TIMING FOR DUMMIES
Ignition timing has to be the least understood, most poorly explained, and undeservedly mystical aspect of engine technology. But it doesn’t have to be this way. Timing is based on scientific facts, not black arts or folklore. This article will 1) give you the scientific facts, and 2) explain why ported vacuum is to be avoided, regardless of what any salesman, manufacturer or grease monkey says. I know, you’re thinking “uh, what’s ported vacuum?” Patience, my pet, all will be revealed.
First a definition: timing is about deciding when to fire the spark plug to initiate the combustion process. That’s it. Now here is the main scientific fact that drives that decision: peak cylinder pressure should occur about 15 crankshaft degrees after TDC for optimum power, economy, and a bunch of other reasons. Gotta trust me here, many scientists and engineers have done the math, and lots of dyno testing has confirmed this. A disclaimer: we’re discussing street cars, not Top Fuel dragsters or NASCAR weenies.
Since the combustion process takes time, we have to start it in advance of the 15 degree ATDC point, and so we talk about timing advance, and we define it in crankshaft degrees before TDC. So “20 degrees of advance” means 20 degrees before TDC.
OK, how do we ensure that this happens for all circumstances of engine speed, engine load, phase of the moon, etc? Well, as it turns out, engine speed and load are conveniently the most important parts of the equation (phase of the moon is only useful if your car is named Christine). The equation:
Optimum advance = f (engine speed, engine load, some other stuff)
…Optimum advance is a function of engine speed, engine load, and some other stuff, most of which we can’t easily measure or compensate for, anyway (compression ratio, temperature, fuel composition, air/fuel ratio, combustion chamber design, etc).
Why engine speed? Engine speed is critical because it determines how much crankshaft rotation we have available to reach our goal at 15 degrees ATDC. The faster the engine turns, the earlier in the rotation we need to fire the plug; this is advancing the timing, and it’s accomplished by the springs and weights inside the distributor, unless you have a fancy new electronic distributor, where it’s all done by magic. Speed advance is often called centrifugal advance, but that is, strictly speaking, incorrect. Centrifugal force is just the mechanism (remember the springs and weights?) used to accomplish speed advance. And since most new cars and many aftermarket distributors no longer use centrifugal advance, the term will probably fade into history.
Why engine load? Loading the engine increases the amount of air/fuel mixture in the cylinder, and therefore its density as it is compressed. A denser mixture burns faster, so we start it later to hit the 15-degree goal. Conversely, a lighter mixture burns slower and so needs to be started sooner. OK, how do we measure load? Turns out that manifold pressure is proportional to engine load (more pressure pushes more mixture into the cylinder), so we use it to adjust timing for load. Since normally aspirated (no blower) engines always have manifold pressure below ambient air pressure, we commonly talk about manifold vacuum, and hence the term vacuum advance is used. The canister on the side of the distributor does the trick. Now pay close attention: the hose from the distributor canister should always connect to a source of full manifold vacuum. And this is where we confront the misguided notion of ported vacuum, which is a vacuum source derived in the carburetor. Ported vacuum was an attempt by the carmakers to meet government emission specs in the 1960’s. It gives no vacuum, and therefore no advance, at idle, which does indeed reduce emissions, but unfortunately makes your engine idle rough, run hot and have a generally unpleasant disposition. Fast forward 50 years and we have figured out how to control emissions without screwing up our engines, but old ways die hard, and some folks still think ported vacuum is the real deal. Smile, give them an 8-track of country music, and they will wander off.
Note: drag racers don’t bother with vacuum advance because their only concern is full-throttle full-load operation. But you need it in your street machine. Vacuum advance makes your engine run cooler, get better mileage, make more power and have better throttle response. That’s a win-win-win-win.
And that is the essence of ignition timing. The timing at any instant is the sum of three parts: 1) initial timing, which is set by twisting the distributor, plus 2) engine speed advance, plus 3) vacuum advance. At idle, your timing will be the sum of the initial timing plus the full amount of vacuum advance (assuming you are using manifold vacuum, as you should). As engine speed increases, the distributor will add (advance) timing. As engine load increases, the distributor will take out (retard) timing.
Now before I wander off, two obscure facts to impress your friends: as engine speed increases, the air/fuel charge gets better mixed and hotter and burns a little faster, offsetting the need for more advance, so at some point, usually around 3000 RPM, we stop adding speed advance. And for those lucky gearheads with blower motors, manifold pressure can exceed ambient air pressure, and we call this condition BOOST (please stop drooling). We often retard timing with increasing boost, under the exact same logic mentioned in the section on engine loading: more manifold pressure, less advance. Vacuum advance and boost retard are two sides of the same coin. Hakuna matata.
I wish I could give you magic numbers to make your engine sing, but each case will vary. How to pick the numbers is a whole other article.
BUT always be alert for knocking, AKA pinging. This usually means you have too much advance. Knocking can be confused with pre-ignition, where the mixture spontaneously ignites due to heat and pressure even before the plug fires. Either condition can turn your pretty motor into a pile of scrap pronto. If you tune your car on a dyno, make sure the operator has a knock sensor and knows how to use it. I didn’t…once.
I’ll be delighted to hear from you; email me at dr.kerrysmith@gmail.com. I don’t have all the answers, but will pretend I do.
Ignition timing has to be the least understood, most poorly explained, and undeservedly mystical aspect of engine technology. But it doesn’t have to be this way. Timing is based on scientific facts, not black arts or folklore. This article will 1) give you the scientific facts, and 2) explain why ported vacuum is to be avoided, regardless of what any salesman, manufacturer or grease monkey says. I know, you’re thinking “uh, what’s ported vacuum?” Patience, my pet, all will be revealed.
First a definition: timing is about deciding when to fire the spark plug to initiate the combustion process. That’s it. Now here is the main scientific fact that drives that decision: peak cylinder pressure should occur about 15 crankshaft degrees after TDC for optimum power, economy, and a bunch of other reasons. Gotta trust me here, many scientists and engineers have done the math, and lots of dyno testing has confirmed this. A disclaimer: we’re discussing street cars, not Top Fuel dragsters or NASCAR weenies.
Since the combustion process takes time, we have to start it in advance of the 15 degree ATDC point, and so we talk about timing advance, and we define it in crankshaft degrees before TDC. So “20 degrees of advance” means 20 degrees before TDC.
OK, how do we ensure that this happens for all circumstances of engine speed, engine load, phase of the moon, etc? Well, as it turns out, engine speed and load are conveniently the most important parts of the equation (phase of the moon is only useful if your car is named Christine). The equation:
Optimum advance = f (engine speed, engine load, some other stuff)
…Optimum advance is a function of engine speed, engine load, and some other stuff, most of which we can’t easily measure or compensate for, anyway (compression ratio, temperature, fuel composition, air/fuel ratio, combustion chamber design, etc).
Why engine speed? Engine speed is critical because it determines how much crankshaft rotation we have available to reach our goal at 15 degrees ATDC. The faster the engine turns, the earlier in the rotation we need to fire the plug; this is advancing the timing, and it’s accomplished by the springs and weights inside the distributor, unless you have a fancy new electronic distributor, where it’s all done by magic. Speed advance is often called centrifugal advance, but that is, strictly speaking, incorrect. Centrifugal force is just the mechanism (remember the springs and weights?) used to accomplish speed advance. And since most new cars and many aftermarket distributors no longer use centrifugal advance, the term will probably fade into history.
Why engine load? Loading the engine increases the amount of air/fuel mixture in the cylinder, and therefore its density as it is compressed. A denser mixture burns faster, so we start it later to hit the 15-degree goal. Conversely, a lighter mixture burns slower and so needs to be started sooner. OK, how do we measure load? Turns out that manifold pressure is proportional to engine load (more pressure pushes more mixture into the cylinder), so we use it to adjust timing for load. Since normally aspirated (no blower) engines always have manifold pressure below ambient air pressure, we commonly talk about manifold vacuum, and hence the term vacuum advance is used. The canister on the side of the distributor does the trick. Now pay close attention: the hose from the distributor canister should always connect to a source of full manifold vacuum. And this is where we confront the misguided notion of ported vacuum, which is a vacuum source derived in the carburetor. Ported vacuum was an attempt by the carmakers to meet government emission specs in the 1960’s. It gives no vacuum, and therefore no advance, at idle, which does indeed reduce emissions, but unfortunately makes your engine idle rough, run hot and have a generally unpleasant disposition. Fast forward 50 years and we have figured out how to control emissions without screwing up our engines, but old ways die hard, and some folks still think ported vacuum is the real deal. Smile, give them an 8-track of country music, and they will wander off.
Note: drag racers don’t bother with vacuum advance because their only concern is full-throttle full-load operation. But you need it in your street machine. Vacuum advance makes your engine run cooler, get better mileage, make more power and have better throttle response. That’s a win-win-win-win.
And that is the essence of ignition timing. The timing at any instant is the sum of three parts: 1) initial timing, which is set by twisting the distributor, plus 2) engine speed advance, plus 3) vacuum advance. At idle, your timing will be the sum of the initial timing plus the full amount of vacuum advance (assuming you are using manifold vacuum, as you should). As engine speed increases, the distributor will add (advance) timing. As engine load increases, the distributor will take out (retard) timing.
Now before I wander off, two obscure facts to impress your friends: as engine speed increases, the air/fuel charge gets better mixed and hotter and burns a little faster, offsetting the need for more advance, so at some point, usually around 3000 RPM, we stop adding speed advance. And for those lucky gearheads with blower motors, manifold pressure can exceed ambient air pressure, and we call this condition BOOST (please stop drooling). We often retard timing with increasing boost, under the exact same logic mentioned in the section on engine loading: more manifold pressure, less advance. Vacuum advance and boost retard are two sides of the same coin. Hakuna matata.
I wish I could give you magic numbers to make your engine sing, but each case will vary. How to pick the numbers is a whole other article.
BUT always be alert for knocking, AKA pinging. This usually means you have too much advance. Knocking can be confused with pre-ignition, where the mixture spontaneously ignites due to heat and pressure even before the plug fires. Either condition can turn your pretty motor into a pile of scrap pronto. If you tune your car on a dyno, make sure the operator has a knock sensor and knows how to use it. I didn’t…once.
I’ll be delighted to hear from you; email me at dr.kerrysmith@gmail.com. I don’t have all the answers, but will pretend I do.