Yes it all depends on how the car has designed. Most cars were tested and rated with 91 octane and then were released with "regualar" fuel required. This was the case with most Toyota cars until recently.

Lexus was almost always rated with premium fuel required. In recent years this has changed

Does anyone find it odd that the Lexus LX 5.7 is rated at 383hp with premium while the Tundra/Sequoia and most importantly Land Cruiser are rated at 381hp with regular fuel?

Would a Land Cruiser make 383hp if you pour premium? I am willing to bet that it would.

 

That's like the previous gen ES350. It was launched with premium recommended for 272 hp. while the 6 Toyota models using the same engine/trans were rated at 268-270 on regular.

 

Exactly. And only Toyota knows for sure if the current ES350 makes more HP with higher octane.

 

With a 2-4 hp difference, makes you wonder why they bother

 

Perception I guess. I think the 07 ES350 was 280hp was it not? Then SAE brought the rating down to 272hp, and then regular fuel brought it to 268?

 

I tried to be careful not to suggest that high octane fuel contained more energy, but apparently I failed. The point that I was trying to make (which I still believe is correct) is that if the fuel is ignited too early or too late, the amount of power that the engine generates from a given quantity of fuel/air goes down, because the pistons aren't in the most opportune location to take maximum advantage of the energy being released. That's what I was referring to with the drop in power/fuel economy.

 

If the fuel is ignited too early, the real problem, over and above any power/economy concerns, will be piston damage. Piston engines are designed (depending on internal engine conditions) to have the air/fuel mixture ignited a few degrees either side of TDC (Top Dead Center)...the point at which the upward-traveling piston reaches its highest point in the cylinder. If the fuel is ignited too late, all that will happen is a power/MPG loss, as the piston is already on the way down when the firing occurs. But if the piston is too short of TDC, still on the way up, the powerful downward-moving explosion (flame front) slams into the upward-moving piston. That is called "knocking" or "pinging". Light knocking, here and there, under heavy throttle and a loaded-engine, probably won't damage anything if it is not continuous and the engine is well-built with strong materials, but ongoing and/or heavy knock/ping can cause serious piston/connecting rod, and possible bearing, damage.

 

I wasn't aware of the 280 hp. rating but very well could have been.

When Lexus started recommending regular vs. premium in the ES350, the economy was poor, gas prices were climbing and most importantly, direct competitors were coming out with regular recommended, so it made sense to give up the 4 hp. advantage over Toyota models, to be more competitive in the market.

 

This is wrong information. Engine speed has nothing to do with compression ratio in many cars. Variable valve timing and phasing can change this, but octane rating has nothing to do with how fast an engine can spin. What you see is high revving engines use high octane fuel, but correlation is not causation. They are using it to achieve higher compression ratios.

I think you did a good job explaining that, but you still seem to be implying that different octane ratings burn at different rates. Inside an engine, this simply isn't true. If a higher octane fuel is ignited at the same angle as a lower octane fuel, they will generate roughly the same amount of power.

Ignition timing varies depending on engine speed. The faster the engine speed, the sooner the fuel must be ignited (generally), and the flame front velocity changes due to turbulence inside the cylinder. Modern engines with computer control can advance the ignition timing to the absolute limit of getting pining & engine knock, which older engines could not due so there were "buffers" in timing.

Pinging is caused by the increase in cylinder pressure from combustion, after ignition, causing auto-ignition in elsewhere in the cylinder, and the two shock waves colliding with one another. True auto-ignition almost never (as in never in a modern car) occurs. If it did, the consequence would be something like an engine exploding, rather than simply the increased wear caused by knock.

Note that this still causes the flame front to reach the piston too soon, robbing the engine of power, as you said.

 

Yes, I know that in modern cars, the computer will retard the timing when it detects pinging. I was just explaining what causes pinging and how it can cause engine damage.

That, BTW, is why American-spec engines sometimes lose a lot of power when using crap-grade Mexican or Third-World gas....a lot of timing-retardaton from the computer.

 

read what you just wrote. It means pretty much what I was trying to say but in a different non technical language. (please do not argue on the semantics. much appreciated.)

RPM determines HP, given that torque is constant through the range.
If they can control detonation (or rather self detonation) under the higher compression and temperature, then they get higher HP. Theoretically. Because as a result of adding the octane booster, energy value is reduced. Which means torque will be reduced, but as rpm increases, you are actually getter more HP.