Dual Plug Heads, or, What's The Alternative Again?

Tuners of vintage Porsches, tuners of vintage VWs, tuners of Harley-Davidsons, pretty much tuners of any air cooled big bore engine being it vintage or modern-ish vintage likes to convert to dual plug heads for better flame propagation, especially in hemispherical combustion chambers. The result is not so much a horsepower increase, but a torque and throttle response increase instead, due to better combustion allowing a point higher in compression ratio and cooler running temps.

But I vaguely remember reading something about someone saying all the trouble to tap for that second plug and then figure out the correct retarded timing was unnecessary if etchings called "flame channels" or something similar were shallowly carved into the surface of the combustion chamber. If I remember correctly, these etched lines went from the spark plug opening to the other side of the chamber where a second plug would theoretically be tapped; their purpose was to somehow speed the spark flame along the chamber so that no second plug was needed.

Anybody know what I'm vaguely remembering, or is my subconscious playing tricks on me?

Now, I'm no engineer by any means, and the only dual plug engine I've owned is the current Ram pickup with the gas engine. But I was curious about the comment of "figure out the corrected retarded timing". Since the dual plug design is supposed to generally offer better combustion characteristics and diminish detonation, why would one have to retard the timing? In some cases I'd think you might even be able to advance the timing due to diminishing detonation. Again...no engineer here, and I'm probably looking at this way too simply.

I think "retarded timing" is a term that can cause a bit of confusion as it is given as a sort of negative to an ideal: it refers to needing less advance in order to attain proper combustion. This is a desirable trait. In general, the less advance an engine requires to have enough time to get complete combustion the more efficient it is burning its fuel and air and the more power it can make. If an engine can run less advance with dual plugs it's an indication that it has sped up its combustion and it will make more torque with the same charge, as less energy is being given away by the rise in combustion pressure before the piston has reached TDC.

In the 80's we squids all thought that the hot ticket to making out GSXR's run better was to install a 5 degree advance wheel on the crank- it did seem to aid extreme high speed running, but some midrange and bottom end was sacrificed. The real tuners knew better and focused on porting and combustion chamber smoothing- all efforts to improve charge filling and turbulence to reduce combustion time- and made their bikes run better everywhere. More advance requirement can be seen as an indicator of poor combustion efficiency.

As for the flame channel notion, I can't speak to it other than to say it's not something I have encountered, and that it is generally accepted that the smoother and more even a combustion chamber can be made the less problematic it becomes. As an example, in early 2-stroke development there were pistons with dividing fences on their crowns intended to control the shape of the incoming fuel and air charge, and to reduce the mixing of the new charge with the spent exhaust gasses. This seemed like a perfectly good notion until the it became clear the fences themselves created even larger problems with heat control, becoming glowing hot spots that caused detonation and seizures. I would suspect channels cut into a head or piston would create their own issues.

But... if anyone out there has direct experience with this I'd love to hear about it.

Are you talking about Singh grooves perhaps?

Haven’t heard anything more on it in years. Was quite a splash, what? Almost 20 years ago. But since then, it’s faded away.

The originator still hawks it:
https://somender-singh.com/

Dave Vizard tried some experimenting with it, and found little benefit;
https://www.speed-talk.com/forum/viewtopic.php?f=1&t=47093&start=105

Yes, thank you, this is the man and his odd technique. I did some duckduckgo.com-ing and found that most folks who have tested Singh grooves on their own have not found them to be of any real world good. Basically, it seems like snake oil at best.

I had dual plug/dual ignition + dual carburetors on an ultralight aircraft 2 cylinder, 2 stroke engine once upon a time.
Safety in redundancy.

As one who dual plugged my airheads back in the 70s and 80s, the rationale was to be able to burn lower octane gas, not any power increase. Yes, there was some efficiency increase, but that was not the motive for most of us. At that time, we were going from 95-98 octane (R+M)/2, to about 89 at best. And often only 87 was available, especially on trips. Add in hot days and detonation was unavoidable. I think that 91 octane and the timing changes from the /5 /6 days addressed a lot of that. The R60 models might still benefit from dual plugging, but unless it is really hot, you probably can just retard the timing to the later specs, and run the highest octane you can find.
EDIT: I was in SoCal when I dual-plugged, at UCR. It made a big difference then and there.

Dual pluging airheads back in the day was something that grew out of racing them. It was found that it also worked to burn nasty fuel from across the big pistons that had grown from 600cc to 900cc. We had yet to get big spark producing coils that would appear later in the early 80's.

Spark became interesting to me thanks to having worked on both Volkswagens and owning a Norton 850. I took the sad little OEM 6 vdc coils and threw them into the garbage after sourcing what Cycle Magazine called, K-Mart coils in their piece on the subject.
I did not retard the timing of the Norton. I could in fact widen the spark gap, because now I had a nice big high voltage spark to just a gap of .060" and produce nice low idle.
Another benefit for a kickstart only machine was that it fired halfway through the first kick. Every time.
And that got me a job as a BMW motorcycle mechanic.

We dual plugged a lot of BMW's. Retarded their timing back to about halfway between TDC and the OEM spot if I remember. I do know I experimented with my 1979 R100RS retarding back to TDC and slowly moving it to get it where I was happiest.
That was a very long time ago.

The high output generic NAPA coil for the right side can be seen ahead of the side cover, behind the carb,


Stock Champion N7Y sparkplugs gapped at .060".

A link to the first page of the piece in Cycle, I still have that magazine.

https://nowels.smugmug.com/Motorcycles/Back-In-The-Day/1974-MKIV-Norton-Interstate/i-SmqFXnM/A

The second plug on BMW airheads was opposite the main plug and a shorter reach, but same heat range plug. The coils were dual output from a Dyna-something. I didn't do the deed to my last airhead until I floated a valve, no rev limiter.

Note the oilheads have dual plugs, the 1150 I had, used standard coil for the bottom plugs and coil over spark plug coils for the center plug. Those coil over plug caps failed about every 30,000 miles or so. That became pretty expensive at around 200,000 miles.

The 2010 990R KTM I have now approaching 80,000 miles has a single plug in the middle of the head for each cylinder.
Seems to work just fine.

I started working part time in a motorcycle shop in the mid-70's. Me and the head mechanic owned '73 Z1's. After doing some research on an aftermarket, solid state ballast resistor, we installed 4 automotive coils. We were able to gap the plugs at .060" like you, and we got equal or better points life. The other guy did an 1170 kit on his Z1 later which really benefited from a stronger spark. My main benefit was better fuel mileage and longer plug life. It's interesting the kind of creative but often simple approaches that work, especially with older designs.

Dual plugs and/or stronger spark can really be a benefit in some cases. My Ram 5.7 pickup is the only dual plug vehicle I've owned, and it seems to work extremely well.

My 76 BMW R100RS I converted to dual plug heads, in part to address the potential for pinging under load without having to reduce the compression ratio. I also updated the ignition system to electronic from Dyna along with the dual output Dyna coils. Made a big difference. On my 2003 Vulcan Classic VN1600 it has dual plugged heads from the factory. Again in part to address the potential for pinging. Those big bore twins have a lot of combustion chamber for the flame front to effectively cover, so two plug flame fronts make for more efficient burn and less hot spots developing in the combustion chamber that leads to pinging or auto-ignition.

Some high performance engines use multi-spark technology; the spark plugs fire multiple times per power stroke to create a more thorough burn.

I believe the dual-plug oilheads actually have different timing curves for the two plugs. My KTM 1290 has a similar arrangement with a central plug and "side" plug in each cylinder head. They have different timing curves. This is all supposedly for emission reasons.

Hey everyone, slightly off topic but I didn’t want to start a entire new thread just for this question. I currently have an R100gs with dual plug heads. I am in the process of simplifying and cleaning up my wiring and would honestly prefer to go back to single plugs. Does anyone sell a kit to thread in and plug up the second spark plug hole? could I simply remove the second the coil and wires, revert to stock timing and leave the lower spark plugs in to block the hole?

My 1972 Yamaha came with two plugs in its head, with only one of them ever being utilized- I suspect the only downside to just deleting the 2nd half of the dual plug system and leaving the 2nd plug as a place holder is a small reduction in compression ratio and a possible hot spot.

I think the dual plug setup is a terrific idea on the airhead motor, and would be reluctant to delete it were it mine- the few I have tinkered with were set up as a redundant system and offered a functioning ignition if one coil failed. I admit the wiring is a bit unsightly, and it could be argued that the 2nd plug isn’t really necessary given the number of airheads happily trundling about without. The most compelling argument AGAINST dual plugging is the relatively high cost of the conversion vs it’s actual benefit. It is an improvement, particularly if one has made other engine modifications, but how measurable of one seems an open question.

JT is in a terrific position to provide a before and after with his machine, as the question is generally weather or not to do it instead of deleting one that had previously been done. Confirmation bias is a real thing, and I suspect most folks who have taken the time and effort to convert to twin plugs are going to claim benefits weather they exist or not. His bike could be run with the dual plug for a time as is and then have the 2nd plug “turned off” by disconnecting the 2nd coil and resetting the timing to stock, and the results compared. If his bike is otherwise stock (or largely so) he could provide some additional input to this question.

Two plugs usually done to get more efficient burn in the cylinder, making for better fuel economy. Two wires I'd leave there.

If that was a 2-stroke, that hole was probably for a compression release.

This has been an interesting discussion so far. Especially about the Singh Grooves, that I had never heard of before. I'm wondering if Kevin Cameron has written anything about them.

For clarity's sake: Early oilheads, eg. -all 1100 and 1150/1200 models up to ~2002? were single spark. The valve covers are shaped different and an easy tell.

Single spark:


Not sure if it was emissions or power that BMW claimed caused the change.

Dual spark:

It was during the 1150 era that the change was made. 1100's ended in 1999 (apart from the 1100S). Early 1150's were still single spark - definitely 2000 and possibly 01 models - but later 1150's had the dual-spark design. Pretty sure there were never any 1200's with single spark (maybe the 1200C but that was an oddball in many other ways).

Here I was thinking it was about dual plugs on an air head. I think they did dual plugs for better burn and efficiency. A number of riders I knew dual plugged their 70s-80s BMW air heads.

I had a 79 R100RS, recently sold, that came to me with the second plug installed, but not connected. I added the spark booster and the dual output coils to complete the process and the pinging was reduced. So the answer to your question in my opinion is that your GS will run fine on single plugs, but you may have to look at the advance to set it to stock specs if the timing was changed for dual plugging. Just leave the lower plug in and disconnect it, no special kit needed.

If you want to read a historical how-to piece about dual plugging check out this link:
http://www.rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf

The guy who wrote this book was on the BMW Daytona race team in the 70s. He continues to work on motorcycles in the Philadelphia area. He is tremendously knowledgeable, occasionally irascible, and usually very helpful via his postings to the Airheads email list at micapeak.com

You might also want to consult the voluminous information on this website run by Snowbum:

https://bmwmotorcycletech.info/technical-articles-list.htm

Depending on the type of ignition you have a coil or coils of different resistance may be needed, and the above link to Snowbum's site should get you started.

“If that was a 2-stroke, that hole was probably for a compression release.”

Aftermarket compression releases were indeed common for the large 2-strokes of this era, but Yamaha fitted as standard to the 360 a separate compression release that opened a small hole above the exhaust port into the exhaust. It was more like a compression reducer, but it helped a little for starting. Kinda. The RT360 has a well deserved reputation for being absolutely awful to start.

My bike came with an aftermarket compression release/lever/cable assembly, but I haven’t fitted it. It was evidently also useful for bulldogging down steep inclines on a dead engine. Ed Hertfielder wrote about that, but It’s not a practice I have employed.

I have seen some speculation that Yamaha included the 2nd plug in the head in order to allow one to have two different heat range plugs on hand- one for highway riding and one for slogging about off road- just pop of the plug cap and move it over- although I haven’t seen any direct confirmation. Another theory was that it gave one a spare plug should your other plug foul (a common occurrence with the oils of the day and the weak spark the bikes had) but it seems to me that the unused plug would likely be just a fouled as the one that was doing the work. In any event, the machines were delivered with two plugs. Weird.

I have upgraded my machine to a 12v cdi ignition and had Harry Klemm reshape the combustion chamber and squish band. The 2nd plug is still there, but it at least has the job of holding a cylinder head temperature ring in place. It is a more reliable to start, but you still have to kick it like you mean it.