So, belatedly on to fuelling. On the FD we have a MAP (Manifold Absolute Pressure) sensor for measing airflow into the engine. It doesn\'t do this directly - we have to tell the ECU how much air is flowing at a given RPM and manifold pressure, and we do this by building a map. Just like the spark advance table, we have a Volumetric Efficiency (VE from here on) table which is used for calculating how much fuel to inject.
Cribbed from Wikipedia: "Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%."
So, to walk through the fuel map as we would when \'mapping\'...
Starting at idle makes most sense, so we\'re in the bottom left of the map. Low RPM and low manifold pressure - the throttle will be shut and the engine has to work hard to move air through the tiny gap left past the butterflies, and we\'re left with a partial vacuum in the cylinders, or chambers in our case, and hence a low volumetric eficiency. It depends on the engine - compression, porting, air cleaner, exhaust and even ignition timing all have a big part to play in determining the VE. Still, a low value here is sensible.
On to the no load range of the map. A slight crack open of the throttle and we\'ll start to build RPM. Volumetric efficiency won\'t be much more so the map will remain fairly flat as we move up the rev range, but will increase slightly if the throttle is opened more.
Time to give it some load. We\'ll look at the cruising area of the map next. This means 2500 - 4000 RPM and low to medium pressure. You won\'t find the VE increases much with RPM under cruise, but the VE map will start to ramp up sightly as the pressure builds towards atmospheric.
The fun part is always Wide Open Throttle. On an NA motor it\'s pretty simple - WOT yields 100KPa, and close to, but not neccesarily complete cylinder filling - 100% VE - and this will be the highest line on your map. Generally you\'ll find it increasing with RPM up to peak torque, and might drop off a little after that.
On a turbo motor it\'s a little more tricky. The 100KPa line is called the transition area - the point at which you actually start making boost. The volumetric efficiency obviously ramps up from here as we\'re pushing more air in than you\'d get if it was at atmospheric pressure. More boost = more air = higher VE.
So, the great question is, how the hell do I know what the volemetric efficiency is to start filling in the bins??? The answer is a wideband lambda sensor. If the lambda is showing lean (greater than 14.7:1) then you\'ve got too low a value in that area of he map, and you\'ll need to increase it. Too rich, and you\'ve overestimated cylinder filling and you\'ll need to lower that part of the map.
I should mention here that target Air Fuel Ratios (AFR) will be different for the vairious parts of the map I\'ve listed above. At idle and cruise you\'ll want to keep it pretty lean for good fuel consumption, so look for the magic 14.7. Some engine will run leaner, others won\'t run lean at all and you\'ll have to keep it slightly rich to keep it smooth. Under higher load up to 100KPa look to make it richer, maybe down to 13 or 12.5 - this is where you\'ll get best power. Under boost I normally go for 12 to 11. Any more and you\'re just wasting fuel. Further than 9.5 will yield a massive cloud of black smoke! If you have to run this rich under boost to stop detonation something else is wrong. Don\'t get tempted to use the fuelling as a band aid for other problems.
I should mention over-run too. Coming down form high RPM with throttle shut will yield the lowest manifold vacuum and lowest VE. If you fancy some lovey pops on over-run, aim for over 16:1 above 3000RPM. Trust me, it\'s good. Th E30 drift car has a straight-through side exit exhaust, and convincingly scares the bejezus out of anyone lucky enough to be on that side of the car. It\'s like a shotgun going off. Looks mighty impressive at night too :Giggle