# 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:** 20\ **Page:** 1
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 5, 2018, 3:48pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/1 "2018-01-05T15:48:23Z")
Small aircraft engines have two spark plugs per cylinder. I have always assumed it was for redundancy, but [this](http://www.boldmethod.com/learn-to-fly/aircraft-systems/why-aircraft-engines-have-two-spark-plugs-per-cylinder/) says it also increases power: > [@](#): > > [With two spark plugs per cylinder,] 1) the fuel air mixture burns more quickly, and 2) the fuel air mixture burns more completely. And when you have both of those things happening in your cylinder, you get more power, which is something almost every pilot is looking for. 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? ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [January 5, 2018, 3:54pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/2 "2018-01-05T15:54:58Z")
[Dual ignition](https://en.wikipedia.org/wiki/Dual_ignition) > [@](#): > > Dual Ignition is a system for spark-ignition engines, whereby critical ignition components, such as spark plugs and magnetos, are duplicated. Dual ignition is most commonly employed on aero engines,[1][2] and is sometimes found on cars and motorcycles. > > Dual ignition provides two advantages:\*\* redundancy in the event of in-flight failure of one ignition system;\*\* and more efficient burning of the fuel-air mixture within the combustion chamber.[1][2] In aircraft, redundancy is the prime consideration, but in other vehicles combustive efficiency is the target. > > Although a dual ignition system is a method of achieving optimum combustion and better fuel consumption, it remains rare in cars and motorcycles because of difficulties in siting the second plug within the cylinder head. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 5, 2018, 4:06pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/3 "2018-01-05T16:06:23Z")
> [@](#): > > Although a dual ignition system is a method of achieving optimum combustion and better fuel consumption, it remains rare in cars and motorcycles because of difficulties in siting the second plug within the cylinder head. 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… ---
**Author:** ![Edward\_The\_Head](https://avatars.discourse-cdn.com/v4/letter/e/ce73a5/32.png) [@Edward\_The\_Head](https://boards.straightdope.com/u/Edward_The_Head)\ **Post date:** [January 5, 2018, 4:11pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/4 "2018-01-05T16:11:11Z")
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. ---
**Author:** ![zwede](https://avatars.discourse-cdn.com/v4/letter/z/ac91a4/32.png) [@zwede](https://boards.straightdope.com/u/zwede)\ **Post date:** [January 5, 2018, 4:12pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/5 "2018-01-05T16:12:25Z")
Alfa Romeo had their “twin spark” engines back in the 80’s: > **[Alfa Romeo Twin Spark engine](https://en.wikipedia.org/wiki/Alfa_Romeo_Twin_Spark_engine)** > > Alfa Romeo Twin Spark (TS) technology was used for the first time in the Alfa Romeo Grand Prix car in 1914. In the early 1960s it was used in their race cars (GTA, TZ) to enable it to achieve a higher power output from its engines. And in the early and middle 1980s, Alfa Romeo incorporated this technology into their road cars to enhance their performance and to comply with stricter emission controls. > In the current Alfa Romeo world the "Twin Spark" name usually refers to the dual ignition engine... Problem with having 2 spark plugs is mainly that it’s hard to find enough room in a modern combustion chamber for the second spark plug, and that with advances in engine design there are really no benefits anymore. Aircraft engines are the exception, as noted, as they want/need the redundancy. ---
**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, 4:17pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/6 "2018-01-05T16:17:37Z")
Two spark plugs is primarily for ignition reliability in cars. Certain combustion chamber designs (e.g. hemispherical) have little-to-no “squish” area between the piston crown and cylinder head leading to poorer turbulence and increasing the chance of a misfire. it does not meaningfully increase power. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [January 5, 2018, 4:32pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/7 "2018-01-05T16:32:38Z")
> [@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… Car manufacturers also spend millions to reduce the bill of materials cost of their cars; adding a second set of spark plugs and the associated wiring would need to result in a pretty significant improvement in efficiency to justify the increased recurring cost (basically: can we charge enough more (including margin) for the increased efficiency to cover the incremental cost). ---
**Author:** ![TCMF-2L](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tcmf-2l/32/16553_2.png) [@TCMF-2L](https://boards.straightdope.com/u/TCMF-2L)\ **Post date:** [January 5, 2018, 4:50pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/8 "2018-01-05T16:50:53Z")
Although it is not the only motorcycle to use twin spark plugs, I present the 1979 Honda NR500 race bike. During the 1960s Honda developed a procession of famous bikes with multiple cylinders and multiple gear ratios to accommodate the narrow power bands these engines had. When both these things were banned, Honda (and the other Japanese manufacturers) boycotted motorcycle racing for a few years. When Honda returned to the top class they decided to deploy a four stroke engine (to match their road bikes) even though all the other teams used two strokes and everyone knew, with the same capacity, a two stroke would always beat a four stroke. In an ideal world Honda would have built a V8 engine but that was still banned. So instead Honda built a four cylinder engine with oval cylinders, 32 valves and two spark plugs per cylinder. It was basically a V8 engine with only four cylinders. Sadly it could not quite beat the two stroke engines and from 1982 Honda switched to two strokes like everyone else. When in 2002 the rules for GP race bikes dictated a return to four strokes for everyone, the rule makers pre-emptively banned oval cylinders. The oval cylinders made a reprise on the extremely exclusive 1992 Honda NR (sometimes known as the NR750) road bike and so did the twin spark plugs per cylinder. Incidentally the primary justification for the oval cylinders was to allow for all the valves and the major technical problem was creating piston rings. TCMF-2L ---
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [January 5, 2018, 6:09pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/9 "2018-01-05T18:09:04Z")
My 1980 Datsun 200SX had twin plugs and oval pistons which I assumed were oval in order to provide room for the two plugs. It was a fun little 5 speed but lost power after a few years and it occurred to me that worn oval pistons are not something I wanted to pay to have fixed. I traded it out in year five. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 5, 2018, 6:13pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/10 "2018-01-05T18:13:10Z")
> [@TCMF-2L](#): > > Although it is not the only motorcycle to use twin spark plugs, I present the 1979 Honda NR500 race bike. BMW has a line of motorcycles powered by an oil-cooled boxer engine with around 600 cm[sup]3[/sup] per cylinder. Since ~2004, they’ve been using two spark plugs on each cylinder. This speeds up the burn time, conferring efficiency benefits via a couple of mechanisms: -In the absence of heat loss to the combustion chamber surfaces (and several other considerations), you’d want all of the mixture to burn instantly at top dead center (TDC) so you can use the entire expansion stroke to extract mechanical work from the burned mixture. A late burn is bad, and so is an early burn, because you end up not using the entire expansion stroke for work extraction. This same logic applies to the initial and final portions of a slow burn: they are poorly timed with respect to TDC. If you use two spark plugs, you start the combustion in two different locations within the cylinder, and the burn duration is reduced, making more of the combustion happen closer to TDC. So for the same mass of fuel/air mixture, you extract more energy. This means the engine is more efficient. In the real world, you do lose heat to the combustion chamber surfaces, so the point at which 50% of the mixture is burned is phased to occur shortly after TDC, rather than right at TDC. Similarly, a faster burn results in higher peak pressures/temperatures, and consequently increased heat loss to the combustion chamber surfaces. So the efficiency gain, while real, isn’t humungous. It’s also lower in engines with smaller bore diameters, for which multiple ignition locations doesn’t greatly decrease the burn time. -Knock is caused by unburned mixture being compressed and heated by the already-burned part of the mixture, and then igniting before the flame front actually arrives. The tricky part is that it’s time dependent. Laboratory studies have used rapid-compression machines that suddenly squeeze fuel/air mixtures past the autoignition temperature, and measurements show that after that compression event there’s a brief time delay before the chemistry catches up and causes a combustion-related pressure/temperature rise. If you use higher octane fuel and subject it to the same process, the time delay is longer. Anyway, the upshot of all this is that for a given octane-rating fuel, you won’t get knock if you can burn it quickly enough, i.e. before the unburned mixture autoignites. On large-bore engines, you can burn it quickly enough…if you use two spark plugs per cylinder. By doing so, you can get away with lower octane fuel…or for the same octane rating, you can build the engine with a higher compression ratio and achieve better efficiency. In piston-engine aircraft, efficiency is nice (for better power-to-weight ratio), IANAP, but AIUI the FAA requires two spark plugs per cylinder, and further requires two completely independent ignition systems, for the sake of reliability. One of the pre-flight checks after the engine is started is to switch off each ignition system, one at a time, and listen for an associated dip in engine RPM. The decrease in RPM confirms that the ignition system you just disabled was in fact working (and then you turn it back on and check the other one in the same manner). The decrease in RPM also demonstrates that two spark plugs per cylinder results in better engine efficiency than one spark plug per cylinder. ---
**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, 6:16pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/11 "2018-01-05T18:16:19Z")
> [@BubbaDog](#): > > My 1980 Datsun 200SX had twin plugs and oval pistons which I assumed were oval in order to provide room for the two plugs. It was a fun little 5 speed but lost power after a few years and it occurred to me that worn oval pistons are not something I wanted to pay to have fixed. I traded it out in year five. it did not have oval pistons. I don’t think any production car engine has ever done that. the Z20 engine had hemispherical combustion chambers so it had plenty of room for two spark plugs. [linky](http://api.ning.com/files/XkLijSk0SNice5PTdas2lm6tpFqVXQRi9HKyB-jI*Y8kO92KyPegy0t9Zkmva*PFzuzM-77iJDvFVxvDOJZT*jINrFA8s2EA/Z22andZ20milageoptionheads.jpg) ---
**Author:** ![EdelweissPirate](https://avatars.discourse-cdn.com/v4/letter/e/8491ac/32.png) [@EdelweissPirate](https://boards.straightdope.com/u/EdelweissPirate)\ **Post date:** [January 5, 2018, 6:29pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/12 "2018-01-05T18:29:12Z")
There’s a another reason some racing cars and motorcycles use two spark plugs. Many race engines are “oversquare,” which is to say the bore is considerably bigger than the stroke. A short stroke means the mean piston speed is lower for a given RPM, and more revs generally mean more power. So race engines often have short strokes to rev high. If you hold displacement constant, shortening the stroke means you have to increase the bore (piston diameter). As you do that, the combustion volume changes shape so that it approaches a thin disc. So the spark plug is now farther from the edge of the piston and the flame front travels a greater distance from the central spark plug to the cylinder wall. The higher you rev, of course, the less time you have to burn all the fuel/air mixture in the cylinder. The greater linear distance from the central spark plug to the edge of the cylinder and the shorter window in which to burn the fuel mean that at some point, the flame front (which starts at the spark plug) won’t reach the edge of the cylinder in the allotted time. By adding a second spark plug, you ensure that no part of the combustion chamber is too far from a spark to burn all the fuel in time. So you can have a massively oversquare bore/stroke ratio and a high-revving engine but _two_ flame fronts, so you get complete combustion. The corollary here is that if you have lots of time in which to burn your fuel/air mixture (because you’re running at low RPM) having two flame fronts really doesn’t solve many problems, so there’s not much to be gained with dual plugs on most passenger cars, as these spend lots of time at 2000-3000 RPM. \ And obviously, if you’re driving a low-revving Diesel engine, you’ve got so much time in which to burn your air/fuel mixture that you don’t need any spark plugs at all. \ ---
**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, 6:36pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/13 "2018-01-05T18:36:40Z")
> [@BubbaDog](#): > > My 1980 Datsun 200SX had twin plugs and oval pistons which I assumed were oval in order to provide room for the two plugs. It was a fun little 5 speed but lost power after a few years and it occurred to me that worn oval pistons are not something I wanted to pay to have fixed. I traded it out in year five. You sure about that? That twin spark engine was the Z20 NAPS-Z, and all the parts catalogs and info I can find show round pistons. Googling ‘Datsun Oval Pistons’ gets me nothing. [Some history on the NAPS-Z Engine](http://720world.com/forum/topics/some-history-on-the-naps-z-engine) [Nissan Silvia (Datsun 200sx)](https://en.wikipedia.org/wiki/Nissan_Silvia#North_America) [Z20e pistons](https://www.amazon.com/NISSAN-200sx-2-0L-Pistons-rings/dp/B00HK1OM2Y) Back to the two sparkplugs thing, my 2016 bike that I rode to work on today\* has two sparkplugs per cylinder. Here’s the marketing blurb about that… > [@](#): > > Independent Dual-Spark configuration > The Ducati Testastretta DVT is equipped with two sparks whose ignition is managed independently: (1) one is placed at the center of the combustion chamber, the additional one (2) on the external side. > > The benefit is to provide at any engine speeds a further more effective twin flame-front that ensures complete and efficient burning (quicker and more complete) of the fuel-air mixture within the combustion chamber across a shorter period of time. > > 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. \*Not really. It’s in the shop. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [January 5, 2018, 6:54pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/14 "2018-01-05T18:54:56Z")
> [@Marvin\_the\_Martian](#): > > Car manufacturers also spend millions to reduce the bill of materials cost of their cars; adding a second set of spark plugs and the associated wiring would need to result in a pretty significant improvement in efficiency to justify the increased recurring cost (basically: can we charge enough more (including margin) for the increased efficiency to cover the incremental cost). I think that explains a large part of why it’s not done. The increased efficiency has been demonstrated in aircraft engines but I don’t know if it’s significant enough to justify the additional costs. 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. ---
**Author:** ![EdelweissPirate](https://avatars.discourse-cdn.com/v4/letter/e/8491ac/32.png) [@EdelweissPirate](https://boards.straightdope.com/u/EdelweissPirate)\ **Post date:** [January 5, 2018, 7:01pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/15 "2018-01-05T19:01:32Z")
> [@jz78817](#): > > it did not have oval pistons. I don’t think any production car engine has ever done that. I’ll second JZ’s point here. One reason non-round pistons are so exceedingly rare is that it’s so easy to make round things to very tight tolerances, and reciprocating internal-combustion engines just _love_ tight tolerances. Drills make round holes trivial, while lathes do the same for round pistons. Sure, most pistons are either cast or forged, but how do you think they make those nice round casting/forging dies? Or drill bits, for that matter? Honda’s engineers designed an oval-piston engine to get around rules written specifically to impede them; this always struck me as an extraordinary feat of engineering badassery. It was also essentially a thumb in the eye of the FIM (Grand Prix motorcycle racing’s governing body), which was trying to force Honda to build and run a two-stroke engine like all of the other manufacturers did. But Honda wanted to compete with a four-stroke engine, and, well, they did. Oval pistons and oval bores make so little sense from a manufacturing and reliability standpoint that Honda’s NR 500 was an ostentatious demonstration of engineering skill and rule-bending that [Smokey Yunick](https://en.wikipedia.org/wiki/Smokey_Yunick#Automobile_racing) would have admired (and perhaps he did). No manufacturer would ever use oval pistons in a production engine. Doing so would be insane, and I love the fact Honda’s Moto GP engineers were crazy like a collective fox. ---
**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, 7:06pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/16 "2018-01-05T19:06:19Z")
> [@EdelweissPirate](#): > > I’ll second JZ’s point here. One reason non-round pistons are so exceedingly rare is that it’s so easy to make round things to very tight tolerances, and reciprocating internal-combustion engines just _love_ tight tolerances. Drills make round holes trivial, while lathes do the same for round pistons. Sure, most pistons are either cast or forged, but how do you think they make those nice round casting/forging dies? Or drill bits, for that matter? > > Honda’s engineers designed an oval-piston engine to get around rules written specifically to impede them; this always struck me as an extraordinary feat of engineering badassery. It was also essentially a thumb in the eye of the FIM (Grand Prix motorcycle racing’s governing body), which was trying to force Honda to build and run a two-stroke engine like all of the other manufacturers did. But Honda wanted to compete with a four-stroke engine, and, well, they did. > > Oval pistons and oval bores make so little sense from a manufacturing and reliability standpoint that Honda’s NR 500 was an ostentatious demonstration of engineering skill and rule-bending that [Smokey Yunick](https://en.wikipedia.org/wiki/Smokey_Yunick#Automobile_racing) would have admired (and perhaps he did). > > No manufacturer would ever use oval pistons in a production engine. Doing so would be insane, and I love the fact Honda’s Moto GP engineers were crazy like a collective fox. I left out that pistons can be designed to be _slightly_ oval when cold (“cam ground,”) but that’s because they expand more along one axis and will become close to perfectly round once up to operating temp. ---
**Author:** ![EdelweissPirate](https://avatars.discourse-cdn.com/v4/letter/e/8491ac/32.png) [@EdelweissPirate](https://boards.straightdope.com/u/EdelweissPirate)\ **Post date:** [January 5, 2018, 7:14pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/17 "2018-01-05T19:14:42Z")
That’s a good point…forged pistons can exhibit a tiny bit of anisotropy, and the half-symmetry of the piston skirts can have a similar effect. Pistons with a little runout when cold can correct for that. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [January 5, 2018, 7:28pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/18 "2018-01-05T19:28:10Z")
> [@Edward\_The\_Head](#): > > … > > 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. Not sure why they just couldn’t fire the same plug twice. Subaru does this, not for efficiency, but a ‘wasted spark’ is fired on their boxer engines as they fire off 2 plugs at a time. Perhaps there is something to be gained by a exhaust stroke spark IDK. ---
**Author:** ![smithsb](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smithsb/32/12107_2.png) [@smithsb](https://boards.straightdope.com/u/smithsb)\ **Post date:** [January 5, 2018, 7:41pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/19 "2018-01-05T19:41:55Z")
My Nissan Hardbody (yes, the actual “nickname” for the trucklette - not me) from 1986/87 also came with twin spark-plugs per cylinder. Lots of wires under the hood. I believe it was touted for emissions and power. In mid-1983, Nissan introduced the Z24 2.4 liter twin spark four-cylinder engine, producing 103 hp (77 kW) - from wiki regarding Datsun trucks. ---
**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, 8:05pm UTC](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921/20 "2018-01-05T20:05:37Z")
> [@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. that’s just the result of using a “waste spark” setup, where one coil fires two spark plugs. you route the ignition wires to pairs of cylinders such that when one cylinder’s plug fires to start combution at the top of the compression stroke, the other fires its plug at the top of the exhaust stroke. the plug firing at the end of the exhaust stroke doesn’t actually do anything since there’s nothing for it to ignite (thus the name “waste” or “wasted spark”,) it simply costs less to have fewer ignition coils. you can have a waste spark system with one or two plugs per cylinder. engines with twin spark plugs fire both at the same time. [Next page](https://boards.straightdope.com/t/two-spark-plugs-per-cylinder/805921.md?page=2)