Should you run premium fuel? Yes and no, explained.

See this topic raised all the time and I thought maybe I could explain it so everyone will understand what is going on. Maybe a moderator could sticky it?

PLEASE take your time and read this thoroughly, even twice if you have to. I realize it's quite a bit to take in for some people without a performance background.


WOT= Wide Open Throttle
TDC= Top Dead Center (Where the piston is at it's maximum height during the compression stroke)

I'm going to use 87 as regular, and 93 as premium. I realize that in different parts of the country there are different ranges, so you'll have to apply it to what you have.
First of all, octane is the measure of the fuel's resistance to burning during the combustion process in the cylinder.
So 87 burns more easily than 89 which burns more easily than 93.

The only reason for high octane is to make maximum horsepower and torque because of it's resistance to pre ignition, otherwise known as knock, or pinging.

Our engines make the most power at high rpms, close to redline, and obvoiusly at Wide Open Throttle (WOT). Under those conditions, pre ignition will occur with low octane fuel, which will be detected by the knock sensor. The ecu(computer) will then retard timing, preventing pre ignition, but also holding back Horse Power and Torque.

The main reason we see timing pulled on a high octane rated engine is because of compression. Most Lexus engines are high compression. The high compression requires that the fuel not "blow up" before it is scheduled to, because the compression itself will cause the fuel to want to burn, even without a spark. Low octane fuel will start to burn too far before the piston reaches TDC. This produces knock or more properly pre ignitin. If allowed to burn uncontrollably, this will stress the piston and rod to the point of failure, as the rod is being phyically forced UP by the crank , while the combustion reaction is trying to force it DOWN. This is how we see rods sticking out of the side of the block.
As a safeguard, the knock sensor sees this pre ignition and informs the ecu. The ecu responds by retarding ignition timing; making the spark occur later in the compression stroke. This will prevent the low octane fuel from burning ahead of time, saving the engine from damage while restricting power.
Over the course of a few drive cycles, the ecu see the continued pre ignition, and places the timing map on a permanent retarded state so that it won't be continually pulling timing from the high performance map.
If you then start using premium, the ecu will see the lack of knock, and slowly start giving timing back. Or if you know you will be using 93 exclusively again, you could reset the ecu which will start out again on the high performance map. (Resetting of the ECU is accomplished by pulling the EFI Fuse in the engine bay)

For maximum power, we want the spark and combustion process to occur at a precise moment before the piston reaches TDC. This is what is known as ignition timing. Too far, and we pre ignite. Too late, and we lose power and torque.

To is it ok to use 87? Yes, if you just cruise along all the time and granny drive, then you can use 87 all the time with no ill effects. If you don't go full throttle to redline regularly, you'll be fine. Just don't expect maximum power if you one day need to floor the throttle to get out of the way of a semi truck.