VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 vacuum advance line diameter question

I have a 216 motor and Im gonna run a new vacuum advance line, my question is that I have some 1/4 inch steel line laying and was thinking to use it. But I wonder if it will cause my vacuum advance to react slower.

I don't think it will affect the operation of the vacuum advance but when you go to change the timing you will be flexing a larger diameter of tubing.

Not so sure about that. Big diff in volume between 1/8 & 1/4 line. If the engine is in very good condition and has a lot of vacuum you might be OK. But, if you have weak vacuum due to bad valves, etc there might not be sufficient vacuum to pull the distributer due to the larger line. I guess you can try it then watch the vacuum advance & see if it moves the distributer when you suddenly raise the RPMs.
Hope someone more knowledgeable than I chimes in on this subject

Dick

My motor is a new rebuilt and the car is in the project state but it does run and I can move it around.

In 1937 and 1938 they used a 1/8" line.
All other years used 3/16"
There must be a good reason for using a small line. Perhaps to regulate or smooth out the vacuum.

My guess is that they used the smaller pipe for quicker response. The vacuum in that line is not that strong so a large pipe will take longer to operate the advance unit though it will happen eventually.
Tony

Interesting discussion!

I was able to salvage and reuse the original 1/8" line on my '37 Master coupe. The replacement line I received from Chev of the 40's was 3/16". The end fittings were too big for the ports in the carburetor and advance diaphragm.

I agree with:
1) the slower response due to the larger line volume (1/4" has 75% more volume that 3/16", 4 times the volume of 1/8")
2) The physical resistance of the line when you rotate the distributor to set the timing. The other day I noticed that the 1/8" line was resisting the rotation of the distributor when I was adjusting the octane selector.

well Im glad my topic has stirred up a pot knowledge and info, Rusty 37 Master I take it that you still run the original motor right? So you just stood with the 1/8 and not the 3/16?

When you say adjust the octane , is that same as timing it or is it more than that?

The octane adjuster adjusts the timing for different octane fuels. When you adjust the timing wind the octane selector off (retard) ans set timing by the embeded ball in the flywheel and pointer on the housing then wind the octane selector for the grade of fuel you are using (usually about 8*).
Tony

I need to learn more on this to make sure my motor is dialed in right. I hope this is covered in my repair manual.

The octane selector was designed so the owner could set the timing with out requiring a timing light. Years ago the octane of fuel varied greatly from one brand to another. If you got a low octane brand the engine would ping under load. If you used a better grade of preimum the timing could be advance more incresing gas mileage and power.
The timing and octane selector is set by placing the octane selector in the center or zero position. The the timing is set with a timing light to the flywheel ball to the zero pointer as recomended nthe shop manual. Then the timing could be reset using the octane selector giving the engine the best economy and performance. This was done by going farther advanced with the octane selector until the engine would ping at 10 or 15 MPH with the gas pedal floored. For an old car today it is almost impossible to make an older ping due to todays higher octane gas. In this case I jusrt set my octane selector to 8 or 10 degrees advanced.

The above is well described in the shop manual, a book readily available at low cost and EVERY OWNER SHOULD HAVE ONE.

Sorry for the delayed response.

The flywheel ball and pointer are set to give you 5 degrees BTDC. The octane selector allows you to advance or retard from that setting.

Yes, I am running the original motor with the original 1/8" line. Everything seems to run great with octane selector set for about 10 degrees. So this means that my total static timing is about 15 degrees advanced. The engine does not knock with that much advance mainly because it is such a relatively low compression engine.

One behavior I do notice is that sometimes when I start the engine when it is at normal operating temperature the engine will kick back and make the starter disengage. It normally starts the next time I try it. I expect I could eliminate that if I backed the octane selector back a little. I run 87 octane 10% ethanol fuel.

The owner's manual also tells you how to use the octane selector.

I have found that the engine may kick-back at 10 Deg. so I have chosen and my advise is to use 8 Deg. to avoid harming the starter.
At that setting the engine was designed to run OK with 76 octane Ethyl gas.

Hi Gene,

I agree that 15 degrees BTDC is a lot of static advance regardless of the compression. I will back mine off a couple of degrees to see what happens.