You're breaking in more than just the engine. Think about the gears inside the tranny and in the rear axle. What break-in does is make all of the internal pieces become microscopically friendly with each other, and this generates heat. Lots of it, in the early stages (after the first 50 easy miles on brand-new axle gears, I'll guarantee that you will NOT hold your hand on the diff cover). Now think about the various clearances, local overheating, and things like ring end-gap (which tend to close up since the ring expands circumferentially before the much more massive block expands a comparable amount).

It's easier to understand the need for a break in period if you've ever spec'ed and built up/blueprinted an engine or two from all brand-new parts or installed gears yourself, and don't like doing things over.

That said, there is some argument that you can make more peak power from a little or hurried-up breakin period. For the short run, maybe so. For an engine to be used in a dedicated dragstrip car, long term might be two seasons or a few hundred total miles . But if the car has any chance of being a street-driven keeper, a handful of bhp at 6000+ rpm isn't worth the risk of early rebuild or the increased oil consumption that frequently follows.


Norm

 

What Norm said. Most people only think of the engine when it comes to break-in, but there are other things to consider, like the gears: they do get really hot when breaking in, and too much heat is not good for the break-in process, not to mention cooking the rear end lube. Same for the tranny. Keep trips to less than 50 miles, then let it cool for like 30 min or an hour. Repeat this process a few times to properly break in the gears.And don't cruise at constant speed, vary speed while driving.
Gotta think about things like the brake pads, too. The pads take at least a few hundred miles to properly seat to the rotors.
All in all, don't just absolutely baby it, but don't beat on it for several hundred miles at least. Full throttle is ok in short bursts, it helps seat the piston rings- but don't redline it out for for a while yet.
Just be reasonable and don't beat it too hard for the first 500 miles or so, and you'll be fine.
Those that say to be the hell out of it or "drive it like you stole it" right off the lot, don't understand the way things have to wear-in and the damage that can be caused by stressing and overheating new parts.

 

all engines (cars atvs bikes) are red lined as they leave the factory to make sure everything is sound and they dont really need a " break in" just as mentioned drive it at random speeds (rpm) to get a good wear pattern on the piston rings and cylinder walls..

old school engines require a break in ( flat tappet lifters )

 

While I agree with Norm and Moose to some extend, I always break in my engine by going into power bend progressively.
In the case of the 4.6, I'd let it rev up by itself to 4000rpm and shift smoothly to help the tranny and differential go into place without force.
After about 200 miles I'd start to go close to red zone occasionally with the same smoothness.

At 500 miles, I'd put this thing sideways and power shift! Woohoo!!
Kiddin'.

 

Why dont you just drive it like a normal car? That might work..

 

Somehow I doubt that this is the case. With an engine whose assembly was on the loose side of tolerance you might be OK, an engine on the tight side quite likely not (and the mfrs are certainly going to be concerned about the scrappage rate).


Flat-tappet lifters should break in withing 20 - 30 minutes, done properly (and note that this procedure can involve breaking the cam & lifters in using valve springs that are much lighter than those that will be used after the break in process is completed).

Other things take longer. I'm guessing that you've never built up an engine using file-fit rings.


Norm

 

and it is true that manufacturers redline thier engines for at least 15 seconds before they leave the factory if you feel it is hard to believe feel free to believe other wise

ive built my share of engines and i wasnt talking about the procedure of the lifter break in i was just saying that that is what is really required for a break in and yes it is a 15-30 minute procedure after that you want to go through the rpm with a load on the engine to "break in"(good wear pattern) the piston rings thats about it..

 

I kinda sit somewhere in the middle. The first 500 miles, I go real easy, varying throttle and engine load. Then, I change all the Lubricating fluids (and I mean ALL... Engine, Tranny, rear...) Then, I spend about 500 miles flogging the ever loving donkey snot out of it. Again, change the fluids and then drive as you will.

This was the method recommended to me by the guy who does all my engine building. He builds alot of all out race engines and stuff, so I take his word for it. Done this on every new car or car part (engine/tranny/rear etc..) since he built my Supra's engine back in 96 and have never had an issue.

 

Look in the owners manual. It should have specific instructions listed as to how to break in your new car.

 

Owner's manual? We don nee no stinkin owner's manual!