Monday, November 24, 2008

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. Tell John I sent you!

Happy Motoring!


Friday, November 14, 2008

When to shift

We’ll start out with something easy. I have heard plenty of times from people that you should shift when the power starts to fall off on the car to be “faster”. But the fact of the matter is that due to gear ratios and the mechanical advantage of the gears, more often than not redline is your best shift point, but to be sure you need to do the calculations

To begin with, lets talk about what makes you “faster.” Faster in this case I will hold to mean acceleration. In a drag race or most other types of races you are rarely holding a constant speed, so it is about maximum acceleration. Want to run a ¼ first or make it down the straightaway fastest? Have the most acceleration. So understanding that we are talking acceleration lets get some of the other variables out of the way.

In simple terms acceleration for a car is described by the equation F=m*a where F is force, m is mass, and a is acceleration. Since we’ll be comparing “within” a car, m remains constant, so the acceleration that we are interested is directly related to the force. The force in this case, is the force exerted at the ground by the driving wheels. The force at the ground is translated from the engine out to the wheels by the transmission. Since, again, we are staying within one car, the radius of the wheel, and the final drive, remain constant so only the gear ratio matters.

For our example, we will look at a 2nd gear to 3rd gear shift. Below is an actual dyno from a Mazda Protégé and the gear ratios for its transmission. From that information you can see that 2nd gear is 1.842 and 3rd gear is 1.310. What this means is that for a given point on the dyno we will multiply the torque value by the gear ratio. Looking at roughly 6,000 rpm’s the torque value is approximately 100 ft-lbs. So in 2nd gear this is translating to the final drive at a value of roughly 1.842*100 or 184.2 ft-lbs (this is not the value applied at the ground as the final drive and the wheel radius are not compensated for, but remember that multiplier, whatever it is, remains constant so only this input value to that system matters for our discussion). But for 3rd gear the input value calculates to 1.310*100 or 131 ft-lbs. That means that 2nd gear exerts 40% more force to the ground at that point than 3rd gear is capable of.

1st gear 3.307:1

2nd gear 1.842:1
3rd gear 1.310:1
4th gear 0.970:1
5th gear 0.755:1
Final Drive 4.105:1

This difference then becomes the critical point of our discussion. Looking again at the graph you can see that at redline the torque has fallen to ~82 ft-lbs. At the peak of torque at about 4,000 rpm’s it is 125 ft-lbs. So second gear at redline creates 151 ft-lbs of torque while 3rd gear at peak torque creates 163 ft-lbs. While this case looks like shifting a little earlier would help you it ignores two important issues:

1)Actual engine speed and where the gears will “pick up” on a shift

2)Other gear ratios

We’ll start with #2. Here are the same values under best/worst case as given above for the other shifts:

1-2: 1st @ redline = 271 ft-lb versus 2nd @ peak = 230.2 ft-lbs

3-4: 3rd @ redline = 107 ft-lb versus 4th @ peak = 121.25 ft-lbs

4-5: 4th @ redline = 79.5 ft-lb versus 5th @ peak = 94.4 ft-lbs

So as we can see the mechanical advantage in 1st gear is so significant that the huge torque fall off still doesn’t matter, but in the closer, higher gears it still looks like shifting early would be better. But now when we go back to point #1 you’ll understand why this is not the case.

To address number one we need to figure out the change in rpm’s. Since at the time of the shift we can roughly assume the speed to be constant, then the rpm’s will change by the gear ratio amount. So in our 2-3 example you calculate it as 6800/1.842=x/1.310 where x yields are new shift rpm which is 4836 in this case.

So now redoing our math, we know we shifted at redline in 2nd at 151 ft-lbs of torque to the final drive, but now when we pick back up in 3rd gear we do so at 4836 which yield a converted torque value of 118*1.310 or 154 ft-lb’s. In other words nearly identical. Now if you shifted earlier to try to obtain peak torque more, so lets say you want to start in 3rd gear at 3800 so you start in and go through the max torque curve portion for this engine, that would mean you’d have to shift from 2nd gear at 5343. This results in starting 3rd gear at 3800 rpm’s with 163 ft-lbs going into the final drive which is nice because you are starting out with more than the 154 we got with the redline shift so we are accelerating harder at the start of 3rd gear, BUT you’ve now cut out of 2nd gear when you were at 206 ft-lbs. So while you've started 3rd gear with more oomph, you gave up a LOT more oomph at the end of 2nd gear.

You can do the math on the other gears and you’ll find that in general the same trend will hold but fades as you get close to the top gears. The 3rd gear to 4th gear shift is essentially a wash with a case for shifting about 100-200 rpm’s early that can be made, but 4th to 5th changes things enough to start to matter due to the overdrive situation. In that case optimum shifting would appear to be at about 6400, or 400 short of redline. This effect is due to the mechanical advantage of the gears relative to one another:

1st has 80% advantage over 2nd

2nd has 41% advantage over 3rd

3rd has 35% advantage over 4th

And 4th has 28.5% advantage over 5th.

So in essentially all cases with this dyno graph you want to shift at or extremely close to redline. This becomes even more apparent when you take a vehicle like the turbocharged Mazdaspeed Protégé that has a flatter torque curve and less fall off toward redline. This also gets into why keeping the torque curve up throughout the power band matters (and that will obviously result in higher horsepower numbers as well). We’ll save torque and horsepower and area under the curves during a gear for another time as that is an even longer discussion.

Thursday, November 13, 2008

It's a start

"But Mom, everyone else is doing it!"

That's about how I felt about blogging, but as more and more customers have needed advice, and the more I talk to friends the more I realize that I might have something of interest to share with others. So here will be the place to do it. I'll focus mostly on automotive topics, it is my hobby and in some ways my passion, but I'm sure I'll muse and meander and that is my way. So check back soon when some real content shows up! This won't be some boring useless personal blog, this will be intended to be a resource and an educational tool if my aspirations are achieved.