no pinging....

You want the most advance without knock...that is the best set up these numbers get you there:nerd:

Lemans guy 04-14-2019 10:10 AM

Also too much idle timing can cause a kickback at startup......so watch it

geeteeohguy 04-14-2019 02:02 PM

I'll add two things here: There is no need to run extremely high initial timing, as all it does is lead to hard starting and a weak top end. I run my '65 and '67 at spec, about 6 BTDC. Also, I have installed the Crane and Accel adjustable cans, and they are JUNK. The allen screw strips after the first use and the can is rendered useless. This happened to me on every one I tried. You are better off using a stock can and installing a limit bar or one of Lars Grimsruud's advance limit brackets that he sells for cheap. Also, reading ANY of Lar's timing articles will put you at the head of the class regarding ignition timing. All cars need and want their own specific curve. I cannot believe that amount of Pontiac guys I run into that think they have to run 16-20 degrees of initial timing, have no manifold vacuum advance, and wonder why they break piston lands and pound their engine bearings into the copper.

bigD 04-14-2019 02:44 PM

"...extremely high initial timing...all it does is lead to hard starting and a weak top end..."

Just curious. The timing at idle, has no effect on the engines power in the upper rpm range, unless your dist has too much mechanical advance. That can be limited by some type of positive advance stop, as previously mentioned in this thread.

geeteeohguy 04-15-2019 03:08 PM


Originally Posted by bigD (Post 882233)
"...extremely high initial timing...all it does is lead to hard starting and a weak top end..."

Just curious. The timing at idle, has no effect on the engines power in the upper rpm range, unless your dist has too much mechanical advance. That can be limited by some type of positive advance stop, as previously mentioned in this thread.

Sure it does. The base timing. Where you actually bolt down the distributor. With no vacuum advance connected. If you set your base timing at 20 degrees BTDC as opposed to 10 degrees BTDC, and then hook up the vacuum advance, and go drive the car, the car will run stronger in the top end and start easier. Timing that is over advanced retards performance.....pistons don't do well when the spark plug fires as it's still moving up in it's bore under compression stroke. Much better when the spark plug fires when the piston is near TDC!

All times are GMT -5. The time now is 11:30 AM.

Powered by vBulletin® Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.

For the best viewing experience please update your browser to Google Chrome