I know this has been posted before several times and I have already read through them. I just had my gears installed and I drove it for 20 minutes(12 miles) under 55 mph on the way home. I have done two other heat cycles over the weekend making sure they cooled for several hours. Each drive was about 15 miles. I have to go on a trip next week and its all interstate. I read in the archives that you should not drive over 65 mph the first 500 miles but the traffic will be blowing by at 80 to 90 mph. Do I need to rent a car for this trip because there's no way I can drive it down there at 65 mph.

Is the speed limit there 80-90 mph? If not id take it on that trip thats a good drive for the break in period plus since ur only goin 65 you wont be getting a ticket. If speed limit is that high i would take another car

There seems to be no real definitive answer to this question ; the best I have seen is two short heat cycles then a longer one and you should be good to go.

I put my car on a dyno with less than 100 miles on a new set of 3.73 gears with three heat cycles on them, so I sure hope those runs up to 150 MPH didn't do any damage! I guess they are broken in now for sure ; I do have some very slight whine but no way to tell if that would have been there if I had gone easy for 500 miles.

I guess if it is bugging you then you should do the full 500 mile break-in thing, just so you won't have to wonder later if you did it right . . . like I am wondering now . . .

The speed limit here is obviously not 80 to 90. I'm just saying that to keep up with some of the traffic thats how fast some drive. I have just heard on previous posts that 65 mph is as fast as you should take it up to during the first 500 miles.

I'm also curious about this. Getting new gears installed tonight so this 'break in period' is something I'd like to get a handle on.

Does it have any similarities to the break in period people talk about when you're driving home in a brand new car? Or does that refer specifically to the internals of the engine? Is it specifically RPM-related for the first X-hundred miles?

Regarding gears, is this "heat cycling" more for the benefit of the hardware itself or the oil? I'm not sure if modern full synthetic oil really needs to go through that type of treatment, as it should already be prepared and capable of doing it's job.

Excuse the novice question, but what exactly does the heat cycling do?

It just seems odd, because I've heard of guys with drag cars - not necesarrily the 8.8' rear, sometimes the Ford 9', and sometimes really steep gears (4.56 and up, etc) - heading out to the track for test and tune the very weekend after they install them.
I know you can't really compare a track car's set up to our street cars, but those guys are beating their rear ends (lol) a lot harder than we tend to, sometimes with little or no break in period.

Of course they're not using their cars for extended driving periods, but I'd still say the stress is there.

I'm not at all trying to question the need to a 'break-in' or ease-in period when installing gears; I'm more trying to question best the method to do so...

Ps - sorry for the thread hi-jack lol..

I had my gears installed by Brenspeed on a Tuesday. Immediately drove 300 miles home. Drove another 200 miles of stop and go traffic over the 3 days. Then took 6 runs down the 1/8th mile.

It's been a year. No problems. No noise.

Been thinking more about this. I'm honestly starting to doubt how necessary it is. Maybe it works, maybe it doesn't. But if someone does the whole break in thing, and they have no whine or problems, of course they're going to attribute that to doing the break in.

I know a lot of shops might recommend it as a "safety net" or as a "just in case" kinda thing, but if they're installed properly, is there any need to break them in?

And related to that, a lot of places even say you should drain and change the differential fluid at 1000 miles (or whatever..).
If that was case, wouldn't we doing that with brand new cars as well, since it's essentially the same thing - everything is new, including the rear end.

I'm picking my car up this evening. I don't plan on pounding it on the way home, but I don't really intend to drive it any different than I normally would...

Just makes me wonder..

There does seem to be quite a bit of "black magic" here, which usually means nobody really knows what is going on.

Generally "break in" means "get the parts mated to each other with the right kind of wear." This means you want to produce some wear, which means you need to put some load on the parts (with very light load they will "glaze over" instead of wearing) and you want to produce the right kind of wear patterns -- in rear end gears, I think this means you want to avoid driving the same speed for a long time because that will cause a wear pattern that is specific to that speed; instead you want to vary the speeds a lot.

Also with the gears I hear people talking about "heat cycles" which implies expansion and contraction ; I doubt this has anything to do with heat-treatment (hardening) of the gears since they don't get anywhere near that hot, probably has to do with getting the parts all seated in correctly with expansion/contraction cycles.

If I had it to do over again, just for my own peace of mind so I wouldn't be wondering later, I would:

1) do two 30-minute heat cycles: meaning drive "normal" for about 30 minutes, avoid the same speed for a long time and avoid super-heavy torque on the rear end (meaning avoid heavy throttle in first or second gear), also avoid high speeds for a long time (not sure what "high speed" is, but I'm thinking about 60 mph) with a long cooling cycle in between each heat cycle (like overninght if possible)

2) do one 60-minute heat cycle -- same as the 30-minute ones above except longer

3) don't do any crazy gear-banging for at least the first 200 miles

I'm planning on getting my gears done at the end of next month. I plan to do whatever the installer recommends me to do. I think that's the bottom line here after all the installer, hopefully, has a warranty and if you don't follow their "break in" then they won't take care of any problems. Just my .02 on it.

Well, my car is not ready yet. I bought the Motive install and bearing kit from AM, but it turns out that it doesn't include axle seals, I had no idea. My installer is grabbing some today for me so it should be good to go tonight. But as of now, my car is up on a lift with the rear end hanging out of it, lol.

Anyway, as it turns out, he also recommends a bit of a break-in period for new gears. As far gear whine goes, you don't worry about that in a drag car - you wouldn't hear a thing anyway. And the gears used in some of those cars (he's currently running 4.88s in his, used 5.13s up until last year) are usually stronger and less prown to reacting to heat, etc.

And as far as wearing them in on a street car, he also recommends the hear cycle thing. He didn't use that term, but it was generally the same idea. He recommended that after I leave his shop, drive home normally (not aggressively), which is approx 20 miles or so, and park the car for a good hour. Take it for another good drive afterwards (or tomorrow), let everything get nice and hot, and let it sit again.

By that point, it should be good to go for any type of regular commute - stop and go, long distances, extended high-speed drives, etc - but give it about 250 miles before you really start slamming it.

I know for a fact that this guy is a great mechanic, so I'm thinkin' I might do what he says. At first I was skeptical about the need for a break-in period, but maybe it couldn't hurt.

that makes sense, what he is recommending is very consistent with the general concensus from my installer, my tuner, and the other posts I've seen on here and elsewhere

Yeah, I'm going with a bit of an ease-in period. I also ended up going with Lucas 75w-140 and a tube of friction modifier.

The car feels good now (went to 3.73 from 3.31) but again, I'm not flogging it yet. First two drives after the installation, basically drove for about 15-20 minutes to get everything hot, then let it sit - overnight the first time, and about 6 hours the second time.

I'm hearing a bit of a different sound when I'm at crusing speed here (approx. 50-60 mph) - I don't know if it's that dreaded gear whine, or just the sounding of higher winding gears. I'm thinking it's the latter.

Like I said, I haven't really pushed it at all, but I can tell it's going to be a pretty big difference.

I think I have that too, it is noticeable if I listen but not overly intrusive . . . I've decided not to worry about it!

Since I put my car on a dyno with less than 100 miles on the new gears, I'm wondering if that caused this slight "whine" and I'll feel better if you go easy on yours for a few hundred miles, and then tell me that you still have that noise!

lol, yeah.. I plan on going a little easy on it for the next week or two. Might as well give it some time, I'm sure it can't hurt anything.

The noise really isn't bad - it's like a faint hum or something, and I really only hear it at 50-55 mph (haven't really driven any faster yet). And it might be my imagination, but it seems to only be there when I let off the throttle and kinda coast. It might be normal, just the sound of higher winding years. And my ears might be exaggerating the fact too, since I've really trying to listen for it. I'll continue to update lol.