VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 octane selector.

What is meant by "octane selector" on a 33 vacuum advance distributor bracket/hold down? I don't understand.

The octane selector allowed the owner to fine tune the timing for the octane of the gas. Back in 1933 the octane could vary a low between different brands. If the engine "pinged" excessivly the owner could retard the timing as necessary or if wanted to spend the extra money for
ethyl or premium grade advance the timing for better power and mileage. Corect use of it is described in both the owners and shop manuals.

I don'tn have access to that information. Are the markings equal to degrees on the flywheel?

The manuals are available from the Filling Station for a very low price.

I bought the manual. There is no mention of the question I asked.

Yes the octane selector makes a great difference in starting, too. Yes it is similar to degrees on the fly wheel. (but you will not find any on there) Placing a timing light to watch the ball and indicator needle through the square on the fly wheel housing will show a difference as the octane selector is adjusted. It is actually retarding or advancing the spark. Make sure that proper lubrication is done to the "slide" below the indicator. This is moved by your vacuum advance. If it hangs up it can reduce your advance. With today's gasoline I have found for my car that +8 is perfect.

I do not have access to a 1933 Owner's Manual. Here is the information from the 1932 & 1934 Owner's Manuals.

1932 Octane Selector

1934 Octane Selector

The instructions may have been printed in late 32 for the 33 models.
"1932 Instructions for the Operation and Care of Chevrolet Motor Cars

EAGLE Series CA Passenger Models"

Copied from the header of the page in the owners manual.

1932 does not have an octane selector but does have a manual cable controlled method to retard the timing for starting.

Rusty,
Actually the 1932 dated owners manual was for the 1933 (CA) models.

What Steve Smith was asking is covered in that manual and further explained by Gene's post. Whether the numbers matched the degrees of advance or retard or are just reference numbers shouldn't make any difference. Set to the position when the idle speed maxes and there is no pinging or hesitation on acceleration.

Too bad that the manual does not specify the type of Octane figure they were using. Research, Motor, etc. had all been established by then. Currently in the US, the pump shows R+M/2 method.
I found a history using an Anti Knock Index (AKI) which might be the same as R+M/2. https://www.energy.gov/eere/vehicle...ne-compression-ratio-and-gasoline-octane
The AKI for 1933 fits squarely in the Zero setting per the manual. If so, then any modern fuel would so far exceed these figures, that perhaps the timing should be advanced a bit, then use the octane selector for fine tuning. Thoughts on that?

The manual also stated that each mark on the octane selector represented two degrees of timing. Therefore advancing the selector to 8 would be 16 degrees of advance. Wow!

Gene has written about octane value in the past maybe he will again.

thank you all. 36Pheaton finally answered my question

Chipper: Thanks for the correction about the online owner's manual.

Steve: I agree that the space between marks equals 2 degrees of timing. I also note that the numbers count by 2 to reflect this. So 8 does equal 8 degrees more advance.

Yes Rusty, that makes sense. It would have been clearer with a picture. Still, with modern fuel, if you set the timing to the proper mark, then the selector would probably end up being turned to full advance. It would seem reasonable to set the timing six or eight degrees advanced, then the selector could start at zero with room left to adjust either way.

Hi 36 Phaeton,

Thanks for the response. I agree that even "regular" 87 octane gas will allow full static advance beyond the 5 degrees set with the ball and pointer on the flywheel for most of these older engines. The relatively low compression and minimal cam timing overlap allows that.

The only limitation I have encountered is "kick-back" during hot starts with 10 degrees added at the distributor. If I back it off to 8 everything is fine.