Dual Distributers

[XLR8928]

Forgive my ignorance, but why do the euro cars have dual distributers?
I dont see any real performance advantage, and it would seem to be a hellava lot more work at tune up time.
-K

[Danglerb]

Only so much energy you can push through even a high quality coil, which is why the modern cars go to coil on plug technology.

Not a big deal really, 2 caps, 2 rotors, and 2 coils, and good for a minimum of 50k miles.

[MPDano]

..and not all Euros either.

[XLR8928]

Originally Posted by MPDano

..and not all Euros either.

And the plot thickens......

[Mrmerlin]

the reason for the dual distributors is for the early LH system this basically breaks the ignition system down to 2 individual 4 cylinder systems.
The 16V heads could only have the distributor attached to the DS head, the 32V heads have a cap on the front of each head.
The small timing belt used for the twin dizzy unit should be replaced every 30K miles

[MPDano]

Originally Posted by Mrmerlin

the reason for the dual distributors is for the early LH system this basically breaks the ignition system down to 2 individual 4 cylinder systems.
The 16V heads could only have the distributor attached to the DS head, the 16V heads have a cap on the front of each head.
The small timing belt used for the twin dizzy unit should be replaced every 30K miles

So was there any performance differences with a twin?

[Mrmerlin]

yes more HP and better engine control, via spark, injection, timing

[Fabio421]

Originally Posted by MPDano

So was there any performance differences with a twin?

There was only a 10 crank HP difference and the distributor wasn't the only thing that was changed. The CIS system was eliminated and replaced with the LH and a MAF. I know that when Herr Kuhn switched an L-Jet car over to a MAF he saw a slight increase in HP as well. The 10 HP could very easily have been attributed to the MAF being less restrictive than that big plate on the CIS.

[Danglerb]

My guess is the dual coil and distributor were to assure no misfires for 50k miles as required by the smog warranty. Any significant failures and EPA makes the manufacturer do a recall and fix all of them.

As for HP, I tend to think a spark is a spark, but I recall from flying a plane with a dual magneto and twin plug heads that power drops during the preflight check switching between single magnetos, but a twin plug could just really suck when both don't work.

[Normy]

The reasoning for the dual distributor system is simple: parts standardization. In 1983, Porsche was about to release the 32 valve engine into production. It was noticed that it would be easy to use the same fuel injection [LH-Jetronic] and ignition [EZF] on both the 16 valve and the 32 valve engines, and the common parts supply would reduce costs. Thus, the 16 valve engine was equipped with two 944 distributors with the second powered off the first by a tiny belt. Apparently it was deemed too expensive to modify the [soon to be out of production] 16 valve heads and cams to carry the distributors on the front of the timing belt cover.

The increased horsepower? Most likely due to the bump from 10:1 compression to 10.4:1. Porsche felt it was safe to raise the compression because the LH-Jetronic's MAF was far more accurate at measuring the air entering the engine [it senses mass rather than volume, hence the name], and as such there was less likelihood of the engine developing a lean-running condition and getting into severe detonation. Where you really see the advantage of this bump in compression is not in the HP, it is in the torque: 295 ft/lb versus the earlier version's 263 ft/lb.

Two distributors? There is another reason. If you take apart these distributors and the body that drives them you DO NOT find any sort of centrifugal or vacuum advance. How does the timing vary? Simple. The EZF computer varies it. With these distributors, the spark does NOT necessarily jump the gap from the rotor to the cap the instant they come close. The spark jumps this gap at some point while the two contacts are near, but not necessarily when they first approach. The EZF decides when the spark should fire, and to make this all work without any mechanical or vacuum advance mechanisms the rotor electrode was designed wider. The reason two distributors are used has to do with real estate. If they used a single 8-wire cap, there wouldn't be enough room to vary when the spark would fire without either making the cap too large or running into problems with misfire due to two cylinders receiving spark at the same time.

All of this was done in order to keep the high performance 16 valve engine in production a few more years while meeting emissions standards and curtailing costs.

N

[MPDano]

Originally Posted by Normy

The reasoning for the dual distributor system is simple: parts standardization. In 1983, Porsche was about to release the 32 valve engine into production. It was noticed that it would be easy to use the same fuel injection [LH-Jetronic] and ignition [EZF] on both the 16 valve and the 32 valve engines, and the common parts supply would reduce costs. Thus, the 16 valve engine was equipped with two 944 distributors with the second powered off the first by a tiny belt. Apparently it was deemed too expensive to modify the [soon to be out of production] 16 valve heads and cams to carry the distributors on the front of the timing belt cover.

The increased horsepower? Most likely due to the bump from 10:1 compression to 10.4:1. Porsche felt it was safe to raise the compression because the LH-Jetronic's MAF was far more accurate at measuring the air entering the engine [it senses mass rather than volume, hence the name], and as such there was less likelihood of the engine developing a lean-running condition and getting into severe detonation.

Two distributors? There is another reason. If you take these distributors and the body that drives them apart you DO NOT find any sort of centrifugal or vacuum advance. How does the timing vary? Simple. The computer varies it. With these distributors, the spark does NOT necessarily jump the gap from the rotor to the cap the instant they come close. The spark jumps this gap at some point while the two contacts are near, but not necessarily right then. The reason two distributors are used has to do with real estate. If they used a single 8-wire cap, there wouldn't be enough room to vary when the spark would fire without either making the cap too large or running into problems with misfire due to two cylinders receiving spark at the same time.

N

Well explained

[XLR8928]

I consider myself duly edified. As for the aircraft comparison Mike, my take on ignition system duplicity is probably in case of a failure. (Can't coast to the side of the road.)
-K

[Danglerb]

Dual plug heads have a real power advantage. The amount of advance required for full combustion at TDC depends on the flame front speed and distance it has to travel from the spark, two plugs and about half the distance.

[XLR8928]

Originally Posted by Danglerb

Dual plug heads have a real power advantage. The amount of advance required for full combustion at TDC depends on the flame front speed and distance it has to travel from the spark, two plugs and about half the distance.

I woulden't mind some 'O those high zoot heads on my '80us

[Danglerb]

Some 911's have dual plug heads, but its very rare in cars.

Basic weakness of all non boost hot rodding with the 928 its that real gains need more rpms and the 928 motor has trouble with higher rpms.

[DanielDudley]

Originally Posted by Danglerb

Dual plug heads have a real power advantage. The amount of advance required for full combustion at TDC depends on the flame front speed and distance it has to travel from the spark, two plugs and about half the distance.

Very true, and on the 911, it is not possible to place the plug in the center of the head, as the valves almost touch at the center. Since this is a true hemisherical combustion chamber with a large volume, the only real way to raise the compression is by putting a large triangular ridge in the piston, which isolates one side of the chamber from the spark at TDC. Good recipe for detonation in a high compression engine, and limits the total advance you can run. That is the principalm reason for twin plugs, to be able to run more advance, for better high RPM power and midrange grunt.

928s do not have this combustion chamber, and thus do not need or have a space for a second plug. Also, in the case of the four valve head, the plug is dead center on a shallow pent roof chamber. These are good heads for flat pistons and high compression. All good stuff for 928 owners.