[Healeys] Vapor lock gas

Bob Spidell bspidell at comcast.net
Sun May 9 13:16:12 MDT 2021

"... what's the general consensus on octane rating when refueling our cars?"

Use the lowest octane that doesn't cause the engine to ping/knock. For a 
stock BJ8's 9.1:1 nominal compression ratio 91 is plenty good, and 89 
generally works OK. If you've got higher compression, higher octane gas 
will be needed. Note with a 'hot' cam with a lot of valve overlap the 
dynamic CR may be less than calculated and, theoretically at least, a 
lower octane gas can be used (but if you've got a hot cam why would 
you?). My Mustang has a 12:1 compression ratio, but with variable valve 
and ignition timing will run on 87, albeit with a loss of power.

In the mountain west, i.e. the 'high desert' areas you often can't get 
better than 89 at most stations anyway because the air is less dense and 
CR is effectively reduced.


On 5/9/2021 11:29 AM, N.S. Bennett wrote:
> Thanks for the link to the MV article on ethanol and vapor lock. It 
> was a good read.
> Although I'm certain this has been debated (many times) before, what's 
> the general consensus on octane rating when refueling our cars?
> I've always used the highest.i can get. But reading that article has 
> once again made me question my practice.
> Interested in the list's thoughts...
> Cheers!
> Nathan
> '66 BJ8

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