High vs Low octane fuel.

[KinLoud]

I've done some thinking about why some engines run a bearing or throw a rod when low octane fuel is used

My personal High vs Low Octane theory (and how it relates to dead engines):
Part One...
Most ecu's will retard the timing when knock occurs. However, it must the take the ecu some period of time to detect the knock and retard the timing.
If the engine does 10 revolutions before the ecu acts, a very tiny bit of damage might occur. If this happens once every time you drive your car the damage will accumulate and might eventually lead to a premature failure of the engine.
Part Two...
Many modern engines are built with a high compression ratio. Often this compression ratio is too high for the fuel that is recommended by the factory. The factory does this deliberately because...
The high compression gives low fuel consumption when the car is cruising, this makes the car more economical. If higher power is used the ecu is programmed to either retard the timing at higher power settings OR the ecu retards the timing when it detects knock OR both.
If it waits for knock to be detected then we might get the problem I mention in part one of my theory.

Comments please (please think before you post... carefully read everything I've written first).

Ken
Hamilton
021 408 863

[sergei]

And if it is knocking and retarding ignition (and sometimes dumping more fuel) engine consumes more fuel, and fuel milage is worse than with higher octane, thus running on higher octane enignes built for high octane is cheaper even that the high octane is more expensive per liter.

[barryogen]

And if it is knocking and retarding ignition (and sometimes dumping more fuel) engine consumes more fuel, and fuel milage is worse than with higher octane, thus running on higher octane enignes built for high octane is cheaper even that the high octane is more expensive per liter.

indeed this is where I am with the 2zz in the Runx(comp ratio=11.5:1), with BP98 I get ~550kms of a 55l tank, with BP95/96(not sure what it is) I get ~510kms, from 91 I get ~470kms.

[RedMist]

Ken, as none of us are privy to the Toyota ECU code it's a little hard to speculate as to what is happening in regards to knock.

I however suspect the following.

Baseline ignition map is built up on a dyno, possibly of several engines then averaged.

Then a knock based map is generated from knock data on running. It modifies the baseline map by several degrees set within limits to ensure bollocks data doesnt remap your car.

Its how this knock data is modified that worries me. I suspect that its not modified frequently and that historical data becomes the benchmark. As such an engine running on Jap gas, in Jap temperatures, in Jap traffic will have a highly advanced knock map. Which isn't applicable to our conditions. When knock occurs here, its retarded via closed loop control, but the event doesnt modify the knock based map, or possibly out of bounds barriers dont allow it to modify it enough. So knock will occur again, under the same circumstances whereas if it had modified the knock map it wouldnt.

The other possibility is that the out of bounds limits, that confines how much the knock map can modify the original baseline map, isn't big enough to cater to our conditions. IE the knock map is sitting at -5 degrees and thats as far as it can go... and its still having to implement closed loop knock control.

Am I talking shyte?