Interesting

Revmaster.

They did my twin-plug heads. There's a whole 'nuther world out there.

Stan, are you running those heads?

More to the point, Stan:  Are you running that propeller??

Yes. Revmaster 049s- they're the only people out there that do the twin-plug thing (that I could find). Great Plains used to do it, but no more.

Nope. Not the turbo either.

Pity, that.

....forgive a bit of a tangent here

    But since 'dual plug heads have been mentioned:

Do both plugs fire simultaneously in one big bang, or is there a miniscule firing lag to reinforce the compression stroke?

Great question, Carl.

Back in the day when people did this, they made an effort to lead or lag one spark before/after the other. Most everything I've read tells me that doesn't really work as well as just firing them together.

Aviation guys have two completely separate ignition systems- twin plugging in aviation is primarily for redundancy.

The idea in a performance application is for a faster burn- the idea being that two flame fronts are better than one at making sure combustion is complete and rapid. Back in the day, Jake Raby indicated (when he was still posting technical stuff on forums) that there isn't really anything to be gained until the bore size increases to about 100 mm-- but I figured it couldn't hurt, and might help.

If the engine can utilize the scheme effectively, it is theoretically possible to run a compression ratio one point higher than would be possible otherwise. I'm running 10.5:1 with an FK8 on 93 octane E10. It's just an ever-so-slight touch more compression than would be absolutely ideal, and I wonder occasionally (as I remember what I went through to get here) if all of the effort was worth the results. My timing map is fully customizable, and tailored to avoid preignition and I consider that to be a critical component in the CR I'm running. It's really right on the edge of too much, but still on the right side I think. Still- it's the best combination so far, and I've had 5 combinations in the car (FK43/9:1 2110, CPR "Torque Grind"/9.5:1 2332, SLR XR310 on 105* LC/9.5:1 2276, SLR302/9:1, 2110, and this one).

You've got to really want to do this, as nobody else wants to put their name on it. The ignition was at least as hard to come to terms with as the build itself. If I had it to do again, I would... but only if I was as full of vim and vigor as I was when I started.

Stan....Were you not afraid of detonation by having two flame fronts coming together at ignition at that compression ratio? What was your logic in doing this.....Bruce

I don't know Stan, why reinvent the wheel?  If I wanted to go twin-plug, I think I'd opt for something more off the shelf:

That thing runs a tad rich when cold, huh?

But what a great SOUND!

Twin-plugging is actually meant to REDUCE the possibility of detonation, Bruce.

Let me take a stab at explaining. I'm lifting some of this from a 2000 paper called: Engine Basics: Detonation by a guy named Allen Cline. Some of this is mine, some is his (so I can't be accused of plagiarizing).

As you know, what ideally happens inside the cylinder when the spark plug fires is not an explosion. It is an orderly flame front moving across the cylinder, building pressure that peaks at 14* AFTC (that's a magic number, and is not arbitrary- it's carved in stone). Shop class in high-school did a poor job of explaining what happens when an engine detonates. We get the idea that detonation is competing flame fronts colliding in the combustion chamber. This is incorrect.

Actually detonation occurs when unburned air/fuel mixtures (called "end gases"), under increasing pressure and heat spontaneously combust, ignite solely by the intense heat and pressure inside the cylinder. This is an explosion, as opposed to a flame front.

Detonation causes a very high, very sharp pressure spike in the combustion chamber but (for a variety of really interesting reasons) it is of a very short duration. In the lab, guys with really cool jobs can look at the pressure within the combustion chamber. With a detonating engine, combustion (and pressure-rise) progresses normally, then suddenly spikes sharply at the point when the detonation occurred. That spike always occurs after the spark plug fires.

That fact is really important.

Detonation is influenced by chamber design, compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the mixture spontaneously combusts (remember: it is pressure, temperature, and a messy flame-front that cause the end gasses to ignite). Almost always, backing off the spark timing will stop the detonation (by reducing the pressure inside the combustion chamber).

The longer it takes to complete controlled combustion, the greater the chance of detonation. In a sloppy flame-font environment, ignition needs to be advanced to the point that detonation becomes very likely unless the mechanical compression ratio is backed way off. The combustion chambers in a Type 1 are not very advanced, even with a good head guy shaping the chamber (the problems are in the architecture). Rule of thumb says that Type 1 engines tend to like about 30* of ignition advance for peak power. Air-cooled engines run hotter cylinder temperatures than water-cooled engines because air is a poor cooling medium. Inefficient combustion chambers and high temperatures mean that we need to back off compression and run better gas to stave off detonation.

That's where the twin-plug thing helps (in theory). Initiating a second flame front in the cylinder means that complete combustion doesn't take as long to finish as it would normally. The ignition timing does not need to be advanced as far to develop peak pressure in the cylinder at 14* ATDC. It also means that a higher mechanical compression ratio can be used since the pressure inside the cylinder is not as great as it would be with the mixture lighting at 30* BTDC.

Twin-plugging really just compensates for shortcomings in combustion chamber (and cooling system) design. This makes an 80+ year old, air-cooled design a really good candidate for the technology. Porsche used it on the later (964 and 993) engines. Datsun (Nissan) and Ford ran twin-plug ignitions in the early 80s in some of their engines. But as combustion chamber design improved, and everything went to water-cooled chambers, twin-plugging became a solution to a problem that no longer existed.

Jake Raby runs something like 20* of advance with his big twin-plug Type 4 engines- that's REALLY maximizing the benefits of twin-plugging. I'm running something like 26* total, but I have a lot of work to do to better tailor the curve to the application. I'd like to have access to a dyno, but I don't.

That's more than you wanted to know, but there it is. This stuff keeps my mind occupied when I should be thinking about more productive stuff, but it's where it wonders as I fall off to sleep at night. I nod off thinking about flame-fronts, etc.

Madness, indeed.

Well written, Stan.

Stan

how does your system work?early rsr's had the single distributor,964/993 use the the mr softee double cone distributor,and my last track car had crankfire ignition.

You can easily squeeze two ignition modules into one distributor and then run each to separate coils and on to two sets of plugs. Here's a pic of that in a Corvair dizzy ( which is larger than your VW example ) but I've done it to smaller Mitsubishi's too.

This guy taught me all I know.....

Hey David here is a better picture!

Well written indeed! Thanks. Years ago I became familiar with dual ignition on Fire truck engines. Primarily Hall Scott and American La France. They were only for redundancy as well. The drivers thought that by using both systems they could get more power. They did, I guess, but we seemed to be replacing pistons right and left for a while. Your post here helped to unravel that mystery after all these years. What we did back then was disable one of the ignition systems to solve the problem. As far as I know, there was never a "failed to start" reported using one system.

Now you know why I asked my question......Bruce

Just another comment. I recall seeing a film made by GM of Ethyl Corp. where they built a special engine with a really thick camera lens built into the cylinder head. They could also change the compression ratio while the thing was running.

They installed a high speed camera and filmed regular flame front propagation in slow motion. It was beautiful to watch! It looked like a Tsunami as it burned and moved across the top of the piston. You could even see the spark plug fire.

The they jacked up the comp. ratio to detonation. That was NOT a controlled burn by any means! It was an explosion and was as instantaneous as the spark from the plug They had to re-shoot at a higher speed to slow down the explosion enough to see more detail. Never forgot it............Bruce

I have a locked out (no advance) 009, modified to accept a Datsun twin-plug distributor cap and rotor (two coil inputs, 8 plug outputs- 1A, 1B, etc.). I use one pick-up as a trigger for on solid state ignition controller (modified CB Black Box) with two coil drivers- this means that both plugs fire in sync with each other no matter what. The coil drivers control two canister-style epoxy coils (A and B), which go back to the distributor.

Crankfire and two Ford Escort wasted spark coils would have been more accurate and way less work. It also isn't as cool-looking as a "bundle-'o-snakes" twin-plug distributor cap sticking up for God and everyone to behold (not that a single person on God's green earth besides me cares).

i care! that is wicked cool looking!i hope you got an extra cap,that looks exspensive.if it makes you happy thats all that matters.

Love that setup, Stan. And I love the reason you did it even more. Why do it the easy way when this insane way looks so cool--and so much more like it would have been done in 1958! 

You do it the hard way because you care. 

That, friend, is what this hobby is all about.

I see some twin spark plug VW heads, ever now and then, advertised on the samba web site.  The idea of two plugs on airplane engines was more to keep the engine running, not to improve on performance.  I have built two engines for experimental small planes in the past. Both were cut down VW cases and we used the flywheel to attach the prop to it as that seemed like the stronger area and gave us the flywheel teeth for start up purposes.  There are several companies ( sorry, don't recall their names0 making two cylinder and small four cylinder engines for the small one person airplanes or experimental licensed aircraft. 

I love the picture of the monster aircraft engine in the Beetle. So funny !  Reminds of my Brother putting in a 455cu. in. Olds engine under a Super Beetle body.  The frame was a shortened Chevy truck, with Olds Tornado front wheel drive, but we only used the suspension system, to tuck the front wheels under the Beetle body.  The radiator was aluminum and we did not have to cut too many round holes in the Super Beetle hood and the car sat close to the stock height of the normal Bug.  Engine had stock 300hp and it was only a stock engine, mounted in front and we sat where the rear seat would normally be, as the engine and automatic transmission took up so much of the interior space.  The Bug really got a lot of attention.  It was super fast, once the tires on the narrowed Chevy rear end, stopped spinning, which it would do , too easily, I all gears.  So, he drove it back and forth to work for two years, then sold the weird car.  I hear someone modified the 455 engine and why would they want more HP, I don't know.  Don't know what became of the beast.  Weight was only 2500 pounds and with 300HP, you figure out how fast it was.  Very quiet, with single muffler and stock exhaust system. You could barely hear the engine running. 

I like the monster aircraft engine the Beetle. So funny !  I was beat by an banged up looking 1967 Bug, on the highway, when doing 90 mph in my 1966 Beetle, which had a top end of 110.  I finally got lucky and the bug pulled off the highway at an exist.  I pulled up next to the bug and the two young men were laughing.  I asked them what they had in the VW Bug, as nobody in a Beetle had ever passed me like I was sitting still, when actually doing 90 mph.  They had installed a stock 200HP Porsche 911 six cylinder engine and five speed Porsche transmission.  I have no idea what the top speed might have been on their banged up looking Beetle.  Awesome engine swap. 

---George K. ---

As I've said before, wicked cool, Stan! And Ed said it best. Al

The real reason for dual ignition in an internal combustion aircraft engine was twofold. Redundancy ( safety ) for sure but it did also improve performance. Same thing for those old Fire truck engines I'm guessing too. They both had huge combustion chambers and with two sparks occurring at the same time, combustion was more complete...   = more power.

Just before takeoff in a small airplane with a combustion engine, the pilot will raise the rpm to 1,800 rpm with the airplane parked into the wind and do a mag check. He will switch from the left mag to both mags and then to the right mag and back to both while observing the DROP in rpm while either on left or right. If the drop ( loss of power ) exceeds 75 rpm or whatever is in his flight manual, prudence says he takes the plane back to the hangar to find out why before going flying.

I've built a 1/2 VW engine for a small plane. The prop goes on the opposite end of the crank that the flywheel goes on. The prop literally becomes the flywheel and we always started them by hand.

Morrey Hummel sold the plans to build these 1/2 VW engines back in the '80's. Others have sold them recently but recommend not cutting the case....the rear of the case ( flywheel end ) actually becomes the motor mount. All of the internals are the same. I once saw a three cylinder VW engine run on a test stand. Two 1,200 cc cylinders on one side and a 1,915 cylinder on the other. Many counterweights on the crank. I asked the guy why he did this and he just shrugged his shoulders and said he couldn't get to sleep one night.

Here's a pic of two old photos of my engine. One on the table and another of it running with an old friend in the pilot's seat.

I still don't know, Stan.

If all you want is eight wires, why not just put in a small block Chebby?

Doh! 

I really like Stan's setup. A LOT!

But I went with the Megajolt/EDIS crankfire. Was it easy? NO! I built it myself, and machined all the parts for the VR sensor and the TPS. I built the unit and fabbed up a wire harness. Hardly plug and play. But accurate as heck. Also it was the only solution available that was accurate AND would work with my 911 shroud. The 911 shroud doesn't allow anything to fit in the distributor hole other than an 009 or similar.

If I ever go dual plug, that will be easy, I'll just add a couple of coil drivers and another coilpack.