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It depends.

If you have not generated enough internal piston pressure, then the process may still be ongoing. But 100 miles should have done the majority (90%) of the sealing process so long as you didn't idle the car for long periods, and so long as you DID take the car up to the 4k limit regularly.

The load (so cylinder pressure) is certainly enough under 4k to do the seal correctly, you don't want really high friction because it generates heat so keeping in manufacturing advice is actually correct.

There is a process used for the FA20 here. I've skipped about about 30 mins of background info in this video to get to his process, but I highly recommend watching it all. He describes a dyno break in, but in the background before this process he talks that its the same for driving the car.
 

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Its a fun topic. I do wish some youtuber did a 3 engine compare, high load drive it like you stole it, moderate load keeping within spec but following a break in process, and low load, taking it easy at all times.
 

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What you all have to take into consideration is the source of the info. That vast majority of people have zero knowledge on the topic and just do what they are told. Of course when your source is the manufacturer they will always tell you to take it easy. After all, if something were to go wrong because you werent paying attention or thought it was a great idea to show all your subie bros how your exhaust pops n crackles when you bang the rev limiter for 20 seconds straight, guess who has to cover it under warranty. On the other hand you talk to any professional engine builder, be it super high performance competition stuff, or just rebuilding an old civic daily driver, an engine builder will always tell you to be "aggressive". The heat cycles and pressure variations going in and out of vacuum are what sets you up for success. Just like new brakes. Most people dont know you are supposed to go out and immediately slam on the brakes from various speeds without coming to a complete stop. It heat cycles the new pads prevents future warping or premature failure. In the grand scheme of things it probably makes no difference at all. I can only speak to my own experiences and I have done PLENTY of hard break ins on all kinds of vehicles. Never had compression issues which is supposedly the only real concern.
I recommend watching the HP Academy video posted above, from the start. There is no BS in there.
 

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Can't find the video anymore, but the GT-R engines are loaded on a dyno as part of the engine build and then taken around a track at WOT to break them in. Here's an article that talks about it: Car Builder>> How The Nissan Gt-r Is Built - Speedhunters

View attachment 3185

I doubt your co-workers hurt anything, unless of course the engine wasn't up to operating temperature, in which case, screw that guy.

Here's a video where Ari builds two identical motorcycle engines, one with a gentle break-in and the other ridden at WOT:


From these you should be able to make an informed decision. In my case I always get cars fully warmed up before getting any significant load or revs, but then get lots of 3-5k acceleration and deceleration cycles, avoid highways, then change the oil to fully synthetic at 500 miles and call it good.
Slightly larger ring end gap for aggressive driving, but pretty much the same.

Great post.
 
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