Explain this new car advice to me...

I recently bought a new car. Asking a few people how I should drive it for the first little while, they both said, “Don’t drive it at the same speed for a long time.” They didn’t know why you shouldn’t do that, just that they’d heard you shouldn’t.

So I open my new car’s owner’s manual and sure enough, for the first 2,000 KM I’m supposed to keep varying my speed. But it doesn’t explain why.


I imagine it woud be to “loosen up” all the parts, and make sure that the car can handle different driving conditions. If you always drive at one speed, the engine and whatnot might “settle” into whatever configuration is needed, and just be stiff for other speeds, possibly causing damage.

But then, I know nothing about cars. This is just a WAG, and the opportunity to post the first reply in GQ :slight_smile:

I think that’s basically right… driving at variable speeds stresses different aspects of the engine differently so the engine gets “broken in” better then just cruising at one speed for an extended period of time. My guess is that it probably doesn’t make that much difference but the fact that all car manufacturers mention it tells me someone thinks it does…

How the thinking goes.

  1. At higher RPMs the piston goes slightly further in the cylinder bore than at lower RPMs
  2. If all the break in time is at constant low RPM, you will get a “lip” in the cylinder wall at the top of piston (ring) travel.
  3. If after break in, you then suddenly start running high RPM, that “lip” might do damage to the rings.
  4. New engines are “tight” and generate more heat through friction than they will later in life.
  5. Running at high for extended time RPM while the engine is tight could overheat parts due to the higher friction and cause damage.

So, taking all that into account, it is good run at high RPM to break in the full length of the cylinder, but with low power, low RPM periods to prevent overheating of any part during break in.

Make sense?

I know nothing about cars, but I bought one within the last couple years, and our dealer said that it’s a good idea to present multiple experiences to the engine and the computer. Of course, my experience in other fields suggests that sales reps often don’t know a thing about what they’re selling.


New engines have tighter “new” tolerances at key areas on the piston rings, bore cylinder areas, bearings, crankshaft, camshaft etc. etc. until they are ‘broke in’; so there are admonishments to avoid high speed driving, towing a trailer, and suggestions to vary the speed. In years past some manufacturers installed a special “break in oil” and instructions not to change the oil until 1500 or 2000 miles had passed.

Modern engines often don’t need any special treatment as they are run in at the factory. But really, if you are buying a new car it’s best to find out. Like as not, wherever you buy the car, they won’t have a clue. A properly broken in engine is quieter, smoother running and lasts longer, big difference. It’s also the reason synthetic oils are not recommended for the first few thousand miles – it may hinder the break in process.

Basically you want to avoid high stress on a tight engine. But, you still want to seat the compression rings against the bore cylinders, for a close mating surface. Cruising at around 10 or 15 mph and then flooring the accelerator until reaching about 40 or 45 mph, release the pedal and coast back down to 5 or 10 mph will do this in short order.

Most people think that the engine bores are smooth and shiny, in fact some if not all car manufacturers go to very great lengths to make sure they are not, and all recommend that you work the engine within a certain rev range with plenty of gear changes etc.

If you run a new engine too fast it will develop hot spots where things are slightly tight and this will accelarate the wear process dramatically.

Conversely if you run it too smooth and steady, the bores themselves will burnish and polish, becoming extremely smooth, which is also not desirable.This is also known as ‘glazing’.
It is very expensive to rectify requiring an engine stripdown, a mild sandblast in the bores and new piston rings.

The smooth burnished surface is not able to hold oil properly and so wear increases, oil consumption increases and emissions become dirty, extreme cases could total the catalytic converter.

Other contributary factor is oil, modern oils such as the close tolerance low wear synthetic types are so good that they can extend the service interavals without any further redesign of the engine, but they also reduce wear hugely and during the running-in period this prevents parts breaking in properly.

It is a good idea to use plain and cheap engine oil for the running-in period and then change to a better grade once this has been achieved.

I have read that one way to speed this up is to drive to the top of a good long hill and descend in a lower gear than you normally would, and let the engine do the braking. This apparently creates a vacuum which splays out the piston rings.
I would also mention that this was recommended by a race bike guru and it may be either nonsense or only applicable to bike engines, maybe someone could comment.

Thanks, guys.

I feel bad now because I did run the car on the highway before I hit 1000 klicks. (I had to.) However, I did vary the speed, pull off at a few rest stops, that sort of thing. I wasn’t exactly burning the highway up either. Hopefully I didn’t kill my engine. Did I?

huh? never heard this, nor does it make sence at all. how is running at a higher RPM make the stroke longer? Unless you’ve got loose parts, but I really don’t see how it can make the stroke longer.

Of course, that’s why it’s called a break in period so the parts work together better.

I don’t get this either, above you say don’t run it at high RPMs now you say do it.

Now for my limited input. To the best of my knowledge the more modern cars do not need this break-in period, I didn’t do anything with mine when I bought it and was not told to do so. Motorcycles on the other hand you ARE told, there’s even a sticker on the tach, or speedo, that tells you what to do.

Here’s what I’ve usually seen, yes I buy more new motorcycles than cars. For the first 600-800 miles, run at less than, X RPMs, usually around 1/3-1/2 or so of the red line. Have the oil changed, and if need a valve clearence done. For the next 500-1500 miles keep the RPMs to around 3/4 max. Again change oil.

Then you may use the engine however. The reason is to get rid of all of the shavings that might still be there. At a higher RPM you might get a shaving caught pretty badly and ruin the engine. Though I wonder how many of the people who buy new sports bikes actually do this. I know it was hard to keep mine under the 5K for the last bike I bought.

My advice would be to change the oil, everything else really shouldn’t matter.

The guys at Car Talk say that “the break-in period is supposed to be a time in which you drive gently and allow the rings to ‘seat,’ or mold themselves perfectly to the exact shapes of the cylinder walls…If the rings don’t ‘seat’ well during break-in, the theory is that your car will burn oil later on, because the poorly seated rings will eventually let oil sneak by and get into the cylinders.” Although they mention that break-in should be done at differents speeds, they don’t quite say why running at different speeds is important.

Edward The Head: I think scotth’s comment about higher RPMs refers to the greater inertial loads at high speeds. At max speeds, the inertial load of the piston can be larger than the firing pressures, causing the con rods to actually stretch. Max inertial loads occur at TDC, so the comment about greater piston movement seems reasonable.

It’s because of momentum. When the piston has a lot of momentum on it’s way up the cylinder, it will travel farther, and the rings, which are seated in a groove around the edge of the piston, will travel even farther because they shift a little bit in the grooves. Contrary to popular belief, metal is not hard. It is soft and cushiony, so you essentially do have ‘loose parts’ as you say. We are talking about differences of .001 inch here, but that’s a big difference when piston rings are concerned.

When you hone a cylinder during an overhaul, it is not perfectly smooth… it should have tiny lines in a sort of cross-hatch pattern. I assume that brand-new engines have this as well. It is important that those little grooves get lubricated, and running the engine at different speeds helps that to occur.

Another reason to run at a multitude of speeds is the computer. Computers these days have a function known as ‘learning’, which basically means that the computer can tune the engine better over time, since it has more data to work with. This is a good thing, but in order for the law of averages to work out, you must run this ‘learning’ process over the whole rpm range of the engine.

Transmissions require a similar break-in period for similar reasons… that’s why you are instructed to drive in all gears as well, or to drive at different speeds if you have an automatic.

I have never seen an engine ‘ruined’ by not doing this… and I’ve never seen any data suggesting that not breaking an engine in properly will reduce it’s lifespan. If such data exists… it would be interesting to see it. The theory and the reality are often two different things, and all I’ve ever heard on this subject is theory.