The Stovebolt Site Forums The Shop Area Engine Shop DISTRIBUTOR TIGHTENING QUESTION

Hi.....my 235 engine has an octane selector on the vacuum advance.
My question is this.............when tightening down the "distributor " are you supposed to do so by tightening down the bolt on the octane selector, or by the slotted screw at the base of the distributor ?????? THANKS, J

Both- - - -center the octane selector and tighten that bolt at the zero point, then loosen the clamp screw and set the ignition timing that way. Then the octane selector can be used to fine-tune the timing if necessary. Back when gasoline octane numbers were not as standardized as they are today, vehicle owners could tweak their timing a little if a tank of gas produced a detonation rattle. Now it's not likely you'll have to do that.
Jerry

Thank you Jerry

One precaution- - - -the clamp also sets the depth of the distributor housing in the block. Make sure the dizzy doesn't get pulled up as you're setting timing, as it can disengage the shaft from the oil pump and still have enough gear mesh to run the engine. Make sure the housing is pushed down to keep the distributor shaft firmly engaged into the oil pump slot.
Jerry

Jerry: Will it help any to adjust this octane selector for pulling a 2000lb. trailer for a little more towing power? Dave

If your hearing is better than mine and you can actually hear a detonation rattle, retarding the timing a few degrees might be in order. I stopped hearing faint rattles years ago, so I have to use one of my grandkids on a test drive to tell me if an engine is detonating. Definitely DO NOT advance the timing for load-pulling- - - -you'll burn big holes in the pistons!

On more dyno runs than I can remember, we found that once the maximum torque "sweet spot" for timing is found, advancing even one more degree robs as much power as a 5 degree retard from that point. Always err on the side of "not enough" timing advance- - - -going too far the other way tries to run the engine backwards and destroys some very e$$pensive parts!!
Jerry

My hearing is the pits also. Need to rely on someone else when it comes to listening for ping. Don't think I have that problem. I was wondering if retarding it a bit with the octane selector would help it pull better. Dave

My hearing is the pits also. Need to rely on someone else when it comes to listening for ping. Don't think I have that problem. I was wondering if retarding it a bit with the octane selector would help it pull better. Dave

The ideal timing setting is where the engine will make a long uphill pull at full throttle and close to rated HP RPM without detonating. I always make those kinds of test drives with the vacuum advance disabled, but that really isn't necessary since the vacuum at the distributor virtually disappears as soon as an acceleration load goes on the engine. The vacuum advance is only operational at light-throttle cruise.
Jerry

Jerry ...I did not realize that the vacuum at the distributor is minimal or non existent on acceleration. I always thought that the vacuum is what caused the diaphragm to advance the distributor upon acceleration.
So, what is causing the advance if not for vacuum ?
I am confused......J.

The way I understand the octane selector is, (your original question, not the one you are looking for now) you set up your timing the way you want it by loosening that sideways screw that keeps the distributor from moving and time the engine perfectly using whatever method. Then set the octane selector in the middle at Zero. THEN if you want a few degrees of advance or retard, you loosen that screw and turn it a tad either way.

If my understanding is off, it's because I time my engines by ear and use HEI innards because the dwell is self adjusting based on RPM, there is no reason to ever change anything.

There are three sources of spark advance:

Initial advance: Set by the position of the distributor housing. Aligning the ball and pointer and adjusting the distributor housing so the points just break contact sets the initial timing. People who think they're smarter than the engineers who designed the engine have all sorts of voodoo rituals to set initial timing.

Centrifugal advance:
A set of weights and springs below the breaker plate moves the distributor cam in the direction of rotation as engine speed increases, allowing the spark to fire sooner at speed so the flame front across the combustion chamber and its resultant pressure wave meets the piston exactly at the top of the stroke. Flame travel is almost a constant speed, with a pretty big variation according to air/fuel mixture. Lean mixtures burn extremely fast; rich ones burn slowly. The centrifuugal advance is set up with different weights, springs, and slots to limit the movement of the weights so the spark advance corresponds to the needs of the engine. Generally speaking, heavily-loaded trucks that spend a lot of time lugged down and pulling hard need less spark advance than lightweight vehicles to prevent detonation while pulling heavy loads. The rate of advance can be altered by changing springs and/or weights. The amount of total centrifugal advance is altered by changing the length of the slot where the weight travels.

Vacuum advance:
At light-throttle cruise, another 8-10 degrees of spark advance can be added for good fuel economy, but only under very light-load conditions. The advance port in the carburetor is designed to provide vacuum to the advance unit only when the throttle plates are partway open and intake manifold vacuum is high. As soon as the throttle is opened for acceleration or load-pulling, the vacuum signal disappears and the distributor goes back to its initial/centrifugal advance system. As the load decreases and cruise conditions return, the vacuum advance comes back into operation for gas mileage.

Very few people actually understand ignition timing. It's very closely related to fuel mixture, load, camshaft profile, and RPM. The research engineers who blew up untold numbers of engines on the dyno figured all this out many years ago. People who weren't even a gleam in grandpa's eye back then think they can throw away all the rules of physics and do their own thing! NOT!
Jerry

Deve you are well on your way to understanding the purpose of the octane selector. The last part of it is that when Chevrolet offered that feature, it was to allow the operator to either advance or retard the timing to account for the wide swing in octane from gas station to gas station, without losing their original pointer-on-the-BB setting.
Carl

Also promoted/provided to assist the driver in situations where the truck was driving at different altitudes. Adjust the octane selector in the different location ("ping" at wide open throttle) and when you get home adjust it back to "normal".

http://chevy.oldcarmanualproject.com/chevyowner/51tom31.htm

If you lived in Ft Collins, CO and had to visit Breckenridge, CO and later visit your mother in North Platte, NE, you would want to adjust timing (and be able to easily return to "normal" timing setting when you return to Ft. Collins).

Jerry.....thanks for making this clearer for me. I printed it out for my files.....J