98 vs 100 octane

[AdrianS]

My JDM 4age (blacktop) is supposed to run on 100 octane (Japanese) fuel.
All we get here is 98.

So : assuming the Japanese 100 octane measured the same way as our 98 (RON), will using 98 RON matter, ie is the blacktop factory tuned to the ragged edge, or do they leave enough leeway for 98 octane ? (Static compression 11:1)

I know I can retard the ignition, but I'm going to start with factory timing.

[TERRA Operative]

Everyone just uses 98 octane. Works fine on factory standard timing.

[Cuzzo]

Yeah if its the stock computer with stock wiring it will adjust itself acordingly.

If its aftermarket ecu, tune it on 98.

[schnitzel]

In Australia the 98 is a minimum figure anyway, a lot of the time the Ron rating is actually higher as it needs to be able to be stored for a period of time and still hold its minimum rating at the time of sale. ( Unleaded fuels will drop rating if stored for to long, in most cases the "shelf life" is only six months for peak performance if stored in non air tight containers)

The major oil companies are currently trialling 100 and 105 octane fuel and hopefully it will be available at the pump in the near future.

[AdrianS]

Yeah if its the stock computer with stock wiring it will adjust itself acordingly.

That's ok if it is "intelligent", ie retards timing just enough. Would be a pain if the ecu sees knock, switches to "bad fuel" mode and pulls all the timing out.
A dyno run is on the cards when the engine is finally in, so I can graph ignition timing then and see what it does.
I would love to see back-to-back blacktop runs on different octane fuels (I've searched all over t'web with no success), but I'd need to find 100 RON fuel in Melbourne. e85 would be interesting, but it adds so many other variables (density, oxy content, etc)

[Norbie]

Not every servo in Japan has 100 RON fuel. The engine/ECU would have been designed with this in mind, ie maximum performance with 100 RON but still runs OK with lesser fuels.

[AdrianS]

Not every servo in Japan has 100 RON fuel. The engine/ECU would have been designed with this in mind, ie maximum performance with 100 RON but still runs OK with lesser fuels.

I know it will be "ok" on 98, but was wondering what the power drop would be.
I worked on an engine dyno for toyota 15-20 years ago, and the 100kw 4age used to go into limp-home / safe mode at the drop of a hat (or sensor). The only way you could tell it was in "safe mode" was a power drop (about 15 kw, as I recall), and the exhaust manifold glowing red-yellow from rich mixture + retarded spark at WOT.

[Cuzzo]

That's ok if it is "intelligent", ie retards timing just enough. Would be a pain if the ecu sees knock, switches to "bad fuel" mode and pulls all the timing out.
A dyno run is on the cards when the engine is finally in, so I can graph ignition timing then and see what it does.
I would love to see back-to-back blacktop runs on different octane fuels (I've searched all over t'web with no success), but I'd need to find 100 RON fuel in Melbourne. e85 would be interesting, but it adds so many other variables (density, oxy content, etc)

How it would work is via the oxy sensor along with the knock sensor.

Then yes it would pull timing

[Norbie]

I know it will be "ok" on 98, but was wondering what the power drop would be.
I worked on an engine dyno for toyota 15-20 years ago, and the 100kw 4age used to go into limp-home / safe mode at the drop of a hat (or sensor). The only way you could tell it was in "safe mode" was a power drop (about 15 kw, as I recall), and the exhaust manifold glowing red-yellow from rich mixture + retarded spark at WOT.

I can't tell you what the power drop would be, but there's no way a 2-point drop in RON would make the ECU go into limp mode or anything drastic like that.

[RyleyMA61]

I can't tell you what the power drop would be, but there's no way a 2-point drop in RON would make the ECU go into limp mode or anything drastic like that.

yeah it'll probably just knocking the ignition timing back a few degrees and maybe increase the fuel a bit. i doubt anyone would notice (unless you're schuey)

... you're not schuey are you??

[AdrianS]

I can't tell you what the power drop would be, but there's no way a 2-point drop in RON would make the ECU go into limp mode or anything drastic like that.

It all depends. Look at 4AGZE issues with over-sensitive knock sensors. Early (1980's) ECU's often had 2 ignition / fuel maps, and would switch to the "safe" map when they detected knock. Later / current ECU's, especially turbos, generally pull just enough timing to avoid knock, and will then gradually increase the timing again to "learn" how aggressive to be.
I was wondering which strategy the 20v ECU uses. As it's a 90's design (mine is '99), I would assume it's a smart strategy. I was hoping someone could say "I know it does xyz, because I tested it under such-and-such conditions", but all I can find are assumptions, with no data to support them.
What got me started on this were all the posts on the net saying how the 20v motors are less powerful than their factory ratings (which may well be true), but no-one says what fuel they were being tested on, or how much difference it makes.

I'm not Schuey, and I'll be happy with whatever power I get, but I have an interest in ECU design & behaviour, and can't help wondering... hence the question.

ps I thought a "point" of octane rating is 0.1, ie 100 octane is 20 points higher than 98. Anyone confirm / deny this ?

[LeeRoy]

There was a thing on the TV the other day about fuel octane and HP.

They chucked a base model VW Golf, performance N/A 4 cyl thing, and an STI onto a hub dyno.

Betwene 95 and BP/Optimax there was no difference at all between the golf, a 1hp difference between the N/A performance 4 cyl, and about 20hp on the STI.

In your case I really doubt there would be any difference at all.

[MR22ZZ]

plenty of blacktops running around on the standard ECU using 98RON and pulling low to mid 90kWs on the dyno and no issues on the road.

[RyleyMA61]

i was only bein funny with the schuey comment, but maybe you should go and do some tests yourself and tell us the difference! bang some octane booster in and go gteching or dynoing or something, and then do it on standard 98!

i'd be very interested in the results, like you said, for tuning and ECU interests. i didn't know the 4a ECUs were so aggressive in their fuel and ignition retardation (lol, borat)

[trdee]

octane number is a ratio, nothing more.