Sorry for dredging up an old thread, was searching for something else and apparently I lost track of this the first time...

Originally posted by ayellowone:
Agreed, but this is very highly unlikely on a stock SC Xterra.
Your basing this on testing (please site source or methodology) or just pulling this statement out of your ass because it sounds good?

Especially since I have tried 87 octane with no audible detonation.
Your engine is just as likely to be damaged from inaudible pre-detonation as it is by audible pre-detonation. Just because you can't hear it doesn't mean it's not going on.

When functioning, your knock sensor can hear it - but I wouldn't want to rely solely on that since there isn't an easy way to tell when your knock sensor is functioning properly either.

It's there as a fail safe. Those are Nissan's words, not mine.

To me fail safe does not equal everyday normal operation.

What you are describing is what would happen to a modified engine (exhaust, intake, pulley, etc.)without the proper reliability mods (MAF sensor, pump, ECU programming, etc..)
Um, it's simple physics. When you compress air, it heats up.

Period. Boyle\'s Law .

A supercharger under boost increases the temperature of the air/gas mixture. If the octane of the gas is low enough and the air temperature present is high enough, the gas spontaneously combusts (pre-detonates, knocks, whatever you want to call it). As was pointed out earlier in this thread, that is the principle that diesel's work off of.

If it pre-detonation occurs at the wrong time (say when the piston is still traveling up to compress the air/fuel mixture before it's ignited by the spark plug) damage can occur.

This has nothing to do with the ECU, MAF, etc. Those devices can not change the combustion properties (octane) of the fuel - only your choice of fuel's octane content can affect that.

So you may get lucky and be lucky for a while, but all it takes is one bad pre-detonation and you just blew a ring, blew a seal, blew a valve, blew a valve sleeve or blew a hole in the top of your piston - that explosion has to go somewhere. When things are working properly, the explosion pushes the piston down and it's converted to work - but if the piston is coming up, something has to give. Usually it's something expensive.

That's also why you have a harder time with pre-detonation in the summer - the ambient air temperature is already higher, so the compressed air/fuel mixture is also pushed higher by your forced induction and you are again pushed to that threshold of temperature of the mix vs. the detonation point of the fuel being used. If they cross, the fuel ignites. That's why cars with higher boost superchargers or turbo's have intercoolers - to cool the compressed air/fuel air, because even with higher octane there is a point of diminishing returns. Eventually the only way to solve the problem is by controlling temperature.

Again, you can not change the laws of physics (at least not easily enough to where it would be practical outside of a lab and for durations longer than fractions of microseconds).

I highly recommend you get a copy of Maximum Boost by Corky Bell . If you are going to be fooling with things you don't understand, you might want to get educated before handing out bad advice to people.
Murderous Fire!