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Old 10-23-2001, 10:36 PM
MikeTangas's Avatar
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Yet another octane question

I read through all the prior octane posts, and couldn't find the answer I seek.

Will the engine run cooler using higher octane?

I ask, because the price of hi-test (92 or 91 whichever) has plummeted here by over $.60/gal. In light of that I splurged and filled with hi-test rather than mid-grade this past weekend. Car is running like a screaming banshee, I mean that 4.5 really likes this stuff. Anyway, I noticed, starting Sunday, that the coolant temp has been consistently 5+ degrees cooler than before (now showing right about 185 rather than 190).

Coolant has shown a constant 190 or so since reworking the radiator, hoses and T-stat. I have run this engine on all grades of gas without any knocking or pinging, seemed to run good on mid-grade too. But the way this thing likes the hi-test, I'll probably just keep burning that - WOW .

PS: Mileage is looking great too! A little over 200 miles, combination driving and still above 1/2 tank.
Mike Tangas
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Old 10-24-2001, 01:15 AM
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well, i'm not an expert, but i don't really see how higher octane would cause an engine to run cooler. a higher octane rating means that the flashpoint (ignition temperature) of the fuel is higher.

by raising the flashpoint, the fuel mixture detonates a little later. without a sufficient octane rating, the fuel could detonate before the piston reached tdc (aka pre-detonation) causing the common problem of knocking.

another common effect of using too low an octane rated fuel is dieseling of the engine when the ignition is shut off.

that all having been said, i can't really see how raising the ignition temp of the fuel would contrbute to cooler water temps. unless it is because the fuel is more completely burned in the combustion chamber and there is less residual fuel making it out into the exhaust before it's completely burned away?

like i said, i'm not an expert
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Old 10-24-2001, 08:09 AM
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The 1973 280's were made to run on leaded hi-octane fuel. I'm sure that timing, fuel mixture & engine condition all are factors in engine temperature. If your engine runs cooler with preminum then you should stay with that.
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Old 10-24-2001, 01:33 PM
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FYI: There's a great article in this month's issue of Car & Driver about octane ratings. Some surprising results!
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Old 10-24-2001, 01:33 PM
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There just happens to be an excellent article in the current issue of Car&Driver. Here is a link to the online version – Enjoy…
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