What octane gas in a '78-'79 351M/400? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 12-25-1999, 10:21 PM
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What octane gas in a \'78-\'79 351M/400?

I'm looking at buying a '78-79 Bronco. Someone told me that you had to run high octane leaded gas in the 351M/400 engine these trucks came with. Is this true or can you run 87 octane unleaded?

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post #2 of (permalink) Old 12-26-1999, 08:23 PM
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Re: What octane gas in a \'78-\'79 351M/400?

I used to own a 78 Bronco with a 351M and drove it for 5 years using 87 octane.Never had any problems.I now drive a 79 Bronco with the 400M and again no problems with that gas.Even used it pulling a trailer with the engine under load and it seems to be fine.These engines are a bit touchy though - and will ping under load if not kept up to snuff in the timing and tuning department. Good Luck to you!

post #3 of (permalink) Old 12-31-1999, 12:07 AM
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Re: What octane gas in a \'78-\'79 351M/400?

I have a '79 351M/400 with a few add-ons (4bbl, intake and headers), and run 93oct Amoco to eliminate light load knock at 8 deg BTDC timing. At this setting, it will knock like heck with anything less. And my local Amoco station sells 93oct Premium for less than the 87oct regular every Tuesday and Saturday (about 1.27/gal)!
Stock 2bbl/iron intake '79 351M trucks were rated to run 87oct unleaded from the factory (they are emissions-equipped), but you have to mind the timing (factory setting 10deg BTDC). If you advance the timing with lower octane fuels to eliminate spark knock, watch your total advance at 3K RPM; you want to keep it within 35-38deg. Personnally, I would spend the few extra cents per gallon to save the pistons and eliminate the question.
This does raise another question: I have heard that lower octane fuels actually burn faster than high octane. Is this true? And, if so, wouldn't retarding the timing be more effective in eliminating pre-detonation? I would appreciate info on this.

'79 Bronco
'89 Trooper
'86 Trooper II
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