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Why 2 plugs per cylinder?


JayW

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How does this arrangement work? Do both plugs fire at the same time with every compression stroke? Why is one darker than the other when plug change time comes around? Why do our Hexheads require 2 plugs while some other bikes and most cars do not?

 

Jay

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Firefight911

2 plugs is about emissions and more complete combustion. Getting the fuel/air charge to burn completely and to get the flame front to be in the fashion that provides the most optimum opportunity to produce the most power, etc.

 

The reason that the secondary plugs are darker is that they do not fire every time. They fire when the computer tells them to. Full throttle, heavy load, etc. Also in conjunction with all the sensors checking teh intake density, exhaust gas make up.

 

Hope this helps to confuse you more!! lmao.giflmao.gif

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I agree with you. Complete combustion and a good flame front is critical at the beginning of the power stroke. The expanding gases push harder on the piston at the top of the cylinder. You don't want the gases firing/burning too late while the piston is already moving down the cylinder. The two sparkplugs spread/ignite the gases more completely when needed.

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For the hexhead bikes the phasing between the plugs is variable. Not sure if the older bikes were also like that or just a fixed timing offset.

 

I would expect riding style to have an affect on the differential coloring between both plugs.

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Dave_zoom_zoom

Very interesting!

 

Does anybody know exactly when the secondrary plug starts to fire & when it stops firing? RPM? Throttle opening? Loss of vacuum? Or whatever??????

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roadscholar

Correct, twin plugs allow better flame propagation and a cleaner burn. But in this particular case it solved a problem that plagued many (lean burning) oilheads, which was surging.

 

In the sixties Porsche found that by twin-plugging their racing (air-cooled, horizontally opposed) engines, power was increased by at least 10% because of the more efficient combustion.

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Thanks for the good answers. Given the expense of new plugs, I wonder if the replacement interval of the secondary plugs could be extended beyond the primaries since they don't fire as often. I suppose it would depend upon the riding style of the owner.

 

Jay

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Correct, twin plugs allow better flame propagation and a cleaner burn. But in this particular case it solved a problem that plagued many (lean burning) oilheads, which was surging.

 

In the sixties Porsche found that by twin-plugging their racing (air-cooled, horizontally opposed) engines, power was increased by at least 10% because of the more efficient combustion.

 

Do you know id simply adding an extra plug per cylinder to early R-bikes would in and of itself add more power? If so, how much power?

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roadscholar

Do you know id simply adding an extra plug per cylinder to early R-bikes would in and of itself add more power? If so, how much power?

 

I'm more familiar with old Porsche race engines than Beemers, but I know many of the airhead performance gurus began twin-plugging their bikes by the early and mid-seventies, and it wasn't to cure surging. grin.gif How they fired the second plug or how much power was gained, I'm not sure.

 

There should be ample data available on this, possibly check the airhead forums. I would think BMW had to have experimented with or used it in racing applications (probably in the sixties also) but I'm just not that versed on Beemer history.

 

Airheads and (old) Porsche motors are similar because they're two-valve heads and there's room for another plug on the bottom. Although because Porsche used smaller combustion chambers and much bigger valves on racing heads, the plugs used were smaller diameter top and bottom for a better fit. Before the days of crank-fire ignitions and seperate coils for each cylinder, for a six cylinder engine they used a Marelli distributor from a twelve cylinder Ferrari to fire the plugs.

 

Oilheads, being a four valve head, I don't know if anyone's tried it or not. The figure 10% I used was a guess, but I think a conservative one based strictly on seat-of-the-pants observation. smirk.gif

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roadscholar

A simpler answer would be, who knows? If you were going to all the trouble to drill and tap another spark plug hole it would make sense to do other head work at the same time, like raise the compression and port (or at least polish) the heads.

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[quote

I'm more familiar with old Porsche race engines than Beemers, but I know many of the airhead performance gurus began twin-plugging their bikes by the early and mid-seventies, and it wasn't to cure surging.

 

IIRC, the main purpose of twin plugging was to help eliminate some of the airhead's pre-ignition caused by the reduction in octane during the late 1970's & early '80's. The alternative was a thicker base gasket to lower compression - naturally resulting in less power and higher fuel consumption.

Tom

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Firefight911
Thanks for the good answers. Given the expense of new plugs, I wonder if the replacement interval of the secondary plugs could be extended beyond the primaries since they don't fire as often. I suppose it would depend upon the riding style of the owner.

 

Jay

 

Every valve change I rotate the primaries to the secondaries and the secondaries to the primaries. No particular reason other than all the plugs get to be the lead dog. grin.giftongue.gifcrazy.gifdopeslap.gifsmirk.gif

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Correct, twin plugs allow better flame propagation and a cleaner burn. But in this particular case it solved a problem that plagued many (lean burning) oilheads, which was surging.

 

In the sixties Porsche found that by twin-plugging their racing (air-cooled, horizontally opposed) engines, power was increased by at least 10% because of the more efficient combustion.

 

Do you know id simply adding an extra plug per cylinder to early R-bikes would in and of itself add more power? If so, how much power?

 

The old bikes were double plugged to aid starting and reduce pinging, not generate any more power.

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roadscholar

The old bikes were double plugged to aid starting and reduce pinging, not generate any more power.

 

Thanks, that's what I get for assuming if A were true then B must be also.

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4-Cyl. aircraft engines (i.e. the Lycoming O-320 found in Cessna 172's) have 2 plugs per cyl. and 2 separate magnetos designated as "left" & "right" that fire them. You can switch the magnetos to either "left" or "right" or "both". The rpm drop from "both" to 1 set of plugs is very dramatic--typical pre-flight engine run-up drop would be from 1700 rpms on 2 down to 1575 on 1 mag. And 2500 is REDLINE on that engine so the drop is a substantial % of full power. On a hot high-density altitude day it could make the diference between getting in the air and NOT. They definitely provided more available power but also one other feature that is nice to have at 12000 feet--redundancy!

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bakerzdosen
You change your valves??? eek.gif

 

No wonder your tech daze take two days. tongue.gif

 

Me? I just adjust the valves. grin.gifwave.gif

LOL. lmao.gif

 

Thanks for catching that first Danny.

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Firefight911
You change your valves??? eek.gif

 

No wonder your tech daze take two days. tongue.gif

 

Me? I just adjust the valves. grin.gifwave.gif

LOL. lmao.gif

 

Thanks for catching that first Danny.

 

Both of you, to your rooms!!! tongue.gif

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The old bikes were double plugged to aid starting and reduce pinging, not generate any more power.

 

 

I disagree. If the design of the combustion chamber does not allow for optimum flame propagation, then a full burn of the charge won't occur. Thus, you have a loss of potential power.

 

Therefore, adding a second plug will aid in completing the burn &, ipso facto, will add power. Read some of Kevin Cameron's articles. He can do a much better job of explaining this.

 

For reference, take a look at Reg Pridmore's R90S race bikes from the seventies. Built by San Jose BMW, they were the pioneers of dual-plug heads on Beemers. When it comes to racing, if it doesn't add power or reduce weight, it doesn't get done.

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roadscholar

For reference, take a look at Reg Pridmore's R90S race bikes from the seventies. Built by San Jose BMW, they were the pioneers of dual-plug heads on Beemers. When it comes to racing, if it doesn't add power or reduce weight, it doesn't get done.

 

Thanks bro', I knew the principle held water, I just needed to get my facts straight. grin.gif

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I upgraded my '82 from single to dual, but I also kicked up compression from 8.2:1 to about 9.5:1. I did pick up about 2MPG (48+ max) and it does feel stronger on a hill.

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