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Octane effects on motor
- Thread starter volnut77
- Start date
Broncobowsher
Total hack
- Joined
- Jun 4, 2002
- Messages
- 35,710
Your lexus is a modern engine tuned for premium fuel. Tuned with hard parts (head design, cam grind, compression ratio) to get that premium car perfomance. Then knock sensors yank it away when it gets a tank of low octane fuel because it senses detonation and it has to back off the power to prevent it from damiging itself.
Now you are comparing that to an old engine designed to run on cheap 87. no knock sensor, engine doesn't have any detonation control (because it doesn't need it). In that case the performance is actually likely to suffer on the higher octane. Higher octane fuels typically have a lower BTU content, so less energy per given quanity of fuel. Normally offset by an engine that can use the higher octane will use it more efficently (more bang from the same quanity of fuel in higher compression ratio).
Higher octane also typically has a lower flame speed. but flame speed is also a factor of compression. So squeeze it tighter and the flame moves a bit faster. (for those playing along at home this is what vacuum advance works against). But if you don't have the compression needed to get a fast enough flame speed then power will suffer as the peak cylinder pressure won't be as high and/or in the right time.
Simply, use the lowest octane you can for the engine where it doesn't detonate and cause damage. Low octane in your Lexus detonates, the computer pull timing and fattens the mix to save the engine from damage. That is why it runs better on premium. Once you reach the level of octane the engine is designed for you can run higher octane and it won't run any better and will actually start running worse. Unless something special has been done to the 5.0, it will be happy as can be on 87
Now you are comparing that to an old engine designed to run on cheap 87. no knock sensor, engine doesn't have any detonation control (because it doesn't need it). In that case the performance is actually likely to suffer on the higher octane. Higher octane fuels typically have a lower BTU content, so less energy per given quanity of fuel. Normally offset by an engine that can use the higher octane will use it more efficently (more bang from the same quanity of fuel in higher compression ratio).
Higher octane also typically has a lower flame speed. but flame speed is also a factor of compression. So squeeze it tighter and the flame moves a bit faster. (for those playing along at home this is what vacuum advance works against). But if you don't have the compression needed to get a fast enough flame speed then power will suffer as the peak cylinder pressure won't be as high and/or in the right time.
Simply, use the lowest octane you can for the engine where it doesn't detonate and cause damage. Low octane in your Lexus detonates, the computer pull timing and fattens the mix to save the engine from damage. That is why it runs better on premium. Once you reach the level of octane the engine is designed for you can run higher octane and it won't run any better and will actually start running worse. Unless something special has been done to the 5.0, it will be happy as can be on 87
Regular 87 octane is fine as long as your engine isn't way souped up.
Steve83
Bronco Guru
Higher octane number = less energy in the fuel = lower fuel economy/power in an engine designed for low octane number
It will also INcrease emissions because it doesn't burn as well, which also means more engine wear due to blowby.
Read the TSBs in this caption:
It will also INcrease emissions because it doesn't burn as well, which also means more engine wear due to blowby.
Read the TSBs in this caption:
- Joined
- Nov 3, 2003
- Messages
- 49,459
Like you, I can tell the difference in some cars. And even back in the day, some engines needed premium to even run decently. But I ran both in my Bronco (stock '71) for years and really could never tell much of a difference between them.
I tended to run Premium for a long time after they removed the lead from it here in CA, but eventually started running low-test again, along with a lead-additive and, like I said, really never could feel any notable differences. And on mine at least, pinging or detonation was never an issue, even with the lowest octane and higher ignition timing.
Paul
I tended to run Premium for a long time after they removed the lead from it here in CA, but eventually started running low-test again, along with a lead-additive and, like I said, really never could feel any notable differences. And on mine at least, pinging or detonation was never an issue, even with the lowest octane and higher ignition timing.
Paul
