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Octane rating for JDM engines

Mon Jul 07, 2014 7:03 pm

So we know the JDM ecu's are tuned for 98RON. What does that really mean. It has been expressed often that Japanese fuel is a higher octane than our fuel. I have read that you can get 101+ RON in Japan. While at a store going out of business I picked up some clearance octane booster. So clearance price plus GOB discount got me 104 boost for about $3 a bottle. I have used toluene in the past,but its quite expensive considering how much you need to use.

Based on this forum I found that 93 octane is likely pretty close to the Japanese RON figures. While some states are limited to 91 octane, you can use a booster method to get it closer to 93.
USA (AKI/PON) --- Europe (RON)

87 = 91
89 = 93
91 = 95
93 = 98

Another link with calculated conversions. This one looks a bit more thought out than the audi post and ranks 94 close to the 98RON and 93 just below.
http://www.pencilgeek.org/2009/05/octan ... sions.html

Some real reviews of octane boosting methods.
http://www.fueltechexperts.com/2008/08/ ... h-is-best/

Calculating fuel ratings here.

Here is a fuel finder as well if your wanting to locate the highest octane near you.

I had feared that running 93 octane was far too low for my engine, when it is closer than what I expected. Also I can gain maybe a RON with the octane booster I bought and feel a little more secure. Especially on race day.

You may consider running different brands of gasoline. I learned recently from some local refineries that there are different qualities of fuel and engines will respond to that. I had been running BP myself, and they told me that BP is actually on the low end. Phillips 66 is one that is on the higher end. It may be worth experimenting with.

If anyone has any data to share on the subject or links please comment. Also, tune at your own risk.
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Last edited by CMS-GT4 on Tue Jul 08, 2014 11:11 pm, edited 1 time in total.

Re: Octane rating for JDM engines

Mon Jul 07, 2014 8:24 pm

I know most octane boosters are junk and only bump you up a fraction of a point, which one are you using? I was planning on using toluene when down in Washington where they only have 92 like a bunch of savages.

Re: Octane rating for JDM engines

Mon Jul 07, 2014 8:37 pm

I have the 104+ brand which they tested at about 1RON increase. Not bad for $3. The issue with toluene is cost and volume needed. You can also get it at paint stores but you need to run a few gallons in your tank for best results. You might try that race gas finder I posted and maybe hit a place that sells race gas to mix in your tanks.

Re: Octane rating for JDM engines

Mon Jul 07, 2014 10:47 pm

I will follow this thread closely as I have a detonation problem (full throttle 5000-7000 rpm) using 91 octane on my JDM 3rd gen engine. My car knocks bad if I use ANY 91 octane fuel available in my city (the highest I can get). The next closer place to me to get 94 octane fuel is a 3 hour drive... I was thinking of installing a Water/Meth kit on my car, to prevent detonation, still using pour gas and possibly raising performance. I still have a lot to read before I make a move, though...

Re: Octane rating for JDM engines

Tue Jul 08, 2014 12:48 am

Yeah toluene you need 1L for every 10L of 92 to make 94, I would only need to run it for one tank worth while I'm on the track. The nearest place on that site is another track, they have race gas at the track I'm going to but it's $$$

Re: Octane rating for JDM engines

Tue Jul 08, 2014 2:30 pm

I just drove about 5000km through the states in my celica running gen 3 efi on a jdm mr2 ecu.

I found it would run well enough to get me back to the land of 94 oct. Even when my options dropped to 89 or 90 oct. But it certainly wasnt ideal.

On my old ECU it was 94 or nothing until i got up into the mountains where it would amazingly start running on 91.

Re: Octane rating for JDM engines

Tue Jul 08, 2014 6:38 pm

Ive been running my car (91 JDM gt-four A) on 91octane since the day I got it, never had an issue (I put 30K miles on the car). I wish I could get some 94 but its extremely rare around here.

Re: Octane rating for JDM engines

Tue Jul 08, 2014 7:45 pm

Does the JDM 185 model list the RON requirement on them or in the manual?

Re: Octane rating for JDM engines

Tue Jul 08, 2014 7:54 pm

I do still have my original manual, but its 6 hours away. I dont recall seeing any mention of required oct but the manual is written like a manga comic book with zero english.

Re: Octane rating for JDM engines

Tue Jul 08, 2014 8:09 pm

CMS-GT4 wrote:Does the JDM 185 model list the RON requirement on them or in the manual?

No clue but these stickers were on the gas door when I got it


If somebody could translate it....

Re: Octane rating for JDM engines

Wed Jul 09, 2014 5:39 pm

Small article on f1 race fuel. http://www.roadandtrack.com/features/we ... =soc_fcbks

Re: Octane rating for JDM engines

Mon Jul 14, 2014 4:48 pm

I found this info online. Some saying octane for jdm 205 is rated 100+ ron.

Ok Just to clear things up for you new guys or those who don't know much about the GT4's ECU the problem is the following.

The JDM gt4 base map is set for 100ron fuel, as such it is setup with a very agressive spark advance curve, i.e. as reves rise the ecu adds a hell of a lot of advance.

Now when you stick 95 ron fuel in the fuel is less able to withstand det and as a result det will start sooner than with 100ron fuel. The 3s-gte is what's called a det limited engine. This means that the maximum torque(which is what you map) you can achive on boost is limited by when the engine starts to det. Given the fact that 95 ron fuel starts to det earlier than 100ron fuel a map setup for 100ron fuel has the potential to cause a lot of det when run on 95ron fuel.

Now the flip side to this that some people will arge is that the gt4 ecu has some pretty advanced knock detection and avoidance built into it, and this will stop the engine detting on 95ron fuel. To some degree this is correct however and this is the big bit in order for the ecu to pull timing and avoid det it first has to detect it, which means it has to be occuring this is massively bad for your engine. Every mapper I know or have ever talked to agreed that it is better to setup the map to aviod det and then have knock detection as an emergency back rather than to map up to det and let the knock detection correct for it.

The reason behind this is that det is very effective at very quickly putting holes in pistions or destroying ringlands, even the mildest amount willeakend pistons and reduce their life considerably.

FYI everyone spouting that JDM gt4's have more power than UK ones, this is down to the mapping for 100ron fuel, as soon as you put a lower grade fuel in the ecu will start pulling timing and reduce the engines power most likely to below that of a UK car if run on 95ron as the JDM ecu will be pulling to be safe where as the UK ecu will be running to map. This is further reinforced by the results of my old UK ST205 which during a GT4OC rolling road day comprehensively beat the majority of JDM cars present putting down 271BHP and 260LB/FT not bad for a stock (Low power!!) UK car...

You pays your money you takes your choice...

Rant over hope everyone looking to buy finds a good example and loves them as much as I do mine... :D

This PDF claims 102RON
A European spec ECU tuned to run the engine on the lower
octane 95RON fuel as opposed to the Japanese 102RON
fuel. This is believed to be a slightly less “aggressive” fuel
map and as such the official power figure of the UK model
is 239bhp compared to the JDMs 255bhp.

http://www.gt4oc.net/forums/filebase/Hi ... _bodhi.pdf

I also found this formula.
( RON Octane Rating x 0.95 = AKI Octane Rating )
98 RON Octane x 0.95 = 93.1 AKI Octane (US measure)
100 RON Octane x 0.95 = 95 AKI Octane (US measure)

Re: Octane rating for JDM engines

Mon Jul 14, 2014 9:58 pm

Nice info! That explains very accurately what's happening under my hood... and that should be fixed!

Re: Octane rating for JDM engines

Mon Nov 03, 2014 6:44 pm

Monkracing found this stuff, and may try it out. I read some reviews online and some claim the described science may be misleading in some respects. The stuff may work, but it may not be the cheapest solution.

I plan on filling up with some shell premium next time I get gas, then I will reset the ecu and see how that does. I may also add my 104+ to get that one extra octane. I will likely save the 104 for any long trips I have in case the best gas is not available.

Re: Octane rating for JDM engines

Mon Nov 03, 2014 11:01 pm

This is a good fuel company who tends to offer high octanes. This like has a formula for mixing gas and the octane result. http://www.racegas.com/article/52

So by this formula in a total of 15 gallons, adding 2 gallons of 110 octane to 13 gallons of 93 you can get 95.21 octane which puts you over 100ron, which I imagine is good enough for our engines.

We often get these two questions:

Can I mix fuels?
What is the octane when I mix them?
The answer to #1 is easy. Yes, you can mix any of our race fuels. But remember – if your engine needs an unleaded fuel, you don’t want to mix leaded with unleaded. Doing so would make a leaded fuel which could still damage oxygen sensors and catalytic converters.

The answer to #2 requires a little math.

First you need to calculate the percentage of each fuel that will be in the final mixture. Then, use this equation to find your octane:

( [ % Fuel A ] x [ Octane of Fuel A ] ) + ( [ % Fuel B ] x [ Octane of Fuel B ] ) = Octane of Mixture

Here’s an example. Let’s say you mix 3 gallons of 110 with 2 gallons of 100 and you want to know the octane of the resulting 5 gallon mixture.

The percentage of 110 in the mix is 3/5 = 0.60 (60%).

The percentage of the 100 octane fuel in the mix is 2/5 = 0.40 ( 40%).

Plugging the information into the equation:

(0.60 x 110) + (0.40 x 100) = 66 + 40 = 106

So the octane of the resulting mixture is 106.

You can also determine the oxygen content of the mixture the same way – just use the oxygen content information we provide in place of the octane numbers.
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