So, with my new carbed engine, I've been thrown into the deep end and am trying my hardest not to sink. At this point I know exactly how to set my initial, mechanical, and vacuum timings to whatever curve I want, but I don't know WHY.

1. Lately what I've been reading has lead me to believe that the goal of a performance engine is to have a much initial timing, and as little mechanical as possible. So, why not just set your initial to 36 and lock the mechanical? What are the pros\cons of the timing being lower at lower rpms, vs just at 36 all the time? For instance, with me, for some reason I need my street avenger's plates open to all hell just to idle. This is uncovering the hell out of my idle transfer slot. One way I could help that situation is to set my initial higher. (like, past 20) Would there be any reason to say "No, that's wrong, your initial should be at 12!" like I've heard from a few people?

2. Now, when all of THAT is said and done, and all that scientific tuning to get the exact timing you want is finished. Why do we plug a tube into a vacuum source and say "go ahead and advance even more when not gunning it!"?

 

I don't know much about mechanical advance, or even carbs, but I DO know why advance is essential.

As revolutions increase, the window for the explosion to occur, and for the spark to happen, grows considerably smaller. The burning of the compressed air/fuel always takes the same amount of time, which is why you need to advance the spark - its the only way to keep the explosion right where you wan it. I think that's the answer you're looking for.

If you simply set your base timing at 36, I'm not sure the car would run. That's when pinging happens. At low rpms, the window for the explosion is very wide, so advance doesn't need to be so severe. In performance engines, not only is running correctly the goal, but getting the max power out of ever piston stroke - the only way to ensure that is with precise timing of the spark in relationship to where the pistion is in its stroke. If there was no varying advance, you could not account for the drastic change of the window between idle and whatever rpm you're revving to.

Does this help?

 

1. At low speed the engine turns through fewer degrees of rotation during the time the mixture needs to ignite and build cylinder pressure, so you need less advance.

2. The mixture in a partially filled chamber burns more slowly than in a fully filled one, so more advance is needed to have enough time for maximum cylinder pressure to build.

 

1. A more scientifically proper way to think of it is, "total" timing is really the "normal" timing; the amount less than that at low RPMs should be thought of as "low speed retard".
2. The "total" requirement is set almost entirely by the nature of the fuel. Virtually ALL gasoline engines, of ANY size, material, usage, country of origin, and so on, need about the same amount of advance (~36°) in their max torque/HP RPM range. A few other things tweeek it a little but it's pretty much dictated by the choice of fuel, which also dictates the range of accepatble compression ratio and alot of other design details.
3. The reason you need a "low speed retard" is because without it, the fuel burns completely long before the piston comes to TDC, and thus tries to turn the engine backwards for that period of time. Spark knock is a problem too but is sort of incidental to the bigger issue.
4. The reason for vacuum advance is because thin mixtures, the result of the high restriction (the throttle) in the intake tract, burn slowly; so for optimum efficiency it is necessary to start the fuel fire even earlier in the cycle.
5. Another reason for "low speed retard" is starting: it's pretty tough for a starter to turn the engine against a cylinder with a full load of air/fuel in it that just burned, raising the pressure the starter has to work against.
6. The reason for a "performance" engine needing less "low speed retard" is because such an engine is usually optimized for high-RPM operation with a cam giving valve opening duration that's WAY too long at low RPMs, which reduces the peak cylinder pressure under those circumstances. Reducing the "low speed retard" (which you put into effect in practice by advancing the "static" timing and backing off on the centrifugal advance) helps bring the cyl pressure back up to compenstae for the cam-indiced loss at low RPMs, thereby helping to recover some of the lost torque from poor efficiency in that RPM range.

 

Many racers lock out the timing because we have no need for low speed retard. Our engines are designed to operate at high rpm for most of the time. Adding mechanical and vacuum advance is more things that can change or wear out unexpectedly.

My distributor is locked out at 37* and even with my high lift, long duration cam, it's hard to crank the engine over with that much advance so my ignition system has a start retard that can retard the ignition up to 20* until the engine is over 500 rpm. That allows me to start the engine without it kicking back and once it's started, the ignition goes back to full advance.

Since I'm not concerned with stop and go or part throttle driving, a timing curve with mechanical and vacuum advance to give better fuel economy and performance in those conditions isn't required but on a street car, it is.

When you're driving around at part throttle then go to accelerate, opening the throttle puts the fuel mixture into a brief lean condition. The fuel system compensates for this but the timing system also changes because the manifold vacuum signal to the vacuum advance changes. Mechanical advance is strictly based of rpm and increases the advance as rpm increases. Vacuum advance an increase or decrease the total based on engine load.

It's all there to keep the engine happy and burn the fuel in the most efficient manner in all types of driving conditions. For drag racing however, the only condition that matters is WOT so mechanical and vacuum advance are not required.

 

Even a fast street car might have the timing locked out. With a loose converter, the engine isn't loaded at a speed where it needs the timing retarded.

 

True. Best performance should have the timing at full advance by 3000 rpm. If the converter is loose enough, your rpms will stay above that most of the time while driving.

 

Awesome, awesome info guys! I've only got a few more piece of info that I need.

1. So, I realize that whatever my initial timing is, I need to make sure that the weight\spring combo in my dizzy leaves me all in with 36 mechanical + initial by 3000 rpm. But, how do I determine my initial? Is it as simple as, "as low as possible while keeping the engine idling the way you want it"? For example, I know that with this thumpr cam it's probably gonna help me to idle without having my primary throttle plates open too far if my initial timing is a tad high.

Basically, is there a procedural way to find my initial timing or is it just some sort of a feel I have to find, then make sure to adjust mechanical for the proper "all in" number?

2. How do I determine what vacuum curve I need? Do I just make sure it starts advancing 1 or 2 psi below my idle manifold vacuum reading, then plug it into a full source, or do I need to adjust how much advance it gives?

 

The optimum timing is something you have to determine by testing. Theres no one value that works for all. At low RPMs, high load (no vacuum) the timing should be between 0-12 degrees, no more than 18 or so, or it can detonate under low RPM, high load (this is your base starting mechanical advance). This isnt quite as critical if your using an automatic trans. With a manual though, this area is important. At idle, it should be around 16-24 degrees, or whatever gives a good idle with optimial engine vacuum. At higher RPMs, high vacuum, the timing should be around 36-40 degrees at 2800-3200 RPMs. At WOT, the timing should start off at the base value used at the low RPM/high load (base mechanical advance in your case), and come up to around 32-36 degrees at 3600-4000 RPM. Bascially you set your springs and vacuum advance so the distributer can more or less deliver the required timing. There should be instructions given on MSD Ignition's website, or the site of whomever made your particular distributor.

I use a "vacuum advance" style timing curve with my fuel injection, simply because its relatively easy to set up. I attached a table of roughly what the timing should look like across the board. The columns are in kPa (manifold pressure), while the rows are in RPM. 35-50 kPa is the area you cruise at, while 100 kPa is WOT, or atmospheric pressure. In your case, the 35-50 kPa area is the high vacuum area as these values are the absolute pressure in the manifold, and not the gauge pressure, which in your case is engine vacuum.

Mine are lower than what yours will likely be since Im using vortec style heads, which burn the mixture a little faster than standard heads. If your running a cam with overlap, then you may need more advance at lower RPM areas to cover for the loss in cylinder pressure.

 

Typically, it's recommend 10- to 12-degrees of initial timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift; 14- to 16-degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”; and 18- to 20-degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.

Less than that and the ignition will be too retarded at low rpm. Too much and the engine will be harder to start and want to kick back.

Once the initial is set, you need to change the mechanical to limit the total. If a stock distributor has 10* initial and 36* total then there's 26* of mechanical. If you now bump up that initial to 20* because you have a big cam, the total also gets bumped up to 46* which will be way too much. The mechanical needs 10* taken out of it to bring it back down to 36*.

 

Based off of my cam http://www.compcams.com/Company/CC/c...?csid=107&sb=2 I believe a good range for my initial would be 14-16. Now, should I use full manifold for my vacuum advance on top of this 14-16 or just used ported? Because right now, at 20 initial and ported vacuum I have to crack the read throttle plates so I can idle with the front throttle plates not exposing half of the damn transfer slot. It seems to me that if I bring it down to 14-16, then go full manifold for the vacuum advance, it would let me re-close the rear plates a bit, and still idle nicely.

 

Manifold vacuum sounds more like what you want then.

 

Awesome info guys, thanks! Now I just gotta wait for these mid june april showers to die down a bit and I can go play with my timing!