There have been a few posts here that referred to a need for high octane due to high compression. While this statement is based in truth, it's not the whole story. Many modern high compression / high performance engines use a knock sensor. If low octane fuel is used (or a crappy tank of high octane) then the knock sensor will detect this during combustion and will retard the timing until the knock is no longer present. This is how the power diminishes from using lower octane fuels in a car that can take advantage of the higher octane rating - from retarded timing. This protects the engine.
The ECU is continually trying to advance the timing for better performance until the knock sensor says to pull some timing back because of knock. This back and forth doesn't happen as often as the back and forth of trying to keep the A/F ratio at stoichiometric during idle and part throttle, so while the low octane will immediately decrease performance by the knock sensor activity, the higher octane might take several miles and/or several WOT cycles to advance the timing back to regain full power.
Which brings me to a question that I don't know anyone here can answer.
Background:
The timing available for the ECU to select from is on a table or tables stored on the ECU. It will constantly try to use the highest values on that table based on inputs from many sensors such as air intake temp, throttle position, knock value, and probably others. The ECU will keep advancing the timing until it hears knock from the knock sensor or until it has reached the highest possible value on the table.
So here's my question:
Does anyone know the NSX ECU well enough to know if there exists "headroom" on the timing table(s) for more timing and most of us have never hit the max value? Are owners in states who have 93 octane getting a little better performance than the owners in states who have only 91 octane at the pumps? Is there another degree or two of timing that even those driving with 93 in their tanks aren't getting to use?
When the NSX was released back in '91, California had 92 octane available. Then some time in the mid 90's we were stuck with only 91. While it's possible that Honda made changes to the timing table(s) for this, logic tells me that they said "just let the ECU do what it was designed to do and the cars using 91 octane will perform a little poorer". If this is the case, using higher octane than what's available at the typical corner gas station could benefit, at least here in California.
Oh, and also... I think that track cars can see a benefit from using higher octane than standard pump gas, especially in hotter climates. I don't mean this to say that the car will provide *more* power, just that it's less likely to lose power. A high-revving, high output, high compression motor driven at WOT for extended periods on a hot track on a hot day ingesting hot air will eventually start to lose power. I believe that no small part of this is due to the timing being pulled due to all that heat causing knock. With higher octane, the resistance to this knock is better and one can eliminate or at least extend the time until power loss is experienced.
just my $.02... which really isn't even worth that...
J
PS: all of the above is based on my experience with other ECUs I've come to know... I'm only assuming that Honda programs their ECUs similarly.
