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85 octane

Old 12-22-2018, 01:57 PM
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85 octane

Ive previously posted that my cars performance and mpg is the same on 85 or 91 octane (lowesr and highest in CO due to combustion in our elevation). Ive been going back and forth between both. However i saw a video on YT that stated if you premix, which i do, that you need to use hi test only. Any validity to that statement?
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Old 12-22-2018, 02:52 PM
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Since the premix slightly reduces octane, I suppose it is on the safe side to use a higher octane fuel when premixing.
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Old 12-22-2018, 02:55 PM
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good question... the octane rating refers to the resistance to pinging and detonation. the reason many run 91+ octane is because the high compression and rotary heat factor. being that premix is there to lube the seals thus creating LESS heat id reason it wouldnt matter which octane you use. but id be curious to see if theres something else im not thinking about? perhaps the premix has a "lower octane" persay, thus requiring 91. deffinatly looking forward to a responce from someone with more experience.
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Old 12-22-2018, 05:40 PM
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Originally Posted by Zeox View Post
good question... the octane rating refers to the resistance to pinging and detonation. the reason many run 91+ octane is because the high compression and rotary heat factor. being that premix is there to lube the seals thus creating LESS heat id reason it wouldnt matter which octane you use. but id be curious to see if theres something else im not thinking about? perhaps the premix has a "lower octane" persay, thus requiring 91. deffinatly looking forward to a responce from someone with more experience.
Renesis is high compression by NA rotary standards, but 10:1 is not that high. My old Accord Coupe V6 has a compression ratio of 10.5:1 and runs just fine on 87. Newer Mazda Skyactiv-G runs at somewhere between 13~14:1. Now that's high compression for an NA gasoline engine.

Given the rest of the engine designs, my theory is that the car is just tuned to run on 91 to squeeze out the most power from the engine.

Premix lowers the octane, but probably not by much if you go with the ~250:1 ratio recommended.
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Old 12-22-2018, 06:09 PM
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fair enough. that makes me wonder though if 91 is recommended for the lower compressions rotarys that premixed? granted our engines stock have very poor heat managment even for rotary standards

Last edited by Zeox; 12-22-2018 at 06:10 PM. Reason: my grammer and typing skills suck
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Old 12-24-2018, 04:04 PM
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I don't think the heat reduction due to premix is noticeable or even measurable. That's not where heat comes from.

The octane reduction is similarly not a factor. Run what you want, especially in rarified air, if you're not pushing the car. If you race it, high octane is cheap insurance.
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Old 12-24-2018, 04:33 PM
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Curious...in what way does hi octane act as insurance? I do drive the car very hard.
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Old 12-24-2018, 05:44 PM
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Old 12-24-2018, 10:05 PM
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Originally Posted by Petscar View Post
Curious...in what way does hi octane act as insurance? I do drive the car very hard.
If you're racing and running hot, always at full load, the risk of detonation is higher. Detonation at low load is meh, whatever. Detonation at high load is instant death.
So there's absolutely no win in saving a couple of bucks per tank by running low octane.
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Old 01-05-2019, 07:04 PM
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Been away for the holidays. Got to ask...detonation???? Are you saying the engine will blow up?
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Old 01-05-2019, 07:11 PM
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Engine detonation means you can have fuel ignite before you properly ignite it with spark plugs. That's very bad for the engine.

Knock sensors are there to adjust the timing so it will ignite the air fuel a bit earlier in the compression cycle, but that reduces power.
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