# What's gonna happen if I disconnect 'single piston'

I saw similar topics but that did not explain my things... As we can see 4 stroke engine with 4 pistons

Is there any chance (only for test) to 'switch-off' two pistons? For this example I'm talking about spark plug/EFI disconnect of two pistons. Is there any chance so car with 4 pistons will works fine with only 2 pistons (as I say 2 pistons without EFI/Spark plug connect). I test it on my second car but disconnecting two pistons gives a result that car vibrated and wasnt 'stable' as normal (perhaps maybe there was knock detonation).

Can someone explain this in simple words and 'science' words?

Thank You.

• Related: mechanics.stackexchange.com/q/911/9884; not sure if it's a duplicate but the combination of all the answers there does have some of what you're looking for and could be helpful. Commented Jul 10, 2016 at 17:15
• Just curious, did you disconnect two consecutive cylinders or two alternate ones? I would think consecutive cylinders (assuming the firing sequence in your image) would just vibrate, but alternate ones may not run at all (since the two missing cylinders would be in sequence).
– Tom
Commented Jul 11, 2016 at 17:06
• I would expect this test to also bugger up the fuel mixture going to the remaining cylinders which explains why it runs rough if at all (in addition to being unbalanced.) Frankly, count yourself lucky you didn't fry an ignition coil or computer playing such games. Commented Mar 28, 2017 at 17:34

As you saw with your test, it is possible to disconnect the ignition on two pistons and still have the engine run.

The vibrations have nothing to do with engine knock or detonation though. The reason it stumbles is because those disabled pistons were expected to sustain the crank's rotation. In the absence of those pistons firing, the crank will experience rotational acceleration/deceleration.

It is a good idea to disconnect the fuel injectors to those cylinders during the test to prevent unburnt hydrocarbons from reaching the catalytic converter.

Don't know why would you need to do that, but yes, you only need to remove spark plugs to decompress unused cylinders. You can disconnect injectors, but the fuel will run out through spark plug holes anyway..

• Not sure I understand the fuel running through spark plug holes part... Commented Jul 11, 2016 at 8:10
• @IhavenoideawhatI'mdoing I think he meant that fuel-air mix escaping through empty sparkplug hole is almost as good as not having fuel injected in the first place. Well, for a short run it should work. Commented Jul 11, 2016 at 8:42
• @Agent_L oh, now I get it. If the purpose is making a cylinder stop working then there is no reason to remove the spark plug if you can disconnect the injector, though. Commented Jul 11, 2016 at 8:53
• @IhavenoideawhatI'mdoing well, the hole will prevent the cylinder from compressing, so it will be more "not working". Plus, if it's a carburetor engine there is no injector to disconnect : ) Commented Jul 11, 2016 at 8:58
• I'm wondering if removing the plug would decrease the vibration. With the spark and even fuel removed the engine still has to compress the air in the cylinder. If the plug is removed it would reduce the resistance.
– Tom
Commented Jul 11, 2016 at 17:00