Ask Away! with Jeff Smith: Vacuum Advance and How it Relates to Part-Throttle Timing

Our tech column seems to be gathering quite a following of guys who want to know more about their cars and engines. One reader posted a comment (below) in response to my answer about ignition timing curves. The original post was roughly a year ago and has resulted in over 20 comments on the answer. All this activity indicates there is still quite a bit of confusion about ignition timing and how it works. Some comments are right on target – others not so much. The following comment reveals some confusion about how vacuum advance works as opposed to mechanical advance. This may have resulted from my answer not being complete enough. So I thought this required us to go a little deeper to shed more light on vacuum advance. The original question and my answer related to a mild small block Chevy with low static compression. Nowhere in my answer did we talk about a 10.5:1 compression engine. But since this gentleman brought it up, we’ll take that ball and run with it!

Here is the link to the original question and answer dated March 27, 2015.

Comment:
If you’re running 10.5:1 compression you will detonate with vacuum advance hooked up.  On pump gas and 10.5:1 compression you can’t have a max total timing more than 28 degrees. I have a cracked cylinder in my garage to show you what happens when you allow the advance to reach 48 degrees or more on a high compression engine. If you have a high performance engine you don’t want to ever see more then 38 degrees. Look up the timing on a 2002 Z06 Corvette. Total max timing on it is 28 degrees and it has 10.5:1 compression.

Here’s my response:

So let’s take a look at a 10.5:1 compression small block Chevy street engine. We’re not talking about a drag race engine – we must emphasize this. This discussion will be mostly aimed at optimizing the part-throttle timing curve on a street engine. We’ll also assume this is a purpose-built engine with aftermarket heads, cam, headers, and a carburetor. It appears there is some misunderstanding about the relationship between mechanical advance and vacuum advance. So let’s start there.

Mechanical (or centrifugal) advance is a system that is purely determined by engine rpm. That system of weights and springs determines not only the rate at which the advance occurs with rpm but also the total amount. The amount is determined by the movement of the pin in the slot. The length of the slot determines how far the pin travels which limits the total mechanical advance. MSD distributors use a bushing that fits over the pin that travels in the slot. A larger bushing reduces the available travel which reduces the total advance. This makes it easy in an MSD distributor to change the total advance just by changing the bushing.

Again, this total mechanical advance is determined by engine speed. The problem with a timing curve determined merely by engine speed is we can have an engine at 3,000 rpm with very little load on it where the throttle is barely open. Or we can have that same rpm with a wide open throttle (WOT). By design, the mechanical curve must be based on a WOT-only situation. There is a huge difference in timing required for optimal engine power at part-throttle compared to WOT. This is where vacuum advance is beneficial.

You can look at vacuum advance as load-based, additive timing. Let’s look at what this means. As an example, let’s use that 3,000 rpm figure. Let’s assume a typical mechanical curve will have achieved complete advance at this rpm. Let’s put the details at 14 degrees initial and 20 degrees of mechanical for a total of 34 degrees. Now let’s put that engine in a street car with a set of 3.73:1 rear gears, a TH350 trans, and 26 inch tall tires that cruises at 70 miles-er-hour. That means the engine will be spinning at around 3,400 rpm with 34 degrees of total mechanical advance. We are also assuming this is a highway cruise on level ground.

One way to make checking advance curves easy is to use a dial-back timing light. This Innova digital light displays both engine speed and amount of advance on the screen. This makes checking ignition curves very easy. The Innova light is PN 3568.

Another important point to make here is that many enthusiasts are under the impression that at this 3,400 rpm highway cruising speed that the throttle is roughly ¼ to half open. I’ve even had guys try to tell me that the secondaries on the carburetor are open in this situation. That is completely not true. I’ve tested hundreds of engines and with the above combination the throttle is probably open no more than 10 to 15 percent. In fact, with deep gears like the 3.73:1’s, the carburetor is operating on the idle circuit. While that sounds fantastic, it’s true. Not all street engines do this – much of it has to do with the size of the camshaft, the gear ratio, tire size, and vehicle speed and again on level ground. An engine with a big cam and tall overdrive gears could be running on the carburetor’s primary main circuit. But most mild street engines like our example will be running on the idle circuit when cruising at freeway speeds.

Why is this important? Because this means there is very little air and fuel entering the engine. In my previous column, I mentioned that this reduces the density of the amount of air and fuel in the cylinder. I was not talking about the density of the ambient air. The density I am talking about is the compactness of the charge in the cylinder. This is an extremely important point. With the throttle nearly closed, it should be obvious that only a very small amount of air and fuel enter the cylinders.

Under this circumstance, we need to adjust the ignition timing to optimize power. If we can make a little more power, we can close the throttle even more and improve fuel mileage. With only a small amount of air and fuel in the cylinder, this less densely packed amount of material will burn more slowly than if the cylinders were packed with more air and fuel.

This is where vacuum advance is beneficial. It allows the tuner to add ignition timing at light throttle when the engine needs it. While this sounds crazy, we’ve seen small block engines with low static compression ratios demand 45 degrees of timing (and sometimes more) in order to be most efficient at light throttle cruise. What this additional timing does is start the combustion process earlier to achieve maximum cylinder pressure. This additional timing is necessary because the less densely packed air and fuel will burn more slowly.

The vacuum advance gives you this advantage because the additional timing is added only under light load. As load increases, the vacuum in the manifold begins to drop. You can witness this by simply watching a vacuum gauge hooked to the engine while you drive. As the throttle opens, the vacuum will drop. At or near WOT, the vacuum will be effectively zero. We typically see about a 0.5-inch of mercury (“Hg) of vacuum at WOT. This means vacuum advance is no longer contributing advanced timing at WOT. This puts us back to the rpm-based mechanical advance. If the engine rattles or detonates at WOT at 3,500 rpm with 36 degrees of total timing, this requires the tuner to retard either the initial or the mechanical advance to prevent detonation. The vacuum advance does not contribute to advance at WOT.

If you need further evidence of adding timing to an engine under light load, all you have to do is look at a typical graph of a fuel-injected engine’s timing curve. With light load, timing is added to improve drivability, throttle response, and fuel economy. Many late-model engines don’t need a lot of advance at high engine speeds because their superior combustion chamber design does not require lots of timing. So when our commenter mentioned that a 2002 Corvette engine only needs 28 degrees of timing – that is true. But as you can see from our accompanying LS ignition map, these engines still add far more than 28 degrees of lead at part throttle for better fuel economy.

This is an LS engine timing map. The vertical scale is load –represent in grams of air mass in the cylinder. The horizontal scale is rpm. Note all along the upper third of the map that the engine needs much more timing – upwards of 36 degrees of timing at light load. The lower right portion of the map represents high load at WOT and timing is around 24 degrees BTDC like we would expect for a well-designed combustion space.

Much of this discussion about timing and detonation is drastically affected by inlet air temperature. Hotter inlet air is more susceptible to create detonation. Our friend Tim Wusz at Rockett Racing gas tells us that for every additional 25 degrees of inlet air temperature, we need to increase the octane rating of the fuel by 1 full point – as with a change from 91 to 92 octane. That should show you how important cool inlet air temperature is to engine performance. So an optimized engine timing curve should also take into consideration the inlet air temperature.

So let’s assume our 10.5:1 compression small block will not detonate at 3,400 rpm with 34 degrees of timing at WOT. The commenter used the example of a 2002 ZO6 Corvette that only requires 28 degrees of total timing. Assuming this is correct, the problem with that comment is that the LS engines as a family benefit from a highly refined, 21^st Century chamber and piston top combustion space where the spark plug is much closer to being centrally located. This means that yes, the LS only needs 28 degrees of total “mechanical” advance. But this comparison is not accurate when you try to apply that to a small block Chevy.

A typical production small block Chevy head offers a far less efficient chamber, which generally means they require more timing to make peak power. In the ‘60s and ‘70s, it wasn’t unusual for a small block to want 40 degrees of timing to make peak power. Sure, there is a limit to how much timing you can run at 10.5:1 compression with 93 octane fuel on a small  block Chevy. But at higher engine speeds, to make best power you will need between 34 and 36 degrees BTDC. The engine might need a higher octane fuel in order to take advantage of this combination of timing and compression.

The commenter stated that you can’t run 48 degrees of timing with a 10.5:1 compression engine. That might be true – an engine with that much compression would probably only require 34 or 36 degrees because it has more static compression. But I also think from his response that he was talking only about total mechanical timing . My point was that with a mild 8.5:1 compression engine with 34 to 36 degrees of total mechanical advance might need as much as 45 degrees of timing at part throttle – such as 14 inches of manifold vacuum at cruise for ideal fuel economy.

So with this added information we’ve either made this situation much clearer or we’ve managed to completely muddy the water. The whole point is that at part throttle, most engines need more timing to run more efficiently.

It’s that simple.