To answer the earlier question about the ECU advancing timing...

Using a semi-closed loop system, The ECU will retard timing as knock appears, and it will advance timing back as knock disappears, slowly but surely until either knock appears or it is back to the default timing. The key here is once the ECU has advanced back to the default value, it will not advance any farther. If there is zero knock on 93 on a sunny day, you are getting the best performance out of the engine possible, and placing a higher octane will not increase power (not getting into the oxygenated argument here). Don't make the mistake that the ECU will continue to advance timing beyond the default until it starts to knock, always giving you the most power possible for each tank of gas... it doesn't work that way.

Just as an example (from a 2008 Corvette) there are High octane and low octane timing tables that are selected based on knock sensor output. Without tuner intervention, the high octane table is going to give you the most timing advance adjusted for commonly available gas octane, but no more. Tuners can and will build Very high octane tables to take advantage of 100 octane race gas. Usually the only differences with these tunes are the timing adjustments. More octane + more timing = more HP

High:
http://images33.fotki.com/v1185/phot...anehigh-vi.jpg

Low:
http://images34.fotki.com/v1195/phot...tanelow-vi.jpg

From what I have heard, 100 octane in Japan is like the 91 octane here and 91 octane there is like the 87 octane here. And since the z is a "world car" the ecu should compensate and performance should improve with the highest octane the car was designed to run on which is 100 octane.

Not sure how true that is

The US doesn't use Octane, we use AKI, average knock index. You cannot compare it to the rest of the world

We do refer to octane, but it's rated a bit differently -- RON+MON/2 = AKI, whereas most of the rest of the world uses RON.

You can get a close approximation on RON vs AKI tho'; typically .95*RON = approximately equivalent AKI.

Example: The Z recommended octane is 91 AKI and 96 RON

96 RON*.95 = 91.2, very close to the AKI value.

Yeah, that's close.

If the RON is 100 in Japan, 95~ AKI

My money would be something like 94 on America's AKI system because that's probably the highest you CAN find at a normal pump in the US (I think I've seen 94 at the pump in Ohio, or Illinois, or Michigan)

All this had to do with octane wars years ago (1970's ??). This ended once they settled on the RON+MON/2 = AKI formula.

I'm sure Nissan considered all this when they put parameters on the ecu so that it could be a world car.

So can a z34 run "100 octane" or whatever it's called from the local 76 station pump here in So Cal and will it improve performance?

When I had my 240sx with an sr swap it ran ok on 91 but felt much livelier when I put in 100 but the vq is a different motor. Sorry for the noob question just trying to learn. It's funny google actually pulls up better results than the forum but I haven't found a thread that confirmed anything

I doubt it. And you'd probably "gain" only if you reset your ECU too

Basically, if it wasn't running full advance, it would be more likely to do so on higher octane fuel.

And I say "more likely" rather than definitely because other sensor data affects how much advance it runs (e.g., oil temp).

Of course, you could always fill it up with a full tank of >100 AKI, and with Uprev try and dial in more advance, then save it as a "track day/High Octane" tune...

Yes, for sure, that is a different story. You can of course do that.

full tank of 100 octane is definitely overkill unless you have a tune for it. but i throw in a few gallons to hit about 93-95 octane whenever i go to the track or do some spirited driving :)

When I track my S14 I advance timing and throw a bit of 100octane in the tank. It seems to pull harder and doesn't knock.

You can buy 100 octane at the pump at certain 76 stations

F = M * A only when the mass of the system is constant. In general it is F = dP/dt, the time rate of change of the momentum. The rocket equation is a good example of a variable mass system, something you should be familiar with from your engineering dynamics class. Jim Nesta - B.S., M.S. in physics and retired college physics instructor.