I have an old carbureted engine with no electronics (beyond spark plug, fuel pump, and wastegate control). At 10psi of boost, it needs 91 octane. In earlier versions of the engine, before the turbo was installed, it needed 87 octane. Unfortunately, the rubber and sealants used are known to be ethanol sensitive, so the engine needs ethanol-free gas. We can only get ethanol-free 90 octane locally, and I wonder about how to think about octane rating vs. boost.

Knowing that 0psi of boost needs 87 and 10psi needs 91, is there a boost pressure which would be safe for 90 octane usage, e.g. 7.5psi, or is that not the way turbos work?

  • Was the other engine setup with lower compression? Early turbocharged engines had compression ratios down to around 8:1 before EFI systems began to replace carburetors. Map sensors for turbo engines measure vacuum and pressure to feed data to the ecm, determining perfect fuel mixtures otherwise we couldn't have the modern muscle cars with several hundred hp, with or without turbos or superchargers. There's a way to remove ethanol (search). Adjusting the blowoff valve to reduce boost requires some knowledge, perhaps from tuning forums.
    – F Dryer
    Oct 3, 2023 at 16:54

1 Answer 1


There is no real (or linear) correlation between boost versus octane. Gasoline with greater octane rating is used in turbocharged (as well as supercharged) applications for the simple reason to help prevent knock. Because turbochargers make a bigger bang when ignition occurs, there is a higher risk of knock due to the higher pressures and higher heat created due to the boost. Your vehicle being old school, it cannot adjust for knock like newer cars can, therefore you need higher octane to help prevent it. It's not that it needs a certain octane, it's that it needs to be above a certain octane. As you suggested, that octane is 91 for your vehicle. If you ran higher octane fuel in it, it would be just fine.

Two things you should realize ...

First, most gasoline sold on the US market today has about 10% ethanol. This amount should be safe for even older vehicles. Running this in your vehicle should not give you any issues. It is recommended you do not go above this amount. Even 15% will start to degrade your vehicles rubber parts. As you've found out, finding fuel without ethanol is a chore these days. (Note: One place you can find it is at marinas ... I don't know if they'll have the octane you need, but they sell ethanol free fuel there ... mind you, you'll probably pay about twice as much as you're used to for it.)

If the above doesn't console you, the second thing you should start thinking about is upgrading your soft parts to the kind which can withstand even 85% ethanol, that way you won't have to worry about it any more.

  • Thanks. Agreed and understood about octane minimums and ethanol's almost universal penetration into the US market. Unfortunately, ethanol-compatible parts are simply not in the cards. Oct 4, 2023 at 0:40

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