Tires are a very interesting element as they have such a significant impact on the handling of your car. Humorously though, they are pretty misunderstood. How a tire keeps the car gripping the road is an extremely in depth and technical issue, so lets preface this discussion by saying we are going to only go in to the broader generalizations and some of the more easy to understand points. It gets very complex when you start trying to approximate contact patch deformation, sidewall spring constant, pressure distribution etc. There are a lot of great books out there, so definitely find them and read them, but the hope here is to get you thinking about a few simple items to begin with.
So far starters… wider is better right? There isn’t a straight answer to that. So lets discuss what happens with the tire first of all. Think of a balloon for starters. If you could put it on a piece of glass and look at the area that is “flat” where the balloon contacts the glass you’ll notice one thing quickly and simply – the more pressure you apply the bigger that area gets. This is a pretty simple concept. When you have a weight applied to an inflated item it has to respond with a force equivalent to the weight to be able to support it. This applies to your tires on your cars as well. As I mentioned before, though, this gets pretty complicated when you really consider how much support the sidewall gives etc, but for now lets ignore that. Most tires for people on high performance cars will run in the 35-45 psi range. So lets choose 40 psi as your inflation pressure. Let’s also use my BMW 318i as an example. It weighs about 2800lbs and is very close to 50/50 weight distribution. So we’ll assume the car is properly corner weighted (corner-weighting is when you adjust spring height and rate etc to distribute the car’s weight evenly across the diagonals and each side/edge of the car) so that the weight is evenly split between each of the 4 tires. This means that each tire is getting 700 lbs of weight on it. At 40 lbs per square inch (psi) that means we need 17.5 square inches of area to support that for my car. Sounds big right? Not really. My car during the summer runs a tire I can only get in a 195 width. So that means it is roughly 7.7 inches across at the point where it touches the pavement. If I’m 7.7 inches across, to get to 17.5 inches means I only have a rectangle that measures 2.3x7.7 inches. Not a lot of area to keep that car on the road is it! So now lets go to a wider tire and see what happens. If I keep the tire pressure the same but go to a 235 width tire my contact patch now measures 9.25x1.9 inches. It is still 17.5 square inches at the road, it’s just wider now. Yep, that’s right, you just went WAY up in tire size and you still have 17.5 square inches in contact with the road! What you have done is changed the shape of your contact patch. Now, this is somewhat misleading because with many of the larger tire sizes you can run a little less tire pressure which will increase your contact patch for real. However, as you reduce pressure you lose some of the structure effects in the tire and you may actually end up with strangely shaped or poorer contact during cornering as you roll on to your sidewalls or have other issues. This is where you need to work on properly balancing the pressure for your tire.
It is also very important to keep in mind tire compound. Often you can achieve much more grip with a smaller contact patch area due to the adhesion factors of certain tire compounds. Tires generate their “grip” in a few ways. One way is simple generic friction between the surfaces and the way they bond to one another. This is greatly affected by compound. A second way is mechanical grip that is generated by interference between the objects, for example, if the rubber complies to the shape of a gravely pavement, you will have some significant vertical walls the rubber is being pushed against and torn away from the tire by. This generates grip as well. The third major grip component is adhesion. Some tire compounds actually are truly “sticky” and will adhere to the surface they roll over. This allows coefficients of friction greater than 1 to be achieved! What this means is that for every 1 lb of force applied down on the tire, it can generate more than 1 lb of force perpendicular to that. This is HUGELY dependent on compound and is a significant factor in the advantage that race slicks will have over street tires (although that changes more and more every year with some of the crazier street tires).
You also need to consider what happens to the contact patch during maneuvers. A tire actually provides the MOST grip at a point when it is “sliding” a little relative to the angle of travel. This is referred to as slip angle and varied greatly depending on tire design. It is often in the range of 3 to 10 degrees though. This matters because as you make your tire contact patch wider you change the amount of difference between the arc radius the inner portion of the tire travels versus the outer portion. This can lead to those portions of the tires actually traveling at different slip angles from one another and it can lead to conditions such that one portion of the tire may be at optimal slip, while another is not. This is one relatively extreme case where wider would indeed NOT be better. Also, as tires get wider things like turn in response and their behaviors under braking or acceleration and the changes in the tread deformation introduces a lot of other variables and possibilities. Again, the topic is very complex so it is hard to argue it fully in a short essay. I’ll try to revisit the topic in the future and get into some of the more specific details for the above, but hopefully this got you thinking and drop at least a few new ideas out there for you.
Happy motoring!
Monday, March 2, 2009
Friday, January 30, 2009
Ideas?
I have a few more topics I want to cover, but if you have any suggestions or burning questions you'd like to see my take on, please do comment and let me know! It's no good if I'm not getting at the issues and areas that you are interested in.
Thanks!
Steve
Thanks!
Steve
Wednesday, January 7, 2009
Air Fuel Ratio - Basics
Now that the holidays are past and our year end accounting is nearly done I have a little time to get another post out for you.
We covered timing before, but as I said previously AFR (Air Fuel Ratio) matters a lot as to how the vehicle will perform. First a simple background on AFR. When it comes to standard gasoline stoichemetric (meaning chemically balanced for both sides of the equation for a chemical reaction, in this case combustion) is 14.7 parts of air to 1 part of fuel or 14.7:1. ANYTHING higher than this (say 15.2:1) is considered a lean mixture, and anything lower is considered a rich mixture. A lot of times people will say when a turbo car is running 13:1 in boost that is running to lean, but what they really mean is that the car is not running as rich as they think it should, because even at 13:1 the air fuel mixture is definitely a rich mixture.
So that all said, what do various AFR mixtures get you. Well obviously the leaner you go the less fuel is being used, but that does NOT mean the vehicle will use less fuel. In fact, as you go leaner than about 15.5 you typically lose enough in power output from the leaner mixture that the additional throttle input and resultant use of fuel to maintain that mixture uses more fuel than if you had simply run a mixture closer to 14.7. So in other words, don’t think that if you managed to safely tune your car to 16.8:1 that you are getting better gas mileage as this is often not the case! Additionally, when you go to mixtures leaner than 14.7 two major things happen: combustion occurs at a higher temp thus heating your cylinders and valves more significantly, and second some of the “bad” pollutant gases increase significantly leading to a more polluting or problematic emissions footprint from the car. This is why most manufacturers design cars to run at 14.7 AFR. In fact in the vast majority of vehicles out there, the only oxygen sensors in the vehicle are all what is referred to as a narrowband sensor which can ONLY tell you if you are rich or lean relative to 14.7 and provides essentially no useful information otherwise. Sorry folks on a budget, but you can not ever tune a vehicle accurately to some other AFR value with a narrowband sensor, you need to drop the money and get a wideband! At 14.7 AFR emissions are typically the most balanced and allows catalytic converters to do their job.
The fact that manufacturers tune most street vehicles to 14.7 AFR is exactly why chipping a vehicle can sometimes get you more horsepower with fuel only changes. 14.7 is the magic number for naturally aspirated vehicles trying to make emissions. However, that number changes whenever you are in boost for a forced induction vehicle, or if you are trying to make power on a naturally aspirated vehicle. The values are hotly contested, and as I alluded to in my last post on timing these values change quite a bit when you work to balance out your timing values with your AFR. But to speak in general rules as guidelines is relatively simple. Gasoline mixtures in MOST modern combustion chamber designs will provide peak power output at AFR’s between 12.3 and 12.8. The value is often narrowed down to be 12.5-12.6 but as I said these values are hotly contested. I personally consider 12.3 to 12.5 to be optimal for turbocharged vehicles and 12.6 to 12.9 to be optimal for naturally aspirated vehicles. So when I’m tuning an NA car I tune wide open throttle AFR’s to 12.8 to 13.0 and then maximize the timing. NA cars are much less sensitive for timing and air fuel balance so this typically works well and is quick and easy. Turbo cars it is much more complicated. Turbo cars I usually tune to about 11.8 and work out the timing. If I can advance the timing fully without detonation issues then I work to see if more can be gotten from leaning out the AFR. This rarely is the case so I settle in near that AFR range almost always.
There are lots of other purposes and uses for AFR though that can yield interesting improvements. Running very rich mixtures can sometimes help with turbo lag and help cool off the valves. The type of injector being used and it’s placement and the atomization of the fuel can also change what AFR is effective in a given vehicle. Sometimes, you need to use rich mixtures to help warm up a car or stabilize an idle, and sometimes a lean mixture will help with rough running on a vehicle. There are a host of options, but we’ll leave that for another post.
Thanks for reading and happy motoring!
Steve
We covered timing before, but as I said previously AFR (Air Fuel Ratio) matters a lot as to how the vehicle will perform. First a simple background on AFR. When it comes to standard gasoline stoichemetric (meaning chemically balanced for both sides of the equation for a chemical reaction, in this case combustion) is 14.7 parts of air to 1 part of fuel or 14.7:1. ANYTHING higher than this (say 15.2:1) is considered a lean mixture, and anything lower is considered a rich mixture. A lot of times people will say when a turbo car is running 13:1 in boost that is running to lean, but what they really mean is that the car is not running as rich as they think it should, because even at 13:1 the air fuel mixture is definitely a rich mixture.
So that all said, what do various AFR mixtures get you. Well obviously the leaner you go the less fuel is being used, but that does NOT mean the vehicle will use less fuel. In fact, as you go leaner than about 15.5 you typically lose enough in power output from the leaner mixture that the additional throttle input and resultant use of fuel to maintain that mixture uses more fuel than if you had simply run a mixture closer to 14.7. So in other words, don’t think that if you managed to safely tune your car to 16.8:1 that you are getting better gas mileage as this is often not the case! Additionally, when you go to mixtures leaner than 14.7 two major things happen: combustion occurs at a higher temp thus heating your cylinders and valves more significantly, and second some of the “bad” pollutant gases increase significantly leading to a more polluting or problematic emissions footprint from the car. This is why most manufacturers design cars to run at 14.7 AFR. In fact in the vast majority of vehicles out there, the only oxygen sensors in the vehicle are all what is referred to as a narrowband sensor which can ONLY tell you if you are rich or lean relative to 14.7 and provides essentially no useful information otherwise. Sorry folks on a budget, but you can not ever tune a vehicle accurately to some other AFR value with a narrowband sensor, you need to drop the money and get a wideband! At 14.7 AFR emissions are typically the most balanced and allows catalytic converters to do their job.
The fact that manufacturers tune most street vehicles to 14.7 AFR is exactly why chipping a vehicle can sometimes get you more horsepower with fuel only changes. 14.7 is the magic number for naturally aspirated vehicles trying to make emissions. However, that number changes whenever you are in boost for a forced induction vehicle, or if you are trying to make power on a naturally aspirated vehicle. The values are hotly contested, and as I alluded to in my last post on timing these values change quite a bit when you work to balance out your timing values with your AFR. But to speak in general rules as guidelines is relatively simple. Gasoline mixtures in MOST modern combustion chamber designs will provide peak power output at AFR’s between 12.3 and 12.8. The value is often narrowed down to be 12.5-12.6 but as I said these values are hotly contested. I personally consider 12.3 to 12.5 to be optimal for turbocharged vehicles and 12.6 to 12.9 to be optimal for naturally aspirated vehicles. So when I’m tuning an NA car I tune wide open throttle AFR’s to 12.8 to 13.0 and then maximize the timing. NA cars are much less sensitive for timing and air fuel balance so this typically works well and is quick and easy. Turbo cars it is much more complicated. Turbo cars I usually tune to about 11.8 and work out the timing. If I can advance the timing fully without detonation issues then I work to see if more can be gotten from leaning out the AFR. This rarely is the case so I settle in near that AFR range almost always.
There are lots of other purposes and uses for AFR though that can yield interesting improvements. Running very rich mixtures can sometimes help with turbo lag and help cool off the valves. The type of injector being used and it’s placement and the atomization of the fuel can also change what AFR is effective in a given vehicle. Sometimes, you need to use rich mixtures to help warm up a car or stabilize an idle, and sometimes a lean mixture will help with rough running on a vehicle. There are a host of options, but we’ll leave that for another post.
Thanks for reading and happy motoring!
Steve
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. www.jandssafeguard.com Tell John I sent you!
Happy Motoring!
Steve
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
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.
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.
Subscribe to:
Posts (Atom)
