advance curves
 

Which of the front mounted distributors has the slowest advance curve?
Alternatively, how does one adjust the advance curve to slow it down?
And not by screwing down the vac brake.

Re: advance curves
 

Yes, I know you want full advance in by around 2000 RPM, but I want to slow the advance down; seems to come on too fast, too soon.

Re: advance curves
 

I set my crab distributors up on a timing fixture; this sets initial advance, also the dwell, both individually and combined. But doesn't do anything about advance curve.....
So, in actual road testing, at certain RPM's/ load, the engine detonates to hell.... Not just the current engine, all the engines I've built and run for the past substantial number of years.

Common denominator in all of these has been my ignition; crab distributors, the same units, rebuilt and set up by yours truly.
So....whilst driving home the other week, after attending a swap meet, listening to the cacophony emanating from the engine, it dawned on me that the dissy was advancing too fast, too soon under certain load conditions. So wondered how I could slow down the advance curve, thought maybe I could drill some holes in the advance weights, which would tend to make them fling out a tad slower....
Next day, got onto it...pulled the dissy, stripped it....playing with the cam/advance/retard assembly, it was apparent that there was bugger all spring tension exerted on the weights; consequently, centrifugal force would cause the weights to fling out to full advance pretty damn quickly.
Another crab gave same results.
Next I stripped a divers helmet dissy with '68' guts. This runs the same size 'small cam' as the crab.However, it definitely had a strong spring tension exerting pressure on the weights. Cool!! That'll slow down the advance.

So, I built up a crab dissy using the '68' cam/weight assembly. Initial testing shows a definite improvement; I still get pinging at certain speeds/loads, but it's much better.
Still room for further investigation/playing, thought I'd put it out here, see what the experts can suggest.

Re: advance curves
 

Have you had the opportunity to use a timing light and measure the "on engine" difference between the setting fixture and the engine?

I do not have a good answer for your overall question. I can only tell you I installed an adjustable timing control so I could take better advantage of leaner, low charge pressure, cruise throttle settings and the crab distributor. With the device near my fingertips I can dial in the necessary retard for hill climbing and go back to higher advance settings for light load.

Re: advance curves
 

I have had several KRW timing fixtures. They are assembled with a pin to properly index the unit. If the pin is missing the parts can rotate out of alignment.

I bought one with the pin missing and out of alignment ... so, it's not theory or a guess. Good idea to make sure, as JWL says, that your fixture is correct.

Re: advance curves
 

The 40 and 68 should have the same curve. The 11A is fastest and slightly more.

Re: advance curves
 

After testing a few hundred crabs and helmets . It appears that the factory advance does in fact come in very early right off idle and all in at 2000-2500. The Ford bulliten books show a early advance curve as well. The rpm band of a flathead is so low i just figured thats the way Henry wanted it???
I hear guys talking about spark knock or ping with a flathead , thats pretty hard to do with good fuel and low compression..

Re: advance curves
 

I'm running 95 RON octane, have 165 lbs compression, and custom Ross flat top pistons.

Re: advance curves
 

Brian if you look in the ford service buletins book, the big thick one with the steel covers,under ignition there is a very detailed list of all the curves,if you dont have it let me know and I will shoot you one.
also I made a machine so I could spin up the dist and its an easy job to map out the curve from there,
What compression ratio are you using,
The vac brake will not alter the curv but will slow it down ,from memory about 5deg,
another thing you could maybe do is fit a space under the little leaf springs to stifen them up.
Lawrie

Re: advance curves
 

You can bend the thinner of the two springs on the weights to slow the curve, just be careful as they can break easily. Usually I am trying to to the opposite. On the early distributors I bend the springs outward to allow a fast curve to mimic the actions of a 11A unit.

I have never had a flathead with spark knock. Are you 100% certain your timing isn't too far advanced?? Like hoop says I have owned KRW fixtures that were off. It sounds like you may very well be too advanced, as not only does the 68 have a slower curve it advances 6-8 degrees (crank) less than the 11A.

This was Ford's idea for the vacuum brake. You can adjust it inward or install a stiffer spring to drastically slow it down, however I only apply the slightest amount of tension on the brake as in my opinion in todays fuels its not needed. I believe Bubba runs either zero or slight tension as well??

Re: advance curves
 

I think part of the problem could be the flat top pistons...during 36, and all subsequent engines ran domed pistons, said by Ford to have better 'quench'.

Re: advance curves
 

Brian look at the 40 or 40 curves, they are a lot slower,but use the larger points cam,
Lawrie

Re: advance curves
 

Yeah, I know, that's why I fitted 68 style; same small cam as crab. I seem to recall you posing a question on here a few years back about how much spring tension should be exerted on the advance weights. Don't know if you ever got a satisfactory answer?

Re: advance curves
 

For whatever it's worth.. I'm another one with 'too much, too fast'. My stock 47/59A/crab gave excessive ping, and I eventually had to replace three pistons due to damage. I understand the reason to be thinking of 68 stuff.

Re: advance curves
 

165# compression. unusually high!

Re: advance curves
 

A couple timing chart pics from the Ford "bible".......

Re: advance curves
 

Bubba, I have seen those before and really don't understand them. They do not match what Ford published in the maintenance manual advance tables and it doesn't match with any of the modern dyno information I've seen. Any insight?

As an example, in a late 50 and 51 advance table the numbers were:

200 rpm 0 degrees
500 rpm 1 degree
1000 rpm 6.25 degrees
1500 rpm 10 degrees
2000 rpm 11.25 degrees

Those are the max values in distributor degrees, so with 2 initial it would be:

200 rpm 2
500 rpm 4
1000 rpm 14.5
1500 rpm 22
2000 rpm 24.5

Which seems a lot more reasonable to me.

Re: advance curves
 

I have been casually watching this thread, and am kind of amazed. Admittedly, I am an "8BA guy", but am kind of surprised about the mention of pre-ignition (ping) in this thread. I have run stock distributors and several Mallory dual points on my engines, and have never experienced "ping". I see from the posts on here (and other boards) that "helmets" and "crabs" are the best distributors for a flathead, but I am now starting to wonder if that''s the truth. I have seen far too many posts about this exact problem. Perhaps there is something else going on here? Just curious.

Re: advance curves
 

Those charts are 32-41 helmets , the load a matic is a totaly different discussion. The load a matic have a ton of advance available , however with venturi its a total different spec ( and a different engine)/I will dig up some specs on those this week as i am presently working on a couple.....

Re: advance curves
 

Tub man i agree to have spark knock with todays fuels will require major compression changes like pistons, heads etc. Or a very incorrect blend of both.....????????