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Discussion Starter #1
Hey Alfa Enthusiasts! I'm still new to this community– we've owned our Alfa for 1 year now and we have 7 months of winter (if that's saying anything). I'm looking for recommendations on octane levels for our '74 Spider. The highest octane we have in Montana is 91... is it worth it to pour in octane booster? Or is it all a waste of money and I should be fine with the octane level of 87? We live at an altitude of 4,793' and we often drive in the mountains ranging 7,000' - 9,000'+. Any insight and help is greatly appreciated!
 

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1973 Spider 2L
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I'd think 87 would be fine. If you want to get really sciencey you could buy an ethonal tester to check and see. I've heard horror stories of e85 being much closer to 65. You might see some benefits to ethonal free gas but I personally just use pump gas and haven't had many issues. I would recommend some fuel stabilizer before you store the car away for the winter.
 

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1974 Alfa Romeo spider
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This is the sticker on the fuel door of my 74 spider. Being an alloy engine, I think the more important thing is to avoid ethanol enriched fuels. Because of the alloy head, the valve seats are already hardened, so there is no problem there. Since the fuel injection made the engine relatively clean emission wise, there was no catalytic converter allowing the use of leaded fuel. I mention this because it means you are use aircraft 100LL. I would try to use a premium octane non ethanol fuel (look at stations that offer Marine or lawn equipment fuel) and add Marvel Mystery oil to the fuel for the fuel injection system. If you have a sympathetic private airport that allows cars to fillup, maybe you could do a half tank of 100LL and half regular non ethanol fuel.
 

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The problem with octane booster’s is they advertise for example, a four point boost, but that doesn’t bring 93 to 97, it generally bumps it up .4 yielding 93.4 octane fuel, hardly worth it, go for a blend of aviation fuel or leaded racing gas for a real bump in the octane rating.
 

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I guess it does to a point (no pun intended) but it’s always good to do your research.
 

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This is a single data point and from a different year car, so I leave it to you to judge the relevance, but the owner’s manual for my 1971 Spider specifically recommends the use of “premium” fuel.
I also think it’s relevant to remember that while today’s computerized fuel injection systems adjust the timing based on the octane of the fuel read by its sensors, the mechanical SPICA system of the 70’s is not that sophisticated.

Add in the altitude and I think that you’d notice a difference in performance. Please report back and let us know.
 

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It's harder to start a cold 105 engine with high octane fuel (e.g., 98 RON) - I've always found 91 RON starts better (from cold) and I have never noticed any difference in fuel economy or driveability between low (91) and high octane (96-98) fuels for my 105 GTV 2000 (1974). I guess if pre-detonation is a problem on 91 then perhaps a higher octane rating might be called for.

You're situation is a little different at 87 which would make me a bit nervous and I think you shiould be on the look out for signs of pre-detonation, or pinginning.

Other people on this list who have experience at your altitude may be better placed to provide advice - my experience is little above sea level - literally and metaphorically.
 

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Discussion Starter #10
This is super helpful @WA-Alfa! thanks for the insight on starting a 105 engine on higher octane... and I'll plan to cross lower octane off my list.
 

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Discussion Starter #11
Thanks @Lokki, I'll definitely keep the octane at 91 or higher and let this you know if I ever have the opportunity to fuel up at the airport.
 

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Gas is formulated by region. In areas like where there are two seasons .. real winter and summer.. the fuel suppliers work magic on the blends.. My cars don't drive over a 300 miles a season so I manage how much gas I replenish as I approach the end of the season I don't want to end up at the end of the driving season with gas that is 2 or 3 months old and then shelve the car for another 4 months. Octane ?/ Always the highest you can buy.
 

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I'd think 87 would be fine. If you want to get really sciencey you could buy an ethonal tester to check and see. I've heard horror stories of e85 being much closer to 65. You might see some benefits to ethonal free gas but I personally just use pump gas and haven't had many issues. I would recommend some fuel stabilizer before you store the car away for the winter.
E85 is essentially race gas in terms of detonation resistance (the reason why you want higher octane in the first place). I'm not sure what an "octane tester" would show, but most tuners will tell you that it has similar detonation resistance to around 110 octane gasoline. It allows a lot of boost and/or a lot more timing/compression than would be available with regular pump gas. The problem with E85 is that it is corrosive (need compatible fuel system components), less energy dense (motor needs more fuel, meaning you need to be tuned for it and will get poor fuel economy), and does not keep well (it's hygroscopic, meaning it absorbs water). I considered tuning for E85 in my Spider, but decided against it because I do not drive the car enough. You really don't want E85 sitting in a fuel tank for more than a month. It's also something I would only consider if you have programmable fuel injection.

As for what your '74 spider needs: that flap on the door says 91RON which is equivalent to 87 on the pump (fuel in Europe is typically rated by RON, but in the U.S., it is the RM/2 method. If you've hotted up the motor, it may need higher octane to avoid detonation.
 
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