Sam Memmolo: Why Do We Need Timing? Because Timing Is Everything

fordengineIn a recent article about high energy ignition systems, or H.E.I., we mentioned timing advance and the critical role ignition timing and the advance curve plays in extracting peak performance from a spark ignited gasoline engine. Now it is time to dive into exactly what timing is and why we have ignition timing advance.

Without trying to get too scientific and confusing, there are some basic Physical Science items we have to establish. Most of these are what your eighth grade science teacher should have ingrained into your head.

coastersticks

Why does the coaster stick to the cold bottle? It’s a very basic example of atmospheric pressure. The moisture from the bottle creates a hermetic seal around the glass and no air gets under the bottle. As you lift the bottle, it creates a vacuum between the glass and coaster. The air pressure under the coaster is greater than that above it (between it and the bottle) and that force presses the coaster to the bottle.

First there is atmospheric pressure. Atmospheric pressure is the pressure exerted by the weight of the atmosphere, which at sea level is roughly 14.7 psi (pounds per square inch.)

Sometimes we see this on the TV weather expressed as barometric pressure. This is just another way of measuring atmospheric pressure in inches of mercury, or Hg. Barometric pressure on a standard sea level day is 29.92 inches of mercury. That is the same as 14.7 psi.

We need to know this, as a baseline for how pressure in a cylinder affects the combustion process that is designed to produce heat energy from the burning of the air / fuel mixture in the cylinders of a gasoline engine.

timingchain

Why We Need Ignition Timing Advance

The air / fuel mixture does not burn instantly. Gasoline takes approximately two milliseconds to burn. Testing has shown that having the burn completed by approximately 15 degrees after top dead center (ATDC), produces the most effective power yield. In order to achieve this power crown, we need to determine how many degrees of rotation the crankshaft turns in 2 milliseconds.

Let’s take an idle speed of 750 rpm. If the specification calls for eight degrees of base timing, the engine builder or manufacturer has determined that igniting the mixture eight degrees before top dead center (BTDC) allows the fuel burn to be completed by 15 degrees ATDC. As the piston reaches the top of its travel and compresses the air / fuel mixture, the spark plug fires eight degrees before it reaches TDC. Add the burn time of two milliseconds and the combustion can be completed by 15 degrees ATDC. The pressure forces the piston down, thereby developing power.

timingmarks2

Now speed the engine up to 2500 RPM and in the same 2 seconds of burn time, the crankshaft turns more degrees of rotation. In order to complete the burn by 15 degrees ATDC, the spark timing needs to happen earlier in the stroke, or more advance is required to produce power.

Most distributor equipped engines have a mechanical advance that triggers the coil earlier as rpm increases. At 2500 rpm the engine may need as much as 15 degrees of mechanical advance. Add the eight degrees base timing to 15 degrees mechanical (centrifugal) advance and you have 23 degrees total timing or total advance. Most street engine distributors also have a vacuum advance. The vacuum advance is critical to adding additional timing for cruising or light throttle / flat road situations. Add 12 degrees of vacuum advance to the 23 we already have from base and centrifugal advance and you end up with 35 degrees of Total Timing.

detonation

Detonation can happen when timing is advanced and combustion happens at the wrong time.

During acceleration, the mixture is richer than when cruising with barely open throttle plates. The lean cruise mixture, less engine load, and lower manifold pressure (higher vacuum reading), can use more ignition timing advance to deliver power, economy and an efficient burn. When opening the throttle for power, there is no vacuum available to the vacuum advance chamber, because the manifold pressure rises dramatically on wide open throttle.

Now you understand why we needed to know about pressure. Manifold pressure rises and vacuum falls as we open the throttle, so under acceleration, we rely completely on base timing and mechanical or centrifugal advance; vacuum is just negative pressure. With a good running engine at sea level, idle speed, warm engine, the vacuum should be around 20 Hg or 20 inches of mercury on a vacuum gauge.

timinglight

These are all approximates, as there are many factors that affect how much and when timing is required. Engine design, compression ratio, octane rating of the fuel, etc. are all factors. That is why dyno tuning is so beneficial.

So don’t just rely on a base timing number. Be sure to tune your engine with the correct amount of centrifugal and vacuum advance. You will add power and fuel economy, and get the most life out of your vehicle’s engine. Good Luck!


Sam Memmolo is a 45 year veteran of the automotive industry, and is well known in business circles from the original equipment manufacturers to the aftermarket and performance segments. Sam hosts and produces his syndicated Sam’s Garage Radio Show, which provides consumers and shop owners with a wide spectrum of advice, industry news, and product awareness, all presented in an easy to listen to entertaining format. Sam is heard on 50 radio stations, and the internet. Sam wrote and co-hosted the incredibly successful TV show Shadetree Mechanic on TNN, which had a terrific run of 190 episodes spread over eight years.  Later he wrote and co-hosted  Crank and Chrome, also seen on TNN. Sam, along with his pal Dave Bowman, set the benchmark for automotive TV programming as Sam became Executive Producer and Host of Two Guys Garage on SPEED. Semi-retired, Sam and his wife Diana live in Alabama with their hot rods and rescued dogs

About the author

Sam Memmolo

Sam Memmolo is a 45 year  veteran of the automotive industry, and is well known in business circles from the original equipment manufacturers to the aftermarket and performance segments.  Sam hosts and produces his syndicated "Sam's Garage Radio Show,” which provides consumers and shop owners with a wide spectrum of advice, industry news, and product awareness, all presented in an easy to listen to entertaining format.  Sam is heard on 50 radio stations, and the internet.  Sam wrote and co-hosted the incredibly successful TV show "Shadetree Mechanic" on TNN, which had a terrific run of 190 episodes spread over 8 years.  Later he wrote and co-hosted  "Crank and Chrome", also seen on TNN.  Sam, along with his pal Dave Bowman, set the benchmark for automotive TV programming as Sam became Executive Producer and Host of "Two Guys Garage on SPEED.  Semi-retired, Sam and his wife Diana live in Alabama with their hot rods and rescued dogs.
Read My Articles

Hot Rods and Muscle Cars in your inbox.

Build your own custom newsletter with the content you love from Street Muscle, directly to your inbox, absolutely FREE!

Free WordPress Themes
Street Muscle NEWSLETTER - SIGN UP FREE!

We will safeguard your e-mail and only send content you request.

Loading