I have come across this term.

  1. What does it mean to retard an engine?
  2. When would you typically have to do such a thing?
  3. What can happen if you don't or incorrectly retard the engine?
  4. What sort of vehicle or machine require engine retarding?
  • Do you mean retarding the engine timing or components related to that? – cloudnyn3 Apr 22 '16 at 11:44

What does retarding an engine mean?

Retarding an engine is essentially a vague, but blunt term. It just means that in one way or another, you are hindering the engines ability to provide driving force, and in some applications you are actually using a device to work against the engine and vehicle.. There are several ways to do this and several reasons for it.

The most famous engine retarder is the Jake Brake. It was designed to open and close the exhaust valves very late to use the pressure from the engine in an attempt to retard the engine. This was done so that large machinery and semi trucks could come to a more quick and even stop. Rather than having to use and heat up their brakes while stopping or traveling downhill, this could be used instead. Here is a video of what it sounds like and a video demonstrating it. https://www.youtube.com/watch?v=qocMoTOVn6Q


Retarding the ignition timing

Retarding the ignition timing can be useful for a few things, but is generally left untouched. Reasons for doing this would be for non-computer managed fuel economy, a car that has minor forced induction and non-computer management, if the vehicle isn't performing as desired, and some engines will actually benefit and gain horsepower from retarded timing.

Hydraulic retarders

Hydraulic retarders are essentially the same design as a torque converter in the sense that they have small veins that hydraulic fluid passes through. HOWEVER they do the opposite, they help slow down the motion of the drive-train. The reason for these can vary greatly. I've seen small cars with them, and typically you see large machinery with them. They serve the same purpose, but are more robust and can do a much quieter and smoother job of slowing things down. These are usually switched on and there are secondary pumps that help assist the hydraulic fluid in stopping what ever it is that you're trying to stop. Generally you would find it just after your drive shaft, because it is usually a stand-alone unit.

What can happen if you don't correctly retard the engine

Depending on your method of retarding the engine, generally nothing catastrophic. Just lack of power and poor gas mileage. I'm saying this assuming you're not adjusting engine cams and things of that nature. Cylinders do not like flame. When you retard the timing without compensating for it, the combustion chamber will ignite late. This actually causes a "fire" inside the cylinder and probably lots of misfires. This will eventually eat away at your valves and cylinder walls. It can also cause the exhaust manifold to heat WAY up.

As far as for the other methods mentioned, those systems typically come on larger machinery and are installed by the factory. IF you tinker with it, it could actually be a huge problem. I've seen Caterpillar do testing on their engines and JUST the right amount will destroy the engine. Trust me, it's way worse than blowing up your Honda at the drag strip.

I hope I covered most of what you wanted to know. Thanks!

  • Add to this, ignition timing retarding can be due to pinking / knocking off the engine, which is that the fuel is igniting partially before the spark. This can cause overheating, con rods snapping and potentially catastrophic problems. This can be due to poor fuel (low octane), poor set up, even with an ecu the map can be wrong, or a few other things. So you'd retard the timing to prevent this. Most modern engines will automatically do this for you! – RemarkLima Apr 22 '16 at 21:27

Advancing or retarding an engine is the term used to describe adjusting the timing slightly to make the spark occur slightly earlier (Advanced) or later (retarded).

Generally the timing is set so that the spark occurs slightly before (in advance of) top dead centre (TDC), because it takes time for the air-fuel mixture to burn and expand - you ideally want the mixture to fully burn by just after TDC.

If the spark occurs too early, you can get the expansion happening before the piston begins to move down, which will cause knocking, and possibly engine damage. If it occurs too late, you lose power as the piston has already moved some way back down the piston - you'll also lose fuel economy and could get overheating.

The exact timing needed depends on a lot of factors, such as the engine speed, load, fuel octane rating and temperature - for example, as the engine gets faster, the timing will need to be advanced further, so that the fuel still has time to burn - the cycle is shorter but the fuel still needs the same amount of time. Equally, as load increases, you need to advance less, as a larger air/fuel ratio burns quicker.

Generally, the manufacturer will specify the amount of advance needed (e.g. 10 degrees before TDC), and adjustments are noted relative to that (so 9 degrees would be referred to as "retarded by one degree").

Most adjustment is done automatically, either be a mechanical or vacuum system in older cars, or by the ECU in modern vehicles. You'd only typically have to adjust it manually if you have changed something, e.g. tuning up the engine or running on a different fuel to normal.

Have a look at the Wikipedia page on Ignition timing for more detail.

  • Higher load need less timing. More air/fuel burns faster. This is why vacuum advances advance at high vac / light load. Similarly, when using forced induction or nitrous, they recommend retarding the timing 1* per 25 HP. From the wiki "The ignition timing is also dependent on the load of the engine with more load (larger throttle opening and therefore air:fuel ratio) requiring less advance (the mixture burns faster)." – rpmerf Apr 22 '16 at 13:20

Retarding an engine is usually done on industrial or fixed installation engines that do things like run conveyor belts or other pieces of equipment at a fixed speed. Retarding the engine simply means to slow down the operating speed of the engine, usually to control the equipment or process they are driving. For example, if you had an engine that was running a beer bottling line, and for some reason the pallets of bottled beer couldn't be moved to the warehouse quickly enough, you would retard the engine to slow the line down so that the warehouse drivers could keep up with production.


In some industries at least, like aviation, we often retard the throttle (i.e. reduce engine output), not just the timing.

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