Why Do Twin Plug Engines Use Two Distributors?

[Oldporsche]

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

[Lorenfb]

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.

[Oldporsche]

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

[911pcars]

"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

[beepbeep]

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?

[911pcars]

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

[cstreit]

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.

[movin]

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!

[beepbeep]

Originally posted by 911pcars
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)?
Sherwood

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.

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

"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

[Geary]

Originally posted by movin
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!

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.

[Oldporsche]

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