What fuel should actually be used? 99, 98 or 95

[Pig benis]

Hello guys

Recently I have gone out and treated myself to a Subaru Impreza WRX (2.0) and now I can see why people buy these things. They are an absolute hoot to drive and I am thoroughly enjoying mine.

However, I have been on the Subaru owner forums to clear up my confusion as to what fuel I should be using. Currently I am filling up with Tesco 99, but from reading lots of different forum posts, I am not entirely sure if this is the best stuff for my car. So I was hoping you guys could clear up any confusion?

I haven't filled the car up with regular pump fuel, but I take it this will make my engine run rough?

Thanks all
PB

[TEKNOPUG]

97 or above as that's the standard RON fuel in Japan and that's what they are mapped for. You can have them remapped for 95, you'll just lose a little power. Or mapped for 98/99 and you'll gain some. Lots of discussion on the web - general advice is Shell Optimax or Tesco Momentum - steer clear of BP.

[tonyb1968]

New Age WRX's can happily run on 95 ron fuel, STI's are a different matter though, if you refer to the hand book it will tell you what fuel the car can run on, there is no issue putting a higher octane fuel in the car, just putting a lower octane fuel in than is recommended by the manufacturer.

[300bhp/ton]

TEKNOPUG said:

97 or above as that's the standard RON fuel in Japan and that's what they are mapped for. You can have them remapped for 95, you'll just lose a little power. Or mapped for 98/99 and you'll gain some. Lots of discussion on the web - general advice is Shell Optimax or Tesco Momentum - steer clear of BP.

If it's a UK car it will not be mapped for JDM fuel.

[300bhp/ton]

Pig benis said:

Hello guys

Recently I have gone out and treated myself to a Subaru Impreza WRX (2.0) and now I can see why people buy these things. They are an absolute hoot to drive and I am thoroughly enjoying mine.

However, I have been on the Subaru owner forums to clear up my confusion as to what fuel I should be using. Currently I am filling up with Tesco 99, but from reading lots of different forum posts, I am not entirely sure if this is the best stuff for my car. So I was hoping you guys could clear up any confusion?

I haven't filled the car up with regular pump fuel, but I take it this will make my engine run rough?

Thanks all
PB

Tbh, the best advise here is to go and read up on what an octane is and what it means.

Stock UK cars of some era's and some model specs are likely rated to run on 95RON.... however that doesn't mean that they wouldn't be better off being run on something with a higher octane rating.

Modified vehicles, imports and some UK models will require superunleaded.

Super unleaded is typically 97 octane or higher. I personally prefer to run 99 octane if I can. As for Tesco vs Shell (the only two 99 octane fuels in the UK iirc). Then it's a toss up.

Some will hate Tesco, because it isn't Shell...

As a rule it is normally accepted that Shell run more additives in their fuel, which is normally seen as a good thing. But Shell V-Power will typically cost more too.

All you can do is try both and see if you notice a difference. Which generally applies to the other superunleaded fuels as well. As certain parts of the country buying 99 octane fuel or even superunleaded can be somewhat challenging.

[vxr2010]

super unleaded the higher octane the better , 95 is likely to damage your engine , if nothing else you could use 95 but don't use the loud pedal as boost and 95 is not a good idea, as above shell v power probably the best , i use tesco 99 , but other sul's will do if i can't use tesco

[Konan]

tonyb1968 said:

New Age WRX's can happily run on 95 ron fuel

My UK WRX on the standard map would still pull a bit of timing on 95.

For a few quid, you can get enough kit to check the ECU for signs of knock.

[Ahbefive]

Just don't run it on 95 and you will be fine.

[tonyb1968]

Konan said:

My UK WRX on the standard map would still pull a bit of timing on 95.

For a few quid, you can get enough kit to check the ECU for signs of knock.

The thing is, you wouldn't know, new age eu/uk wrx's are mapped for 95 ron fuel so will run on that happily, those saying they won't need to understand that its a compliance and has had hundreds of hours testing.
It does not mean you won't get a bad tank of unleaded but then again you also get bad tanks of super unleaded/unleaded put into the super unleaded tank at the garage, which if you have an STI is a little more worrying!

[Konan]

tonyb1968 said:

The thing is, you wouldn't know, new age eu/uk wrx's are mapped for 95 ron fuel so will run on that happily, those saying they won't need to understand that its a compliance and has had hundreds of hours testing.

It's a little difficult to see what you're saying there. Wouldn't know what?

[tonyb1968]

You wouldnt know when the ecu has pulled the timing back.

[vxr2010]

the ecu reacts quickly to pull timing but i've heard from tuners not quick enough so damage already started or done, so at least if you are higher octane you are better off plus better fuel economy , i run a na v8 and yes you can 95 it if you want but runs much better on sul , further to that in the usa were cars not set up for the us fuel and look at the frequency of there engine failures , it's a quick car with a slightly fragile engine so i would suggest sul all the way , if you can't and put in 95 then drive like an old fart

[tonyb1968]

Ok lets go through this again.
If a car is mapped for 95 ron then it can happily run that and produce the given power output, ie 225ps.

You can run a higher octane fuel with no issues, issues arise when you run a lower octane fuel in the car than the specified grade.

Ps issues with the 2.5 in the USA were not fuel related, yanks use a MON system and we use a RON system for rating octane, MON numbers are lower than RON for the same octane

[Ahbefive]

You keep saying that but the OP hasn't even said if his is a newage or classic. I certainly wouldn't run a classic on 95 and personally wouldn't make a habit of running any petrol turbo car on 95 either.

[DuncB7]

There's a reason devices such as Knocklink exist.

This could be a classic case of RTFM. The manual will advise which fuel is suitable. If it has been tuned, advice is obviously to run a higher RON fuel.

It's a relative figure which expresses the resistance to combustion under compression. Increased boost pressure and high temperature conditions are perfect for detonation. I speak from experience.

Don't get drawn into the Shell vs. Tesco argument. Negligible difference if any. You'd see more gains from emptying the ash tray.

[300bhp/ton]

Ahbefive said:

You keep saying that but the OP hasn't even said if his is a newage or classic. I certainly wouldn't run a classic on 95 and personally wouldn't make a habit of running any petrol turbo car on 95 either.

+1

[Konan]

tonyb1968 said:

You wouldnt know when the ecu has pulled the timing back.

I said that mine would pull timing, which would indicate that I did know.

For example (not my car!): This was taken from a WRX that was on 95 AND had running issues.

I wouldn't expect to see anywhere near this much pulled just for the fuel, but I'd always see a little more than could be accounted for by noise pulled that would clear up when run on SUL instead. Never anything terrible enough to pull the IAM back, but enough to make me choose super over 95.

[tonyb1968]

Ahbefive said:

You keep saying that but the OP hasn't even said if his is a newage or classic. I certainly wouldn't run a classic on 95 and personally wouldn't make a habit of running any petrol turbo car on 95 either.

Read my first post....

DuncB7 said:

There's a reason devices such as Knocklink exist.

Knocklinks are are pile of poo, KS3 is a far better solution because you can actually set the damned thing up so you can hear when it hits said freshold and not wait to see some lights that can be set off by a dip in the road (had both, KS3 wins easily).

Konan said:

tonyb1968 said:

You wouldnt know when the ecu has pulled the timing back.

I said that mine would pull timing, which would indicate that I did know.

For example (not my car!): This was taken from a WRX that was on 95 AND had running issues.

I wouldn't expect to see anywhere near this much pulled just for the fuel, but I'd always see a little more than could be accounted for by noise pulled that would clear up when run on SUL instead. Never anything terrible enough to pull the IAM back, but enough to make me choose super over 95.

If it had running issues then super unleaded wouldn't save you, no fuel would as the ecu is getting incorrect info to start with from what ever part is faulty (MAF/O2 sensor etc)

[DuncB7]

tonyb1968 said:

DuncB7 said:

There's a reason devices such as Knocklink exist.

Knocklinks are are pile of poo, KS3 is a far better solution because you can actually set the damned thing up so you can hear when it hits said freshold and not wait to see some lights that can be set off by a dip in the road (had both, KS3 wins easily).

Interesting. Must research this in more detail.

[tonyb1968]

DuncB7 said:

Interesting. Must research this in more detail.

phormula.com is where you need to look

Added:- It also shows you what you hit on a journey which a knocklink doesn't, not the cheapest bit of kit but well worth it on a modded car or JDM import

Edited by tonyb1968 on Thursday 20th October 15:59