Pouring unleaded versus premium
#16
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.
Last edited by Toys4RJill; 05-14-14 at 07:42 PM.
#17
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.
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.
#18
Exactly. And only Toyota knows for sure if the current ES350 makes more HP with higher octane.
#19
#20
#21
geko29 did a good job in paragraph 2, so I won't reiterate that. But paragraph 3 is incorrect again. Different octane ratings have nothing to do with energy density, just as they have nothing to do with how fast fuel burns. The whole reason higher octane fuel will not increase the power your engine generates is due to the fact that the energy density does not materially change between octane ratings. Engines that require higher octane fuel simply operate more efficiently, as in they extract a higher percent of the energy contained in the fuel. The efficiency of an engine burning gasoline is directly related to the compression ratio, and higher octane fuel allows an increase of the compression ratio before autoignition of the fuel.
#22
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.
Last edited by mmarshall; 05-15-14 at 09:02 AM.
#23
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.
#24
goes back to HP vs Torque.
A higher octane fuel enables the engine to spin to a higher rpm without misfiring (ie igniting on its own before the spark plug fires). This will result theoretically in higher HP. Now if your car is not misfiring and spin easily upto the limit, I suspect you will discover any gains.
A higher octane fuel enables the engine to spin to a higher rpm without misfiring (ie igniting on its own before the spark plug fires). This will result theoretically in higher HP. Now if your car is not misfiring and spin easily upto the limit, I suspect you will discover any gains.
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.
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.
Last edited by Infra; 05-15-14 at 12:28 PM.
#25
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.
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.
#26
goes back to HP vs Torque.
A higher octane fuel enables the engine to spin to a higher rpm without misfiring (ie igniting on its own before the spark plug fires). This will result theoretically in higher HP. Now if your car is not misfiring and spin easily upto the limit, I suspect you will discover any gains.
A higher octane fuel enables the engine to spin to a higher rpm without misfiring (ie igniting on its own before the spark plug fires). This will result theoretically in higher HP. Now if your car is not misfiring and spin easily upto the limit, I suspect you will discover any gains.
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.
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.
Last edited by chikoo; 05-16-14 at 10:21 AM.
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