AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
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Well if you got points, what are the chances that your top bushing in the distributor is sloppy?
If the bushing is sloppy the point gap will change. and this will change the Ignition Timing.
If the VA is changing, that can also change the point gap, if the bushing is pooched.
As can a bent distributor driveshaft.
And we haven't even talked about the bushing of the advance cam......
Nor about how end-play affects the timing.
My guess is that you need to rebuild that distributor.
My guess is that as the distributor shaft and/or distributor cam turns, the Point-gap is closing, and/or the rotor is randomly firing to the wrong tower.
You can measure a changing gap with a dwell-meter on a running engine.
Checking the rotor to tower phasing, is not overly complicated.
You asked about where to hook the VA signal to, and why. IDK if this was ever properly explained so I'll try.
The VA has the job of bringing in extra timing, for Part Throttle operation and/or for fuel-efficiency during cruising. Generally they are factory preset to around 7 to 15 degrees, but can be modified to go as high as 22>24.
Your mechanical advance System is generally set to deliver a maximum of 34>36 degrees of Power-Timing at some rpm higher than 3000, usually around 3500.
As for Idle-Timing, Factory setting used to be, from TDC to maybe 10* at most, with 5* being typical. The Factory distributor was designed to marry those two points, adding timing with rpm. This is your basic Power-Timing curve. It can only be right at WOT. At all other throttle settings it will be dead wrong, and there is nothing the factory could do about it.
I can almost guarantee you that on your many decades-old engine, your WOT Timing will NEVER be right, except maybe at one tiny rpm window, but lets leave that alone for now..
Now, lets talk about Idle-Timing.
Most of our engines will idle faster, the more timing you give them, cuz the cylinder pressure is rising, which makes more Idle-Power. Up to a point. You may find that your engine rpm-peaks at 25*, or 30, or 35, IDK but the point is the same, Your Idle-Timing can never be set there because the Idle speed will be too high and all that Idle-power really smashes the transmission guts, and then the car jumps ahead and maybe the brakes can't even hold her back. And even if you try it, chances are she will detonate with the slightest increased load.
So, you gotta Idle her down to get a handle on things.
Let's say that by testing you find 5* satisfactory. It's just a number, follow along. So Now, the Tuner has to marry 5* at say 700rpm to say 35* at 3700rpm. Ok so that's gonna be a range of 30 degrees over over 3000 rpm which equals 1 degree per 100rpm. This is not supposed to change.
Now lets say you have a 2000stall TC. When you punch it, your Timing will climb from 5* at idle to 5+13= 18* @2000. The motor is no power house, but at least it ain't detonating.
Now, let's say you are granny driving at quarter throttle, and the VA starts bringing in timing at 1700 rpm Lets say by 2000 it has brought in 10 degrees. Thus, your timing at stall will be 18+10=28, that's great.
Ok now, lets put the VA on full manifold and change NOTHING else.
Lets say your VA brings in 15*
Immediately your Idle-Timing is 5+15=20, and the Idle-rpm shoots up. When you put it into gear, the Transmission jumps the car ahead, and you slam the brakes on, and the idle goes from say 1200 to 800 I'm just guessing. With your factory lo-stall TC, it is already pulling pretty hard. Now remember, the timing is already 20* and change. Now, you take off granny style and by 2000 stall, the timing has increased another 13 degrees same as before for a total of 33. It may be that a factory 2bbl 383 is ok with that, IDK, I'm a small-block guy.. But it may also be that YOURS is not ok with 33* at stall..
Now suppose you lay on the gas pedal, and the manifold vacuum begins to drop; but it doesn't drop fast enough, and now the engine starts rattling the pistons. What's the solution?
Well, it never did that with the VA on the sparkport, so you put it back.
But right away you notice that the engine has lost throttle response and even went soft on the bottom end. You correctly deduce that the engine liked that 20degrees to 33 degrees SOME of the time just not ALL of the time, or maybe it would be fine with a little less. Can this be fixed? Sure. It begins with a two-stage advance curve, that winds up real quick, then slows down to prevent early-onset detonation.
Anyway, once you get all that figured out, then you can start tuning the VA, which begins with determining the ideal cruise -timing, lol, which could be in the 50s at 2400rpm. More fun.
Finally, all numbers in this post are for ease of math, I made them up, so I could do the mental math; therefore do not look to them for targets. lol.
Every engine is just a little different. The cam and the compression ratio, and the throttle opening/load, are constantly playing a little dance with cylinder pressure, and the tuners job is to hit as many correct timing values as possible, as often as possible, without getting into detonation, and that is as good as it gets, until you change something.
I hope I made this understandable, I mean I really tried.
If the bushing is sloppy the point gap will change. and this will change the Ignition Timing.
If the VA is changing, that can also change the point gap, if the bushing is pooched.
As can a bent distributor driveshaft.
And we haven't even talked about the bushing of the advance cam......
Nor about how end-play affects the timing.
My guess is that you need to rebuild that distributor.
My guess is that as the distributor shaft and/or distributor cam turns, the Point-gap is closing, and/or the rotor is randomly firing to the wrong tower.
You can measure a changing gap with a dwell-meter on a running engine.
Checking the rotor to tower phasing, is not overly complicated.
You asked about where to hook the VA signal to, and why. IDK if this was ever properly explained so I'll try.
The VA has the job of bringing in extra timing, for Part Throttle operation and/or for fuel-efficiency during cruising. Generally they are factory preset to around 7 to 15 degrees, but can be modified to go as high as 22>24.
Your mechanical advance System is generally set to deliver a maximum of 34>36 degrees of Power-Timing at some rpm higher than 3000, usually around 3500.
As for Idle-Timing, Factory setting used to be, from TDC to maybe 10* at most, with 5* being typical. The Factory distributor was designed to marry those two points, adding timing with rpm. This is your basic Power-Timing curve. It can only be right at WOT. At all other throttle settings it will be dead wrong, and there is nothing the factory could do about it.
I can almost guarantee you that on your many decades-old engine, your WOT Timing will NEVER be right, except maybe at one tiny rpm window, but lets leave that alone for now..
Now, lets talk about Idle-Timing.
Most of our engines will idle faster, the more timing you give them, cuz the cylinder pressure is rising, which makes more Idle-Power. Up to a point. You may find that your engine rpm-peaks at 25*, or 30, or 35, IDK but the point is the same, Your Idle-Timing can never be set there because the Idle speed will be too high and all that Idle-power really smashes the transmission guts, and then the car jumps ahead and maybe the brakes can't even hold her back. And even if you try it, chances are she will detonate with the slightest increased load.
So, you gotta Idle her down to get a handle on things.
Let's say that by testing you find 5* satisfactory. It's just a number, follow along. So Now, the Tuner has to marry 5* at say 700rpm to say 35* at 3700rpm. Ok so that's gonna be a range of 30 degrees over over 3000 rpm which equals 1 degree per 100rpm. This is not supposed to change.
Now lets say you have a 2000stall TC. When you punch it, your Timing will climb from 5* at idle to 5+13= 18* @2000. The motor is no power house, but at least it ain't detonating.
Now, let's say you are granny driving at quarter throttle, and the VA starts bringing in timing at 1700 rpm Lets say by 2000 it has brought in 10 degrees. Thus, your timing at stall will be 18+10=28, that's great.
Ok now, lets put the VA on full manifold and change NOTHING else.
Lets say your VA brings in 15*
Immediately your Idle-Timing is 5+15=20, and the Idle-rpm shoots up. When you put it into gear, the Transmission jumps the car ahead, and you slam the brakes on, and the idle goes from say 1200 to 800 I'm just guessing. With your factory lo-stall TC, it is already pulling pretty hard. Now remember, the timing is already 20* and change. Now, you take off granny style and by 2000 stall, the timing has increased another 13 degrees same as before for a total of 33. It may be that a factory 2bbl 383 is ok with that, IDK, I'm a small-block guy.. But it may also be that YOURS is not ok with 33* at stall..
Now suppose you lay on the gas pedal, and the manifold vacuum begins to drop; but it doesn't drop fast enough, and now the engine starts rattling the pistons. What's the solution?
Well, it never did that with the VA on the sparkport, so you put it back.
But right away you notice that the engine has lost throttle response and even went soft on the bottom end. You correctly deduce that the engine liked that 20degrees to 33 degrees SOME of the time just not ALL of the time, or maybe it would be fine with a little less. Can this be fixed? Sure. It begins with a two-stage advance curve, that winds up real quick, then slows down to prevent early-onset detonation.
Anyway, once you get all that figured out, then you can start tuning the VA, which begins with determining the ideal cruise -timing, lol, which could be in the 50s at 2400rpm. More fun.
Finally, all numbers in this post are for ease of math, I made them up, so I could do the mental math; therefore do not look to them for targets. lol.
Every engine is just a little different. The cam and the compression ratio, and the throttle opening/load, are constantly playing a little dance with cylinder pressure, and the tuners job is to hit as many correct timing values as possible, as often as possible, without getting into detonation, and that is as good as it gets, until you change something.
I hope I made this understandable, I mean I really tried.
Last edited:
wish you all a great weekend.
