351 4v question

[leomust68]

I was speaking to a couple of people and havent really got a good answer . Now I know the information for 71-73 cars is a bit rough . On a 1973 351 cj motor with dual advance distributor why did some recieve the dual advance and some didnt ?

 

[Don65Stang]

Because the engineers knew that some engines would be operated at higher rpms than other engines based on transmission and rear gears. Those engines got dual points so that there would be some overap where one set covered the initial dwell period and the second set covered the later period. At lower rpms one set worked fine. At higher rpm the dwell decreased to a point where it hindered performane. A second set of points that overlapped increased the dwell time.

I don't remember what the exact criteria was to get dual points at the moment.

 

[leomust68]

That is interesting . So it didnt have anything to do with the car being automatic or manual . The original engine in my 73 was a q code but had single points as well as advance . I bought a 73 q code motor that had dual points but the car it came feom was an automatuc as well . Just curious ive tried google for more info but havent really got any solid input

 

[Steve73QMach1]

The 1973 Ford Shop Manual has the following description of the dual breaker points:

"The 351CJ engine with a manual transmission is equipped with a dual-diaphragm dual breaker point distributor. Two sets of pivoted breaker points are used because a slightly longer dwell is thus possible. This results in a better high tension spark. On this model, the breaker plate is mounted on a ball bearing and is designed to pivot concentrically around the distributor shaft. This results in a constant dwell angle."

Here is the specification sheet from the manual showing that the 351CJ with manual transmission was the only one to get the dual point distributor. I have a 4 speed Mach 1 and it has the dual point distributor. Click on the picture below and go full screen make it easier to read. Footnotes 5 and 6 apply to the 351CJ with a manual transmission.

 

[mchandford]

So then what are the benefits? Is the dual points idea outdated? does it increase performance in these other engines?

 

[Don C]

You can still buy aftermarket dual point distributors for the same reason, longer dwell. However, most electronic ignitions, like Pertronix conversion, provide more dwell, as do complete electronic distributors/controllers, like Mallaory or MSD.

 

[will e]

Points came out before electronic ignitions. The dwell is controlled by how long the points are closed. During this time the coil is saturating. Once the points open the coil discharges to the distributor sending the charge to the right spark plug.

So the single point rides on a cam and opens/closes 8 times. The faster the distributor is turning the less time it spends being either open or closed for each spark. By running a second set of points you can increase the time the circuit is closed. The first set of points is closed a few degrees before the second set, it then opens a few degrees before the second set but the curcuit is still closed. When the second set of points opens then the circuit is open and the coil discharges to the spark plug.

It really only helps at higher RPM's. (Although there are arguments that dual points are good for longevity).

But modern electronic ignitions are much better at managing this. I would only run dual point if I was doing a concurs restoration. If I was trying to just appear 'period correct' I would upgrade my stock distributor to electronic.

 

[MotoArts]

I was speaking to a couple of people and havent really got a good answer . Now I know the information for 71-73 cars is a bit rough . On a 1973 351 cj motor with dual advance distributor why did some recieve the dual advance and some didnt ?

Are you asking about dual points... or a dual vacuum advance??

A dual advance distributor was used in the "later" cars for emissions... it pulls the timing towards "retard" at idle for reduced exhaust emissions.

Dual advance went with open chambers and dished pistons, welcoming the beginning of the end...