Why Do Twin Plug Engines Use Two Distributors?
 

I have a question about twin plug 911 engines.

Why do they need a 12-way distributor (as in the RSR) or two 6-way distributors (as in the 964)?

Both plugs for a given cylinder fire at the same time, right? So why can't they be fired by the same distributor contact, using a 1-into-2 ("Y") sort of plug wire?

I thought about this, the only answer I could come up with is maybe a single coil doesn't put out enough voltage to fire two plugs?

How would you split the juice that forms the spark? The path of least resistance...One wire would always get the most juice. The coil(s) are another matter.

John,
The major reason for that is that the spark will take the path of least resistance ie through only one plug.
Tim

edit: now it would be interesting if you could find a plug out there that doesn't ground to the engine but grounds to a second terminal. You could wire them in series that way but I doubt you'd find the right plugs for it.

We had a thread going about this last year right about this time...

THese guys are right... One plug or the other would spark, but not both.

If this is true, how do wasted spark ignitions work?

wasted spark ignitions are different in the current path. In a normal distributor ignition system the spark goes through the spark plug into the engine and back to the ignition box. In a coilpack ignition the current path is into one plug into the engine, into the other plug and then back to the coil pack.. that way they only need half the amount of packs. because of the distributor you can't do that with a normal ignition system.

No.

Wasted spark ignitions doesn't have any distributors. Every plug has it's own coil, but secondary windings are bi-filary winded on same coil. That means two secondary windings on same coil with only one primary.

That way, you fire one coil and both plugs give spark, but each plug has it's own secondary...

Goran,
What I meant was that you can't send the spark current through a 2nd plug when you have a distributor, only 1 wire at a time has a full path back to the ignition box. With coilpacks you can run the return through a differnt plug.
hmm.. a diagram would be in order.

Yupp...there are two different schools there, one with basically two secondaries for each plug and other with one bigger secondary that is grounded trough plugs on each end. Both work :-)

The sparks occur at two different times, before and after the normal timing mark. This has the effect of a longer spark in the cylinder, which increases power and burns the mixture more efficiently (and, I would imagine, hotter). Racers use this setup often.

Nope. I don't know what happends with OEM 964 dual plug setup (but i woulöd be really surprized if they had different timings for plugs) but all wastespark coils are triggered with one lead and will fire both plugs simultaniously. As far as i can see it's the only optimal way. Trick with dual plugs is to initiate flame front from "both sides" so it burns faster. That way you don't need as much power-sapping advance on ignition. Firing two symetrically positioned plugs in same cylinder at different times would do little good as it will prolong flame-front burning.

Actually, wastespark coils aren't really wasting any spark when used on twin plug heads beacuse both sparks are going into same head, as they should. On the other hand, using wastespark coils on twin-plugged heads and ECU that is actually made for wastespark will actually waste sparks...effectivly firing four plugs at once...

Getting complicated? :-)

Thanks for the correction, I'll have to have a "talk" with my source.

:confused:

My english is not so good and i'm in hurry but I'll try to explain this quickly:

1. Twin plugging - ignition of mixture using two spark-plugs, usually positioned symetrically in the head. Twin-plugging is the (bandaid) way of solving problems of long burn time associated with having two valves per cylinder and one offset-positioned plug. Not used on engines with four valve heads and centrally positioned plug. With TP, mixture will be ignited from "two sides" simultaniously and flame front won't need to cover as much distance and whole sheebang will ignite (Observe: ignite, not detonate!) faster...which will allow you to run more advance and thus use more energy by allowing pressure to press pistons downwards longer time.

2. Direct fire - modern ignition that doesn't have any distrubutor. Used on almost all new cars. There is one coil per plug and all timing is done electronically, sending current to each coil when it's time to fire that particular plug. That way all mechanical stuff (dizzy, menchanical advance etc.) is removed and you have true solid-state system that is infinitely adjustable by software.

3. Wastefire - Direct fire with only two coils, but each firing two plugs. That way manufacturers will cut amount of wires in two. One cylinder will fire when it should and another during exhaust phase (when it actually doesn't matter).

4. Wastefire coils - looks like ordinary coil but it has two outputs and fires two plugs at once when energized. Plugs aren't connected in parallel but in series, with coil inbetween.

5. Using wastefire coils on dual-plugged engine with non-wastefire ECU (like using Motoronic on twin-plugged Porsche engine) - it looks like ordinary direct-fire system but instead of having one coil per plug you have one wastefire coil and two plugs per cylinder connected to each of six leads. ECU doesn't know there are two plugs, it just energizes wastefire coil instead of ordinary one and it fires two plugs simultaniously instead of one.

6. Using wastefire ECU with wastefire coils on single-plug engine (like using SDS on crochety original SC engine) - you have three wastefire coils and three leads from ECU...it will fire two plugs positioned in different cylinder at once. One of them will ignite mixture, other will do nothing.

7. Using wastefire ECU with wastefire coils on twin-plugged engine (SDS on twin-plugged 3.6 for instance) - Now it get's interesting. You have three leads coming out of ECU, each connected to two wastefire coils that are connected to two plugs each, effectively firing four plugs at once. Two will fire in on cylinder and do some good, other two will just fire and do nothing. Otherwise same as 6.

8. Dizzy - Mechanical device prone to failure, not used on new cars. It rotates and distributes high tension lead to each spark-plug on engine. That way, one coil will need to fire as many times as there are cylinders. When used in conjuction with twin-plugging (as in 3.6) you will have two coils and two dizzy's, connected together with a belt that snaps and turns your car into single-plugged one when you least expect it.


Get the picture?

There's a bunch of info on this in the Engine Rebuild book too...

-Wayne

Well done, I appreciate the info.

cause 12plug distributors are a PITA to find

It's very simple.

1. All 964/993 twin plug systems fire at exactly the same time.
2. Wastefire - as mentioned, plugs are in series so energy stored in coil
field is divided between plugs
3. Ideal - two seperate coils to store energy for each plug and provides best ignition

Ouch, my head hurts.

So it sounds like a waste fire system is a potential alternative ignition system when you twin plug a Motronic 3.2? You don't have to source the 964 dual distributor and expensive Andial splitter, or the even more expensive RSR distributor?

I'm out of my depth, so apologies for the naive questions. I'm going to look in my copy of the Engine Book now.

You need to have multiple coils for a wastefire system which is NOT
compatible with the 3.2 DME that has a single output. Like the Mercedes
Benz or GM 6 cylinders, you'll need three coils and three DME timed
outputs.

I think you guys have several ideas here mixed together.

You have a waste fire system mixed with twin plugs.

Twin plugs are used because the speed of the flame front is not fast enough to get all of the way across the combustion chamber. Also with some types of pistons the crown of the piston is so high that it actually cuts off a portion of the combustion chamber and that portion contains air fuel combustion mix that needs to be fired also.

The two spark plugs in the single cylinder are fired essentially at the same time. They are fired in parallel to each other, not series. (Actually the 964 distributor has one rotor phased slightly behind the other). The original 12 plug distributor fired the two plugs in the single selected cylinder simuletaneously.

The wast fire system is not necessarily used with twin plugs! The trigger mechanism fires two plugs. However in our 4 cycle motors there is only one cycle that the ignition does any good. That is the compression cycle. The other plug is firing a cylinder thats on the exhaust cycle, thus the firing of that cylinder's spark plug is wasted.

In a waste fire system our 6 cylinder motors only need three triggers of the ignition is necessary for it to run.

I hope that clears up a little smoke.

David Duffield

You could use a wastefire coil system on a dual plug system.
This would require 6 coils with dual series outputs on each coil.
It's still not as good as having independent coils fire each plug.

yep you are correct.

The waste fire system is a more simple concept. I can't think of any other particular advantages to it. I guess that it confuses some gear head that's looking at your motor.


David Duffield

"Twin plugs are used because the speed of the flame front is not fast enough to get all of the way across the combustion chamber."

David,
Perhaps this could be another way of saying twin plugs reduce the distance the flame front must travel for complete combustion. The longer the flame front, the more tendency for detonation; you don't want the flame front to travel too fast. That's primarily a function of the A/F ratio.

Poor combustion chamber design, excessive compression pressure and/or temperature, insufficient fuel octane and excessive spark advance can all lead to spontaneous combustion (aka detonation). The actual distance the flame front must travel is a primarily a function of combustion chamber design and where the spark plug is located in the chamber.

Sherwood

Simply put:

1. you can use wastefire coils with both wastefire and direct-fire ECU on your dual-plugged engine.

2. there is no particular disadvantage with running wastefire ECU...just get less clutter but it will waste more energy too.

3. there is no real-life advantage of running two coils instead of wastefire coil per cylinder in dual-plugged engine. with wastefire coil being connected "in series" with plugs with winding in-between, either both plugs fire or none will.

Clear as mud?

http://forums.pelicanparts.com/uploa...1074310172.jpg

BTW, nice info on this thread.

Goran,
I've got a question regarding wastefire coils. Since the "wasted" spark occurs on the exhaust stroke of the paired cylinder, what encourages the spark to jump on the firing cylinder or both cylinders with equal ease? Wouldn't the spark tend to jump easier on the plug with less compression resistance (the cylinder not on it's firing stroke)?

I've also heard that ultimate spark voltage is limited on systems with shared coils when compared to individual coils. True?

Sherwood

I can't speak for all systems, but the electromotive has two posts per coil pack. However I think each post, while triggered at the same point, are essentially separately wired... They look like one unit, but have concentrically wound "innards" that discharge separately.

Interesting observation though Sherwood... Electromotive is a great company and I'm sure they would be happy to answer this question if someone called them.

I thought Nissan (Datsun) did this already with their 200SX model back in the ancient 80's. They didn't need no stinking two distributors!

Hello Sherwood!

Yes you are correct, arc will jump easier in exhaust cylinder. But as you can see from the schematics, plugs are connected in series so both must fire (in order for circuit to be closed) or none of them will. With other words, only thing that happends when both plugs fire is that one that fires in exhaust phase will "shorten" easier and thus deliver more "juice" to other one that is in it's combustion phase.

"Standard" shared coil needs to be energized more often than coil that just fires one (or two) plugs. There is just less time for it to store energy and it might thus deliver less joules per ignition at high RPM's

The Nissan cap & rotor were perfect for 4-cylinder 912 dual-plug conversions -- especially when Nissan switched to Zytel material for the rotor. The early plain plastic rotors would fly apart at around 6,000 rpm.

I believe that one of the advantages to twin plugging is that the burn is completed in less crankshaft degrees, thus requiring LESS ignition advance.

In horizontally opposed engines, some theorize advantages to firing the lower plug 2-3 degrees earlier than the upper plug due to fuel puddling (gravity). I don'no -- beyond me.

Next day and this thread is still here.

The dual plugs have a couple of good advantages.
Yes, it shortens the flame front. This reduces pre-detonation and allows you to make up for crapola gas. Here in this local, we have cheap gas prices compared to the rest of the nation. We have 3 major pipelines run through this county. Thus, gas is sold as is, right out of the pipeline with zip additives. It varies in quality from station to station and day to day.

The dual plug motor doesn't need as much ignition advance for power.

The other thing is that the dual plug motor has a torque curve of a table top. It pulls right through the mid range and doesn't seem nearly as "peaky" as most of the single plug motors I have delt with.

There seems to be some movement in ignition systems mfg's, that will help produce a "reasonably priced" stand alone distributorless ignition system.

I found at least one website that is working on the Ford ignition system. Megajolt Lite as and offshoot of the Megasquirt EFI development effort.

BTW, I was just looking at the Ford Racer #999. This car set several records while driven by both Henry Ford and Barney Oldfield. It has two spark plugs per cylinder.

David Duffield