I just bought a 65 gt. The 289 is stock except going .30 over. The carb also has been replaced with a Holly600 cfm. Question is what fuel does everybody use? 91 octane, any additives for unleaded, etc. Thanks for any help.
If you got hardened valve seats when the motor was bored over, I would just use the highest octane unleaded that they have in Utah. (91 is the highest here.) If the heads have never been rebuilt, I would consider a lead substitute. I blew out an exhaust valve once because I didn't. I learn stuff the hard way.
I would also look to see if you can find gas stations in your area that carry 100% gas (no ethanol), it will burn better and give you better mileage to boot. Some states have 'outlawed'??? 100% gas so you'll have to find out. The ethanol will also slowly dry out your rubber and gaskets in the carb and fuel lines...
Octane requirement is more a product of altitude and compression ratio. Most of these motors were built to pretty low compression (8.0-9.0:1) by todays standards except the HiPo, Boss, etc. engines.
As Snake said, compression ratio determines octane requirements, the carb has hardly anything to do with it. You didn't mention anything about what CR you have. If it was done any time recently, a reputable shop would have installed hardened valve seats during an overhaul to +0.030. With hardened seats there is no need for any lead. It is illegal in the USA to sell real lead additives for street use and IMHO the 'lead substitutes' you can buy are nearly worthless so I wouldn't waste money on them. Its not a major problem, but if you have no hardened seats and you develop a problem you can always pull the heads and have hardened inserts installed later; its not an expensive job. My original C-code 289 has no hardened seats and I have yet to think it needs any although if I ever re-do the engine I will add them. Its runs on pump regular and has never used any additives. That is with 9 deg BTC timing which is slightly more than the factory recommended 6 deg.
You live at an elevation somewhere around 4400 feet above sea level. That has a huge impact on your octane requirements as well as the gas sold there. Your highest 'premium' is likely only 90 but you may not even need that. For a normally aspirated engine with no computerized controls, i.e. an old Mustang, the octane requirement decreases by 1.4 Research Octane Numbers for every 300 meters of altitude. Above 5900' the decrease is even steeper: 2.5 RON/300 meters. That means your octane requirements in Bluffdale are nominally 6 octane numbers lower than if you were at sea level. So the 90 octane in your premium pump is the same as if it were 96 at sea level. No one sells 96 octane in sea level USA! At 6150' my octane requirements are 9 octane numbers lower than sea level.
It depends upon what ignition advance you use, but at 6150' I use pump regular at 85-86 octane in another 289, 9.5:1 (also +.030 which raises CR slightly) or about the same as using 94 at sea level. Unless you have a higher CR I expect you can also use 86-87 regular which I expect is what they sell in Bluffdale. More octane than needed will only cost you more at the pump, the engine could care less.
As far as ethanol goes, if you aren't using it now you will be shortly. Federal rules are forcing more of it onto the market and I doubt you can find anything for sale that doesn't have it. Its super high octane so higher octane gas may be that way only because it has more ethanol.
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