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MegaManual Index -- Adding a Turbocharger to Your Engine -- Intercooling -- Water Injection -- Wastegates and Blow-Off Valves
Plumbing Your Turbocharger -- Other Considerations -- Making a Turbo Exhaust Manifold
MegaSquirt® Hardware/Software Considerations -- Turbo Tuning with MegaSquirt® -- Setting the VE Table on a Turbocharged Engine

MegaSquirt® and Turbocharging

This section looks at adapting MegaSquirt® to a turbocharged engine. As well, there are a number of tips on adding a turbocharger/MegaSquirt® combination to an engine that was previously naturally aspirated.

Note that if you have a MS-II™ controller, you should also read the configuration and tuning documents, as well as the latest MegaTune information, for things like spark retard and IAT retard, etc.

The MegaSquirt® itself does not need any modifications to be used on a turbocharged engine, provided:

With MS-II™, you will be able to set the advance anywhere you like (so you can retard it from the peak levels as boost rises), and there is also an intake air temperature retard table.

What is Boost?

Boost is the pressure in the intake manifold above ambient air pressure. However the ambient air pressure depends on the weather and your elevation, so it changes from hour to hour. As a result, we will always use absolute pressure (in kiloPascals - kPa).

For example, at sea level, on a day with an ambient air pressure of about 100 kPa, boost is anything above 100 kPa. A value of ~200 kPa would represent a boost pressure of about 1 bar, or 14.5 psi. However, in Calgary, Alberta, Canada, at an elevation of 3750 feet, the same 200 kPa is on top of a typical ambient air pressure of 90 kPa, for a boost of 16 psi. If a low pressure front had passed through, the same kPa might be even more 'boost'. But the pressure inside the engine is that same in all cases. As a result, absolute pressure in kPa is a better measure of how much the engine is working than 'boost' values.

However, for those that must convert, here is a calculator to convert from MAP kPa to PSI of boost (if given the ambient pressure):

Ambient Air Pressure (kPa),
Manifold Absolute Pressure (kPa),
PSI boost

However, there are a number of issues concerning the sensors, coding, and other hardware/software issues for turbocharged engines you will need to consider, and these will be covered later in this section of the MegaManual.

Adding a Turbocharger to Your Engine

There are many considerations when turbocharging an engine. Fortunately, adding programmable fuel injection (such as MegaSquirt®) at the same time makes some of these considerations easier to deduce.

This manual will not cover the all of above considerations. An excellent book covering many of these topics is Turbochargers by Hugh MacInnes, published by HPBooks, ISBN 0-89586-135-6. Another more recent book is: Maximum Boost Designing, Testing, and Installing Turbocharger Systems by Corky Bell. Some on-line turbocharger selection and power calculators are at Ray Hall Turbocharging.

Using the above references as guides, you first need to choose a suitable turbocharger(s) for your engine.

This manual will cover the design and construction of the turbocharger exhaust manifold, plumbing the turbocharger (and intercooler, if used), and tuning the turbocharged engine with MegaSquirt® EFI.


The purpose of an intercooler is to cool the air charge. The turbocharger heats the intake air when compressing it. The temperature (T2) that the intake air gets heated to depends on the:

The relationship is:

T2 = T1 + ΔT = T1 + {T1×[(P2/P1)0.283]-T1}/ηc

Where T2 is also in absolute temperature.

For example, if T1 = 21°C = 21+273 = 294°K, P1/P2 = 200/101.3 = 1.97, and ηC = 76%, then:

T2 = 294 + ΔT = 294 + {294*(1.97)0.283]-294}/0.76 = 294 +{294×1.212-294}/0.76

= 294 + 62.3/0.76 = 294 + 82 = 376°K, or 376 - 273 = 103°C

The air can then be cooled by an intercooler after the compressor (sometimes called an 'aftercooler'). To make the calculations easier, you can use the following calculator to determine how hot the air will be based on the pressure ratio (the compressor output pressure ÷ input pressure) and compressor/intercooler efficiency:


Ambient Air Temperature (°C/F),

Ambient Air Pressure (kPa),

Turbo Compressor Efficiency (%),

Outlet Pressure (kPa/PSI),

pre-IC temp.

Intercooler Efficency (%),

post-IC temp.

Hot intake air is detrimental to power and will increase the chance of detonation. An intercooler reduces the intake temperature by pushing the air through a heat exchanger (like a small radiator) that absorbs some of the heat from the intake air. Placing an intercooler in the air path between the turbocharger compressor and the intake manifold provides two advantages:

Not all intercoolers are created equal, however. The intercooler efficiency depends on the design of the intercooler. The two critical factors are:

Regardless of the degree of thermal efficiency, if too much boost pressure is lost to a restrictive intercooler, the intercooler may actually decrease performance.

There are two basic styles of automotive intercoolers:Air-to-Water, whereby the intake air is cooled by water (usually the engine coolant), and Air-to-Air, whereby the intake air is cooled by (ambient) air in the same fashion as an engine radiator. Many intercoolers are available form factory vehicles, and those off of Volvos, Saabs, and Ford 2.3l seem popular. Larger intercoolers were often used in pick-up trucks.

The Volvo intercooler is very large and efficient. The Volvo part number is 317319. Its overall width is 28.75 inches (73 cm) at the top where the inlet/outlet are located and 23.125 inches at the bottom with the tanks. The core measures 17.75 inches (45 cm) wide X 17.00 inches (43 cm) tall. It is 1.25 inches (32 mm) thick. One of the inlet/outlet tubes is perpendicular to the core, the other is at approximately a 30° angle. The inlet and outlet tubes measure is 2.5 inch (6.35 cm) O.D. and 2.25 inch (5.72 cm) I.D. These often go for around $150 at salvage yards.

Saab intercoolers are commonly available from wrecking yards and eBay. They come in a number of sizes, depending on the model.

A large intercooler came on the 1993 Dodge Ram with the Cummins Turbo diesel engine. It is available from your Dodge dealer under PN 52004274 or 637714. It weighs 20 lbs, and is 37" wide, 12-3/4" high. The core thickness is 1-5/8". The inlet/outlet inside diameter is 2-1/4", with an outside diameter of 2-5/8". Apparently these are available from dealers for about $200.

Another “giant” is the HUGE intercooler from a 1999 Ford Power Stroke Diesel engine. It is Ford part number F81Z-6K775-BAand cost about $300. The Ford intercooler is 39" wide, 18" tall, and 2.5 inches thick. The actual core measures 18" tall, 30" wide, and 2" thick. It has 3 inch inlet and outlet connections, which are about 35 inches apart (center to center).

You can see pictures of many OEM intercoolers at DDG - The Intercooler Identification Page.

Water Injection

Water injection is useful in preventing detonation in supercharged engines that produce more than about ~10 psi of boost. The water injection system is a completely separate system from the fuel injection system.

Typically, the water is mixed with methanol to improve its cooling effect. Mixtures up to 50% work well. Amazingly enough, regular windshield washer fluid works well, as long as you buy the blue stuff. If you can find one that says 50% methanol, 50% distilled water, that is great. Or you can add methyl hydrate to your water.

To set-up a water injection system, you can use a fluid tank, small pump (windshield washer pumps work well), and a Hobbs switchto inject fluid into the inlet tract. This switch closes at a specific pressure, and is installed in the intake manifold. One Hobbs switch is NAPA part# 7011577. This corresponds to a Hobbs part number #76052. It is a normally open 2 terminal switch, factory set to 15 psi. However it is adjustable from 14 psi to 24 psi. Both lower and higher pressure rangesare available from Hobbs 5000 series pressure switches.

An alternative is the Summit Racing “Oil Pressure Safety Switch”. It is made of steel and finished with zinc plating, for just $13. It opens at 7 psi (non-adjustable). Instead of being plumbed to the oil system, you can screw it into the manifold and use boost pressure to turn it on/off. Similar switches are available from Holley (PN 12-810, $20), and Mr. Gasket (MRG-7872, $13).

You will want to plan and wire the water injection system very carefully. If you have an electrical problem that causes the injection system to stay in the open position while the engine is not running, you can have a "hydraulic lock" which can seriously damage your engine.

To adjust the water flow rate to be suitable for your engine, you need to install a restrictor (typically ~0.030" to 0.050", 0.8 to 1.3 mm inside diameter) in the water line from the pump to the manifold.

Wastegates and Blow-Off Valves

A wastegate is a valve that allows the exhaust gas to bypass the turbine whenever the boost pressure is above the maximum desired level. The wastegate prevent the turbocharger from over boosting the engine, which would destroy it. Many modern turbochargers have the wastegate built into the exhaust turbine outlet, and require just a vacuum hose from the compressor or manifold to operate. Other turbochargers are built without integral wastegates, and require a separate wastegate plumbed into the exhaust manifold to vent excess exhaust to the exhaust system downstream of the turbocharger.

A blow off valve (BOV) prevents excessive pressure in the inlet system. While this sounds superficially similar to a wastegate, it operates differently, and is used for a different purpose. The blow off valve is situated in the inlet tract before the throttle blade(s) and is used to prevent surge from excessive pressure in the compressor housing when the throttle is closed suddenly while the turbocharger is at speed. The blow-off valve opens at a few psi above the maximal boost level, and prevent damaging excessive pressure fluctuations (called surge) in the compressor.

Common OEM BOVs are the Bosch part numbers:

The Porsche part number is 993.110.337.50, and they call it an “air cut-off valve”. 1996 9000 Saab Turbos use the Bosch PN0 280 142 103. You might also consider Bosch PN 0 280 142 110(Saab dealer PN 4441895 - called a “bypass valve” or “compressor over-pressure valve”), which offers a higher boost threshold. All of these have connections for 1" inlet and outlet hoses. The 0 280 142 103 BOV costs about $37. In Canada, you order Saab part number 30544792, for about $75Cdn.

There are also numerous aftermarket BOVs, search eBay or use google.

A well-engineered turbocharger set-up needs both a wastegate and a blow-off valve if high boost levels will be used.

Plumbing Your Turbocharger

A turbocharger requires a number of tubes to connect it to the engine intake manifold, exhaust manifold, air cleaner, exhaust system, oil supply, and possibly to the engine cooling system (depending on your particular turbo). These tubes must flow well, withstand significant temperatures, boost or exhaust pressure, and a lot of vibration in the inlet tract before

Sharp bends cause a restriction in the flow of air, so try to put the turbo where the bends in the exhaust and intake tracts will be least restrictive. This will require compromises with exhaust tubing length, etc., and you are looking for the best overall solution.

On the oil supply to the turbo a somewhat restrictive fitting on the pressure side is generally okay. This is because most engine oiling systems can supply much more pressure than the turbo needs. However, a restriction on the drain side of the turbo can back up oil in the cartridge and cause many problems. The returning oil can be whipped into a froth by the speed of the bearings and the slight blow-by from both the compressor and exhaust housings. If allowed to build up, this oil can be pushed past the seals into the compressor section when not under boost. To prevent any problems, the drain line should be at least 3/4" (19mm) inside diameter, and must flow continuously down, it should not have horizontal or up-hill sections. In addition, the return line must not feed the return oil back into the crankcase below the sump oil level.

For the turbine side, the exhaust gas entering the turbine will be at a much higher temperature and pressure than in the exhaust system after the turbo. You try to compromise the least number of these things, given the space you have to work with. This may require relocating some under hood components and/or protecting others.

Other Considerations

The turbo should not be positioned too near any engine, steering, brake, or other components that will be affected by radiated heat from the turbo or its plumbing. Note that the engine also moves on its mounts as torque is applied to the wheels. You will need at least 2" to 3" (50mm to 75mm) of clearance. This applies to the compressor housing as well as the turbine (exhaust) housing.

The compressor housing is relatively cool while the engine is running due to inlet air cooling, though this temperature is increased by the inefficiency of the compression (this is why we have an intercooler, after all!).

However when you shut down the engine, exhaust heat will soak through the cartridge and heat the compressor side nearly as hot as the turbine housing. When an engine shuts down, there is little to no air movement under hood. The entire turbocharger assembly can get very hot. You might consider making or purchasing a turbo insulating shield(s). These can be made from sheet metal. Properly designed, they work very well and also cover the turbine housing to make it look better.

A turbocharger is driven by exhaust gas velocity. When temperature of the exhaust gas is allowed to drop before reaching the turbine, its velocity drops as well. Thus heat retention in the exhaust system between the cylinder head and turbocharger is important. Locating the turbocharger as close to the exhaust ports as practical will give best turbo performance. Sometimes is it not possible to locate the turbo very close to the engine. In that case, making the manifold (or header) from a material with a strong fatigue resistance will allow you to insulate the exhaust manifold, retaining exhaust heat.

When deciding where to mount the turbo, consider the effect each possible mounting location/orientation on routine maintenance. If you have a solid cam that requires periodic valve adjustments, it would be nice to be able to remove the valve cover(s) without removing a turbo. Make sure you can get to your spark plugs without too much trouble. When you are mocking up a manifold, plugs and wires should be in place and checked for enough clearance. Also check the engine oil dip stick for adequate access to check the oil. One idea is to take a few pictures of the engine bay before taking things apart as a reference. It is easy to forget the little things that make a big difference.

Making a Turbo Exhaust Manifold

In making a turbocharger manifold, the first thing you will need to do is plan carefully. The manifold needs to be designed so that the connections are the best compromise you can make them. You will need to connect:

Each of these is affected in turn by the exhaust manifold design, which establishes the location and orientation of the turbocharger itself.

All of the connections, and the turbocharger/manifold/wastegate assembly must be planned so that all the associated components:

The best way to ensure that the installation will work is to plan carefully. To do this you can:

These will be very helpful to avoid expensive mistakes later on, and if you are having the manifold welded for you, the person doing it will appreciate the drawings and mock-up as a guide to assist them.

Your next decision will be what material you will use.



304 Stainless
321 Stainless

Thin walled mild steel header (generally 16 to 18 gauge) tubing is inexpensive and easy to work with, but is not strong and will fatigue (and crack) quickly from the high temperatures and mechanical stresses a turbocharger will put on it. Thin mild steel also dissipates heat quickly, reducing the turbocharger efficiency. You can wrap the tubes with insulating material, but more heat means the metal fatigues and breaks more quickly.

Stainless steel tubing is an excellent choice for both reliability and heat retention, but you will also need stainless steel flanges and the skill and tools to weld it all together. The extra cost of stainless, and the difficulty of welding it, makes stainless steel a poor choice for some people.

An alternative to stainless steel is to used thick-wall mild steel. The mechanical and thermal properties can be substantially improved with thicker material. Schedule 40 butt weld pipe fittings are a practical choice. They are inexpensive, readily available from many local plumbing supply houses and very easy to weld.

The turbocharger exhaust manifold can be constructed from Schedule 40 steel weld "els" (short for elbows) and Schedule 40 steel tubing. The 0.125+" thick weld fittings are very strong and last “forever” if built right. The els are available from most industrial plumbing and gas fitting shops and are very inexpensive. They are made from a high strength, weldable alloy steel and have a nice V at each end to pour in a good, strong weld. They are available in 45° and 90° bends and standard and long radius styles. Use the “long radius” elbows whenever possible as they are less restrictive. The els are available in different inside diameters (ID) to match the pipe sizes below:

Nominal SizeActual IDApprox. ODWall
1" 1.049" 1.315" 0.133"
1¼" 1.380" 1.660" 0.140"
1½" 1.610" 1.900" 0.145"
2" 2.067" 2.375" 0.154"
2½" 2.469" 2.875" 0.203"
3" 3.068" 3.500" 0.216"

Straight schedule 40 NSP tubing is used between els. These pipes have a very thick walls so the turbo manifold will probably weigh almost as much as a regular cast iron exhaust manifold.

Turbo flanges should be at least 5/16" (8mm) thick, but up to 1/2" (12mm) is even better. The problem is the heat of welding, which can easily warp them. The actual operating heat is less of a problem as it is more evenly applied.

If you decide use to fabricate flanges from the thinner material, clamp the flange in a vise. This will dissipate welding heat from the flange faster. An even better idea is to bolt them to an old turbine housing, if you have one available. If you are not a professional welder, thicker flanges are easier to work with. Make your turbo flanges at least 1/16" (1.5mm) thicker than the turbine housing thickness (at the mounting holes).

Layout the flange on the material, and mark all the centers of the port and bolt holes with a center punch. Then use the center punch to mark the outline of the flange with a punch mark every ¼ inch (6 mm) or so. This will allow you to remark the flange at any point, like after it has disappeared from the fluids used to cool the hole-saw! A drill press and hole saw are used to cut the holes. A hand drill can be used, but will be slow and frustrating... For rectangular holes, 4 smaller holes may have to be drilled and then hand filed to obtain the correct shape. An oxy-acetylene torch or plasma arc cutter will make this job will go much faster! If you are making your own exhaust manifold head flanges, 5/16" (8mm) thick flanges are a good choice. You can use an exhaust manifold gasket as a template for cutting the flange.

If you cannot, or do not want to make your own flanges, there are a few places you can buy them. Turbo Techniques, at, sells very nice ½" (12mm) thick flanges, as well as most other things you will need to complete your turbo system. For example, they have a T04B flange for $28.00 as PN 20167. They also have some outlet flanges for starting your exhaust system fabrication.

You may be able to purchase ready-made head flanges from Hedman, Hooker and other manufacturers that offer flanges. Often they have flanges for most popular American engines that are 5/16" (8mm) thick.

If suitable flanges are not available for your engine and you do not want to make your own, Headers by Ed will custom cut flanges to your specifications. Stainless Works offers a flanges for popular American V8 engines in stainless steel for about $125, and they will cut custom flanges to your gaskets or drawings in material as thick as 3/4" (19mm) thick for mild steel or ½" (12mm) for 304 stainless steel.

Only you can decide how best to configure your turbocharger manifold. If you buy a good selection of 90° els, you can cut them later to any lesser angle, or weld two together to get larger angles. 90º elbows are quite a bit cheaper and easier to find than either 45° or 180° els.

With a turbocharged engine, the oxygen sensor should be placed after the turbo, where it doesn't see the high exhaust gas pressures that affect the accuracy of the sensor. If you decide you want to have an oxygen sensor before the turbo (generally not recommended), you will need to weld a bung into your manifold. It should be as close to the turbo as possible, so that it can read exhaust gases from all cylinders. Your turbocharger may have an oxygen sensor fitting on the outlet housing, and this is fine to use.

You do not need additional plumbing on the manifold (other than for the exhaust ports to the turbocharger's turbine, of course) if you are using a turbocharger(s) with an internal waste gate. A flange that matches the turbocharger exhaust housing flange can be welded directly on the collector.

Some turbochargers use external waste gates. For these, you must fabricate a mount for the wastegate before the turbo so excessive exhaust gases can bypass the turbine.

Note that you can have the manifold(s) coated for corrosion resistance and heat retention. Google the internet for "header coating" to find companies that provide this service. Alternatively, if you have the right tools (sand blaster, spray gun &, compressor), you can do it yourself with Caswell's Black Satin (BHK) 2000°F exhaust system coating.

MegaSquirt Hardware/Software Considerations

Injector Sizing - Injectors for turbocharged engines need to be 10% - 20% larger than for a naturally aspirated engine of the same horsepower. This is because turbo engines have a lower BSFC(brake specific engine consumption = the amount of fuel need to produce one horsepower for one hour on an engine dynomometer at WOT [wide open throttle]), and they benefit from a rich Air/Fuel mixture that cools the piston and valves and provides a measure of anti-detonation. As well, a rich mixture that continues to burn in the exhaust heats sin the turbo up faster.

Use the chart below as a guide when selecting injectors for your turbocharged engine.

Injectors Rating Required for Specified Horsepower
for Turbocharged Engines
in lbs/hr and (cc/min)
Number of Injectors
10065 (682)32 (335)16 (174)13 (139)11 (115)-
15097 (1016)48 (508)24 (254)20 (208)16 (174)12 (128)
200-65 (682)32 (335)26 (277)22 (231)16 (174)
250-81 (855)41 (428)32 (335)28 (289)20 (208)
300-97 (1016)48 (508)39 (405)32 (335)24 (254)
350--56 (587)45 (474)37 (392)29 (300)
400--65 (682)52 (543)43 (451)32 (335)
450--73 (762)58 (613)48 (508)36 (382)
500--81 (855)65 (682)54 (567)41 (428)
550--89 (936)72 (751)59 (624)44 (462)
600--97 (1016)78 (821)65 (682)48 (508)
700--112 (1174)90 (948)74 (784)58 (600)
800--130 (1364)104 (1086)86 (902)64 (670)
based on 0.50 BSFC and 85% duty cycle

MAP sensor - boost range, alternate sensors. The MPX4250AP was supplied with the “turbo” units from the first group buy, and ALL the units from the second and subsequent group buys, is in the Digi-Key catalog on page 1098. This is a 2.50 bar absolute pressure unit. That means it measure from a full vacuum to 2.50 bars (i.e., up to ~1.5 above normal atmospheric pressure). The stated pressure range is from 2.9 to 36.3 absolute psi, equivalent to ~21.6 psi of boost.

To use higher boost, click here for information on using other sensors with MegaSquirt, or search the distributors' sites for their 4-bar MAP sensors for MegaSquirt® controllers (~44 PSI).

IAT sensor - this should be located in the intake system somewhere after the compressor and intercooler. It needs to sense the temperature of the air going into the cylinders. You can put it before the throttle body or in the intake plenum, either location will be fine. Your intake temperature sensor must be the “open-element” type. Regular closed sensors are shielded within a brass housing, and will respond too slowly to the rapidly rising intake temperatures that occur when boost comes on. See the Sensors and Wiring section for more details.

SD vs. Alpha-N For turbocharged engine, you must use the speed density control algorithm only. Turbocharged engines do not have a linear relationship between throttle position, rpm, and fuel requirements, and thus cannot use alpha-N.

Turbo Tuning with MegaSquirt

To set up the fuel curves for your turbocharged engine with MegaSquirt® EFI Controller, you have a number of parameters to work with. The most important of these are the Req_Fuel value and the VE table (8x8 or 12x12 volumetric efficiency table). You are aiming to achieve 11.0-12.5:1 air/fuel ratios at full boost, and 15-17:1 under light loads for a turbocharged engine.

If you did not have MegaSquirt® EFI Controller, you would need an add-on fuel controller, a rising rate fuel pressure regulator, or some other trick to add extra full when the boost comes on. With MegaSquirt® EFI Controller, though, you simply program in longer pulse widths via the VE table.

Once you get the engine started and idling, you proceed with tuning it.

When you start tuning, make sure you:

To tune all the parameters of MegaSquirt® so that your engine runs the best it can, you will need to do the following:

  1. First, learn to use MegaTune,
  2. Next, set the constants,
  3. Get the engine started and idling,
  4. Then set the PWM criteria,
  5. Then set the cold start and warm-up enrichments,
  6. Then set the VE table,
  7. Set the acceleration enrichments,
  8. Check that certain resistors are not getting too hot while driving.

These are the same steps as for a naturally aspirated engine, and the procedures are the same for a turbo engine except setting the VE table and spark advance table. All the other steps are covered in the Tuning section of the manual for MS-I/MS-II, so follow the instructions there for those steps.

Having a wide band O2 sensor makes tuning much easier. With a turbocharged engine, the oxygen sensor can be placed either in the exhaust manifold, or downstream of the turbo. Frequently, the outlet housing that bolts to modern turbochargers (and that cover the wastegate port), provide both a 90º bend in the exhaust path (making the fabrication of an exhaust system a great deal simpler) and also often have a suitable threaded bung for an oxygen sensor.

Setting the VE Table on a Turbocharged Engine

To set up the fuel curves for the engine with MegaSquirt, you have a number of parameters to work with. The most important of these are the Req_Fuel value and the VE table (8x8 or 12x12 volumetric efficiency table). You are aiming to achieve 12.5-13.1:1 air/fuel ratios under full throttle, and 15-17:1 under light loads for a naturally aspirated engine. Boosted engines may require a richer mixture under power.

To get the VE table set, adjust the boost as low as you can, if you have an adjustable wastegate. Turn on datalogging in MegaTune. Take a few easy drives up and down the street (avoid boost entirely is you can), keeping the revs down and the throttle light. Adjust the VE table according to what MLV tells you. A bit more tuning, and you are ready to go a bit harder. Increase the revs, or increase the boost a bit. Do not go harder if there are any problems [typically a back fire means too lean, sluggish revving means too rich].

Volumetric Efficiency (VE) entries in 8x8 or 12x12 MegaSquirt VE table actually are VE * gamma, where gamma is the (stoichiometric AFR)/(actual AFR), and VE is expressed as a percent (i.e. 65 represents 65% volumetric efficiency at 14.7:1 AFR) - unless you have selected 'separate AFR & VE tables' in MS-II™.

For MegaSquirt® (and most MAP based EFI controllers), VE (= percentage cylinder filling) is relative to manifold pressure, according to the following equation:

mass air in cylinder = (VE x MAP x cylinder volume) / R(constant) x MAT

So mass air increases with boost (MAP), as we all know, but VE, the efficiency of the engine geometry and valve timing, does not keep rising. There are many reasons for this, one of the most important being residual exhaust gas in the cylinder, which displaces the air you are trying to stuff in there. The mechanism by which EGR enters the cylinder is the intake/exhaust valve overlap. As boost increases, this window between intake and exhaust allows more residual gas to flow back into the cylinder because the exhaust backpressure rises at very high boost, and can become greater than the intake pressure. So in practice, VEs are never much greater than about 130%. There is more on the MegaSquirt fuelling equation here:

You can tune your engine to a stoichiometric mixture with NB O2 sensor, then use a little math to “correct the mixtures”. For example, if you get a stoichiometric mixture with 65% VE at a certain RPM and kPa, then to lean the mix to 16.0:1 you need:

65% * (14.7 / 16.0) = 60%

To richen an 80% VE entry to 12.5:1 from stoichiometric:

80% * (14.7 / 12.5) = 94%

Note that with MLV, you will get stoichiometric mixes if you set the “crossover voltage” to 0.45-0.50 volts with a narrow band O2 sensor. This is where you should have the EGO switch point set on the Enrichments page in MegaTune as well. You can then adjust your VE to reflect the VE table suggested by MLV as described above to get other mixtures.

However you cannot use a narrow band O2 sensor to guide you under boost or at high rpm. You can seriously damage your engine if you try! You will want to be sure of running rich mixture under high load/high RPM conditions. This makes a narrow band sensor somewhat less useful. As a starting guide, make sure you have at least 0.8 - 0.9 volts from the sensor under wide open throttle (WOT). Start with very rich mixtures under load, and lean them cautiously to reach your target.

To tune your VE table, you must proceed with caution in the upper ranges of boost and rpm. Do not rush yourself, and jump ahead of a proper procedure. You can destroy your engine if you do not “sneak up” on the proper VE numbers. To start tuning the VE table, warm the engine to full operating temperature first. Install new spark plugs, then go for a “spirited” drive.

Let up on the throttle immediately if you hear the rattles of detonation, or if boost rises higher than you planned. Then remove and inspect your spark plugs. Look for evidence of detonation on the porcelain nose of the spark plug that surrounds the center electrode. Detonation will show as "salt and pepper", tiny flecks of carbon and/or aluminum that indicate detonation has occurred. (Note that those tiny specks of aluminum are bit of your pistons that are being destroyed - so you will want to pay attention to them, and fix it as soon as you can!!)

If there are no rattles and no “salt and pepper”, increase the boost by a few psi, and repeat. Check the spark plugs after each drive. As you continue to increase boost, you will eventually either hear detonation (let off the gas immediately!) or you will have evidence on the plugs that it has occurred. At this point, increase the VE at that point of the VE table, decrease the timing (in the 12x12 Spark Advance table if you have MS-II™, otherwise by whatever means you have!), or reduce your boost levels. Do not continue to operate an engine the shows signs of detonation, even if it is brief.

Do not retard the timing excessively to combat detonation. If you retard the timing too much, the exhaust gases will get very hot, causing the exhaust manifold to glow bright red hot, and there might be damage to the exhaust valves, turbine wheel, catalytic converters, and exhaust manifold. It can also cause engine compartment fires!

In OEM turbocharged applications, the engine tune is carefully planned so that exhaust gas temperatures will not exceed around 1600°F/870°C. If you find yourself taking out more than about 0.3° to 0.4° of advance for every kPa above 100 kPa (~2° to 3° per PSI of boost) to deter detonation, then you should stop removing timing and add fuel instead. Add fuel via the VE table in 2% to 3% increments until you are sure the detonation is avoided and the exhaust temperatures come down.

You can set RPM and MAP sensor 'bin values' on the VE table wherever you want them, but they must be in the same order as in the table supplied with the software. MAP sensor values can be between 0 and 250 kPa for MegaSquirt® controllers sold since 2002. Put the bin values so they cover entire rpm/boost range of your engine. That is, you want to cover from your slowest idle speed to your red line, and from the MAP kPa value at idle or deceleration (whichever is lower) to full boost. Evenly spaced values work well, but you may choose different values to suit your combination.

Generally, VE table numbers above 100% are used only to richen mixtures. Even a turbocharged engine capable of 20 lbs/in of boost will generally not have extremely large VE numbers. The addition of fuel for boost comes through the MAP term in the fuel equation:

PW = REQ_FUEL * VE * MAP * E + accel + Injector_open_time

Thus increasing the VE at higher boosts makes the mixture richer, but it would not have run leaner simply because of the higher boost.

In essence, the mass of the air is computed using the ideal gas law (PV = nRT where the pressure P is a function of VE and MAP, the volume V = cylinder displacement, the air temperature Tis a function of E, R is the gas constant, and we are looking for n, the mass of inducted air) and then that result is combined with a characteristic number for a given injector.

If you get the injector opening time correct, and the REQ_FUEL accurately represents the flow rate of your injectors, then the VE entries will be close to the VE*gamma noted above. However, if your opening time is not right, or your REQ_FUEL is not, then the numbers will be skewed by the amount the values are in error. In general, except for when you are first trying to get your engine started, use the calculated value for REQ_FUEL and do not change it.

The tuning numbers within the 8x8 or 12x12 VE grid are gamma values composed of (lambda * VE), where lambda is the product of [air/fuel ratio divided by stoichiometric] multiplied by VE, which is the volumetric efficiency. Values beyond the table bounds are extrapolated at the boundary value, so the surface beyond the table is "flat". Note that you can change the RPM and MAP bins to suit your operating ranges.

Here is a sample VE table for a hypothetical turbocharged, intercooled, and water injected 250+ horsepower 4 cylinder 2.0 liter engine with 500cc/min injectors capable of 6500 rpm and about 20 psi of boost:

idle and cruise - lean~stoichiometric - 14.7:1WOT and boost - richnot often used


Note in the above VE table that VE values continue to rise slowly above 100 kPa. The intent is solely to richen the mixture as boost increases (an effect similar to that given by a rising-rate fuel pressure regulator). For example, at 2500 rpm, we have a VE of 79% at 100 kPa, and 94% at 250 kPa. The 79% represents the VE of this engine if it were naturally aspirated, and lets assume is has an AFR of 13.0:1 at that point. By raising the VE value to 94%, we get:

AFR(250) = VE(100) / VE(250) * AFR(100)

AFR(250) = 79% / 94% * 13.0 = 10.9:1

This very rich mixture accomplishes a number of things:

Having an O2 sensor makes the driving part of the setting up much easier, as you can datalog and use megaLogViewer to get the VE table set up with a few easy drives up and down the street, a bit more tuning, and you are ready to go a bit harder. You do not go harder if there is any problems [typically a back fire means too lean, sluggish revving means too rich]. Read the Datalogging and MSTweak3000 section for more information.

Do not get hung up on actual AFR numbers - for the example above to work, everything else must be dead on, including the injector offset, injector battery voltage correction, REQ_Fuel for your injector flow rate, and air temperature correction. It will get you close enough with the resolutions we are working with, but remember that the only AFR you can nail down with a NB O2 sensor is 14.7:1, everything else is an estimate from this point. If you have a WB-O2 sensor, then you can read the AFR directly from the sensor/controller output voltage and use those results to tweak your VE table.

On a turbocharged engine, it is very easy to achieve too much boost and too little fuel, which can seriously damage your engine. In order to safely tune your engine with MegaSquirt® EFI Controller, you need to be especially cautious about tuning in high boost/high rpm ranges. Where possible, limit boost to lower levels with reduced wastegate settings (for those that have adjustable wastegates) until you are satisfied that you have “mapped” that boost range satisfactorily. Then increase the boost a bit, perhaps 1-2 psi, and map that range.

Have someone ride with you and bring up the tuning screen. See where the "dot" hangs around when you are under load - this is where you need to focus on tuning. Use the up-arrow+shift to richen the VE values - enrich (with increased VE number) the four corners around where the dot is - give each corner five up-arrow-shifts, and see if this helps. Turn off the O2 closed-loop mode by setting the step size to zero. Watch the O2 gauge on the tuning screen and use this as feedback for rich and lean. The 02 gauge may move to fast from rich to lean to be able to tune. Another strategy that works is to turn on EGO correction, and then tune using the EGO correction gauge rather than the EGO voltage gauge. If correction is below 100%, then raise VE to raise correction and so on.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
©2004, 2008 Bruce Bowling and Al Grippo and Lance Gardiner. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.