While trying to investigate some rattling noise in my single cylinder motorcycle engine, the question of whether valves are closed during engine braking jumped into my head. I was assuming that the camshaft still opens and closes the valves but no juice is getting into the cylinder(or very little). However, the not to be trusted chatgpt answered that the intake valve remains closed during engine brake which sounded strange to me as I don't see how is that possible.

  • Are you asking about a specific motorcycle engine?
    – HandyHowie
    Jul 25 at 16:32

1 Answer 1


In a "normal" 4-stroke engine, the valves continue to function the same no matter the throttle position. The camshaft has to continue to rotate pushing the valves (in whatever manner they do that ... ie: SOHC or DOHC or OHV config). Most cylinder deactivation schemes work by allowing the intake valve to stay closed on certain cylinders, but this really has nothing to do with engine braking.

What creates the engine braking is by vacuum caused when the throttle body is closed. It can't get any air and basically no fuel is introduced. It is a fairly large draw on power not being able to draw the air in and therefore acts as a brake on the entire system. The drive train pushes the engine to continue turning, but the vacuum causes the whole works to slow down.

I am curious as to whether ChatGPT was actually considering a "Jake Brake" (Jacobs Compression Brake) which is used on diesel engines. I wouldn't think so, because it works by opening the exhaust valve at the top of the compression stroke. Who knows what AI comes up with and how.

  • @MTA - I had already typed out my answer before I saw your comment. Good to know someone else was thinking the same. Jul 25 at 18:06
  • 1
    Good that we have a complete answer now. +1
    – MTA
    Jul 25 at 18:22

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