Slant Six Forum
https://slantsix.org/forum/

Distributor Recurve Confusion
https://slantsix.org/forum/viewtopic.php?t=59204		 Page 1 of 1
Author:  SpaceFrank [ Mon Feb 29, 2016 12:14 am ]
Post subject:  Distributor Recurve Confusion
As described in this thread, I finally determined that the engine in my racecar was getting about 35 degrees of mechanical advance by 4500 RPM- way too much, which explains why we kept blowing head gaskets after shaving the head. Before anyone asks, the vacuum advance line was disconnected while measuring the advance curve.

I found this old post by Slant Cecil describing how he measured and modified a distributor's governor (or "advance rotor") to limit the total mechanical advance. He measured the slots in a stock 15R governor (which allows 15 x 2=30 degrees total mechanical advance vs. the crank) and found them to be .475" long. Subtracting the width of the pin (.240") gave a total allowed movement of .235". He then did some math to determine the final desired length of the slot and either brazed or welded it to match. The key assumption here is that the allowed movement of the pin in the slot is directly proportional to the total mechanical advance.

So I disassembled my distributor, an electronic type originally bought from FBO back in 2006 or so, in order to modify the governor plate. Considering that I measured at least 35 degrees of advance with my fancy digital timing gun, I expected to find my governor plate with longer slots than .475"; in fact, they should have been at least .514" if Cecil's math holds.

That is not what I found. My governor has already been modified to limit its mechanical advance. It is stamped "15R", and its original slots appear to have been about the same as the .475" measured by Cecil. Except these slots have already been shortened by a tack weld and filed out to about .435". The pins in my weights are also about .240" in diameter. Using Cecil's math, this should make this governor the equivalent of about a "12.5R", allowing about 25 degrees of mechanical advance.

Just to check myself, I disassembled another dizzy I had hanging around. Its governor is stamped "R13.5", and its slots measure about .450". This checks out with the math Cecil used.

So how am I seeing 35 degrees mechanical advance from a distributor whose governor slots are only long enough to allow 25 degrees? I'm confident in my arithmetic and my measurements. Is there something I'm missing that could be increasing the amount of allowed mechanical advance?

Just for reference, here's a picture of both governors I have on hand. The left one is from the race car (Mopar EI distributor from FBO) and the left one is from a spare points distributor I had laying around.

Image
Author:  emsvitil [ Mon Feb 29, 2016 1:22 am ]
Post subject: 
dial back timing light or timing tab/multiple marks on balancer?


The mark spacing may not correspond to correct degrees
Author:  SpaceFrank [ Mon Feb 29, 2016 1:44 am ]
Post subject: 
I did dial-back measurements with a digital timing light, and the single mark on the balancer has been verified to line up with zero on the tab at TDC. So I'm pretty sure the advance numbers I recorded are correct, unless I royally screwed something up, or this brand new timing light is defective...
Author:  Charrlie_S [ Mon Feb 29, 2016 4:10 am ]
Post subject: 
Are you measuring just the mechanical advanve, or total, ie, mechanical plus initial (10 initial plus 25 mechanical= 35)
Author:  Dart270 [ Mon Feb 29, 2016 11:02 am ]
Post subject: 
I agree you are likely measuring total = initial + mech advance swing. I could certainly see 35 deg total causing blown gaskets with even a mildly built motor in an endurance race.

On a racecar (yours or mine), I would limit mechanical swing to about 10-15 deg so you make initial timing 15 deg and then have final between 25 and 30 deg. This will give you more consistent idle and low speed, and better throttle response. 15 deg initial will not hurt anything.

Slant on!

Lou
Author:  SpaceFrank [ Mon Feb 29, 2016 12:21 pm ]
Post subject: 
No, the 35 is just the mechanical advance. Here's the data I collected with my timing gun. Initial was apparently set around 2 degrees when we were screwing with it at the last race, and 37 was the total where I stopped measuring at 4500 RPM. Even if the jumpy measurements at low RPM were throwing me off, I'd need to have started with at least 12 degrees initial for the governor measurements to make sense.

Image
Author:  Dart270 [ Mon Feb 29, 2016 1:52 pm ]
Post subject: 
OK, got it. So, your dist is giving more than is stamped on the governor or your measurements indicate. There can be slop in several of those parts. Maybe get a different distributor, or just weld up the slots further. I usually drop the swing by more than half when I weld mine up. I just cut a piece of 0.040" or 0.060" sheet and balance it in there, then spot (MIG) weld it on both sides. Less messy and less grinding needed (often none).

The stock springs in there (big one, notably) create a super slow curve. I usually go to the hardware store and cut my own springs to have same diam as stockers but wire gauge about the same as the light stock spring or a bit heavier. Get ride of the big heavy spring. There are threads on this site that have more precise info, of course, but I have my own system and have not looked too much at those. DusterIdiot and bigslant6fan are two experts.

Cheers,

Lou
Author:  emsvitil [ Mon Feb 29, 2016 2:54 pm ]
Post subject: 
Is the vacuum plated locked down?

Maybe it's slightly rotating.........
Author:  kesteb [ Mon Feb 29, 2016 9:46 pm ]
Post subject: 
Do you have a msd or similar ignition box? It is my understanding that some timming lights are not acurate with those types of boxes. I know with my setup, I never had stable reading at idle. I just bought a Summit light that is "msd"compatiable. We'll see how this one works.
Author:  SpaceFrank [ Mon Feb 29, 2016 11:51 pm ]
Post subject: 
Thanks for the info, Lou. That does sound less messy than building up a weld bead just to grind it down again. I have a couple spare points distributors, so I can grab one of the lighter springs from there.

The vacuum plate wasn't locked down, but it was still attached to the arm from the vacuum can (which had no vacuum line running to it, since we don't run with vac. advance on the race engine). The spring in this vacuum can feels pretty stiff, so I doubt it would be wandering too much with no outside force on it. Still, it would be good to eliminate this as a possible source of "slop." There are some unused holes on the upper plate, so I could drill straight through the bottom plate and run a self-tapper or two through both plates to keep them fixed. Then I could just remove the vacuum can altogether. Is this what people normally do when they race without vacuum advance?

Any ideas on what other sources of slop there could be in the system?

It's an aftermarket Mopar-style electronic ignition, not MSD. So that shouldn't be a problem for the timing light.
Author:  Rick Covalt [ Tue Mar 01, 2016 3:39 am ]
Post subject: 
Quote:
Then I could just remove the vacuum can altogether. Is this what people normally do when they race without vacuum advance?


One option is to get a Lean burn distributor off a 80's truck that had no Vacuum advance to start with. But I would think just locking it down with a screw should do the trick also.

Rick
Author:  Badvert65 [ Tue Mar 01, 2016 4:39 am ]
Post subject: 
You won't be able to just remove the vacuum can completely. It has the tab that locates the distributor cap. I have used pieces from the lean burn distributors and tried using cut down vacuum cans (with a screw locking plate in the distributor). Just be fore warned, this opens up another can of worms dealing with distributor cap/rotor tip alignment at the point of ignition firing.
-Matt
Author:  Dart270 [ Tue Mar 01, 2016 5:16 am ]
Post subject: 
I would just disconnect the vac hose and leave the can on there. If no can/arm, you need to lock it down with a screw/bolt.

Lou
Author:  Dart270 [ Tue Mar 01, 2016 5:28 am ]
Post subject: 
I'm not sure I would worry much about extra slop unless you can feel something broken when you manually actuate the mech advance. I would just weld metal in the slots and adjust empirically. Starting with another dist would be a good idea, though.

Lou
Author:  Old6rodder [ Tue Mar 01, 2016 9:50 am ]
Post subject: 
I've taken the opportunity to cobble up a screw adjust mech, using the vac's arm & mount plate, to replace the vac can on race engines.
This gives me the ability to adjust advance quickly and minutely from the outside with just a screwdriver.

I'm thinking of doing a thumbscrew style next time. Yeah, I know ...... too much time on my hands (and far too lazy). :lol:
Page 1 of 1 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/