Why two distributers?

In a two distributor system, the the rotor shaft can run half normal speed OR can have adjustments spaced out for greater procision. Coil energy is divided amungst 4 plugs and can build and deliver energy faster than serving 8 hungry gaps. Breaker float can be a problem at high rpm and this virtually elliminates that, and provides for more accuracy and less jitter in timing. Ideal is every cylinder having it's dedicated spark plug, coil, and control...and this is not found on many high proformance motors.

Obvoiously 2 distributors carries is burdened by reliability issues, greater setting labor, and cost.

 

I agree that using two or one distrubutor caps is not a whole lot better one versus the other, but some disadvantages for using a single cap are:

1. the spark plug wires for one bank get a little longish -- so routing is a bit messy and the wires for the two banks have slightly different electrical characteristics, and

2. to fit the 8 contacts inside the cap and not have cross-firing, the physical size of the cap needs to be somewhat bigger.

Nothing inside the 328 distributors affects WHEN the firing events occur, so even if you did rotate them (and you can't), it wouldn't change the ignition timing -- it's all controlled by the Microplex ecu setting the appropriate delay from when the TDC sensor signals occur. So on a 328 you really can't have "the ignition timing could be wrong for half the engine" (although 1/2 could go missing if a bank has a problem). That is possible on a carbed US 308; however, for that model they had a good reason to use the two distributors because there weren't modern ecus available, and they couldn't easily fit the 4 point sets inside the euro 8 cyl single distrubutor.

 

It is also about coil charge/dwell time. Your Chevy V8 with its redline at 5500 rpm has just enough time to adequately charge the coil to make a spark. Your Ferrari V8 spinning at 8000 has far less time to saturate the coil for a good spark. Your Ferrari V12 spinning at 8000 - fuhgetaboutit. (Is my math right? That's a plug firing every .00125 second?) The answer is to use two coils to get your saturation time back up for good spark. How to use two coils and one distributor? Easier to use two distributors.

Sort of. As long as they are perfectly matched in the timing curves with their springs and weights and lubrication, etc.

 

I agree with you guys that there is a very valid technical reason for using the two coils, but this doesn't require using two distributor caps -- in fact, carbed euro 308 use two coils and one distributor.

 

Marelli made 12 cylinder distributors with 4 sets of points at that time that worked just fine (when new and set-up correctly).

 

I said "easily", not "because they absolutely couldn't" -- also a 12 cyl cap tends to be larger than an 8 cyl cap.

 

Early 308s have 1 distributor and 2 coils, the number of coils in not related to the number of distributors but rather to their design.

 

I'm not so sure about that, Stratos. Original blueprints for the 308 engine (F106AL000) show two distributors, and pictures of early manufacturing (1973-74) of the 308 engine show two distributors. IIRC the Euro 308s got single distributors in 1977-78.

 

I believe Ferrari used two distributors from early on for both availability and relaibility reasons.
The twelve cylinder cars used two six cylinder distributors because that is what was available
in post war Italy from Marelli, the major manufacturer of such items. Since both the Columbo
and Lampredi V-12's were a two-cam engines, it was an easy matter to drive each distributor
off one of the camshafts.

Maserati and Ferrari also used two distributors with dual plug heads on four and six cylinder engines,
for reliability. This is still done on virtually all certified aircraft engines. You can have one fail and
still get where you're going.

The coil dwell/ satuation etc. and RPM arguments are so 1955! Tomorrow and every weekend around
the world, thousands of racing V-8 engines will turn 7500-9500 RPM continuously for 500 miles. In the
case of the Corvette Team, 24 hours at LeMans, Daytona etc. They have ONE distributor and ONE coil.
Their cylinder pressures are considerably greater than any 328 or 355 going down the road at 75 MPH.
Ignition failure/ breakdown is almost unheard of.

With today's electronic, computer controlled ignitions, it's surprising they use a distributor at all. It's a
lot of uneeded monkey-motion. Crank sensor triggered, individual coils, located the the cylinders would
make far more sense. Ask Delco-Remy, Bosch, and the F-1 guys.

 

Having personally revved big Pontiac V8's to 8000 rpm with points ignition, the agument there isnt enough dwell time doesnt entirely hold water. But you need a better ditributor than the garbage they came with, more like a Mallory or Accel dual point distributor.

In the early 308, as far as I have been able to determine, each distributor was a dual point unit with points 180 degrees opposed. Whether to cut costs, or perhaps because it was found unnecessary, the cars went to a single unit with two sets of points offset 135 degrees, each point set operating one bank, and a dual rotor two plane cap with 8 plug wires.

For 1977 and to meet US emissions, they reverted to the dual distributor, but for entirely different reasoning. In the US dual distributor cars, each distributor has two sets of points specified as R1 and R2, with the second set (R2) opposed at 185 degrees. The two points sets are isolated from each other by insulating the R2 points base and running a ground wire to a throttle controlled microswitch. This gives the result of retarding ignition timing at idle so the car could pass emissions a bit better, and that they could not fit 4 sets of points into that distributor. Once enough throttle was applied, the throttle switch cut the R2 points set and the car reverted to R1 only and ran as a single points ignition. Basically its a dual point ignition, but by dropping the second set, and having the second set opposed an additional 5 degrees, they achieve a full 10 degree retard when set up properly.

In a true dual point, each point set can run slightly less points gap, and between the two sets, which are slightly spread from each other, broaden the effective dwell period. This puts less strain on each set of points, longer dwell makes a bit more power, and between the two allow higher rpm with better performance. But only slightly. The 308 running only R1 points makes just as much power and revs just as well as anything else.

But perhaps the best reason for dual distributors, they look really cool.

 

Ferrari 12 and 8 cylinder caps and distributors are exactly the same size. All Euro C4's had 4 sets of points and fit quite well.

In the cars with actual distributors the bodies were the same part.

 

All 4 cycle motors ignition rotors and distributor shafts turn at 50% of crankshaft speed. It is just the way things work.

 

winner!

You get a much hotter spark at high rpm with 2 coils. You can get away with 1 on a V8 but point life would be short with the coil you'd want to run. On a 12 there really isn't a choice...not if you want to make hp anyway.

 

The cool thing about using two four cyl dist caps for two coils (which is on some 308s) instead of a single distributor cap that has two levels in it to distribute two different coils (which is on some 308s) is that you can use a standard four cylinder distributor cap that is widely available and cheap and already tooled up for that has distributors already being made them, etc. How easy and smart to use an Alfa dist cap and just use two of them! Brilliant!

Of course Ferrari used a distribtor on the early 308s shared only with the Jalpa, apparently, whith a cap that is different from everything else on the planet. How cool is that?

 

There are technical reasons to use a single distributor which have been described above.
Using 2 is a convenience/compromise.

 

Geez -- what is with you guys! I didn't say there ARE larger, I said they TEND to be larger. If they are not in this case - fine.

OK, if the C4 distributor is such a great design, why do you think that Ferrari didn't use it for US carbed 308? And why haven't you sent a letter to Ferrari, Spa demanding that they change back?

 

Actually, I think it's the other way around... crankshaft 50% of cam (rotor) speed...

 

Actually not. On all 4-stroke engines, the distributor turns at half the speed of the crankshaft.

 

Agreed. Just this evening I confirmed this empirically . I started putting an electromotive in my carb 308. I know to some it is heresy. I do like the look of the dual distributors. Anyway, my wife always helps me. The first time we found the PM1-4 mark the rotor wasn't near #1. I had to come around another rotation on the crankshaft before we found it. I have the urge to pull the cam covers to be sure but I know we're there. (It's hard to see just removing the oil filler cap.)

Seamus

 

A four stroke engine is such because the piston makes four "strokes". Up, down, up, and back down again. In order they are: intake, compressionpower (going down), exhaust (going up),. It is specifically at the top of the compression stroke (actually just before) where where the ignition spark occures and propels the POWER stroke. Only by turning half speed can the ignition and cam timing stay on concert with the two revolutions of the crankshaft, which are thus required to occomplish four strokes.