VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Octane Selector Setting

I'm going to do a tune up on my 235. After reviewing my shop manual, I see it recommends setting the "octane selector" to zero before adjusting timing. I assume this pertains to running regular gas.

I'm running 91 octane (as that's the only gas available in my area without Ethanol). Should the octane selector be set differently when running a higher octane than 87 (regular), and if so which way??

Just curious if anyone has played around with this setting.

Set the octane selector to the center position. Then set the timing to the recommended point. Then set the octane selector to the full 10 degrees advanced. If the engine turns over "hard" reset octane selector back to 8 Deg.
When your car was new it was desighned to run on 70 octane regular gas. As you can see your 91 octane is much higher than even preimum was when the car was new.
Preimum at that time was called ethyl and contained tetraethyl lead to raise the octane. The regular grade of gas did not contane tetraethyl lead. It was unleaded.

Thanks Gene. Yeah--I wasn't sure whether to advance or retard and by how much. You've answered my question.

I assume if the engine still turns over hard @ 8° I should nudge it back to 6, etc until it starts easy--right??

Right....but if that is necessary there is another problem.

So it shouldn't be advanced less than 8°??

(I've had it at zero since I got it running and it always started right up at zero. ???)

Rick,
It will start fine at zero, the higher the advance, the higher the chance of a slower, more labored, slower, sounding crank.

Yes, it can be advacnced to 8. That is where i run my '50 235. If I go to 10 the starter "binds" some times.
If yours binds at 6 there is something wrong with the starter or cables or connections. Same for binding at 8 Deg.

As I have said before, it has been a long time since I have worked on these cars. What is an octane selector?

It is a device located under the distributor that enabled the owner/driver to adjust the timing according to the grade and quality of the gas being used. Years ago gas varied very much in octane from brand to brand and when you got a low grade the engine would "ping". Also if you used "Hi-test" or "Ethyl" (now called preimum) (ethyl was the only grade that contained lead) you could advance the timing for better performance.The octane selector was marked in degress s the owner could tell where he was going and could experiment and still get back with not using a timiing light. Used from 1934-1962.Some other GM cars also had this feature.

How important is it to disconnect the vacuum advance line when setting the initial timing ?? I believe the manual tells you to do that but I'm guilty on not doing that every time.

Dick

No mention of disconnecting vacuum line. See link below.
The stock carburetor, has no vacuum to the vacuum advance at idle speed.
The vacuum port in the carb is above the throttle plate.
So, no vacuum at idle speed.

http://chevy.oldcarmanualproject.com/shop/1949_53/06engine/6_029.HTM

Very important to have the engine at slow idle when setting timing with a timing light. `If runnning faster the throttle plate could be exposing the vacuum advance port in the carb. throat or the advance weigts could be activated.

Ok I see the "degrees" markings on the plate where the distributor goes into the block with the bolts hold it in position

What I don't see is the "octane" adjuster with any kind of markings on it. Are you talking about the big silver colored knob which mounts horizontally below the distributor cap? If so where are the markings?

The big silver colored knob is the lubrication device for the distributor shaft. If you unscrew the knob you will see it is filled with grease (or should be) and when the knob is turned down tighter every 1000 miles with each chassis grease job. You should read the owners and shop manual sections pertaining to lubrication. There are many things that require frequent lubrication, items that have beenn long eleminated on modern cars.
There is a little "point" that points to the degrees. It is on the plate on top of the degree plate.

Gene is correct.......check out your Owner's Manual and your shop manual for instructions. Your shop manual should even have some illustrations that will help you.

Dads51 - and others. Please do not take this personally but the first thing you should do when you get a different car, old or new, is study the owners manual. Just the care and feeding can prevent future problems and help you to enjoy the features built intoo the car or truck.

Gene,
I fully understand your post and agree that is what should happen. However, being males that traditionally don't read the instructions before trying to "fix" something, "open" something, "find" something or "operate" something you might be suggesting something foreign to most males. They prefer to "get it done" quickly so they can enjoy it or or or other pleasant activity. Secondarily they might ask directions or call someone or check the internet for manual, video or picture. Last option is buy the book, wait for it to arrive and READ it. Before I get bombarded with comments, Yes I know that there are exceptions! Just don't personally know many.

BTW Owner's Manual, Parts Book, Repair Manua, Service News, Factory photos all are helpful.

I have read the manual for my 2009 Silverado but will not try to remember it all as the book is an inch thick.

Back in the '50's, when I initially got involved into the vast world of Chevrolet, first as a hobby, then as a lifelong source of income, there was no Internet or You Tube.
Folks were not a fingers touch away, to answer questions.
There were, however, Chevrolet factory manuals, owner manuals, service bulletins, etc.
Those same sources continue to be available today.
Those manuals quickly became my "Bibles", still are today.
When a "new" or "different" vehicle came into my life, the factory / owners manual pertaining to that acquisition became my homework, for that night.
My questions and concerns still come up, but my "Bibles" continue to be my initial source of information.

I would bet it is more than an inch thick with half the pages being warnings.

I agree! Whenever someone buys an old Chevrolet to either restore or just to have fun with, the first thing that he should do is purchase every literature piece that is available in reprint for that vehicle, including an owner's manual, a parts book and a shop manual. When working on a vehicle I have found these literature pieces invaluable. I use my parts books and shop manuals just about every day. Couldn't survive without them!

Thanks Chev Gene, the way you guys were talking I thought there was a knob or something that had degrees on it.
I do have the manuals but the picture doesn't show close ups or point out anything with indicators.

Perhaps you may not be aware that until this restoration project I have never even put in a single spark plug in a auto engine. Maybe twenty or so oil changes and tire changes in my 57 years but that's about it. So it's a bit over whelming and frustrating at times.

Now give me a Rifle, shotgun, bow, Skill saw, Real estate or insurance contract and I'm right at home!

It truly has been a learning experience. I have forsure learned never again!

Just happy to help with yor 1951 Chevrolet education.

After my living with this stuff for the last 65+ years I think everyone knows the ins and outs.

http://chevy.oldcarmanualproject.com/shop/1949_53/06engine/6_029.HTM

Page 6-29 in the 49-53 Shop Manual
Lower left, with the numbered markings, is the octane selector.
Loosen that bolt to adjust. The cylinder above right is the lube knob for the distributor shaft.
See "Ignition Timing" above the picture.