hope2

That would have been simple for me but my timing light was working intermittently, very intermittently. It is a induction Mac tools light from the 70's.

MikeM

00fxd

Heyyyy.. I resemble that remark

lars

The answer to that is in the definition of "dwell." Dwell is the number of degrees of distributor rotation that the points remain closed. The distributor fires every 45 degrees of rotation (fires 8 times for every single revolution), whereas the crankshaft has a cylinder fire every 90 degrees of rotation. So there is a 2:1 relationship. For every 1 degree of distributor "action," there is a change of 2 degrees at the crank. A 1 degree change in dwell changes the crankshaft timing 2 degrees. I have actually verified it on my test engine.

EDIT: This post is incorrect - please scroll down to my next posts for correction and factual test results.

Lars

seb67

You might want to back away from your bar!!

Critter1

Nope. You might want to test again. Read my previous posts.

One degree of DISTRIBUTOR rotation changes the crankshaft timing by two degrees.

One degree of POINT DWELL change changes crankshaft timing by one degree.

65tripleblack

Changes in dwell angle affects both ends of the points cam...........the "opening" side and the "closing" side. It is logical that any change in dwell angle must be split between both of these. Bottom line is that the "opening" event moves by 1/2 the dwell angle change, as does the "closing" event.

lars

You're right.
If dwell, for instance, is decreased by a total of one degree, the one degree dwell change comes from the points opening a half-degree earlier and closing a half-degree later. But it's only the half-degree opening point that affects timing, and with the 2:1 ratio, the half-degree opening point change changes crank timing by one degree. So, effectively, there is a 1:1 relationship, with a one degree change in timing for each one degree change in dwell, even though there is a 2:1 relationship between distributor degrees and crankshaft degrees.

Duke has asked me to test to see how many turns of the points dwell screw result in how much timing change... heading down to the test engine to check that right now - will post results.

Lars

survivor66

Exactly. And 65Tripleblack explains it very clearly in the next post. Phew!

lars

A quick test run on the test engine to demonstrate this. Here is the test setup (dwell meter is reading "44.9" (i.e.45) since the engine is not running in this photo):


(There is tape across the carb airhorn because the only running carb I had available this morning was a Q-Jet with no choke - the tape acts as a choke during warm-up for the testing)

Bottom line: A 1/2 turn of the points dwell screw results in a timing change at the crank of 11 degrees. It's a straight-line relationship, so you can divide it out in any increment you want, and it works the same in both screw directions. In rough terms, turning the screw 1/4 turn will change timing 5.5 degrees, and turning the screw only 1/8 turn will change the timing roughly 3 degrees. If engine speed increases when turning the screw, you are advancing timing and decreasing dwell. If engine speed decreases when turning the screw, you are retarding timing and increasing dwell.

Additionally, I tested the 1:1 relationship of dwell change to timing change: Testing as I had done before, changing the dwell 1 degree can appear to change the timing 2 degrees, especially if that's what you're anticipating (as I had done earlier). It's difficult to actually interpret only a 1-degree or 2-degree timing change at idle due to normal minor timing fluctuations. So I changed the dwell by 5 degrees to induce a much larger timing change: It would then be very easy to tell if the timing changed 5 degrees or 10 degrees - there should be no mistake. With a 5 degree change in dwell, timing changed 5 degrees. Not 10. No doubt about it.

Lars

Critter1

Thanks Lars...

We'll get everyone on board with us, eventually.

Maybe even ol Duke will get out his old slide rule and figure out that I'm right, and have been from the beginning of this discussion.

I don't think the guys over at NCRS agree with me/us yet though. They'll get it figured out.

MikeM

I'm thinking the fat lady has sung.

lars

I just edited my post above with some additional info from some more testing I just did.
Lars

SWCDuke

Thanks, Lars for spending the time today to do the testing. Now we can step aside to let everyone who was "right" crow about it.

Duke

Critter1

Now someone should go over to the NCRS board and straighten out those folks. Most follow Duke like he's the pied piper and I doubt he will correct it and admit he's wrong.

It should also be mentioned that the correct answer came from here.

Critter1

You should have admitted you had it wrong way back in the beginning of this thread.

wmf62

that will never happen....
Bill

DansYellow66

Wow - almost two weeks to get to the bottom of this thread that took on a life of it's own. Thanks to LARS for saving us from ourselves.

Frankie the Fink

Good luck with getting them to believe that, more than a bit of contempt over there for the CF...

MikeM

I coulda' sworn it was over starting with post #2 and post #8 on 12/17/16?