So far, the focus of this book has been naturally aspirated engine calibration. Although forced induction has been occasionally mentioned, it deserves some more focused attention. The aftermarket performance industry is flush with ways to add power to an engine. Few of these methods come close to the potential that forced induction offers for power increases. Many OEM vehicles are built with superchargers or turbochargers from the factory, which often leaves the door open for the enthusiast to simply increase the pressure ratio in the search for power. At the end of the day, the concerns are the same. Any time more compression happens to a gas, its temperature rises by some amount. How much temperature increase comes along with the compression is a direct result of the amount of compression happening and the efficiency with which it is being done. Not all compression methods are created equal.
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It is important not to confuse manifold pressure with cylinder pressure. The two are linked, but the relationship between them is unique for each engine combination. Cylinder pressure is the primary deciding factor when considering how much less timing is needed under boost compared to the same engine under atmospheric conditions. Looking at load instead of boost gives a better representation of what the engine wants. Increases in airflow cause load shifts, not boost. Boost is just a measure of restriction between the engine and compressor. If exhaust backpressure or camshaft design is changed to increase flow, boost may actually decrease (without a change in drive/pulley ratio) while airflow and load increase. This condition may require a reduction in timing to prevent knock at the higher load even though boost has been reduced. The only way to know for sure is to test under load.
As mentioned before, extra fueling is used to improve combustion stability and reduce temperatures under high load. The increase in available airflow may also require revision to acceleration enrichment requirements to accommodate the higher instantaneous increases in airflow on tip-in. Supercharged engines should still be able to operate normally at low and mid load. The addition of forced induction alone does not require that the engine always be operated rich. The normal procedure for part throttle mapping remains and λ = 1 fueling for most operating conditions should still be targeted to preserve fuel economy and emissions.
Part load spark calibration is again almost identical to that of a naturally aspirated engine. MBT is not affected by the supercharger at part throttle. MBT remains the ignition target at part load. There will just have to be a blending into the new higher load regions of the map.
Probably the most widely used compressor in the performance aftermarket is the centrifugal pump. Companies such as Vortech, Paxton, and ATI produce countless systems for almost every performance vehicle on the market. These compressors are driven by the accessory drive belt, linking their impeller speed directly to engine speed. Because these compressors offer little increase in flow at low speeds, they are overdriven through both pulley ratio and internal gear sets. Still, the centrifugal pump design requires relatively high speed to reach its potential. Actual engine boost output from a centrifugal supercharger increases with engine speed. Drive losses for a centrifugal supercharger also vary depending on the load. At idle they only require a few horsepower to turn, but losses increase with speed. A typical after-market street supercharger can take anywhere from 30 to 50 hp to drive at full load. This is very real power that must be transferred from the crankshaft to the supercharger, and it must be taken into account when calculating fuel system requirements. Luckily, the drastic increase in airflow more than outweighs the parasitic losses at high load.
As impeller speed increases, so does pumping volume and efficiency… to a point. The result is usually a wide-open throttle boost curve that increases almost linearly with engine speed. At low RPM, there is almost no added manifold pressure. Therefore, engine calibration at these speeds can be done with a leaner lambda and more aggressive spark advance below peak camshaft efficiency, just like on a naturally aspirated engine.
Even with relatively little increase in engine load at low throttle position, air is still being pumped by the compressor at low speed. This pumping volume is still greater than the engine’s airflow requirements at idle and low load, so pressure builds between the throttle blade and compressor. The added pressure on the compressor blades creates drag on the accessory drive that can make idle control difficult. A bypass valve is used to divert this pressure back to the inlet side of the compressor. The valve is actuated by a vacuum diaphragm connected to the intake manifold. When the throttle is opened and manifold vacuum disappears, the bypass valve closes and forces all compressor output to be routed directly into the engine.
One word of caution for mass airflow based systems is to ensure that all flow only passes the metering element once. It is a common mistake to route bypass air back into the inlet upstream of the MAF sensor, causing double metering during bypass. This makes proper MAF calibration all but impossible. Likewise, if the bypass is placed after the MAF element, it must not vent to atmosphere. Dumping metered air back to the atmosphere results in a rich condition as the PCM assumes all metered air is actually going into the engine.
As engine speeds increase, boost pressure at WOT also increases, resulting in loads exceeding 100%. Load continues to increase all the way until redline unless otherwise restricted. Unless the drive belt slips, centrifugal superchargers develop maximum boost at redline. Cylinder pressures should steadily increase with speed as well, with moderate increases near peak torque and substantial improvements at high speed where the efficiency of the centrifugal pump becomes greater.
Because of the variable load increase, a centrifugal supercharger system rated to deliver 10 psi (0.7 bar) of boost only does so at maximum speed. If this maximum speed is 6,500 rpm, there may only be 5 psi (0.35 bar) of boost present at the torque peak near 4,500 rpm. This tells us two things. First, engine load increase near peak torque (the point at which cylinder pressure would normally be highest) is significantly less than what is seen at redline. This means that less timing retard is necessary at this point in order to maintain good engine durability and knock resistance versus red-line. Secondly, increasing loading means that the high-RPM pumping efficiency of the newly supercharged engine system is extended. An engine that would otherwise lose volumetric efficiency near 6,000 rpm may now continue to develop useful power to 6,500 rpm or higher. This is due to the ever-increasing manifold pressure overcoming the loss of ram tuning effectiveness. When peak load is reached at high engine speed, more timing must be pulled to avoid knock. The increased inlet temperature that comes with boost should be compensated for in the calibration. This is done by adjusting the IAT timing function accordingly to pull timing at high temperatures. For this reason, the IAT sensor should be placed such that it always measures actual manifold air temperatures in supercharged applications.
Most efficient centrifugally supercharged engines can be safely calibrated to operate with a lambda close to that of a naturally aspirated engine for short durations. However, in the interest of durability, a slight enrichment with the resulting reduction in combustion temperatures is desirable. The hotter the actual inlet air temperatures are anticipated to be, the more enrichment should be used. I have typically targeted λ = 0.8 (11.7 a/f) as a starting point for all conditions in excess of 100% load with centrifugally supercharged engines. Some intercooled applications with lower inlet temperatures may be just fine at λ = 0.85 (12.5a/f), but I still prefer to stay on the safe side for street-driven vehicles. On one centrifugally supercharged, non-intercooled drag race application, I found best power at λ = 0.89 (13.1 a/f)! This engine actually worked very well with no signs of knock and terrific performance, even if it was only for 10 seconds at a time. Obviously, the short duration of load in the drag racing environment allowed for the hotter burn without engine damage.
Generally speaking, charge temperatures dictate how rich the engine must be run under load to preserve durability. Since centrifugal super-chargers only produce peak boost (and temperatures) for a brief period of time near redline, their need for charge cooling is not as great as other superchargers. If a charge cooler is used, target lambda can be higher. Since there are usually diminishing returns on power increase from enleanment, a slightly rich mixture can be kept with the power being found in ignition timing. The engine’s knock limit is usually the limit on how much power can be made at a certain boost level.
Positive Displacement Superchargers
The positive displacement supercharger traces its heritage back to the earliest days of automotive performance. The compact design is often integrated with the intake manifold to save space, making for an attractive OEM option. Ford, GM, Mercedes, and others have employed positive displacement superchargers to increase performance in dozens of car models. Eaton, Autorotor, and Whipple manufacture these units, which are used in various OEM and aftermarket installations.
The principle is simple. For every rotation of the positive displacement (PD) supercharger, a fixed volume of air is moved through it. When the PD supercharger is sped up with a pulley drive similar to the centrifugal supercharger, more air is moved than the engine normally displaces. The result is almost instant boost when full throttle is applied, regardless of engine speed. Because the ratio between blower displacement and engine displacement does not change, boost can remain almost constant across all engine speeds.
There are a couple different varieties of positive displacement super-chargers. The most common are Roots style such as the Eaton Model 90 found on the Pontiac Grand Prix or Model 112 on the Mustang Cobra. Top Fuel dragsters also employ a much larger variant of the Roots compressor design. A more efficient variant is the Lysholm Screw compressor such as the Autorotor and Whipple designs. The increase in efficiency comes from a more axial flow through the compressor with a different blade design, yielding less heat at the same pressure ratio as well as increased flow volume.
Parasitic losses to drive a positive displacement (PD) supercharger are higher than the equivalent centrifugal supercharger. Drive losses increase with speed for PD superchargers as their efficiency drops. The caveat is that PD superchargers become less efficient as speeds increase. Attempting to overdrive a PD supercharger can often lead to immense increases in outlet temperature without the desired mass flow increase due to efficiency losses. An overdriven Eaton supercharger on a Ford Lightning can take over 80 hp to operate!
Due to the increased low-speed pumping, idle loads are more pronounced with positive displacement superchargers. An appropriate change must be made to idle speed targets and spark values to maintain stable idle when adding a positive displacement supercharger. A bypass valve is also mandatory to prevent overheating the intake charge and excessive crank loading even at idle. The throttle blade is usually positioned upstream of the compressor section, so bypass happens entirely behind both the IAC and MAF sensor. This makes the calibration of the MAF and IAC functions similar to that of a naturally aspirated car of larger displacement.
WOT loads for positive displacement supercharged engines can easily exceed 100% even at speeds just above idle. This makes low speed spark mapping more critical than with a centrifugal supercharger. The added load means that cylinder pressures rise naturally without the need for extra spark advance below peak camshaft efficiency. Off idle, high load ignition timing requirements for PD supercharged engines are usually surprisingly low.
Nearing peak torque, cylinder pressure rises rapidly, which requires significantly less ignition lead. Unlike centrifugal superchargers, PD units have full boost at the same point as peak camshaft efficiency, leading to even higher cylinder pressures at the same peak boost levels. This added cylinder pressure translates to a lower ignition advance at the knock limit near peak torque.
After peak torque, when camshaft efficiency decreases, cylinder pressures begin to drop again in a similar manner to naturally aspirated engines. Obviously there is still boost, but timing can still be added to make up for the decreasing engine efficiency at high speed. Although timing does not approach the same total values as a naturally aspirated engine at WOT, the increase with speed after peak torque is similar. Practical engine speed ranges are usually not increased as a result of adding a positive displacement supercharger.
Because of the tendency to run much warmer than their centrifugal counterparts, PD superchargers require additional help cooling the intake charge. Just like with centrifugal superchargers, inlet temperature should be monitored after the compressor to allow for proper adjustments to ignition lead or fueling. Most OEMs use the preferred method of charge cooling with an intercooler to reduce inlet temperatures. A water-to-air cooler can often be packaged as part of the supercharger and intake manifold assembly, saving precious underhood space.
If intercooling is not possible, extra fuel can be used to accomplish the necessary combustion temperature reductions. To keep combustion temperatures in check, I usually run a positive displacement supercharger richer than its centrifugal counterpart. A starting target of λ = 0.78 (11.4 a/f) usually works best. Again, the slight increase in fuel usually has little impact on power output, but really helps stave off knock. The richer mixture burns cooler in the chamber, but ignition timing can also be increased slightly to maintain stable combustion and good power. This usually gives the best overall safety margin without hurting power. Some properly intercooled positive displacement engines are perfectly fine at λ = 0.85 (12.5 a/f), but little power is usually lost with some added enrichment that can go a long way toward avoiding knock with the occasional heat soak or tank of bad fuel.
Turbochargers are the other compressor so often embraced by OEMs. A turbocharger is not driven directly by the crankshaft, but rather by the otherwise unused heat and velocity of the spent exhaust gases. This indirect coupling gives the turbocharger some unique performance characteristics.
The first difference is parasitic loading. Since the turbine section harnesses the power of the exhaust gases, it presents almost zero load to the engine at idle and part throttle. This makes idle and part-throttle calibration identical to that of naturally aspirated engines.
The boost curve is the second, more noticeable difference. It is the energy in the exhaust stream that powers the turbocharger. Placing the turbine section in this flow of hot gases turns heat and velocity of the gas flow into mechanical work that drives the compressor. The more exhaust energy that is available, the more compressor energy that is available. The buildup of this energy to the point where the compressor flows more air than the engine would on its own is known as “spooling.” Many factors go into deciding at what engine speed this point occurs at WOT, primarily turbocharger sizing relative to the engine. While we do not go into detail here about how to ensure best system efficiency and quick spooling, it is important for the calibrator to keep thinking about this during the tuning process.
Spooling is dependent upon having enough exhaust gas energy (either heat or velocity) to drive the turbine. If the turbocharger does not respond quickly enough, more exhaust energy is needed. This can be done by adding mass flow or adding temperature. Mass flow is dictated by engine hardware at WOT, so little can be done in the calibration to improve this. Temperature can be increased by retarding ignition lead or richening the mixture slightly to allow more combustion to happen outside of the chamber on the way to the turbine. If gases are still burning and expanding as they pass the turbine, more power is harnessed there rather than by the pistons. Although it results in a slight decrease in cylinder pressure, a quicker spooling and the resulting manifold pressure rise quickly offsets this. Likewise, if spooling occurs too quickly, response is softened by advancing ignition lead or throttling airflow.
Once the turbine has harnessed sufficient power to drive the compressor to the desired manifold pressure, gases are partially routed around the turbine section through the wastegate to maintain a constant energy transfer to the turbine. The result is a boost curve sloping up to the desired level during the spooling period and holding steady until redline, assuming a proper flow match to the engine.
The turbocharger uses the more efficient centrifugal compressor design, but without the limitation of being linked to engine speed. By allowing the compressor to be driven into its peak efficiency range much sooner, overall system efficiency is increased. The power to drive the compressor registers only as an increase in exhaust backpressure to the engine. Depending on catalyst and muffler design, this may not even be any greater than the naturally aspirated equivalent. As a result, turbocharger installations often yield parasitic losses of less than 15 hp, with almost zero loss at cruise and idle. Another side benefit is that the rotating blades of the turbine section tend to break up individual exhaust pulses, quieting the exhaust note of the vehicle. A lower-restriction muffler can then be used without the usual noise penalty, again improving system efficiency.
Much like the positive displacement supercharger, WOT loads from the turbocharger represent a magnification of the engine’s naturally aspirated tendencies. Although low-speed high-load operation is somewhat limited by spooling characteristics, peak boost is still often available well before peak torque. To compensate here, timing must once again be reduced near what would normally be peak cylinder pressure near peak cam efficiency. After this peak, timing can once again be increased with engine speed as long as inlet temperatures and cylinder pressures permit.
Unlike the positive displacement superchargers, turbochargers continue operating closer to peak efficiency all the way to redline when sized properly. This means that inlet temperatures do not increase as dramatically as with the PD superchargers at high engine speeds. Even with the increased efficiency, turbocharger systems typically include some form of charge cooling to account for the longer time duration that the engine can be exposed to boost. The effectiveness of this charge cooling largely dictates the spark advance requirements to avoid knock at WOT. Again, actual inlet temperature should be used to adjust ignition advance.
Even with significantly lower heat soak than positive displacement superchargers, turbochargers require some amount of fuel enrichment for combustion temperature control depending on intended loading. The longer the engine is expected to perform under boost, the richer target lambda should be for durability. Drag race turbocharged engines may be set to run as lean as λ = 0.85 (12.5 a/f), but road racing or street applications are better suited to operate around λ = 0.8 (11.7 a/f) to cool exhaust valves and manifolds.
Timing should not be excessively retarded with a turbocharger at WOT. The resulting increased exhaust gas temperatures can be hot enough to make the entire manifold and turbocharger glow red or orange. If this is the case, ignition timing should be advanced to reduce exhaust temperatures. If the knock limit is reached before exhaust temperatures are acceptable, more fuel enrichment is required. Try incrementing lambda down by approximately 0.02 and run the test again with the increased timing. It is acceptable to see some amount of color change even with a cast iron manifold and proper tuning under extended loading, but it must not be left unchecked. Most OEMs limit exhaust temperatures to approximately 1,600 degrees F (870 degrees C) to aid durability. Excessive heat can quickly damage valves, catalysts, turbine wheels, manifolds, and surrounding engine compartment items.
The final form of forced induction to be discussed here is the use of nitrous oxide gas injection. Nitrousoxide (N2O, “nitrous”) is a molecule that is 33% oxygen, roughly double that of the air we breathe. Nitrous oxide is stored in high-pressure cylinders as a liquid. When released through a valve to ambient pressures, its temperature drops to well below freezing. The result is a double benefit of both higher chemical oxygen content and lower charge temperatures. The use of nitrous oxide in internal combustion engines was pioneered by German fighter pilots who needed extra help with climbing power during dogfights. By injecting the gas into the engine with the appropriate fuel, power was substantially increased. Nitrous oxide breaks down inside the combustion chamber to allow more raw oxygen molecules to interact with more fuel, creating more heat and cylinder pressure. This increase in pressure and heat is quite large, so it can usually be applied only for a brief period. Nitrous oxide works incredibly well for short-duration events such as drag racing or sprints where heat buildup is not much of a concern.
In modern performance systems, nitrous oxide is stored remotely in a high-pressure cylinder that feeds electronic solenoids metering flow to the engine. The nitrous oxide gas is only injected when extra power is needed at WOT. Extra fuel to accommodate the added oxygen is added through either a secondary feed near the nitrous oxide injection point or by artificially increasing flow through existing injectors. Additional injector flow may come from an increase in fuel-rail pressure or pulsewidth. Care should be taken in the placement of the nitrous oxide injection point. It is possible to freeze MAF and IAT sensors if the injection nozzle is too close or aimed directly at the sensor.
No change to any engine system or performance is necessary when the system is not actively flowing. Calibration at idle and part load are completely unaffected by its presence. Prior to the addition of nitrous oxide, the engine should be completely calibrated at WOT naturally aspirated. This determines future fuel and spark needs when the nitrous oxide is added. If the engine does not operate properly in naturally aspirated trim, it can hardly be expected that performance will improve with the addition of nitrous oxide.
The actual nitrous oxide system is triggered electronically via a relay. There can be a series of switches daisy-chained to limit activation to the desired time. All systems employ some form of arming switch to enable. A WOT switch is also mandatory to prevent accidental engagement at part load. Because nitrous oxide is so effective at increasing engine torque, it should not be used any time torque is being limited by throttling. Introducing nitrous oxide at part throttle causes serious damage because the calibration has the engine operating near λ = 1 and MBT. The addition of massive amounts of extra oxygen and fuel under these conditions can send cylinder pressures well beyond the mechanical limits of the engine components, even before knock occurs.
The metering of nitrous oxide delivery is regulated by flow through a fixed orifice (jet) at the instantaneous pressure ratio. Its volumetric flow is not linked to engine speed or load. This means that conventional nitrous oxide delivery systems provide an almost immediate increase in oxygen supply and the resulting torque increase. Mechanical limits of the engine must be considered again when choosing a point at which to begin the addition of nitrous oxide at WOT. It is not advisable to attempt to triple the torque output just off idle, as the resulting strain can cause internal engine component failure. The engine must be stabilized under WOT conditions before nitrous oxide is added. This typically means achieving a minimum engine speed of roughly 2,500 to 3,000 rpm. At this point, the onset of torque is smoother and the likelihood of component failure is much lower. For vehicles equipped with automatic transmissions, it is common to set the triggering point for nitrous oxide activation above the stall speed of the torque converter to better control engine loading. Many systems incorporate an engine speed sensitive switch in series with the master arming switch to ensure proper delay.
The addition of the extra oxygen to the intake charge represents a very real increase in load and volumetric efficiency, whether the PCM registers it or not. As a result, spark and fuel needs are similar to an equivalent supercharged engine seeing instant full boost. The difference with nitrous oxide is that there is no real ramping up of load, but rather a step change.
Target lambda varies depending on how much nitrous oxide is to be used. The chemical makeup of nitrous oxide is different from the air, therefore its stoichiometry changes even when burned with the same fuel. The stoichiometric air/fuel ratio for nitrous oxide and gasoline is approximately 7:1, so the target air/fuel ratio should be adjusted accordingly to avoid a lean condition. Since airflow is directly proportional to power potential, target ratios can be calculated based upon what percent of power comes from air versus nitrous oxide. Each fraction of power production is multiplied by its own target ratio. The two are added to find the new target ratio. If a 300-hp engine is found to operate best at 12.7 a/f and 100 hp of nitrous oxide is to be added:
Base engine fuel maps should not be adjusted at this point. Doing so is only detrimental to normally aspirated engine performance. Instead, nitrous-specific fuel additions only should be adjusted. This typically means adjusting jet size much like on a carbureted engine until an acceptable ratio is reached at all speeds. Some more advanced aftermarket EFI systems allow for nitrous specific fuel adders that can increase pulsewidth for additional delivery. While this method gives a good starting point, the actual ratio may need to be adjusted if combustion or exhaust temperatures become unacceptable.
Ignition lead must be retarded slightly to compensate for the higher combustion temperature and faster burn rates. Most nitrous oxide kit manufacturers agree that a retardation of 2 degrees for every 50 hp dosage increase is a good starting point. For example, a “150-hp” nitrous oxide kit would require 6 degrees of ignition retard at the onset of the tuning process. If lambda is correct, timing can be carefully added to increase engine output until the knock limit is found. This testing should be done with bottle pressure near the intended maximum usable range to ensure that cylinder pressures are near peak during testing. Advancing timing during testing with bottle pressure below target maximum may lead to detonation when the vehicle is run later with a warmer or full bottle.
Much like with the positive-displacement superchargers, timing ends up being lower near peak torque and can increase as engine speed rises. The increase in engine speed and natural airflow due to pumping has the effect of decreasing the percentage of nitrous influence on the power production at higher speeds. This means that the majority of ignition retardation needs to be near system activation and torque peak. Just like the naturally aspirated map, the engine benefits from a slight increase in timing to compensate for increasing speed. Because of the tremendous temperature drop associated with the evaporation of nitrous oxide in the intake charge, ambient temperature compensation does not need to be as aggressive as with a supercharger. In high humidity, nitrous oxide equipped cars can operate with slightly more timing than their supercharged counterparts due to the latent heat of vaporization of the water vapor in the intake charge. The water molecules absorb some of the combustion heat and energy during the cycle, lowering burn temperatures.
Nitrous oxide can be used in small amounts as a cooling agent for supercharged engines as well. Small dosages (typically less than 50 hp) can substantially drop intake charge temperatures on otherwise heat-soaked charges. These charge cooling doses should be used sparingly, as they add tremendous amounts of cylinder pressure in short order. Fueling should be added accordingly, if not almost excessively. Timing should only be added in very small increments to prevent accidentally venturing into detonation and breaking components without warning. The line between best power and detonation becomes very thin when multiple forms of forced induction are being applied simultaneously.
Some turbocharged race applications use nitrous oxide to instantaneously increase torque during the spooling period. This is only done on small engines with blatantly over-sized turbochargers attempting to make large amounts of power. Again, dosages are usually relatively small (usually less than 100 hp), and taper or shut completely off by the point at which full boost is achieved. The nitrous oxide provides for greater mass flow through the engine and exhaust, effectively spooling the turbo like a larger displacement engine at the same speed. To this end, timing is again retarded significantly and excess fuel enrichment is used to maintain combustion stability.
Written by Greg Banish and Posted with Permission of CarTechBooks