It's probably been asked a thousand times but I can't find a search feature on the forum...

Engine is a '59 Fairlane 292- as far as I know it is stock, so according to my book, it's 8.8:1 compression.  I know a lot of people subscribe to the thought that they only run 91-93 octane in older engines.  I've been running 93 in this since I got it but don't think I need to.  There is one school of thought that said to divide your CR by 10 to get the octane you should be running.  That doesn't work past 9.3:1, though.  

What are your thoughts?  

You should be able to get by with 9.0-9.2:1 on 87 octane with your Y, and with a more modern head you can run up to ~9.7:1 compression on the 87 octane. I run regular in my Fox Mustang GT with 9.7 and no issues with pinging, but I have ported aluminum heads, etc.  If you have 89 octane, try that and save a few dollars.  Joe-JDC  

JDC

Don't you mean divide the octane rating by 10 to get CR?

My two cents worth on the subject.

Carbureted engines are in a different playing field versus those that are fuel injected when it comes to gasoline with added ethanol.  Also keep in mind that the dynamic compression ratio is a more accurate means of determining the octane requirements for an engine rather than the static compression ratio.  The static compression ratio is the number that is often mentioned when talking compression ratios and that number is quite variable when it comes to the fuel octane requirements.  Only by injecting the camshaft specs and to a small measure the connecting rod lengths into the compression ratio calculation can the octane requirement be more accurately determined.  With those extra values you have stepped forward from the static compression ratio to the dynamic compression ratio.

Pay me now or pay me later.  Cheaper costs at the pump does not necessarily mean the fuel will be less expensive in the long run.  What must be kept in mind is that the 87-89 octane gasolines have a higher ethanol percentage than the premium grades.  Most gasoline pumps say “Contains up to 10% ethanol” and not “Contains 10% ethanol” and that’s a clue that there’s something else going on in the back ground.

Using the latest blends of premium gasoline in the older carbureted vehicles has more to do with the reduced amount of ethanol in the premium grades rather than the octane rating.  Running the 87-89 octane gasoline with 10% added ethanol in the older vehicles is an issue from both a shelf life perspective and with the corrosive nature of the ethanol itself in the fuel systems and the engines.  By using the premium grades (90-93 octane), the ethanol content is reduced to the 3-5% level which in turn reduces the harmful effects seen by using the 10% and higher levels of ethanol laden gasoline.  The higher level of ethanol in a carbureted vehicle is also a contributor to reduced fuel mileage and in some instances, hesitation and/or drivability issues.

The compression ratio comes into play only when the dynamic compression ratio gets on the high side.  A 292 with 9.0:1 static compression ratio using a stock 1958 and the later model low duration camshaft can have a dynamic compression ratio as high as ~8.5:1 and that’s still too high even for premium grade fuel.  Simply taking that same engine and installing an Isky E4 camshaft suddenly makes that same engine 87 octane fuel capable.  But these lower grades of gasoline will be maximizing the mandated limits for added ethanol which is not going to store well in a vehicle that is not having its fuel refreshed every 90 days or less.

Lorena, Texas (South of Waco)

A calculator on is on the KB Pistons or UEM Pistons site if you know your engine internal dimensions. I have used it many times to determine how many cc’s I need on a piston top for compression ratios when having custom pistons made. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I just watched a video on engine masters earlier today.    They dynoed 1 engine on different grades of gas starting with 87 and going up to racing leaded fuel.   Then did it with E85.   Not quite real driving,  but interesting  non the less

That "up to 10% ethanol" might soon be 15%, because President Biden lifted the ban on 15%. If we have to modify our fuel systems for ethanol compatibility, E85 might be the way to go, if you can find it. I know of only one station in my town that sells E85. Of course, it will adversely affect your mpg, but it's cheaper (so far). As far as benefiting the environment, it seems the jury is still out. 

Once my engine is rebuilt, the dynamic compression ratio should be below 8:1, but I will run 91 premium, the highest readily available in California. Most stations no longer offer 89 mid-grade. 

1960fordf350- I just watched that same episode!  That was kind of where my question came from.  They seemed to get no benefit from pump gas over 87 octane for their application and I've often wondered if 93 was overkill for mine.  I'll keep with the 93 just for cheap "insurance."

Using more than your engine requires is just putting money down the filler tube. Depending on which grade, you might need more/less advance.

I’ll suggest that you perform an ethanol percentage check on your fuel and then use that as a determination if you would prefer to run the premium grade over the lower octane grades.  It’s more about the ethanol percentage than the octane rating when it comes to using the premium or higher octane grades in the lower compression ratio engines.  By design, the premium grades have a lower percentage of ethanol than the lower rated octane blends.

Lorena, Texas (South of Waco)