Fuel, Power Output and Reliability

The first thing most people get attracted by is the power output from the engine, which is much greater than the 215bhp export version. The last MY2000 Japanese WRX is quoted in standard 4 door form as 280PS compared to 215bhp for the UK/export specification turbo. Note PS, this is not equivalent to bhp, and 280PS is infact 276bhp (see also below regarding the New Age WRX). There is also the little realised fact that the 276bhp is obtained on Japanese Super Unleaded fuel which is much purer and higher octane than that normally available in the UK and rated at 100RON. To make you even more envious, the Japanese also have fuel rated at 102RON to gain that little bit extra!

It was certainly true on the earlier "280PS" models that running an unmodified WRX engine on super unleaded (now only 97RON) would not give 276bhp but more like 270bhp due to the knock sensor restricting the level of boost attainable. I have seen a modified WRX STi 3 type R on a rolling road and it achieved the quoted power output but had a modified rear section silencer and free flow air filter, as well as running on the older SUL of 98RON. If 95RON were to be used on a standard car then the boost level would be further reduced and with a probable under 260bhp being achieved. John Hunter, an IWOC member, had his WRX STi-R tested on a rolling road when running on 95RON. Output at the crank was 254bhp. There is thus a danger of a frugal WRX owner finding himself becoming prey of UK specification owners who have carefully set up their car with free flowing air filters, exhausts and reset ECU's, and definately those running any ECU conversions. John retested his car again with 98RON and has found much to his delight, power output at the crank much nearer the advertised 276bhp! See his pages for further details.

The newer version 5 and 6 engines seem able to produce a more reliable 280bhp (yes bhp) on 98RON but it seems their ECU mapping is extremely critical. The state of tune seems that prolonged use of 95RON fuel will actually damage the engine. There have been many confirmed cases of pistons, valves and bore damage from owners who have used purely 95RON fuel. The officially imported Australian STi3 models have also been reported to suffer from locally available fuel. There is also at least one known case of a car in the UK being run on 98RON fuel, at a trackday, on a hot summers day, which ended up in a broken engine. Even earlier WRX are not safe with an owner managing to destroy an MY96 WRX engine during one weekend on 95RON unleaded. A combination of air density, temperature and poor fuel has been known to cause knock on even standard grey WRX (not just STi).

As a matter of safety it is therefore recommended that all grey import vehicles which have not been remapped for lower octane fuel be used on 97RON Super Unleaded at minimum, and prefferably Super with an added octane booster or Shell Optimax. During trackdays where the engine will be at maximum performance an octane booster is a MUST. Also any modification that improves breathing, either an air filter or exhaust modification must now be taken into account by the use of an octane booster because the chances of detonation will be increased.

An explanation of the effects of fuel octane from Pete Croney of Scoobysport "As the cylinder compresses the charge, the charge heats up (universal gas law). 95RON will self detonate at a lower temperature (or pressure) than 97RON. If the charge is reaching that temperature before the piston gets to TDC, then it will knock. This puts extreme loading onto the con-rod. If the ECU detects this knock, it will up the fuel going in, to try and cool things by A) the cooling effect of misting the fuel, B) the dulling effect that over fuelling has on the efficiency of the detonation. It will also retard the timing, in the hope of delaying the burn. If the ECU has reached the limits, ie so much fuel is going in that it is risking both no explosion and "bore wash" (petrol running down past the pistons and diluting the oil), then there is no more it can do to stop the pre-detonation."

I also know of a 1994 WRX which was unable to pass its MOT because of emmisions when running on 95RON for exactly the same reason. The amount of unburnt fuel being recorded was unacceptable. Retaking the test but with 98RON super unleaded the car passed.

Another thing that is apparent is the different power outputs from the Japanese WRX's over the years. As mentioned above I have seen an STi 3 type R put out around 280bhp. The latest STi version 5 are putting out around 280 to 290bhp with 300bhp seen on one example with the same sort of air filter/rear silencer modifications. Conversly the earlier 95 and 96 WRX are in the region of 250 to 260bhp. In fact the first STi had only 250PS as released, which surprises some people as they have bought the car under the mishaprehension that it was a 280BHP model! Similar reductions are seen also in the lower specification WRX models with some early examples only producing 220PS which is identical to the UK spec. Also note that the 5 door versions usually produce less than their 4 and 2 door counterparts. This is due to the mapping of the ECU being aimed more at producing more torque lower down in the rev range to take into account the higher payload capacity.

MY98 WRX (copyright © Martin Buck)

The New Age WRX from Japan has reset the power output levels to an extent. The outgoing WRX had 280PS, but the New Age WRX model is back down at 250PS (247bhp or 184kW). The STi model retains the 280PS with some added benefits.

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Page last updated 16th April 2001
Copyright © 1997/8/9/2000 Steve Breen