silver67

My 270 hp (dual carb) 57 came with a vacuum advance single point dist installed with no vacuum line installed to the can. I have the original dual point no advance dist. (seems ok) I would like to put the correct one back in. Any thoughts on why this was done and what are the pros and cons of going back with the dual points? Thanks in advance.

wmf62

depends.... you get better coil saturation with dual points, but you don't get the benefit of the maximum total advance (probable better mileage and cooler running) you get with a vacuum distributor.

i'd stay with the single point vacuum advance distributor.
Bill

Plasticman

I agree, and would get some quality Accel or Mallory or Echlin points. And if concerned with higher RPM operation, set the dwell to 32 degrees.

Plasticman

jim lockwood

This is clearly due to the longer overall dwell that two, slightly time-staggered sets of points can produce. What I don't understand is why one doesn't simply increase the dwell of a single set of points to achieve the same result. The coil certainly doesn't know or care what's producing the extended charging time.

Jim

MikeM

When you close up the points to increase dwell, they'll burn quicker/easier due to arcing.

wmf62

i thought the condenser was for that purpose
Bill

Plasticman

That is exactly correct. Sharing the increased dwell time and current between 2 sets of points increases their life. Also, having one set "make" contact, and the other "break" the contact also increases their contact life (half the total arcing as seen by a set of single points), as well as the period where both sets are made, reduces current flow by half.

I have been running the same set of single HP Delco points for about 7K miles set at 32 degrees dwell with no sign of wear. Understand that is not very long in today's 100K emission warranty mindset, but for an occasional "weekend" vehicle, it is not terrible at all.

Increased dwell will increase the rpm of the onset of point float at high rpm, due to the lessening of the actual height and movement of the movable contact away from the stationary contact. However, as noted, increased dwell time does increase total current "on time" for both the points and coil, so they are at increased risk.

I used to be able to check dist. operation up to 6500 rpm with my Sun Machine (using a separate tach), and found several aftermarket single point sets (see my initial recommendations) are capable of operation at least to that range without point float (if the dist. point cam is in good condition). However, I had to recently do some "jury rig" repair on my Sun Machine drive, and now it only sees 5450 rpm top rpm (I need to look at that again).

Plasticman

MikeM

I ran a 270 distributor for years on the street. The primary set would outlast the secondary set 3-1. I would just change one set of points and adjust the other. Back then, those points cost about a buck apiece and a condenser was around $.60.

Plasticman

Bill,

You are correct, but the capacitor is limited, and will not totally suppress the arcing. I can show you if you are interested, on the Sun Machine, how much arcing occurs. Note also, that if the cap was not there, the engine will run poorly due to the amount of arcing that occurs, which causes a loss in full energy being released into the coil.

Plasticman

jim lockwood

I understand the idea of current sharing, but for that to be truly effective, the contact resistances of the two points sets would need to be well matched. Otherwise, the "better" set will tend to hog the current. In absolute terms, it wouldn't take much of a mismatch either. Say one set of points has a closed resistance of .05 Ohms and the other has a resistance of .1 Ohms. Not unreasonable numbers. The set with .05 Ohms resistance is going to conduct twice the current flowing through the other set.

As far as make and break situation... what Mike wrote about having to change the "breaking" set of points more often makes all the sense in the world. At the leading edge of point closure, there is little current that initially flows. At the trailing edge, when the 2nd set of points opens, the coil is trying to do everything it can to sustain the current flow, and hence you get the arcing. It would appear, then, that the "breaking" set of points in a DP distributor would have to be changed about as often as the single set of points because it's subject to the same arcing conditions.

IOW, I'm not seeing an advantage to the dual points yet.


I get this; the reduced amplitude of the contact movement increases the ability of the contact to follow the cam.

This phenomenon makes extended dwell for a single set of points look more attractive than the relatively shorter individual dwell of the dual points.


The coil doesn't care whether the extended dwell time and increased saturation comes from a single set of points or two sets of phased points. Either way, it sees the same total conduction angle.

And since the conduction of current isn't nearly as stressful to a set of points as the interruption of current flow, I'm not seeing where that second set of points is yet helping. Seriously.

I realize that a long time ago some very bright engineer thought there was some benefit, otherwise dual point distributors would never have come into being. But after considering the mechanical effects and the electrical phenomenon involved, I just don't see it.

Jim

Plasticman

With the point contacts made from identical materials, and operating under essentially the same conditions, I would not expect as much of a resistance differential as you state. But even if there was that much, it would still be less total current than a single set of points through either set.

There will be a minor amount of arc when the points are initially closed, but I agree that the majority is when they are opening. And I won't disagree that if it is contact failure that is a concern, that the break contacts will fail before the make contacts. However, I have had more points failure due to spring failure (spring for the movable contact arm) than I have of the contacts, so my point sets typically get changed out way before the contacts have worn to the point of concern.


Dual points are set at a nominal total dwell of 34 degrees, whereas the max dwell setting for single points is 32 degrees (per GM). If you believe in following their gospel to the letter, then the dual points have the advantage of increased dwell (increased coil saturation time to provide a stronger spark at high rpm), less total current through the contacts, and also as a back up (call it a limp home mode) if one set does fail.


As you can see (from my previous statements), I am a fan of a single set of points (within limits), and they fit my requirements: Sufficient rpm capability for my application, enough dwell to provide the high rpm energy required to "fire" my engine, and the addition of a vacuum advance for increased fuel economy and low load advance for better thermal management.

Of course that is correct, but I was speaking in relation to increasing the total dwell on a single set of points, and the possible ramification when doing so. Increased dwell (from either single points or dual points - it does not matter) will increase the heat the coil sees. If the coil is in marginal condition, has a high rate of previous failures (more than one), and/or is located in a high heat area, then the addition of more dwell will decrease it's mean time to failure.

Current related stress is cumulative over time, so any sharing between sets of points will increase their overall lifespan.

See all above.

Plasticman

silver67

Gentlemen thanks for all the info. Since my vacuum advance isn't hooked up and that is where I would apparently see the most benefit does anyone have a suggestion where I can/should tap the vacuum from? I want to get the coil back under the ignition shielding, but could live with it if this setup operates better. This car is just a weekend cruiser and I'm not concerned about fuel economy, but don't want it running hot either. The PO installed the coil outside the shielding because the vacuum can was taking it's space. Without the vacuum hooked up is it operating that much different than the DP would under normal driving conditions?

MikeM

Try putting the distributor in "wrong" by facing the vacuum advance toward the left side of the engine. That may clear the ignition shield and you can put it back on. Somebody here has a picture of it.

Best I remember, there is a vacuum tap on the baseplate at the front of the carb you can tap into.

The vacuum distributor will give you better throttle response and economy in the "cruise" mode. That's where they work best. The dual point works best when you keep your foot in it all the time.

The dual point distributor won't cause overheating. If you overheat, the root cause will be something other.

Frankie the Fink

You should draw the distrib vacuum from a NON-ported source (meaning full manifold vacuum); typically this is a take off located below the throttle plates at the carb base or a similar location. You can 're-index' the distributor spark plug routing if necessary to allow the non-stock distrib to fit completely under the shielding. The picture shows how I oriented my Pertronix billet replacement distributor to fit under the shielding after removing my dual point version.

JohnZ

There's a hex plug in the rear of the rear carb baseplate that's a full manifold vacuum source; replace the plug with Paragon's #755 vacuum fitting (the nipple part of the fitting screws into the hex part, so you don't have to remove the carb to install it).

jim lockwood

My "take away" from having this discussion and having thought about the single vs dual point comparison is this:

1. A dual point distributor adjusted for 34 degrees of dwell offers no electrical advantage over a single point distributor adjusted for 34 degrees of dwell. The coil sees the same degree of saturation in both cases.

2. Because of reduced contact movement amplitude, a single point distributor adjusted for 34 degrees dwell could conceivably rev higher than a dual point distributor set for 29 and 29, 34 total degrees dwell, all else being equal.

3. The point set in a single point distributor might experience detectably more rapid wear than the trailing point set in a dual point distributor, and will almost certainly wear more rapidly than the leading point set.

Jim

Frankie the Fink

Pretty much my take Jim...which is why I didn't agonize too much over snatching out my original dual-point, mechanical unit and putting in a Pertronix replacement dizzy with lifetime bearings and internal solid-state points conversion (plus a vacuum advance).

CJS

Just look at what GM did a few years later. They put Single points and vacuum advance on the SHP engines. Dual points went away forever. Also note your idle will be much stronger with all that vacuum added to the initial.

Dan Hampton

Excellent discussion. Back in the 60s the dual point distributors supposedly had the advantage of "assisting in coil saturation".

Here is a piece I picked up from Mallory re: dual point distributors.

Looking at an 8-cylinder single points distributor, the dwell may be 30 degrees with an open time of 15 degrees. Here an increased dwell runs into decreased open time. When the 8-cylinder Mallory Dual Points makes 33 degrees dwell it is down to only 12 degrees open. You might see where this trend is going. This is why 12-cylinder engines commonly use two 6-cylinder distributors. For various distributors dwell can be anything from 30 degrees down to 60 degrees, and open circuit can be anything from 60 to 12. A V8 engine might run 6000 rpm with a single distributor making 400 sparks per second, and you still get a hot spark, so it's no big deal for a 4-cylinder to do 200 sparks per second (or more if you have a heavy foot).

David Schuster

I had a 59 270 H.P. Corvette and found the dual point distributor gave up a fair amount of gas milage compared to the single point, vacuum set up that I replaced it with.

Also had a 271 H.P. 65 Mustang which also had dual points. On a long trip from Dallas to Detroit the rubbing block broke off one set of points.
I removed the bad set and continued on the rest of the trip with one set.
So I guess I discovered one advantage. Redundancy.

Dave