here I made some additions, I hope you dont mind.
OK, now that we understand what all of these words mean, lets move on to actually tuning your engine. All tuning starts with ensuring that the system is working as is should. This means checking that you have adequate sized injectors, a good fuel pressure regulator, proper fuel pressure, a good fuel pump, clean fuel filter, no kinked hoses or lines, and no leaks in the fuel system. Then, we want to make sure the intake system (everything from the back of the MAFS [IF YOU HAVE ONE] to the cylinder head) is sealed and not leaking. Leaks in the intake system cause all kinds of hard to fix problems that make tuning a pain. The path air takes from the filter to the exhaust system is called simply "air path". The best way to determine if your air path is sealed up is to pressurize it using a home-made boost leak checker (insert link here to DIY boost leak checker using PVC and tire valve stem). Pressurize your air path to 15-20PSI and find all the leaks.
Next is the ignition system. MAKE SURE YOUR PLUGS LOOK GOOD AND THE GAP IS SET!!! A word about gap here. The larger the gap the better the performance, however, too large of a gap and spark will blow out when resistance in the cylinder climbs high enough. A spark starts combustion by breaking the bonds between Oxygen atoms, the same way lightning does, which allows the chain reaction of combustion to take place in the presence of an appropriate electron donor such as gasoline or ethanol. It is worth noting that liquid fuel will not participate, in fact liquid gasoline will not burn. It is only the vaporized form of gasoline atoms that participate in combustion. If you light a puddle of gasoline on fire it will not explode suddenly, but rather, burn steadily, as the gasoline molecules gradually evaporate and then burn when they reach oxygen atoms in the air, to produce heat and energy. Higher fuel pressure and better injector nozzles are more likely to give a finely atomized spray which leads to better performance. Spark gap tuning: The way I tune spark gap is to run the plugs with the stock gap and gradually turn up the boost until we blow out the spark. THEN you gap the plugs down. Typically you will need about .028" gap on a 400rwhp sr20det engine using stock ignition system. If you run an MSD or other ignition amplifier you may increase your gap to as large as .055" and this will result with improved performance and economy. This is SOO important. I'm amazed at how often this is overlooked. Check your plugs, check your plugs, check your plugs! Next, if you have them, check the plug wires, cap, rotor, and coil. Now check that the ignition timing at the crank actually matches what's happening at the crank. Once again, this is a very simple step that is often overlooked. If the timing is off +5°, that can easily be the difference between a little knock from too much timing, and a bent rod, or a hole in a piston, or a blown HG from serious detonation. Last, but not least, is the oil. Make sure it's fresh, AND OF THE CORRECT VISCOSITY!!! There is little that will thin oil faster than a few pulls on a dyno.
(Note:I have a personal rule, that I don't do more than 4 pulls without shutting the motor off and letting the coolant and oil cool down for 10-15min or so. If you're REALLY confident in your engine cooling and oil cooling system, this doesn't have to apply, but be careful) Good advice, never run an engine on the dyno back to back several times without a significant cooldown period.
Now that we're confident the motor isn't going to have some mechanical failure, we can start the actual tuning process. I'm not going to get very detailed here, just cover the basics. First thing is to get the motor idling and up to NOT. Unless you're using a stock ECU, or a standalone that allows for closed loop at idle, you may start with your AFR'S around 12:1. This will vary for different motors, so just treat that as a starting point. Once you acheive a stable idle at around 12:1 you will want to lean it out quickly, to around 14.5:1 or leaner. The more lean you run the engine the more efficient it will be, the more heat will be produced, the better economy you will acheive in general. We will also want to adjust the timing so that it gives a stable idle. What you may notice is that as you adjust the timing and fuel, you'll have to re-adjust the the other. This is normal. There is a relationship between timing and what AFR is showing up at your sensor. I'll explain why later. Just give the motor what it wants. It took me a long time to stop questioning why different motors like different things. If it runs well, and it sounds well, and it makes good power, it's right. The books and numbers don't matter. Results matter. It is worth noting at this point that just because an engine starts to idle rough at leaner air fuel ratios does not make it a bad place to be. Some engines with large camshafts will idle smoothly at 14.5:1 and lope at 15.5:1. There is nothing wrong with leaving it at 15.5:1 and letting the engine lope, in fact this is desirable to many individuals. You must remember that the oxygen sensor is reading the oxygen content, not fuel, so an engine with a large camshaft is likely to pass oxygen into the exhaust system during overlap, giving you a false lean reading on your wideband. The same thing happens if one cylinder is misfiring, the wideband will read lean because the misfire is passing un-used oxygen into the exhaust system. So again, results matter. If the engine has HKS 272 cams in it, and lopes steadily and happily along at 16:1 I would leave it there.
Ok, so now you should have the car idling well. First we want to set up a quick timing map. In most systems, you want to start with some sort of base map to work with. Building a timing map from scratch can be excruciating depending on the system. The timing should slowly ramp up from idle to around 30° (If I don't specify, all timing will be given in degrees BTDC) and reach maximum timing around 2500 rpm. Once again, these are just starting points. Your final cruise timing for most vehicles will be about 37* btdc. The process to finely tune ignition timing is given next (advanced):
Advanced timing tuning for best economy:
First clarification about the word average, When I say average I mean when driving the car like a normal daily driver out of boost, with 50% highway commutes.
Now, on to the gory details,
To achieve this kind of economy requires patience to say the least. There are several factors affecting fuel economy on all cars;
First let us tackle the tuning. My average air fuel ratio while cruising is 15.4:1, and I have developed a sort of "checker board hypothesis" whereby the fuel map gradually dips from high 14's (14.9:1) to mid 15's(15.7:1 max) in cruise situations, and to ensure engine longevity the fuel map resolution in those areas has been enhanced (a typical feature of many stand-alone ecu is you can decide what sort of resolution you would like by changing pressure/mass breakpoints) such that a slight movement outside of the cruise range (even just a steep hill while cruise control is set) will drop the air fuels quickly to about 14.5:1 which is much closer to the average closed-loop air fuel ratio you would see when using a narrowband sensor (they tend to flip from 14.3 -> 14.9 quickly). In the past I have used a simulated wideband analog output to keep my ECU in closed loop at these air fuel ratios ( 15.5:1 @ .002volts, 14.9:1 @ 1.002 volts) but I have found open loop to be just as effective when the engine is mostly stock (mainly because of the great vacuum a stock engine will produce, makes each load point more well defined)
Now, all of that is fine and dandy, but it does not have that great of an effect on fuel economy as you might expect. And I have discovered this because I data-log everything for hours of driving for years and years on different cars. The best way I have found to measure fuel economy is by comparing injectors duty cycle, engine vacuum, and road speed all at once. If you also include the injector flow-rate, you can write an algorithm to generate a real-time fuel economy display from this data, but it is un-necessary to simply find out if your changes are saving you gas. The more important tuning aspect of fuel economy is ignition timing. Finding the best timing for your highway cruise at different load points. And here is my method.
Load your car with luggage, full tank of gas, passenger, weight it down as heavy as it can be. This will ensure your load cell is just about as low-vacuum as possible for a highway cruise, this will ensure that if you tune your timing then later on decide to add all of this weight, you do not run outside of your intended cruise load cell and into the part throttle acceleration zone.
Now, set your cruise control to eliminate the movement of the tps to keep the acceleration enrichment out of the picture as much as possible. In my case, I simply disabled my TPS enrichment feature for this tuning session. Cruise along at a speed low enough that wind resistance will not factor in (yet). I usually start at 65MPH. Find your load cell, remember the vehicle is heavy, on my car this cell is right about 10" of vacuum at 65 MPH. Set your timing to a low number, something like 30* that is much too low to be the final result, and drive for 20~ minutes while logging your fuel injector duty cycle and engine vacuum. In my case, injector duty was about 8%. Now, advance your timing. I used 35* next. And repeat the above proceedure. My injector duty cycle dropped from 8% to 7.6% average, and my air fuels became slightly richer in fact, allowing me to remove even more fuel. If I had an EGT gauge I would have also noticed the temperature drop at the exhaust manifold around 50*F~. This is all a very good sign that the additional timing was a welcome change. You can now see where this is going; repeat the proceedure, adding timing and logging fuel injector duty cycle, and look for the signs of a welcome change, that is, reduced injector duty cycle, lower EGT, increased engine vacuum. At some point you will encounter diminishing returns, and then the application of the vehicle will tell you what to do next. For me, safety is number one, so I never run off into the deep end for that extra .2% (it took another 12* BTDC of timing to drop the injector duty only .2% so clearly the additional timing was unwelcome). My final timing is right at 37* btdc for this engine, some have appreciated as much as 45* btdc (big cam(s), big manifolds, large displacement engine usually)
Now, it is important to realize that before you start playing with the engine's tune, there are several other factors to consider when trying to get the best fuel economy, they should be taken care of first.
1. Rolling resistance. Do your brakes drag? grease the backs of your pads. Grease your wheel bearings. Spin your driveshaft by hand and feel it, does it feel rough or notchy? Push your vehicle by hand, often, get a feel for how easily it rolls. Update and maintain all of the aspects of driving straight, such as the alignment, toe and caster. Consider the tire compound.
2. rotating weight. Use the lightest wheels you can find. I also have a light weight flywheel (but not too light!). Anything in your drivetrain you can afford to have lighter is a fuel saving benefit. You do not want the rotating assembly too light on a street car (no knife edged crankshafts) though.
3. ride height / wind resistance. Speaks for itself, is your car suitable for high speeds? Get it lower to the ground if possible.
and finally, the way you drive is very important. use your brakes as little as possible. Every time you brake, you waste the energy that was extracted from the fuel you burnt and send money out the window. Engine braking for long distances pissing everybody off behind you that wants to speed up to the stop light is essential. My car will engine brake with zero fuel input from 60mph roll for about 1/4 mile.
/end timing tuning for best economy
Now for the fuel at WOT. On a turbocharged vehicle, We're going to aim for about 12:1 at WOT all the way to redline. A naturally aspirated engine will tolerate 13:1 or even leaner. Most LS-X engines will run 13.6:1 for instance. It depends heavily how much power per unit displacement you are making. This will probably take a few pulls to even get the car to make it to redline. Watch the AFR'S closely. For right now, if you see anything leaner than 13:1, get out of it. Add some fuel where it leaned out, and any rpm and load point above that an equal PERCENTAGE more. It's important to keep the idea in your head that if you change the number at a given load point by "5", you can't just add "5" everywhere above that. You need to find out what percentage change "5" was for that load point, and apply that change to every load point above that. Most standalones will let you do this easily. If they don't, it will just take a little longer to do the tuning. Once you've got the car capable of making a pull all the way to redline, start trying to get the AFR'S evened out so that your AFR'S look like a line around 12:1 all the way to red line. For now, we're done. There is more we can do with fuel, but for now, lets move on to timing.
The timing is a bit more time consuming, and you have to be careful here, especially if you're on pump gas. (excluding E85 and other alcohols, that is a "whole 'nother can o' worms", as they say)Basically, what I do, is start from an ignition timing map that I know needs to have timing added to it. I will start out by adding about 2 degrees of total timing at full load, and see if, and where it makes power. Usually it will make more torque every where. Continue to do this until it stops giving back SIGNIFICANT increases in torque. Then back it off 2 degrees. Also, listen VERY CAREFULLY for knock (if you can. Most exhaust systems will drown out any knock). ESPECIALLY AROUND PEAK TORQUE!!! (watch your knock sensor and look at the torque graph. If it starts getting jagged peaks and valleys that is another sign of too much timing. Always use Smoothing:0 when tuning on a dyno.) (insert example photo of a jagged torque from too much timing). Knock is most likely to occur at peak torque, because that is when cylinder pressure is highest and octane is the weakest link. Sometimes (almost always) you have to pull the timing back around peak torque, and then bring it back up afterwards. What you may also find, is that after you've ramped the ignition back up after peak torque, you may have to start advancing the timing again around 6K and ramp it up as the RPM's increase. This is happens to some motors. If you're motor likes it, then so be it. If not, then that works too. Most SR20DET engines will be happy around 9* of timing through peak torque on a given 280rwhp engine with mostly stock components, leading up to about 12* of timing by 7000rpm. Rarely on pump gas will any sr20det engine require more than 12* of timing anywhere during WOT, unless your boost is ultra low, say 4psi. always start tuning timing with the lowest boost you can, for me this is 0psi, you disconnect the wastegate arm so the flapper just hangs open and the engine will not build boost. This way you can tune the engine's 0psi area through WOT, which would be impossible otherwise. You will want as much as 15-18* in this area (NO BOOST). Once you re-enable the wastegate arm and build boost, usually you are stuck with 7psi minimum, and I recommend starting with 12* and making a pass, then adding 3* for a total of 15* and making another pass at 7psi just to see for yourself what the engine does. It should not pick up much more than 7 horsepower. Remember that anytime you add timing, it will always give you more torque and power somewhere, even if the timing is dangerous. It is important to realize that additional timing will almost always yield improved torque output (you are beginning to spike the cylinder pressure, which will generate more torque, but it will also hammer the rod bearings and put strain on the head gasket). That is why we tune for SIGNIFICANT increases in torque, like a gain of 15-20+ for an additional 2* of timing advance input.
Alright, for now, that is a nice, safe, map that will get you down the road and not blow anything up. When we start tuning for peak power, turning up the boost, and other changes, I'll go into more detail about what to do, and what's actually happening inside the combustion chamber.[/quote]