Engine Mapper s Subaru Tuning Guide V1.1

Engine Mapper’s Subaru Tuning Guide Introduction My name is Graham, I’m an Engine Mapper based in North Wales. I’m generally residing on Group B Motorsport’s Dyno Dynamics dynamometer, or in the engine room. My background has

been based around both the computer software and control systems side of engine electronics and the physical motorsport engineering behind internal combustion engines and their performance efficiency. I feel that the large majority of customer’s make un-informed decisions about tuning parts and companies to use, based on reputation solely that doesn’t necessarily directly represent quality of work. So I’ve put together a basic guide to understanding Subaru tuning and modifications,

hopefully helping my customers and others to make more informed decisions about key products including mapping. Some prior knowledge of internal combustion engines, engine management systems and sensors as well as computer software would be extremely useful, however I will try and cover most bases in my explanations. Obviously a lot of what included in this guide is based on my experience and my opinions, thoughts and reasons. Please bear this in mind as others will have different views. Anything not factually correct can be taken up with me by email: [email protected] I welcome constructive critism, suggestions and corrections to make the article as accurate as possibly.

Contents Fuels and additives……………………………………………………………………………………. 3 Fuel pumps………………………………………………………………………………………………..4 Injectors…………………………………………………………………………………………………….4 Fuel pressure regulators……..…………………………………………………………………....7 Intercoolers……………………………………………………………………………………………….8 Dump Valves…………………………………………………………………………………………… ..10 Exhausts…………………………………………………………………………………………………….10 Spark plugs………………………………………………………………………………………………..13 Airbox vs induction kit……………………………………………………………………………….13 Suggested typical upgrade routes………………………………………………………………13 What is engine mapping?...............................................................................15 Mapping terminology and what it means…………………………………………………..16

Fuels and additives So let’s talk a little about fuel and additives and the basics surrounding that. You’ll find that the majority of UK WRXs can run 95 RON fuel, (manufacturers recommends 97+), obviously the minimum RON rated fuel in the UK. The STIs dependant on year and model tend to have to run 97+RON and it is recommended that any JDM Impreza is run on 99 RON only, although this is not strictly necessary, (I’ll cover this in a minute).

So what is RON – In simple terms, the higher the RON number, the more resistant to detonation* the fuel is, (see the mapping section for detonation definition). So the higher compression you can run in an engine on that given fuel, higher temperature and more ignition timing available before the fuel detonates. You will find that the higher octane race fuels will generally have a lower calorific value or energy rating per litre. So unless the engine can utilise the increase in RON and compensate for this with increased timing, it may not be a worthwhile swap. However 99 octane pump fuels like V Power or Tesco 99 have a higher energy rating and therefore will produce more power than 95 octane pump fuels. Going back to my comment at the beginning of this section, I don’t believe that JDM cars require any

special treatment regarding fuel. In my experience of the Newage, (2001-2011), imports, there has been no issue running them on 97+ octane fuel. We know the Japanese have 100 RON fuel, however much like most mass- production companies, the maps aren’t that c lose to the limit that 99 RON fuel will magically start destroying them. So much so, that on a standard JDM STI I am able to usually increase the timing by 4-6 degrees before hearing the on-set of detonation. Sense says run the highest octane fuel available, but don’t panic that you should be running 100 RON.

Again the use of additives is detrimental to the calorific value of t he fuel, so octane booster should only really be used where necessary. Methanol and such additives are being used by greater numbers now to incr ease performance. The benefit of an additive like Methanol works in a number of different ways and has great advantages from its use. It’s burn rate is slower than petrol on its own, which would be a disadvantage but due to its higher RON rating it can be used as an octane boosting additive to allow addition of ignition timing in the map. It burns cooler and much cleaner when it does burn, along with it being cheaper per litre than petrol it brings great benefits. The biggest benefit is its knock resistance. On a typical 20% meth mix, it will be very hard for a standard compression Impreza to detonate on the fuel, so the mapper has the ability to keep advancing the ignition timing until the performance starts to drop away. This is something that we call maximum brake torque of a fuel, or MBT for short. On a

standard petrol like Vpower you will always reach detonation before reaching MBT, so therefore you cannot ever extract the full calorific performance of the fuel. The other thing to think about when we are talking fuels is the stoichiometric point of any fuel. The chemical reaction between standard petrol and air in an ideal combustion process would have 14.7 parts of air to 1 part fuel, giving it a stoichiometric point of 14.7:1. We c all this lambda 1. However, on full throttle at full boost we might target something like 11.4:1 air-to-fuel ratio to obtain maximum power. Methanol has a completely different stoichiometric point of 6.4:1. Mixing 20% of meth with 80% petrol nets an overall stoichiometric point of 13.04:1, so f ull throttle you might target something like 10.1:1. This is an average demand of 13% more fuel due to running methanol. So attention must be paid as to what injectors would need to be run in order to get the c orrect fuel mixture when using methanol.

Fuel Pumps Fuel pumps are a very simple topic to cover, especially talking approx sub-450bhp applications. The fuel pump is the key component in the fuel system that keeps the integrity of the system. Without a consistent flow and pressure from the pump, the injectors would not be able to fire with such accuracy and your engine would suffer fatal consequences. My rough rule of thumb as to whether you should do something with your pump works like this: Is your car over 5 years old?

Yes – Replace the pump with an uprated pump, (Usually Walbro

255LPH), be careful of cheap or fake pumps that are not up to the job. Are you looking for over 280bhp on your WRX? Yes – Replace the pump with an uprated pump. Are you looking for over 300bhp on your STI? Yes – Replace the pump with an uprated pump.

(Prodrive STI cars generally come with an uprated pump, just ask yourself the age related question still though!). Over approx 450bhp, other factors will come in to your fuel system equation. The usual route is a parallel fuel rail feed system to reduce the pressure drop along the injectors, an in tank uprated pump with an external swirl pot feeding usually a Bosch 044 externally mounted pump which in turn feeds the rails.

Injectors Injectors are a critical point in the modification process that will allow you to reach your end bhp goal. I see many questions asked about size of injectors and the bhp limit. The problem is, although there is a consistent relationship between injector and bhp, the relationship doesn’t have strict

rules to it. There are many factors that will dictate the duty of the injecto rs versus the power output

of your car and why that is. There are many calculators online that g ive you information regarding injector size and bhp limitations but fundamentally there is a far more complicated equation needed to develop the fuel amount required per bhp. Variables to this equation which cannot be answered in customer specific setups are: 

Combustion chamber design

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Combustion burn rate.

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Combustion burn temperature.

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Cylinder head efficiency.

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Exhaust efficiency.

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Turbo efficiency and pressure.

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Lambda fuel target on full boost.

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Amount of ignition timing.

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AVCS advance, (Variable valve timing).

All of this means that there is not a simple answer, or direct proportional answer to injector cc/bhp, but based on experience I can give you my rough guide to injector size and average limit bhp: All at fuel pressure regulator 3bar, vac pipe disconnected: 380cc = 270-280bhp (1993-1996 Impreza WRX) 410cc = 300-310bhp (2001-2003 Impreza WRX) 440cc = 320-330bhp (Many years of STI and WRX top and side feed) 550cc = 380-400bhp (Newage STI and WRX mostly) 650cc = 460-480bhp You can find a table of estimated bhp here http://www.scoobypedia.co.uk/index.php/Knowledge/EstimatedHorsePowerFromInjectors These bhp limits are based on petrol only being used. You would have to factor in extra capacity if using Meths or other additives, (see f uels above).

After-market injectors and modified injectors I thought I’d add a little bit in here about injectors just in case you’re thinking of getting some

modified or after-market injectors. When thinking of injectors, it is very important to consider the spray pattern that the injectors you might buy are putting out. Spray patterns and atomisation are much more important to the good running of a car than you might already know or think. Let me give you an example. This is a picture of some PE 650s, which also have the same spray pattern as modified classic yellow 440 injectors when they are modified to 850cc and twin spray:

The problem with these types of spray, is that they are very concentrated in their spray pattern. The more concentrated the fuel spray is, the harder it is to thoroughly mix the fuel and air, (atomisation), so the less efficient the mixture will burn and potentially more fuel will have to be added, making the fuel consumption and atomisation much less efficient and the fuel delivery a lot less linear in it’s application when tuning. This means throttle response and std ECU drive -ability is

effected greatly. After-market ecus cope better with non-linear fuel flows as you can tailor the engines fuel demand as a raw factor, rather than using a cross reference between air flow, target lambda and rpm like the original ecu calculates, where you are on the fuel or ignition tables. Ultimately the answer is to stick with a type of injector that sprays in a fine mist with a conical type spray lending itself to the most efficient atomisation and best cylinder fill and burn. On a side note I have tested both the modified classic yellow injectors and the newer blue/pink top feed injectors. The older classic injectors suffer greatly with the above negative effects, so are not recommended in my personal opinion. The modified top feed injectors work very well, however when modified, it still effects the linearity of the fuel demand, so they are only recommended on an after-market ecu.

Fuel Pressure regulators You see a lot of these fitted to cars as a matter of course. They don’t increase performance in any way, only control the fuel pressure that feeds the injectors by controlling the return feed to the fuel tank. The standard fuel pressure regulator i s adequate for the majority of customer’s needs right up

to 500bhp, (injectors and fuel pump permitting). The major advantage in having a good quality fuel pressure regulator, is the ability to up the fuel pressure by a certain percentage if you find that your injectors are reaching their limits when you are close to your bhp goal. As an example: You have 440cc injectors on a classic hitting 330bhp @ 99% injector duty @ a fuel pressure of 3bar. In order to reduce this duty to 90% you would want to add 9% fuel pressure approx.. So raise the pressure to around 3.3bar and you would expect at 330bhp to be hitting a 85-90% injector duty when tuned to suite the raised fuel pressure. This is effectively making the injectors of larger capacity, i.e 440cc @ 3bar fuel pressure would then be 475cc @ 3.5bar pressure. The negative impact on raising the fuel pressure is three-fold potentially. The raise in stress on the fuel pipe fittings, pipe itself and all the fuel system and the increased stress on the fuel pump. The other critical equation for the fuel pump, is the fuel pressure vs the fuel flow. All pumps have a maximum capability in terms of litres per hour, most are rated at a nominal 43.5psi. When the pressure is increased, the flow reduces and vice versa. An example below:

Intercoolers Intercoolers on the Imprezas are a hard debated subject especially when it comes to the differences between front mounted intercoolers and top mounted i ntercoolers. The classic range of intercoolers on the STIs are generally capable of just over 300bhp-330bhp without issue, but when seeking more power you need to start thinking STI 7 onwards, i.e. larger. The WRX clas sic intercoolers are not so good, the very old V1/V2 slanty type are fairly useless at keeping the temperatures down, so this is almost always the first place to start to modify on one of these. On the newage cars the intercoolers tend to be much better. WRX intercoolers tend to do 350bhp quite happily and STI intercoolers have seen over 400bhp without problem. Ultimately though the intercooler determines how cool the air charge temperature will remain under hard use, so m ore efficient/bigger intercoolers will net better power output for the same amount of mods. An intercooler must always be spec’d to suite the expected power and turbo output, rather than just

going for the largest thing you can buy! Fitting front mount intercoolers brings another world of issues for the majority of Subarus. The original management system runs the airflow calculations being drawn into the engine from the

MAF sensor, (see MAF section for more information). When fitting a front mount the air box requires relocation or removal and fitting of an induction system. Induction systems with open cone filters lend themselves to allowing water and liquids into the MAF and potentially destroying the integrity of the sensor. They also change the characteristics of the air flow being read by the MAF. Although a front mount intercooler is always the recommended route in terms of air charge cooling, I recommend that the induction system is very well thought through because of the knock on effects it can have. The above problems with the MAF sy stem don’t then apply when using after -market management systems like Simtek/Syvecs/Link etc that are mafless. The MAF sensor is no longer read and instead a MAP, (Manifold Absolute Pressure), sensor is used along with an ai r intake temperature sensor to provide speed density calculations to the ECU, (see mapping section). Speed density/mafless on the original ecu? Well this is something that is offered predominantly by EcuTek, but can be done by us open- source mappers also on some variants. I don’t particu larly like the implementation used in order to run mafless on an ECU that was always designed to run a MAF based airflow calculation, so I tend not to recommend it and suggest that if a customer would like to go mafless that they should look at a Simtek/Syvecs/Link etc.

Dump Valves and Blow off valves These are fitted purely to redirect the turbo pressure which builds up in the intake system, post turbo, pre-throttle plate when the throttle is closed quickly, i.e when you lift off. In all of the production based vehicles with MAF sensors reading ECU load it is highly likely that any dump valve will be of the recirculating type. Meaning the air pressure built post turbo, pre-throttle will be dumped back into the turbo inlet side, post MAF. This is a key point to note regarding what dump valve you choose and why. On a MAF system, all the air that enters the engine will travel through the MAF to be measured for the engine management to work out how much fuel is needed for the combustion process. When you lift off and the pressurised air back up against the back of the throttle plate, the dump valve, (on a standard recirculating system), will dump it back into the intake system, so the same total volume of air that the MAF has measured remains within the system and will still be consumed by the engine. Fitting an externally venting dump valve, or a vent to atmosphere, (VTA), valve will cause a number of problems when you lift off and general engine running, as that dumped air has been measured by the MAF, but is then not used by the engine, although it expects it. The other main issue regarding externally venting dump-valves and MAF sensors, is that the dumpvalve will always open under vacuum, i.e when not is boost, (positive pressure). This means that on idle and cruise, an external venting dump valve will always be letting in extra air post MAF , (essentially an air leak). This means that initially it will run leaner than it should until the ECU learns

some corrections, (standard ECUs only). This problem is not applicable when running a mafless ECU, like a Simtek, Syvecs, Link etc. My advice; on a standard ECU stick with a recirculating dump valve for drive-ability and general integrity of the engine management control systems. As said before, if you are running mafless, then running an external dump valve won’t matter or not.

Exhausts Exhausts are a fairly simple topic to cover. I’m not going to go into brands and differences in that

respect, but will cover sizes and designs in a brief outlook. Catalytic convertors in the exhaust system are the biggest restriction inherent in the Impreza exhaust design to reach over 300bhp especially on a 2001-2006 Newage WRX. The up-pipe has an inbuilt catalytic convertor which has a common habit of collapsing after a number of years of use and generally is very restrictive. This would be the first place to start on your newage WRX. STI, Hawkeye models and hatc hbacks don’t have these cats and neither do the classics. Removing just the up-pipe cat will not be an MOT failure. That brings me on to the two questions that need to be asked related to exhausts. Do you want to be MOT friendly or not MOT friendly. If the answer is that you want to remain MOT friendly, then you would need to keep either the down pipe cat or the mid-section cat in the system to pass an MOT. The use of a sports cat can also be an MOT pass, but they are hit and miss as to their effectiveness and require some heating prior to the emissions test. So this is your biggest limitation in terms of a change of exhaust. Whether you will need a catalytic convertor or whether you can find yourself a friendly MOT inspector. The most effective change in the exhaust system aside from the up-pipe cat removal on a WRX is a de-cat turbo back system. A 2.5” system from turbo backwards is an absolute minimum if seeking around 300bhp. To suite 35 0bhp or more it is recommended that you then go for a 3” system turbo backwards and for 400bhp or more 3.5” is recommended.

Ultimately on a turbo car, the exhaust post turbo needs to be as free flowing as possible. The crucial part of the exhaust on an Impreza is the headers/manifold including the up-pipe. Up-pipe and manifold diameter is very critical for turbo spool and exhaust velocity. Having too small an up-pipe and manifold will result in good spool but a restriction on top end power and boost. Vice-versa and spool will be adversely effected but high-end power should be increased to a point based on what the turbo can withstand and produce. Selecting a compatible setup in terms of headers an up- pipe is a bit of a mine field. So I’ll give you a few ideas as to what header designs work and what doesn’t.

Designs that do: HKS style:

Equal length:

Designs that don’t:

Unequal length:

Equal length:

Suppliers that are tried and tested:

Roger Clark Motorsport Williams Motorsport Lateral Performance Harvey Smith Silverstone Autosport Scoobyclinic Headers bring good benefits regardless of power output as they increase the volumetric efficiency of the engine with smoother exhaust extraction. W ith a well-designed up-pipe a tubular setup will be much better than the originals without the restriction at high exhaust flow. The up-pipe really is the key to make the turbo work, along with reasonable spool! Wrapped or un-wrapped? Well it’s quite simple really, if your exhaust is of good quality then wrapping, or even ceramic coating

can be of great benefit, increasing exhaust gas temperature and which in turn accelerates gas velocity and aids spool. It does also reduce under bonnet temperatures, but inevitably will m ake your exhaust crack if it has a weakness in strength or quality!

Spark plugs Another easy topic to cover with simplicity. Sub 300bhp you are fine in all the Impreza turbos up to 2005 with PFR6B, standard NGK plugs. Post 300bhp – 450bhp you will need PFR7B plugs, and then after that it’s worth considering 8 graded temperature plugs.

With the 2006 – on cars they use long reach plugs, so ILFR6B standard and IL FR7B upgrades normally as above. The temperature of the plug denotes essentially at what operating temperature the plug is most suitable for. So more boost and power means hotter cylinder temperatures, so will need a higher numbered plug, or what we call a colder plug.

Air box vs induction kit The air box on the classic Impreza range is fairly good and only starts to become problematic at over 300bhp. On the newage 2001-2005 the original air box is fine for 400bhp.

If you can avoid adding an induction kit then all the better when running the standard ECU. (See MAF explanation for more information in mapping section). The hatchback airbox is quite restrictive and needs to be considered when looking at running 350bhp+.

Suggested Typical Upgrade Routes 1993-1996 WRX turbo models: 

ESL daughterboard, Apexi FC, (With AVCR), or Simtek ECU to make it mappable.

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TD05 16g turbo is capable of 340bhp with supporting mods.

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Decat turbo back 3” system.

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Port turbo wastegate to reduce boost creep.

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Must upgrade intercooler, ideally FMIC or STI 7+ intercooler.

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380cc injectors as standard need upgrading to 440cc for more than 280-290bhp. (STI models fitted with 440cc).

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Uprated fuel pump.

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Uprated 3 bar map sensor for running more than 1.2bar of boost.

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Panel filter.

1996-1998 WRX turbo models: 

ESL daughterboard, Apexi FC, (With AVCR), or Simtek ECU to make it mappable.

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TD04 turbo is capable of 280-290bhp with supporting mods. (STI turbos are IHI and depending on model are capable of 300bhp+).

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Uprated fuel pump.

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Panel filter.

1999-2001 Classic WRX turbo models: 

Ecutek remap, Apexi FC, (With AVCR), or Simtek ECU to make it mappable.

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TD04 turbo is capable of 280-290bhp with supporting mods. (STI turbos are IHI and depending on model are capable of 300bhp+).

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Uprated fuel pump.

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Panel filter.

2001-2005 Bugeye/Blobeye:

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WRX needs decat up-pipe.

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Uprated fuel pump.

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WRX has TD04 turbo is capable of 280-290bhp, 280-300lb/ft torque with supporting mods.

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STI has VF35 turbo capable of 320-340bhp, 320-340lb/ft torque with supporting mods.

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Panel filter.

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Std ECU is remappable via open source or Ecutek mappers.

2006-2007 2.5L Hawkeye: 


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Uprated fuel pump.

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Panel filter.

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2008-2010 2.5L Hatchback: 


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Panel filter.

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Induction kit needed for 350bhp+.

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2008-2011 2.5L Saloon: 


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Panel filter.

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Induction kit needed for 350bhp+.

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What is engine mapping? Perhaps a better question to ask would be how can engine mapping help you? And why should you care what it does in technical terms? Engine mapping or remaps, as they are more commonly known, is all about optimization of the basic functions of an internal combustion engine. Two main aspects on naturally aspirated

engines that are in focus whilst mapping are ignition timing and fuelling a third if you factor in the use of systems like Vtec, Mivec etc. When mapping a turbocharged vehicle, ignition and fuelling are again the two main aspects in addition to a wastegate control system to control turbocharger dynamics and on the later production models a type of variable cam timing like AVCS or Mivec again. Optimizing these aspects is the main game when engine mapping is concerned, mainly to produce maximum torque and power, either to customer specifications or engine limitations. As an example, the Impreza engine and mapping set up, (regardless of year and type), are all tuned to be efficient in maintenance and safety, as such in areas of high boost pressure they are mapped to run rich. Mainly due to the varied ambient environment and use each vehicle will be subjected to, so it works as a safety margin before reaching lean conditions and detonation, (explained in the next topic). This means that the manufacturer’s large safety margin in terms of

fuelling and ignition timing can be trimmed through the remap process to increase the efficiency of the engine and produce more power, even before altering boost levels. Understanding these principles, even just the basics, will help you with understanding potential risks, problems and general failures and will aid you to reduce and potentially stop yourself from spending large amounts of time and money trouble solving your engine problems.

Mapping terminology and what it means Air to fuel ratio (AFR/Lambda) – Literally as it means, the ratio of air to fuel that is subsequently burnt and expelled through the exhaust. Stoichiometric ratio is said to be 14.7(air): 1(fuel) where an ideal petrol is completely burnt. This is also referred to as lambda = 1. Greater than lambda = 1 is said to be lean, lower is said to be rich, (14.7> lean 14.7< rich.) However in practice an engine can run up to 15.7:1 quite safely on low load a nd cruising and vice versa high boosted engines will need anything from 11-12:1 to obtain safety and efficient power. Note: If you ever receive a dyno graph with your fuelling on it from me, you will see it is in Lambda and not AFR. That’s because AFR changes with different fuel mixtures, whereas the lambda point of

any fuel is always the same. A sore trip up point for those that tune in AFR and forget to richen the mixture when running methanol or other additives.

Detonation – An erratic form of combustion caused by a variety of conditions e.g. excessive heat, or excessive pressures which cause an auto ignition, or an ignition too early, of the combustion mixture. There are other contributing factors that cause detonation, but these are the most common issues when mapping.

MAF – Mass air flow, a sensor usually just after the air filter using a heated wire element which works as a positive temperature co-efficient resister. The air passing across a hot element has a cooling effect changing the resistance of the element which is then converted into an output voltage to give the EMS mass flow. The most common method used on road cars today and doesn’t generally need changing unless you dramatically increase the air flow demands of the engine.

Speed Density – A term that is used when an engine management system, (EMS), takes its primary load from a map sensor in the plenum chamber and reading the air intake temperature, (AIT, post intercooler on a turbo car). Most after-market EMS use this type of load due to the restrictions/limitations of MAF and ease of use with a map sensor and AIT sensor, (Speed Density).

Det Cans – These are headphones attached to some sort of frequency separating amplifier which is then connected to a knock sensor which is attached to the engine. This allows the mapper/man wearing the cans, (headphones), to hear the combustion in the cylinders acutely and listen for detonation.

Narrowband Lambda Sensor – The lambda sensor is situated in the exhaust and is used to regulate the fuelling on low load and idling to hit lambda 1 for emissions regulations and fuel efficiency. It is called a narrowband sensor because it cannot read a large range outside of lambda 1. Typically older sensors will literally switch between lean and rich to give the reading.

Wideband Lambda Sensor – These are typically found on dynos and mapping lambda sensors like the Innovate LM-2. However they are now m ore commonly installed on newer vehicles so the fuelling control from the ECU can be more accurate and allow a learning ability.

To be continued in V01.1…….

Engine Mapper s Subaru Tuning Guide V1.1

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