# Two spark plugs per cylinder **URL:** \ **Category:** Factual Questions\ **Created:** [January 5, 2018, 3:48pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921 "2018-01-05T15:48:23Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [January 5, 2018, 8:19pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/21 "2018-01-05T20:19:01Z")
> [@TriPolar](#): > > The increased efficiency has been demonstrated in aircraft engines but I don’t know if it’s significant enough to justify the additional costs. Anything you get is a side benefit of reliability. There are two separate ignition systems for flight safety. They are also not connected to or dependent on the electrical system - some airplanes don’t even have them - since they run either on magnetos (ask your great grandfather about his Model T sometime) or solid-state systems. An airplane engine has room for 2 plugs per cylinder since they have separate air-cooled jugs covered with fins, like a motorcycle engine, instead of having them all jammed together in a single block with coolant passages getting in the way of things. > [@](#): > > In addition, those aircraft engines aren’t required to meet the pollution standards of automobiles and the improvements may not be the same when accommodating engines and pollution control systems under those requirements. They’re generally exempt from any pollution standards. The main concern is lead, and that appears to be on its way to finally being removed from aviation gasoline. ---
**Author:** ![busterpickle](https://avatars.discourse-cdn.com/v4/letter/b/258eb7/32.png) [@busterpickle](https://boards.straightdope.com/u/busterpickle)\ **Post date:** [January 5, 2018, 9:24pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/22 "2018-01-05T21:24:15Z")
Oooh… learned some stuff today without having to dig through thousands of posts. It seems that two plugs matter if the benefit out-weighs the cost. The benefit being power or efficiency or emissions. But, there’s going to be that trade-off in cylinder head being used for valves versus spark plugs. Would some or all of this be why BMW came out with laser ignition? Is that ignition system on the cylinder head smaller than a spark plug so you can have two or more lasers causing spark allowing for greater spark coverage? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 5, 2018, 9:32pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/23 "2018-01-05T21:32:13Z")
> [@electronbee](#): > > Would some or all of this be why BMW came out with laser ignition? Is that ignition system on the cylinder head smaller than a spark plug so you can have two or more lasers causing spark allowing for greater spark coverage? I don’t think laser ignition is in production. IIRC the main benefit to it is that it doesn’t require as much power/current to operate than an electro-magnetic ignition coil. I think some of the reporting on it is misconstruing the efficiency claims; as I read it it would potentially be 27% more efficient than coils/spark plugs, and some sites seem to have misinterpreted that as making the _engine_ 27% more efficient. which is unlikely. ---
**Author:** ![Pork\_Rind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pork_rind/32/111_2.png) [@Pork\_Rind](https://boards.straightdope.com/u/Pork_Rind)\ **Post date:** [January 5, 2018, 10:20pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/24 "2018-01-05T22:20:54Z")
> [@jz78817](#): > > engines with twin spark plugs fire both at the same time. Not always. From my quote above… > [@](#): > > At low engine speeds the two plugs work simultaneously, while at high engine speeds the two plugs are managed independently with a dedicated ignition timing. ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [January 5, 2018, 10:32pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/25 "2018-01-05T22:32:47Z")
> [@Edward\_The\_Head](#): > > I don’t know for how long, but Ford put them in the 4 cylinder Mustangs. I believe they started in the mid-late 80s. So there are cars out there that have this, but it doesn’t seem very common. > > On one of the [Mustang forums](http://forums.vintage-mustang.com/vintage-mustang-forum/359294-what-s-4-cyl-8-spark-plugs-why.html) they said this: > “One plug in a cylinder fires in the compression cycle and the other plug fires during the exhaust stroke”, which comes from the manual. They didn’t have two plugs per cylinder, the exhaust stroke mentioned was on another cylinder. I had one of those cars. Firing two cylinders at once simplified the ignition system somewhat. Toyota did produce a lot of twin plug 4 cylinders back in the late 70s, though. ---
**Author:** ![KMS94](https://avatars.discourse-cdn.com/v4/letter/k/ad7895/32.png) [@KMS94](https://boards.straightdope.com/u/KMS94)\ **Post date:** [January 5, 2018, 10:36pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/26 "2018-01-05T22:36:46Z")
> [@Crafter\_Man](#): > > Car manufacturers spend millions of dollars every year to achieve fractions of a percent improvements in efficiency. If two spark plugs per cylinder could provide an instant boost in efficiency, you would think they would figure out a way to do it… Both of my current vehicles have 5.7 liter Hemi V-8s (Chrysler and Dodge) and both have two plugs per cylinder. I do not know for what particular reason, just pointing out at least one manufacturer does make them currently. ---
**Author:** ![Fubaya](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@Fubaya](https://boards.straightdope.com/u/Fubaya)\ **Post date:** [January 6, 2018, 12:35am UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/27 "2018-01-06T00:35:38Z")
Can’t believe nobody’s brought up Splitfire spark plugs yet. Those came out when I was a kid, but I thought I smelled a scam even at that age. > [@KMS94](#): > > Both of my current vehicles have 5.7 liter Hemi V-8s (Chrysler and Dodge) and both have two plugs per cylinder. I do not know for what particular reason, just pointing out at least one manufacturer does make them currently. Hemis have emissions problems because the hemispherical shape has areas where unburnt fuel can hide. The dual plugs are for getting a more complete combustion and lowering emissions. In other words, it made sense for them financially. ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [January 6, 2018, 12:56am UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/28 "2018-01-06T00:56:52Z")
> [@Crafter\_Man](#): > > … > Does this mean an aircraft engine is more efficient – and thus gets better gas mileage – with two spark plugs per cylinder? And if so, why don’t automobile engines also use two spark plugs per cylinder? My Hemi Challenger had dual sparkplugs. It was for emission reasons, not power. But, 16 plugs at about 8 bucks each was a turn off. It also was not a 4 valve engine. ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [January 6, 2018, 2:52am UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/29 "2018-01-06T02:52:29Z")
As an aside, for a very short time, top fuel motors had _three_ plugs per cylinder. I think NHRA banned it rather than it not working. [Previous page](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921.md?page=1)