But the gist of it all is twin plugging WORKS, provides more power ( ever drive a 964 with a broken dist belt). Porsche did it on there early race cars.. and the collective Porsche braintrust blows all of us out of the water onsome of these subjects..

Im sure POrsche has quite a bit of internal data and documents regarding twin plugs........... it would be dry reading Im sure but quite informative.

In a previous thread titled Why do lean cars run hotter? I attempted to put it in lay-terms.

Here we go:

Capturing the "BANG"

Air+fuel+spark=BANG
gunpowder+spark=BANG . . .that all seems simple enough.
Or simplistic?
Pressure is the missing ingredient in the above equations.
For example; If you make a little trail of gunpowder (or pour a trail of gasoline) on the ground and light it, you get no "BANG" ; just a fizzle or a flame.
If you take a bit of the powder and wrap it in paper (firecracker) that little containment will cause the pressure to rise fast; which in turn causes the chemical reation to go even faster =BANG.

Pressure management:
The piston engine wants to have a fairly fast burn, but not to the point of "bang" (detonation)
Think about the forces you put on a bicycle pedal. You wouldn't want to jump on the pedal only at TDC, and sit back on the seat after 15° of rotation.
So too, the engine wants to capture the burn pressure to extract a nice even push (work, hp).
The engine geometry is infuencing the burn rate by the changing volume (effecting pressure by giving more space).
The advance is infuencing the burn rate by where in the geometry (bicycle pedal) the burn starts building pressure. Too early and the pressure rises fast, puts some of its work into pushing the piston, and a lot of work in to make heat for the head.
So there *is* a lot of balancing going on there beyond just lighting it off.

since getting a 993, I have done a lot of research about the car and Porsche's (at that point in time) latest and greatest advancements. I thought reading about how the secondary ignition is fired microseconds after the first to burn what was not burned from the primary ignition. Maybe I dont remember correctly.

:confused:

If I recall thats what the top fuel cars do since they have trouble burning the fuel they pump through those things. I realize they burn 90% nitro-methane, which burns (explodes) at a much slower rate than gasoline, but none-the-less it is a internal combustion mechanism.

Thank you all for contributing!

Chris

Im gonna buy either a twin plug 3.6 or a single plus, what gives here, they are both rated at the same horse power, is it worth the money for me to spend the extra on a twin plug motor, Kevin

Kevin, as little or as much as I understand here, one thing is accepted. Twin plugs allow for higher compression ratios and higher CR is more HP. So, it's like the old adage, "How fa$t would you like to go?"

Now here's a thought - several types of cars use multiple coils (one per plug). For example, the RX-7 had 6 coils, 2 for each rotor. Presumably fired by low voltage pulses from the electronic ignition... See where I'm going here?

Maybe some potential to hack together a working system... In essence you'd be going from a mechanical (distributor) to electronic ignition system. Scavenge parts from a couple RX-7s, plug in a Motec engine control unit - might as well upgrade to electronic fuel injection while you're at it...

Never mind! Sounds expensive too...

-Boyo

Yeah Island!!!
 

I think you summed it up beautifully in lay terms Island. The bicycle analogy is right on the money. It shows the correlation between timing and a the burn rate of fuel to extract maximum power.

Advance is a compensation for the burn rate of fuel not being fast enough to complete on the combustion stroke right?

To much advance and detonation occurs from too quick a rise in tempurature on the compression stroke.

Twin plug to the rescue. Quicker controlled burn rate at a later timing.

I never quite completely got it until now.

Thanks
Bobby

P.S. It also shows why higher octane achieves the same thing.
Your a genius Dr. Island!!!

Boyo they also do this with a kit that uses a crank trigger, ots of Porsche use this at the track, Kevin

So, if I can turn the discussion to the practical side . . .

- How much does it really cost to twin-plug a 3.2? What with the parts, the machining, engine drop, assembly, I'm wondering if we're talking $2,000 or $6,000?

- How much more HP and torque does it give, on its own? Assume the 3.2 has the stock US C.R. (9:1?) and you don't increase it.

- How much more HP and torque does it give, combined with a C.R. increase? Assume you now increase the C.R. to something that works with Calif gas, say 10.5:1?

- Beyond HP and torque, are there other advantages of twin-plugging? Better MPG, cleaner emissions, etc?

- Beyond the initial cost and the added cost at tune-up time (replacing 2x the plugs, wires, rotors, etc), are there disadvantages of twin-plugging? Trouble with Calif smog, have to relocate heater duct or other things in the engine compartment, etc?

Great thread..... v. interesting.:p

following on from jyl.... does twin-plugging affect reliability and can it result in increased engine wear (assuming that engine internals ARE modified to deal with the incresaed C.R) ???

Can't comment on most of the questions except price...

Typical machine price -

Modify heads for twin plug - $330-350 (seen as high as $670 for changing both plug sizes from 14mm-12mm)
Modify lower valve covers - $50

Ignitions:

RSR Twin plug diz - $unobtanium
Electromotive - $1500
Modified 964 dual diz w/ MSD (rennsport) $1900-$2500 (depends on donor parts)
Programmable systems $1000-How much you got?)

Labor 0-? depends on if you disassemble

So I would figure on $2000-$3000

I've done this and this is the parts costs:
- Andial splitter box: ~$600
- Machining heads and valve covers for twin plug: ~$400
- Used 964 or 993 distributor: $250-400
- Custom chip: $450
- 2nd set of plug wires: ?
- 2nd coil: ?

Knowledgable people will say that there is a power improvement and other knowledgeable people will disagree. My take on twin plugging is that it will allow you to increase compression and thus make more horespower but I think it is a waste of money on a stock motor.

One down side is that I have to remove the headers to change my lower plugs.
See my web site for what was involved in my own twin plugging project.

-Chris

What About Those JE Pistons Anyway?
 

Chris, thanks for the good content on your website.

Sorry about the pistons.

A slight detour of topic - I occasionally see stories on this board of JE pistons failing but don't recall ever seeing such a story about Mahle pistons. Is this because the JE pistons are inferior? Or because hot-rodders / budget racers tend to use JE pistons (for cost reasons?) so those pistons are more often put "in harm's way"?

Thoughts? Maybe you've talked to other racers, etc, about this.

Re: What About Those JE Pistons Anyway?
 

I haven't updated my web site in a while but I am not prepared to lay the blame on the JE pistons. From what I can tell, the motor went lean. Possibly because of a fuel tank venting issue. Mahle's do fail but perhaps they are used by more competent engine builders who avoid the problems that would make any piston fail.
-Chris

Just to add some more inflection to an already complex discussion...

People often say that twin plugging adds HP. Most of the knowlegeable people I've seen or read say that it improves torque, but doesn't make a big difference at peak HP. This is confirmed by the data mentioned above. In all the data that I've collected; Peak HP seems to be overwhelming related to the amount of air moved through the engine at high engine speeds, so it's a function of engine capacity, engine speed, cam spec's and porting and valve size. Note that neither ignition timeing nor number of spark plugs appears has much of an impact on the flow of air through the engine.

Increasing the CR does increase the torque somewhat, but if you look at the changes in torque/liter of the different 911 motor's in the 70's, you won't find a big drop in the torque/liter as the CR's dropped rather dramatically. So there are obviously other things that factor in with a greater impact, such as cam timing and intake/exhaust tuning as well as combustion quality.

Which now brings be back to partially rebut myself...

Twin plugging should improve the combustion quality by ensuring that more of the fuel/air mixture gets burned. I think that this was the primary motivator in the case of the 964 since the improved burn quality significantly reduced emissions. We shouldn't underestimate the importance of emissions to an auto manufacturer such as Porsche. Anyhow, Hemi-headed engines such as the 911's, Ford's SOHC 2.0, Jag Straight 6 and Alfa's inline 4-Cylinder have always responded well to twin plugging because the large combustion chamber requires a large "intruder" on the piston crown to get a reasonable CR. This intruder gets in the way of the flame front. They also lack a "squash" zone which also improves the combustion quality. ("Modern" penteroof 4-valve heads have none of these problems.) The very large bore of the 964 also contributed to the poor combustion quality since it just took sooooo looonnnggg (in engine terms) for the flame front to reach the extremes of the cylinder from the single plugs off-set position. As far as 964's with broken distributor belts, keep in mind that the distributor timing was set for twin plugs. When the belt breaks, you are left with a single plug engine with the spark timing far too retarded. I suspect that if you were to take that hypothetical 964 and advance the timing (as a "battlefield repair") that you would gain back a lot -- but not all -- of the missing performance.

Just some more things to consider.

So what about aftermarket multiple spark ignition systems? I believe manufacturers such as Saab are using this technology as standard on their engines.
I suppose if the sparkplug is in a poor location then multiple sparks will have little benefit.

What interesting reading this thread makes- thanks to all contributors for clarifying the twin spark theory.

I agree with jluetjen. Unless you have variable cam timing, the benifits are, maybe a bit, in the mid-range, and mostly for emissions. But only if match w/ a higher compression ratio.

From the thread The effects of higher CR is this chart, showing the relationship between CR and thermal efficiency.
(just one more factor in understanding the engineering balancing act)

http://forums.pelicanparts.com/uploads/thermeff.jpg

Actually the reality is different. This I can state as a fact after installing an MSD on my 2.0 (With S pistons). MSD's do make a difference under certain conditions. As far as I can tell these are:

1) Sub-obtimal or large combustion chamber designs (such as a 911's).

2) Relatively slow engine speeds. Since MSD's only do their MSD thing at speeds of 3000 RPM and below, I guess that this is the range.

Why: As the fuel/air mixture flows into the chamber, it (hopefully) does not sit still. This is know as "Swirl" and "Tumble". In the range of head designs (from Hemi at one end to bath-tub chambers like in a small block V8 at the other. We'll ignore Heron Heads), Hemi heads apparently have great flow and poor swirl and tumble charactoristics. Bath-tub and 4-valve Pente-roof designs apparently have much better swirl and tumble charactoristics. The same applies to squash zones which come into play as the piston approaches TDC. All of this movement inside the chamber ensures an even distribution of mixture within the chamber as well as affecting the flame front. Even in a poor hemi-head's chamber, there is some movement. So if the ignition does not initiate a good burn on the first try, the 2nd or 3rd time that the the plug fires will often work better if the conditions around the plug are different. As the engine speed increases, there is less and less time for multiple sparks, plus the increased gas velocity apparently increases the mixing. MSD doesn't do much with engines on the other design extreme since the mis-fire much less. Basically the extra sparks are waisted on already burnt mixture.

BTW: 911's hemi heads and big valves flow a lot of air, but have compromised combustion design -- especially the 2 liter cars with S pistons. On the other hand, when Keith Duckworth designed the Cosworth DFV, he consiously chose to use smaller valves then he could have in order to improve the swirl and tumble charactoristics. The increased velocity passed the smaller valves helped with this. The result is that the engine had excellent torque/liter and pretty much pushed out all of the other F1 engines for over 10 years with the exception of the Ferrari 312 engine.

Just to add to Chris' (Bennet) comments, I think you need a second Bosch CD box (or two MSDs and sell your one Bosch CD box to recoup the cost!).

Looking at some of these prices, I start to feel a bit less crappy about how much I spent...

I have a 964 dizzy modified to fit a 3.0 case and with a chopper plate inside to provide a crank signal for a MoTeC. I did none of the work, but incessantly pestered the guy who did.

If I was just after twin ignition, what would I do....

....wait for it (I have been thinking about this).

To back it up, here is my understanding:

- no point in twin ignition unless you have high compression. I don't see the benefit on a stock Carrera (I think you would fail smog too). For a rebuilt 2.7 or SC with high compression pistons (and in particular more exciting cams) I see a benefit, in particular because a high compression piston with deep enough pockets for S cams and beyond has a REALLY high dome (have a look for Chris Streit's pic of his new pistons). Thus you need carbs, MFI or fancy EFI with individual throttle bodies (cause these cams won't run well with CIS or airflow governed EFI).

- if you want twin ignition, you need the heads and lower valve covers modified - approx US$400

- then you need the cheapest way of getting a pair of sparks to each cylinder. It seems this is Electromotive (~$2000) unless you are good at DIY and can find a cheap 964 distributor and donor SC one (for points, advance mechanism and drive/gear, forgotten the name of the bit at the bottom). You still need two CD boxes then (ie MSD)

But what I would do is think about a fully mapped ignition with a crank angle sensor. Ignition only assumes you have carbs or MFI. It isn't too expensive - eg:

NZ made IgnitionLink is approx US$500 (NZ$990) plus 2x ignitors at about US$110 each and a PC link at $40. Issues are figuring out the crank angle sensor and finding suitable coil packs (eg the RX7 idea above). This could be CHEAP as well as good - I can see under US$1000 and it should be pretty easy to program (just copy the factory advance curve for starters.....)

I might give it a go on my BMW (but single plug) as the feeling of accomplishment would justify the $$$.

Other stuff I can think of:

I've used a full set of 964 plug leads.

My lower spark plugs are "little" ones.

I still run 98RON (93 your gas) with 10.3:1 c/r.

Wow, this has turned into quite a discussion. Without going into too many details the following statements are true with respect to twin-plugging on the 911:

- The 911 head does not give good combustion in it's original form. This is because the spark plug is off to the side. Theoretically, it should be in the center of the head, like it was on the 959 (four valve heads).

- Twin-plugging a stock engine will buy you almost no performance improvements. Twin-plugging is an upgrade that needs to be combined with either:
a) an increase in compression ratio or total compression from forced induction
b) a signifianct increase in the piston diameter (like 100mm or greater). This is because the time it takes the flame front to travel across the piston is significant at these larger diameters.

Curiously enough, the factory did not twin-plug the Turbo cars, yet I would suspect that they could have benefitted from it. Cost savings?

The additional power from the 964 does not come from the twin-plugging, but instead from the higher compression. The twin-plugging allows you to run that higher compression safely and effeciently, giving you the power increase...

-Wayne