Every time I hear people talking about how the “warm-up” myth has been “debunked,” it’s usually closely followed by information that isn’t necessarily “incorrect”; it’s just – not correct. It’s time to set things straight.
Here’s the deal – there are two very distinct methodologies regarding the “correct” method to properly letting your car warm up from a cold start; one says it’s a good idea, the other says (to some varying degree or another) it’s not necessary.
Warm Up? Or Crank-N-Go?
Without telling you how you should or should not live your life, I felt compelled to offer my two cents, because 98% of the time I hear this argument, it’s misses the most fundamental aspects of the warm-up cycle! So here’s my take on engine warm-ups – something I do religiously.
When people tell me that an engine is “modern,” and therefore it doesn’t need to warm up, I would like to know exactly what this means? I’ve been a line mechanic for the better part of my working life; I’ve worked on aircraft and countless cars. Many, many cars. Even those so-called “modern” ones they speak of.
“Warming up your car does absolutely nothing…more harm than good…false information.”
When someone tells me their car doesn’t need to warm up because it’s “modern,” I ask them, “What exactly makes it modern?” This is where I usually stop getting intelligent responses. It’s not because they’re not intelligent people; it’s just because the automobile is extremely complex, and you can’t reasonably expect the average person to have any clue what goes on after they turn the key.
How It Really Works
The fact is, a heck of a lot goes on after you move that key into the start position. The, “My car is modern,” argument isn’t necessarily incorrect, but it leads to incorrect assumptions about the rest of the warm-up process. Your car is “modern,” therefore it undoubtedly incorporates modern fuel injection technologies, able to meter fuel by the precise drop. The computer varies this amount of fuel, based on a complex matrix of variables that we’re not going to discuss here, and delivers just the right amount to make your car run like a champion, albeit a tired champion. Your modern car doesn’t need to warm up, right?
Carburators Have Nothing To Do With It – Anymore
In reference to the carburated days of mechanical chokes and bimetal springs, this argument is valid; my 1964 Nova needs at least a good five minutes for those loosey-goosey, worn-out pistons to snuggle on up onto their cylinder walls. And herein lies the “lost” argument – the effects of heat expansion.
More from Art of Gears
Regardless of what anybody tells you; regardless of how “modern” your fuel delivery system is, or how well your ECM can adjust spark timing to perfectly fire that frosty fuel charge into an ignition stroke – a cold-start is a cold-start, any way you cut it.
The Missing Facts – What Nobody Mentions
Take this Washington Post article into consideration. The pinnacle of Chris Mooney’s argument is the aforementioned “carburetor,” and the fact that it’s a bygone era. He then goes on to cite some statistics about emissions, and how “half of all U.S. greenhouse gasses are attributable to idling.” For one – he should be riding a bike if he’s that concerned about greenhouse gasses.
For two – he owes you, his audience, a fair and comprehensive perspective on idling’s effects ON YOUR CAR if he’s going to write an article telling you not to do it.
Number 1: Expansion Coefficient
His argument isn’t wrong, per se – but what about thermal expansion? He touched on absolutely none of the internal processes taking place right there in your own engine. Reading his article, you’d never have guessed that pistons can actually be cut into an elliptical shape, due to the expansion coefficient of the metal; why? Because the piston doesn’t expand evenly, and the thick slug of metal around the piston pin is subject to a different amount of heat than that of the thin skirts.
Furthermore, aluminum pistons in a cast iron block take additional design consideration due to the differing expansion coefficients. A tremendous amount of effort is put into calculating the clearances of an engine, mostly for operation at its theoretical operating temperature – why? BECAUSE IT MATTERS!
Number 2: Ridiculously Extreme Internal Pressures
The engine warm-up myth gets even more interesting when we consider pressures and forces that must be sustained by the rotating assembly. Theoretical physicist, C. Johnson, does a much better job of explaining all the math here, but to distill 10,000 words into a paragraph – ignition pressure in an internal combustion engine rises anywhere between three and a half to five times the compression pressure. If your engine produces 120psi (which is fairly standard), ignition pressures will peak at around 500psi. That’s 500 pounds per square inch.
If you have a 4″ piston (with a surface area of approximately 12.6 square-inches), multiplying the surface area of the piston (12.6 squre-inches) by the peak ignition pressures discussed above (500psi) would find the piston exposed to a force of approximately 6,300lbs. – every single time that cylinder fired! This force is directly transfered through the pitson pin, to the rod, crank, and everything else holding it all together – including all your bearings!
Number 3: Residual Oil Is Your Only Line Of Defence
Oil viscosity at operating temperature is a completely different argument, but no matter if it’s cold or hot, it doesn’t get to from the oil pan to your bearings by the time you can throw your seatbelt on, and jam it into reverse! Whatever thin layer of film is remaining between those metals the moment you turn the key needs to be able to lubricate the journals until your oil pump can send life-sustaining lubrication to all critical areas of your engine.
It’s just a fact, the lubricative properties of engine oil are negligible until a hydrodynamic film of pressurized oil can coat bearing surfaces. Even when you let your car “warm up” properly, uncoated bearings usually show severe wear after around 100,000 start cycles.
One Ten-Thousandth of an Inch is the Only Thing Protecting You
A final thought on oil and bearing clearances, as it solidifies the whole deal with a sobering fact – you only have 0.0015″ clearance for oil to lubricate the most critical of engine components. The informative Engine Builder article (linked above) outlines general bearing clearances – by viscosity. 5W-20 (one of the most-standard oils used in “modern” automobile engines), has the tightest tolerances, as it is the thinnest oil. Under operating pressure, all is well. But those critical, first few cranks, void of fresh, pressurized oil, is not, by any far stretch of the imagination.
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Crank-N-Go If You Must
This is not to say that you need to sit through half of “Master of Puppets” (an eight-minute song) before you hit the road in the morning. As a matter of fact, it’s not saying anything. You can get in your car, slam it into gear, and be halfway down the street by the time your crankshaft turns over for all I care. You have to live with it. But for what it’s worth, you have the right to at least hear the things nobody ever seems to mention.
How long you “need” to warm your car up is completely up to you. I let my 2014 Mazda warm up for 30 seconds to a minute, sometimes longer, you know why? Because that’s what I’m comfortable with. If you’re good with a quick crank-n-go, that’s ok too. But next time someone says you don’t have to warm up your car, because it’s “modern,” go ahead and ask them what “modern” means, and enjoy a demonstration of their mechanical comprehension in action.