3.91 break in procedure, 2 1/2 hr drive home from shop

[ssmike]

I'm ready to have the GM 3.91's installed and I know there has been some threads here regarding break in procedure. My question is, if I have a shop do the swap but they're a 2 1/2 hour drive back home, is it ok to drive that long right after a gear swap. I understand that it is recommended to drive for a half hour or so then let it cool, then another half hour and so on. So what do I do if I have a 2 1/2 hour drive? Maybe drive 45 minute segments? It'll be a LONG drive home! Any advise is welcomed!

[Scott@Bjorn3D]

Are you getting just the gears put in or an entire rear-end built by GM. If just the gears then yes I would allow for some cool down. If complete rear-end breakin would be like when you got the car new.

[ssmike]

Originally Posted by Scott@Bjorn3D

Are you getting just the gears put in or an entire rear-end built by GM. If just the gears then yes I would allow for some cool down. If complete rear-end breakin would be like when you got the car new.

Just the gears. Swapping stock 3.42's with GM 3.91's, nothing else.

[Scott@Bjorn3D]

Drive maybe half way and stop and get something to eat, vary speeds not driving at the same speed all the time.

My entire rear-end is being put in as we speak.

[ssmike]

Originally Posted by Scott@Bjorn3D

Drive maybe half way and stop and get something to eat, vary speeds not driving at the same speed all the time.

My entire rear-end is being put in as we speak.

Very cool! 1LE?

[blazzin1]

You can still do the heat cycles (as recommended). Just drive for 20 min. or so, and then pull over and stop for 10-15 min. Do that about 3 times and you're done. Makes for a "looooong" ride home, but by the time you get home, your gears will be properly heat cycles and ready for some real action. I've also heard to "take it easy" for about the first 500 miles or so....not sure how necessary that is???

[Scott@Bjorn3D]

Originally Posted by ssmike

Very cool! 1LE?

Yup full 1LE rear-end to keep my drivetrain warranty.

[10aSSee D]

I'm in the process of breaking in my Gforce 9 inch with 3.90 gears and it states to drive the car under light load for 50 miles and then let it cool down (this will take longer than 15 minutes) and then do it again. Next take it out under medium load the same way as before twice. By now you should have about 200 miles on it and you can do normal driving for another 200 miles, no wot pulls. After that you are good to go!!!

[Mr_Draco]

Originally Posted by ssmike

I'm ready to have the GM 3.91's installed and I know there has been some threads here regarding break in procedure. My question is, if I have a shop do the swap but they're a 2 1/2 hour drive back home, is it ok to drive that long right after a gear swap. I understand that it is recommended to drive for a half hour or so then let it cool, then another half hour and so on. So what do I do if I have a 2 1/2 hour drive? Maybe drive 45 minute segments? It'll be a LONG drive home! Any advise is welcomed!

Personally I would look into removing the rear end myself and taking just the rear end. There is no way I would do this and if I REALLY REALLY had to I would get a hotel room at the half way point.

The cool down cycles take hours, not minutes. Remember the gears get really hot and so does the fluid. Hot metal surrounded by hot fluid takes a long time to cool down. I've done a couple rear end fluid changes a couple hours after a car has been driven for 30-45 minutes and the fluid still be hot enough to burn my hand. If you follow the directions, it will take you all day to not even cover half the distance.

[ssmike]

This is why I'm concerned! I want to make sure I don't overheat them and cause whine and short life. The shop owner says that it won't be a problem so that's why I'm throwing this concern out there! I'll wait for more opinions!

[ssmike]

Originally Posted by Mr_Draco

Personally I would look into removing the rear end myself and taking just the rear end. There is no way I would do this and if I REALLY REALLY had to I would get a hotel room at the half way point.

The cool down cycles take hours, not minutes. Remember the gears get really hot and so does the fluid. Hot metal surrounded by hot fluid takes a long time to cool down. I've done a couple rear end fluid changes a couple hours after a car has been driven for 30-45 minutes and the fluid still be hot enough to burn my hand. If you follow the directions, it will take you all day to not even cover half the distance.

I thought about removing it and bringing it in to a shop but I'm having other suspension mods done at the same time. I'm not up to crawling back under the car yet since I injured my neck a couple months ago. There's a couple local shops that could probably do the work but I don't have the warm fuzzies about them. The shop I'm going to is a good reputation but I'm not sure he's really thinking hard about the break in procedure!

[10aSSee D]

Going through the heat cycles is very important for longevity and so it doesn't whine.

[Mchlgrrsn]

20 to 30 minute intervals. Change the diff fluid at 500 miles, than bombs away. Avoid hard shifting for first 500 miles
Failure to follow this can result in gear whine, permenantly

[NinoSS]

JRE installed my 3.91's. I live about 1 hr 45 min away. Ted had his son take my car out for the first 20-25 minute run, then we let them cool down. He then told me to take it easy for a couple hundered miles, then I should be good. He said change the diff oil at about 1000 miles.. Mine are very quiet BTW...