Spark Timing - Basics

Timing is often one of the scariest things there is to adjust on a car in large part because you end up adjusting it blind, especially street or track tuning. What is worse, is that you can adjust it all you want on the dyno, but once you hit the street what you did has almost no basis in reality for actually being safe for driving the car. Conditions simply change too much and too rapidly to really be accurate at all on the street with a dyno based tune unless a sophisticated road simulation dyno tune has been done (but open up that wallet!). This results in nearly every tune out there done for customers of most institutions being on the conservative side for timing and considerably so. There’s a fix for that though, but I’ll get to that sales pitch in a minute.

Timing on an engine in the case we are talking about is simply about spark timing (not cam or valve timing). Timing is measured in degrees of crank rotation relative to the top dead center (TDC) position of the cylinder in question. Since most systems only have single cylinder timing, meaning the ECU can’t set different timing values for each cylinder, it is usually considered to be the same for all the cylinders in question whether you have 3 (i.e. a Geo Metro) or 12 (i.e. my personal lust car the V12 Vanquish by Aston Martin) or something ridiculous like the 16 of the Cadillac Sixteen concept car. Timing is also usually defined as being advanced or retarded. When you say timing is advanced it means before top dead center (BTDC) and retarded is after top dead center (ATDC).

Which immediately brings up an interesting point. Normal timing is before the piston has compressed the mixture fully, meaning you are starting that burn while the piston is traveling toward the spark plug. Bad idea right? No, not really but it does get into discussions of detonation and pre-ignition which we will discuss a little bit in a minute, and in depth in a different post. The reason this is okay or works is due to the fact that unlike many people think, the gasoline mixtures does not explode, but rather burns in a controlled manner. The spark initiates a flame kernel at the spark plug which then spreads out toward the walls and the mixture burns and expands in a controlled manner along the flame front that travels through mixture and the space it occupies. As it does this the pressure increases within the cylinder much like it does as the piston is coming up, just at a new rate when both are in effect. As long as this pressure does not get so high that the mixture spontaneously ignites throughout (one form of preignition) everything is fine!

So what is a common timing value? Timing in most cars runs anywhere from as much as 50-60 degrees BTDC to as little as 5-10 BTDC. That’s a general range for street cars, it can vary widely based on engine type, cylinder head design, spark plug design, fuel type, aspiration type, charge modifications like water or methanol injection, and many other factors. Additionally, this all changes moment to moment in a given example based on engine speed and engine load and density of the air fuel mixture (commonly referred to as the “charge”). The higher your load on the engine and/or the more dense the charge, the lower the timing is, or the closer to TDC (e.g. 5°BTDC versus 15°BTDC). Also, the slower the engine is turning the lower/less advanced your timing is. So extending that to the obvious, a light load, low density charge, at a high engine speed is your most advanced timing situation typically. So for example in a naturally aspirated high compression engine with a nice dense charge coming in at 2,000 rpm’s and 0 inHG of vacuum may run something like a 18° BTDC versus that same engine at 6,000 rpm’s and 20 inHG might run about 55°BTDC.

So, obviously timing can be spread across a wide range of values, so how does one choose the timing? What you choose for timing depends on a number of things including reliability, gas octane, environmental factors like temperature and elevation, emissions desires, and response of other aspects of the engine. When speaking to maximum power output, which is what I assume most of my readers would be interested in, there is one general simple rule (which like all general simple rules does not hold for every case, nor truly give you an exact answer) is that you want to have your peak cylinder pressure occur in a range of 12° - 16° ATDC. This situation is typically regarded to give the peak torque value for the engine and thus the maximum output. However, many times this can not be achieved due to limitations of the engine and design that can lead to pre-ignition and/or detonation or due to other limitations such as emissions. Most tuners will attempt to estimate this by simply pushing timing forward on an engine until they see some torque fall off on the dyno and the pull the timing back a few degrees. This is rather imprecise and in fact often dangerous as there are many cases where detonation will have already set in and begun threatening the engine even though you can’t hear it or detect it on the EGT or dyno. Furthermore, this only takes that one moment in time in to consideration and thus, you could be dangerously wrong, or wholly inadequate at other points as well.

Timing is also effective for smoothing out various driving situations. Sometimes cars will buck or get unstable outputs from the engine at low throttle openings and slightly lean air fuel ratios. Adding a little timing will often help smooth this out. Timing can also be used to strengthen idle and will move it up a few 100 rpm’s if there is not an idle air control solenoid interfering. Additionally, running very retarded values of timing right as you enter boost on a turbocharged car can also help contribute to improved spool up of the turbo (at the price of significant heat being dumped into the exhaust, fuel into the catalytics, and heat exposure to the exhaust valves and turbo components). At cruising speeds, maximizing timing will yield your best gas mileage as well and should be balanced with a stoichiometric to slightly lean (14.7-15.5) AFR as suits the vehicle best. So, as you can see timing has significantly more effects than just being a path to maximum output on your engine.

Timing also needs to be balanced against air fuel ratios for a good tune. Sometime the peak power of a vehicle will occur at a slightly more retarded timing value that allows the fuel mixture to be made more lean (relatively speaking). However, I have found in most cases that working to maximize timing has yielded better results on the engines I have had. So I’ve run air fuel ratios on the richer side and worked to get more timing out and had good results in the past. However, while I start with this approach it is important to work the other direction and optimize AFR at the cost of timing as every car is different and there is no general rule on this for every car or every situation, so it is really the call of the tuner.

I mentioned it briefly before, but timing does affect your emissions. What your timing is will affect the levels of carbon monoxide, hydrocarbons, sulfur oxides,and NOx that the vehicle produces. Advancing your timing will reduce some of the emissions while increasing others, and the same for retarding your timing. So when you have to deal with the emissions timing does need to be a consideration, although it typically does not have as significant of an impact as AFR and the overall effectiveness of your emission systems (or lack thereof) on your vehicle.

Lastly, before I end this first pass of discussion on timing the sales pitch I mentioned up in the first paragraph. There is a device out there that takes away the blindness factor of tuning timing and makes your car MUCH safer on the road. It is called a J&S Safeguard system. It is made by a husband and wife team out of California that have been in business for I believe a couple decades now. The device is well known in some circles, unheard of in others, and misunderstood in its usefulness by many! The J&S Safeguard is a simple device that has the ability to retard timing on EACH individual cylinder depending on when it detects detonation. This is unique for several reasons. First, they use a rather sophisticated algorithm and filtering to be much more accurate than most systems out there. Second, even most factory and race systems retard all the cylinders as a whole, so you lose output if one cylinder is more finicky than the others, the J&S reduces or eliminates this fact. Third, the system is much simpler and more accurate that others. For example, the detonation prevention system on EMS’s like the AEM or the Haltech have you set a noise threshold for given rpm’s and conditions. If the motor is noiser than that it thinks it is detonating. The J&S actually knows when and where to look for detonation relative to your spark event and also adjusts dynamically to the engine background noise. By doing all this it achieves much better accuracy than anything else I have ever seen. It is also ridiculously simple. Most install are between 5 and 15 total wire connections and setup involves a simple procedure to set the sensitivity of the system. Once set, the J&S in many ways acts as a “wideband” sensor for timing. You wouldn’t tune your AFR’s blind, so why would you tune your timing blind when you can get this? It’s also typically 500-600 dollars which is cheap insurance for your engine. If you ever lost coolant, froze an injector, or any number of other things the J&S would cover you, without it… boom. It has the ability to show timing retard on an LED array for each individual cylinder, and it also dynamically feeds the timing in and out on each cylinder, so as detonation gets close to happening the system pulls out the timing and when the conditions pass the timing is fed back in, all with no input from you! I’ve used the system on my car and on customers’ cars as well. I’ve checked it against noise canceled ear phones and dyno systems and more and it has been vastly superior in all cases. While I do sell them, I also encourage you to go direct to them. www.jandssafeguard.com Tell John I sent you!

Happy Motoring!

Steve