Bobbyz

iTrader: (1)

trying to get my timing just about right and i just cant seem to comprehend the whole thing. i know how to set initial but the whole all in at xxxx rpms get me

motor is a zz4 350, speed demon carb, msd 6 al, 1 light 1 medium spring and adjustable vacuum can.

i had the intial timing set at 16 degrees ran ok then i tried holding the throttle at 3200 rpms and bringing the timing to 32 degrees and let the initial fall where it wanted about 16 again ran the same all ported vacuum. now i tried full manifold vaccum and the timing shot up to 40 degrees at idle and no power off the line but bat out of hell after 2000

i know everything is trial and error but is there a base line anyone one knows of that i can set everything up at with full manifold vacuum?

Rolling Thunder

My question is why would you want to run full manifold vacuum to your vacuum advance? I know of some cases where this is appropriate but thats limited to some fairly radical cars. It sounds like you were going about it right originally with the ported vacuum. Whats happening now is by attaching it to manifold vacuum its working in the opposite manner that you want it to. Max advance at idle where there's max vacuum and when you stomp on it the vacuum drops retarding the timing when you would want it to advance. Later as you get into the high RPMs vacuum builds again advancing the timing again probably causing you to feel it pick up again. With ported vacuum the vacuum is 0 at idle and when you open the throttle vacuum increases helping advance the timing. Cars with really radical cams can run strait manifold vacuum because the vacuum at idle is so low its more appropriate than the ported vacuum but most people don't fall into this category. Idk maybe im just reading this wrong but It seems to me running manifold vacuum isn't the best plan.

305sbc

iTrader: (3)

Rolling thunder is incorrect on this.

Manifold vacuum is what you want to use for performance.
It works exactly how it should:
High load = less advance
Low load = more advance

That's excatly what vacuum advance is for - to add advance under high vacuum (low load) conditions. At WOT the vacuum advance should not be a factor at all, because it should not be activated or functioning.
At WOT and zero vacuum, you should get no vacuum advance.

The RPM added advance is handled completely by the centrifugal mechanism. As RPM increases the advance increases.

For performance you disable the vacuum advance completely to set up your timing curve. 10* to 15* is usually a good point for intial advance (idle with no vacuum advance). You then use a dial-back style timing light to set up your centrifugal advance.
The one weak & one medium spring sounds about right for most performance applications. The springs control the rate of advance, but NOT the limit (amount) of advance. Distributors either use metal tabs or different sized bushings for limiting devices. The dial-back light will show you have much centrifugal advance you have and how soon it all comes in.
With 15* of initial advance you will want to limit the centrifugal advance to 20*. The two added together gives you a total advance of 35*. This is your timing for WOT runs. Your car may like a tad bit more or less total, which you can adjust using limiters in the centrifugal mechanism. Anywhere from 18* to 24* is a typical amount of centrifugal advance.

When this process is done, you then hook up your vacuum advance mechanism and adjust it for your idle, part throttle, and deceleration modes of operation. If your initial advance is set to 15*, then you will want the vacuum advance to add between 10* and 15* of extra advance, to give you 25* to 30* advance at idle. You may have to lower this a bit to prevent knocking at part throttle. You can't drive around while looking at your timing with a light, so you have to listen for knock when you push the gas for part-throttle driving. Vacuum advance mechanisms can be adjustable, as in you adjust the rate that the advance is added by the vacuum applied. You may also have to limit the advance mechanism to prevent knock at part throttle. I like to limit the travel of the advance mechanism to give only 15* maximum advance at maximum vacuum (decel). I have it add between 10* and 15* at idle and cruise conditions.
When you load the engine the vacuum drops, so the advance is reduced, but your centrifugal advance is still there functioning to add advance per RPM.
That's why you set up the centrifugal first. Even if the vacuum advance were unhooked, you should still have no problems during a WOT run.
The vacuum advance should not be a factor at all during WOT. It is like icing on the cake and is set up after you already have a fully functioning spark curve for WOT.

If the vacuum advance is hooked to ported vacuum, then it acts much like the centrifugal mechanism by adding more advance under load. You don't want this as it makes power tuning very complicated, and negates the advantage of the vacuum advance increasing engine efficiency during low load conditions - the biggest advantage to running a vacuum advance in the first place.

A ported vacuum sources shows increased vacuum as the air-flow through the carb or throttle-body increases (high load).
Manifold vacuum is opposite and shows higher vacuum as the throttle is closed (low load).

In many factory applications a pseudo vacuum advance mechanism was used with a ported vacuum source for emissions applications where smaller timing numbers were needed for some low-load driving conditions. This would cool the burn (less power & efficiency) to lower the emissions, and the entire timing curve was set up around the presence of a functioning EGR system, which also served the purpose of cooling the burn in the chamber.
Unless you're trying to replicate this factory goal, you don't want to use a ported vacuum source for your vacuum advance.

No offense to Rolling Thunder, but this is a very common point of confusion as people try to copy the factory emissions set up for spark not understanding the theory behind it.

Spark advance is a mechanism to start the spark sooner to make up for a slow burn (low load). It increases efficiency and power under these conditions. As load goes up, the spark needs to be delayed due to the faster burn in the chamber. This need is completely independent of engine speed, which needs its own advance mechanism to match the decrease in available burn time (window) for efficient combustion. Load is completely different from engine speed, and is why a vacuum advance or equivalent is needed in the first place.

I don't know if this information is in a tech article or a sticky, but it really should be.

Rolling Thunder

Very interesting post 305sbc I know I learned something today lol. I agree this info should be in a sticky somewhere a good read for anyone getting serious about doing their timing.

305sbc

iTrader: (3)

Thank you for not taking offense.
If you tune your TPI's PROM you will see a timing adder table separate from the main timing table. It is for power enrichment (PE SA) spark advance and basically does the same type of function for emissions reasons.
When in PE mode your AFR goes from 14.6:1 to approx 12.5:1. The richer AFR creates a slower burn situation. That's where the PE SA table comes in to start the spark sooner and restore some power.
The reason it works is because the main timing table had been compromised for emissions purposes in the first place.
Many performance tuners elect not to use the PE SA table and set it all to zeros to make the job of tuning easier.
When factory emissions standards are not a concern you can simply use the main timing table to fulfill the functions of both centrifugal advance and vacuum advance with no need for the PE spark adder.
With a speed-density (SD) system, the main timing or spark table is a 3-D table based on engine load (vacuum) and engine speed.
With a MAF system the load is calculated into an LV8 term, but the idea is the same.
There is also a table to adjust the spark based on coolant temp, which is an advantage over a non CC distributor.

The main disadvantage to a CC type distributor effecting spark advance is rotor phasing, since the pick-up point no longer moves in relation to the rotor. With certain rotors there can be a large gap between the cap terminal and the rotor tip when at full advance. Also, when the full advance approaches 45* (the half-way point between cap terminals) crossfires can start.

oamhmad

iTrader: (1)

I'm sorry to bring this thread back from the dead, I realize it's over a year old. There is some very good information here, until I came across this statement....

I will have to respectfully disagree, both ported as well as manifold vacuum show the exact same vacuum as the air-flow through the carb or throttle body is increased or decreased.

I too was under the same assumption until I verified this with a vacuum gauge.

The difference is that ported vacuum is zero at idle while manifold is not. as soon as the throttle blades move past idle both ported as well as manifold behave exactly the same.

I find a lot of people misinformed about this. The 2 sources of vacuum are one and the same for all throttle positions except for idle.

So this leads me to question some of the other information posted in this thread, leading to further head scratching

camaronewbie

Here's how I determined mine ...

I set initial timing to a spot where the engine ran well, and adjusted my idle screw accordingly for a decent idle - this happened to be about 11*BTDC. Then, a few weeks later (as I studied more) I got a timing tape for my harmonic balancer - this allows me to see timing at any given RPM.

So then, at 3000 rpms (I can't tell you why 3000 is the magic number - it's what I read) ... I read my total timing off the timing tape - I was getting about 24*BTDC TOTAL timing at 3000 RPM's. I had read that you want 32 - 36 at 3000 RPM's - so I advanced the timing up, while motor was running 3000 RPM's, until I got to 34*BTDC. This ended up being almost 18* BTDC initial timing.

Then, I went for a drive - at WOT I got spark knock (pinging), telling me my timing was too far advanced. So, I backed it off 2 degrees (down to 16 initial), and rode again - again spark knock at WOT, and so adjusted down another 2 degrees (now 14 initial), and rode again. This last time, no spark knock at WOT, so I left it there, locked it down, and readjusted carb idle.

The end result was 14* BTDC initial, which gives me 28* BTDC total at 3000 RPM's. Not the 32-36 I read was where I want to be, but it's as close as I can get with the weights/springs that my distributor has in it.

BTW - this was ALL done with NO VACUUM connected to the distributor at all.

So now, for me - I need to get a timing advance curve kit and install into my distributor - this includes lighter springs and heavier weights. Less spring resistance and/or heavier springs will allow the mechanical advance (working with centrifugal force) to advance the timing farther and faster. I just have to take the time to get the kit, and play with the various weight/spring combos until I get to those magical numbers (wherever they come from).

I have heard of folks using manifold vacuum on the distributor rather than ported. I'm guessing that maybe, this allows them to have more timing initally without physically turning the distributor, and thus they can reach the magic numbers faster easier without getting the spark knock from having the base timing physically set too hight (via turning the distributor). But I don't know. Part of me thinks the timing would be too high (thus getting spark knock) regardless of whether the timing was initially set higher by rotating the dist or set higher by connecting a higher vacuum source. But also, when WOT occurs, there is no vacuum - so timing would actually reduce from lack of having a vacuum advancing it BEFORE it gets advanced mechnaically (weights and springs). So maybe, they have a very low base timing without vacuum, and are running quite heavy weights and very light springs, and using manifold vacuum to achieve a decent base timing - thus they can achieve 34 very easily before 3000 RPM's.

I don't know - it hurts to think that hard!

And, to add to the confusion, I know some folks that don't connect any vacuum to their distributor at all!

AND ... I also see distributors available that don't have vacuum advance on them, nor are they computer controlled, but they are pre-set according to YOUR specs.

OK - time for some Excedrine!