Fuel Octane - Vintage Mustang Forums

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post #1 of 6 (permalink) Old 03-13-2008, 11:59 AM Thread Starter
Join Date: Sep 2003
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Hi All,
What with gas prices ( ) the way they been going up lately, I would like to know which grade of gas Mustangs need:

Originally, my Stang had a 289 2 barrel carb
Now, it has the same engine with a EB intake manifold and 650 CFM 4 barrel Holley carb.

I've been using premium (octane 91, as I remember). Could I use a lower octane gas?

What engine or other parameters dictate which grade of gas to use?


65 Coupe - White w/ Red Interior - Holley Manifold and 650 CFM Carb - Dual Flowmaster2C - 289AC - Dunlop GT Qualifiers - SS Power Drum brakes
97 Mitsu (her daily)
01 Solara (my daily)
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post #2 of 6 (permalink) Old 03-13-2008, 12:04 PM
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i would say test the waters.

Try 87 for a tank and see if it starts knocking.

If it does, try 89.

If 89 knocks and detonates then you can try an octane booster, but thats just as expensive as just buying 91 octane

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post #3 of 6 (permalink) Old 03-13-2008, 12:10 PM
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I have to mix race fuel, either 110 or 114 with 93 pump gas in my GT 500; goal of around 100. I tried straight lead but one quart per tank does not do the trick. BTW the engine is a 427 sideoiler, and the CR is 11.1 to 1.
I run 93 in the 289 HiPo.
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post #4 of 6 (permalink) Old 03-13-2008, 09:39 PM
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The biggest thing on what octane you need is what compression does your engine have?If your down around 9 to 1 you can get by on the cheap stuff. Get up to 10 to 1 you better get the 91 octane. Up around 11 to 1 you better be mixing in some racing fuel. Also backing down your timing will help if your engine starts to ping.

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post #5 of 6 (permalink) Old 03-14-2008, 04:39 PM
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Originally Posted by rtmc67shelby
I have to mix race fuel, either 110 or 114 with 93 pump gas in my GT 500; goal of around 100.
I run 93 in the 289 HiPo.
I do the same. I do 1/2 CAM2 and 1/2 93 pump gas.
But CAM2 is getting way out of hand$$$$$$$

1966 Mustang Coupe
Ford Racing 5.0 HO Roller Motor/B303 Cam/1.7 RR
TFS TW 205 Heads/Ceramic Coated Tri-Y Headers
Edelbrock Performer RPM/Holley 650 DP
8 qrt Milodon oil pan/Griffin aluminum radiator
Mallory Distributor/MSD/Holley Electric Fuel Pump
2.5" Exhaust with H pipe/2 chamber Flowmasters
Aluminum Drive Shaft + Drive Shaft Loop
Auburn 3:55's rear/SSBC disc's/C-4 wTCI 2800 stall
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post #6 of 6 (permalink) Old 03-14-2008, 09:15 PM
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I use lead additive and Premium gas. I can use unleaded but the premium lasts a bit longer. I will use about half a tank driving 20 miles with unleaded. With premuim, it lasts almost 3 times longer.
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