The Impreza Turbo engine loves the best grade fuel you can put in. As the charge is compressed in the cylinder there is a heat build up (Universal Gas Law) and is this heat build up is so great the fuel will pre-ignite before the spark plug fires. This preignition is also known as pinging/pinking/knock. The ECU will try to over come this by over fueling, or retarding the ignition but it will finally reduce the boost pressure. The various fuel grades available have different properties in this respect with the higher octane grades having a higher preignition temperation, and thus higher boost capability.
Here in the UK we have normal unleaded with an octane rating of 95RON and Super Unleaded with a rating of 97RON. Previously this was 98RON but with the introduction of Lead Replacement Petrol it has been reduced by 1 point, and with noticeable effect on fuel mapping. The reasoning is due to the fact the same base fuel is used for the two products. This does NOT mean that LRP can be used in the Impreza! Just as lead would contaminate the Lambda sensor and catalysts in the exhaust, so will the addatives put into LRP. Things are looking up though, with Shell introducing Optimax into the UK with an octane rating of "over 98RON".
The differences on my car (both the standard 208bhp MY97 and the STi based P1 276bhp engine) between 95RON and either the 97RON or 98RON super unleaded is felt between 2000 and 4000rpm. The car is much more eager to pull in the lower rev range. These seat of the pants figures have been seen on recent IWOC rolling road visits where peak power around 6000rpm is seen to improve by about 5bhp but lower down the rev range a much greater improvement is made.
The adaptability and self learning nature of the ECU allows it to change with respect to certain conditions via its outboard sensors around the engine. Unfortunately the effect of this learning process, which is going on continualy, is that it may learn some bad habits! Early models of MY96 and older needed the memory to be reset to take advantage of changing from normal to super unleaded as the ECU remembered the optimum settings for the lower grade fuel. This also meant that is a bad batch of fuel was used then it was this which was remembered. Newer MY97 to MY00 models relearn a bit better, the MY99/00 being faster than the MY97/98 but this process can be speeded up by a reset also. There is no mileage in resetting the New Age WRX because of major differences in the ECU/software.
There are three ways to reset the ECU.
For me the last is much simpler. To reset the ECU manually at home, disconnect the battery for around an hour and then reconnect it. There is a back up capacitor inside the ECU which retains the memory for the running parameters which eventually goes flat during this period. One point, ensure that you fill up with a full tank of petrol and run a few miles before the reset because of the old fuel still in the fuel line! Better still wait until tank number 2.
Super is a given for the STi based engines and all grey imports which are mapped for the Japanese 100RON fuel. The ECU in these vehicles have not got enough adjustment for the lower grade fuel and can lead to failed pistons, valves and worn bores. Likewise do not expect to undertake any engine modification and expect to then run on 95RON unleaded as it is truly a false economy and potentially expensive mistake! If you are looking at pushing the envelope with respect to engine output you MUST really consider sticking to the highest RON due to the extra ceiling with respect to engine knock and detonation.
The standard JECS ECU as used in the Impreza is very good at compensating for most general first step modifications such as air filters and exhausts. It should be mentioned though that Subaru are constrained by the fuel maps that can be used due to emmisions regulations and having to take into account the variabilities of mass produced engines. Engine reliability is also a factor where the lowest common denominator owner has to be taken into account.
The actual map does a lot more than just the fuel. It controls the ignition and boost pressure as well. As mentioned above the three are related due mainly to the Universal Gas Law and thus can be altered in many different ways to achieve many different effects.
The Prodrive Performance Pack includes an ECU replacement along with a 2 stage centre and rear box exhaust and a piece of silicon intercooler piping. This is a straight drop in item which needs little or no set-up as like the standard ECU it is designed around mass produced engines. For more information see the Prodrive upgrade section.
The Possum Link ECU upgrade is a drop in replacement for the standard ECU. To get the most out of it though the fuel map must be extensively programmed for the engine it is connected to. It is all down to the skill of the programmer as to whether the car drives like a dog, a rocket ship or a ticking time bomb.
The programmable nature of the Link also allows it to be used to replace the ECU in a Japanese import which has been designed arouned 100RON fuel. With the Link in place a fuel map to take into account 95RON may then be made.
Like the Link this is also a drop in replacement which is mapped by a specialist but it is far more sophisticated (and costly) option. This is the ECU of choice for use in a Group N rally car. The number of maps available and their accuracy is far greater than the Link, and the number of sensor input/outputs available is greater. The result is probably the ultimate in Impreza Turbo ECU upgrades available to the public. For the techno geeks (and rally teams) out there the Motec can even datalog the cars performance to then be downloaded onto a PC.
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Page originally created 23rd September, last updated 20th October 2001
Copyright © 2001 Steve Breen. Copyright © of photographs remain with source. Used here by permission and may NOT be copied for any purpose.