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DesmoSouthwest tech: Engine warm up

Posted on October 03, 2013 by acruhl

overheated_engine_piston_skirt_damageAfter reading about this subject on the internet, I can’t believe how contentious this issue is. I don’t understand why anyone would be against engine warmup?

I think part of the reason is because engine warm up is not a very well defined thing.

Disclaimer: I’m not a scientist, and I haven’t done scientific experiments to quantify what I’m writing here. Everything I’m writing about comes from experience with building engines.

Let me make it easy to start off with though…

Is engine warm up important?

Yes it is.


For the longevity of your engine.

Now it’s time to define what engine warm up actually is. I haven’t read a very good definition, I think it’s best to start out with what it is not:

o Starting the engine and immediately driving away
o Holding the engine at high rpm with the throttle just after startup (high RPM to me means anything over about 2000 RPM).
o Wildly revving the engine up and down just after start up.

Since I didn’t really find a good definition of what engine warmup is, I’m going to go out on a limb and make my own definition (relevant to the bikes at we ride):

When the engine is cold (after sitting for more than an hour or so), it’s 30 to 60 seconds of idling after you hit the start button, without touching the throttle unless it’s necessary to keep the engine running. Then drive gently for another 30 to 60 seconds or so. And it’s a good idea to not accelerate too quickly in the first few minutes after a cold start.

That’s not so hard, is it?

(This won’t be a perfect strategy for every engine, and I don’t claim that this amount of time is enough to get all engines to operating temperature. But it should be enough to get some level of heat into critical parts so that they are not operating cold.)

We’re motorcyclists, so this should be exceedingly easy to follow. Hit the start button, put on your jacket, helmet, and gloves, and you’re past the first step.

I’ve found 3 common reasons why some people think engine warmup isn’t necessary, none are very good:

1. Impatience
2. It wastes fuel
3. It’s bad for the environment.

#1 can’t be fixed. Impatience is unfortunate.

#2 is nonsense because very little fuel is used during startup. A minor amount of aggressive driving would use more fuel than a few minutes of warmup.

#3 is especially insidious though. In this era of “green is good”, sometimes the truth takes a backseat. I’d argue that having to put your car/bike into the shitheap years early because you didn’t take proper care of your engine is much harder on the environment than a few extra seconds of the engine running. If people are that concerned about the environment, they should stop driving!

So what is the technical reason for warming up your engine?

Your engine is made of metal(s), and metal expands as it heats up. It was designed to run properly when hot.

Anyone who has ridden a two stroke motorcycle and had the engine seize has experienced this first hand! The aluminum pistons got hot quicker than the cylinder bore and grew larger than the bore. The bore was too cool, and the pistons got themselves stuck.

Another reason why warm up is good is that it gives the engine a chance to distribute oil before any significant load is applied. This is a good thing.

I’ve heard from knowledgeable people that engine warmup isn’t as important anymore with modern engines. I tend to agree to some extent, but motorcycle engines are different (because they put out so much power for their size). I believe that there is some short period of time when the engine is started cold that a large load should not be put on it. I think in general this is true of any engine.

I would love to get feedback from anyone who can tell me why engine warmup isn’t important!

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