rjben

OK I know I should be using SUL or Optimax because of the higher RON, but why?

Am I correct to assume that that RON and Octane are the same thing? It would appear that the higher the RON, the less the ignition retard, does this been that octane in some way allows petrol to reach a higher pressure before deting?

Why is it bad to use lower RON in the JDM cars? Can't these retard enough to cope with the lower RON?

Cheers,

Rob

EvoRSX

RJ,
For most things you want to know have a look at the following link.

Andy

http://www.dynopower.freeserve.co.uk...reignition.htm

Steve 555

Sounds like an election campaign

Mike Tuckwood

OK, that links a bit heavy if you haven't already got an insight into it.

Simply put, RON (Research Octane Number) is the resistance of self ignition under load and or heat without a spark.

The spark ignites the fuel air mixture and develops what is called a flame front. This should propogate out from the spark in a controlled manner, a little like a wave from dropping a pebble into water.

Lower octane rated fuel (RON) is often not good enough (stable enough) and self ignites due to the pressure and heat generated in the cylinder before the flame front from the spark reaches it and "explodes" before it should do.

This cause a secondary flame front (or multiple flame fronts), which then "crashes" into the first flame front causing a shock wave/pressure wave (this can cause stresses up to 60 times higher than the normal ignition reaction).

JDM cars are mapped for the higher quality fuel available over there and consequently aren't as able to pull the ignition back far enough to take into account the lower quality (lower RON rated) fuel available in the UK.

Hope that helped?

Mike
Scooby Mania

rjben

Mike,

Yep, this helps. The article was good as it gave me an insight into the causes of knock and the effects of knock which are very usefull when going down the path of cheap performance mods!

I think I need an EGT, AFR and Knock sensor before I get my FCD.....Sounds like an extract from a NASA hand book!

Cheers,

Rob

john banks

Rob, what model year do you have?

The STi and P1 maps I have looked at for 1999/2000 cars have low and high compensation maps for detonation for fuel and ignition, but the values in them are not as broad as the UK cars, in some cases the low and high octane maps are the same, giving them far less ability to retard and increase fuel in response to knock. This is worrying given that they are mapped for 100 RON, the knock sensor sensitivity setting in the ECU is lower and a lot of Scooby maps are quite aggressive with the ignition timing already even when using their intended fuel.

rjben

Hi John,

I have a UK97. I only ask about JDM cars out of interest.

I'm looking at getting at getting a FCD to allow me to boost to about 18ish, but I want to get a lambda link to ensure that my mixture is OK, and possibley the HKS AFR to allow me to tweek the mix a bit (I'd only make it richer by the way). I was also thinking of a knock link just to be on the safe side.

Will I need a downpipe to take advantage of these extra mods? I don't want to go done this root as I'm concearned about imissions. I've currently got a dawes with Magnex BB and and decat CC and Blitz induction kit.

Regards,

Rob

john banks

Your safest bet is an ECU remap. I can happily say this as I can't sell you one and suggest you see www.brdevelopments.com and consider a Link ECU - more expensive, but if you want it done properly...

Reasons being -

There is no point in making your car richer. You would need to take fuel away to make more power, but you would probably want to richen up the spool up area and advance the timing, but lean out the mid and top. With the basic HKS AFR you are changing fuelling across the whole range - this is pretty useless to be honest. The Super versions are too limited in resolution to be useful IMHO.

The AFR gauges are not as good to map by compared with datalogging as they cannot show you small lean spots so well.

The EGTs will probably still be cool, and a Knocklink might not show anything, but by the time these are going off you have arguably lost the plot with your map. By the time your knocklink lights up your car will have retarded a lot and has run out of retard or there are other unfavourable conditions resulting in knock and you have probably gone way too far for your map. If you can see on your datalog the ECU retarding a few degrees then you can address it and put it right before it becomes a problem. A knocklink is a bit after the event and doesn't tell you WHY knock has occurred, and the fuelling will be richened (at least with JECS ECUs) and the timing so retarded you have probably lost power by the time it happens. Just my 2p but the info you get from a remappable ECU is so superior. I used to mess about with piggyback devices but it is tuning blind in comparison to a proper ECU.

Of course you can get around all this as some have by unplugging the knock sensor, but you'd be either a genius or very stupid indeed depending on how lucky you are to do this unless you have some way of addressing the knock.

I did think EGT would be very useful as a tuning aid, but it seems not to be so far - just sits in between 700 and 750 in the headers even if I get -5 knock corrections, and the ECU corrects knock with still quite low EGTs. This might be because on balance my timing has been too advanced and the fuelling too rich and I am now redressing the balance. Because I have a new turbo and fuel pump there is a lot of work to be done to the standard map to make it play nicely.

[Edited by john banks - 27/05/2002 14:17:22]

rjben

Cheers John,

You are right as usual! I should go the the Link but I really just want to play around with the settings for myself....out of interest really (and probably significant terminal expense!).

I'm a little confused as to why I would want to lean the mix....I thought in general terms, as more boost is applied, the mixture should get richer. I thought this allowed cooler running and helped reduce knock caused by high temps rather than knock caused by low ocatane or too much pressure....?

I understand your point about the knock sensor. I guess what is needed is a reading of the current ingnition status as well as a knock sensor, then we could tell if we had no knock on advance, or if we had no knock becuase the ECU had retarded the ignition...if you know what I mean.

As you can tell John I'm very new to all this, I've only had my Scoob for 2 months, but I'm enjoying learning about how to tune them. At the moment I'm just trying to break the 16psi barrier with the confidence that my mixture is rich enough and that I'm not going to melt anything!

Regards,

Rob

john banks

Yes you are right about the fuelling, but Subarus are SUPER rich, I would rather run a sensible boost figure with slightly leaner (but still very and appropriately rich) fuelling and make the same power. Leaning out makes more power as does advancing the ignition or increasing airflow - all within reason.

rjben

OK,

So how can I tell if my ignition is retarding? I assume that if I lift my boost and the mix (checked and not altered by AFR Meter) is OK, the only thing I need to check is that my ignition is not retarding due to knock.

Is there a device I can use to check advance? If so, can I just reset my ECU, check advance, then up boost and check advance again? I suppose any retardation (made up word!) would indicate knock?

Thnaks again John,

Rob

john banks

Use a Select Monitor if you can find a local specialist with one.

rjben

Thanks John.