Looking for info on the mechanical advance in my distributor. 1977 400 motor in an F150. It is the original factory distributor and I need to know what the maximum mechanical advance is as I've destroyed 4 pistons in my high compression build due to detonation. I've looked at the weights themselves and there are no numbers stamped on them. Any help here would be greatly appreciated. .

 

There are 2 positions for the slot that lets the weights move (down below the breaker plate)
That is where you will see the advance numbers, is by the slots
You have to disassemble the shaft to change positions
It has been a long time since I had one apart but 13 and 17 degrees come to mind
One slot allows for 13 degrees of mechanical advance, and the other slot allows for 17 degrees (or something like that)
The fuel you are running, is it high octane? (add octane, use some racing fuel mixed with regular gasoline to give you a mix of about 95 octane and try it)
Racing fuel is cheaper than pistons
Those 351M- 400 motors ping and run on, even when timed correctly

 

As manic stated there are 2 different length slots in the distributor and those can be welded up or ground out to give a longer or shorter curve. The number on them is in distributor degrees so 13 would equal 26 degrees of curve when checked with a timing light.

The reason for having a timing "curve" is really based on a few factors. Being able to crank the engine when it is hot without kicking back and how the overall VE and detonation sensitivity changes with RPM. As RPM increases the engine's tendency to experience detonation at WOT goes down and above about 3000rpm full advance is usually no problem. There will be a full advance setting that will make the best torque and HP and this IS NOT necessarily the maximum amount of timing that it'll take without detonating.

Then on top of the centrifugal advance curve you have the vacuum advance which has the sole purpose of adding timing when running at part throttle to improve fuel economy. A lot of the stock cans and most aftermarket ones are adjustable. What I've found tends to work is to get the full advance timing figured out, say that's 34 degrees. Then play with the curve so that it can get there at around 3000rpm or so. Then try it with the vacuum hooked up. Does it tend to ping at part throttle? Does it surge or break up from having too much timing? If so adjust the can to limit the amount of advance that it can pull in. Most of the time I try to get 5-8 degrees and no more. Many times the vacuum advance can pull in 15 or more degrees which is too much.

 

I put a quick curve kit in at least 35 years ago, I think was maybe by Mr. Gasket. It used lighter springs, to let the weights swing out earlier.

Now to set timing, I unplug the ported vacuum, plug the hose, set timing at or above 2,500 rpm, at 34 degrees BTC "all in" (I can see it stop advancing in the timing light). I marked my damper long ago to 45 degrees. My centrifugal is all in before 2,500. Then I reconnect ported vacuum (have tried manifold vacuum, but then I get "run on" at shut down and low speed pinging) to the distributor. I check it every 5 or so years. I do know the advance starts soon enough above idle that I don't trust using the initial timing numbers quoted in books.

I was maybe earlier recalling a brass limiter collar of sorts, but I think it was maybe part of an adjustable vacuum can kit that I once tried, not sure if it's still there? I'll have to look now.

 

I don't have any information on a 77-truck distributor but the ones in 71 & 72 cars with 400's were 24 degrees @ 4000 rpm with a maximum of 28 degrees at 4550 rpm. That in itself shouldn't cause a detonation problem unless you are running a bunch of initial timing.
It is possible depending on the cam you chose that you have a dynamic compression ratio that is not compatible with pump gas. Use this calculator and figure out what your dynamic is. If it is over 8-1 you are getting close to all 91-93 oct can handle with most ordinary combinations.
Wallace Racing: Dynamic Compression Ratio Calculator
You need to use seat to seat numbers to get the most accurate answer.

 

To answer your question would require you post the part number on your distributor. But if you want to recurve yours and I'm assuming you have a Duraspark here is a link to some good instructions on how to do it. Duraspark_distributor_recurve_instructions_index (reincarnation-automotive.com)
Also, on the Ford vacuum canisters most are adjustable but only for the amount of vacuum it takes to start them moving and how much vacuum it takes to get to full travel.
Here is a picture of three different canisters the number stamped on the arm is how much advance they will pull in, in camshaft degrees double that for crank degrees. The amount of travel is determined by a slot in the arm, and it cannot be changed. Ford and the aftermarket graciously stamped the arms so we can pick one that works for us. These are 12, 10 and a 4 degree cans. There are others but I have found the 10 degree cans most useful. If you want a 4 degree you will have to get it from a Ford dealer it is from a 429 in a 1994 medium duty truck.

 

These are 12* crank advance.

Works really well, came on fe's in 74-76 f250. With how I've got it dialed in on my 460 it adds about 10*

https://www.rockauto.com/en/moreinfo...&pt=7232&jsn=7

 

I suspect 71/72 engines would have a lot more total than 28 degrees. I run 38 (without vacuum) in my performance build (at a mile above sea level). Be nice to know more about your build for good advice. If all else fails Smow Performance water/methenol works quite well. (triggers on low vacuum only)

 

4 degrees initial (approximately)
13-17 for the mechanical (approximately_
13 for the vacuum (approximately)
That is 30 total for a typical factory timing curve
Right, going racing you want around 38 total once all in
I have lighter springs in the Duraspark I am using and run about 12 degrees initial at 5000 feet attitude

 

The best thing to do (IMO) is find the OEM curve in the SHOP MANUAL and have the DIST ran on a DIST MACHINE to compare curves.

You modify after ascertaining the DIST ADV is correct or worn.

You might find the problem being a previous owner guessed on how to modify the curve. MECHANICAL EMISSION engines (w/ high mileage) don't tend to this type of modifications without others being done.