3.91 gear talk

[camguynj]

Originally Posted by judkins75

My dealer quoted me a price on gears. $429 for gear kit and $693/6.3 hours install. I think I'm going to pull the trigger this winter. I've read a lot of post on 3.91, but does anyone have more up too date info and ride info on using 3.91 gears on a daily driver...

That’s a very good price for a dealer install. But two things before you commit, will the dealer give you a warranty (if not walk away) and ask if the tech has experience in installing them, if they say yes demand to speak to the tech. Also get your old gears back so you can sell them.

[christianchevell]

The deal is and has been over time of the threads on here those using higher numerical gears the higher you go; of course the easier it is to eat the gears, the size of our differential being 8.6" say compared to a zl1 at 9.9" is big...….and the size of the teeth on the gear get smaller and smaller the higher the numerical gears number go's and that's a basic fact for any race car rears any where usually.

The 3.91 gear being stock on the 1le is cheaper due to having been produced for GM, and also I have read over the years of those drag racing of scattering 4;10/4;11 gears and going to the 3.91 and maybe launching more in the 3500 area for optimal launch and less towards the stock launch control at 4500 which is hard and more likely to loose traction.

Our pumpkin is aluminum and needs help to remain strong as forces from hard launches need good trailing arms and toes, a good LPW reinforced cover can help as in better reinforcement like say jannetty's bearing install, or even better things like a different type rear like a true trac. AS known wheel hop can be a killer.....

[G8One2]

Too much traction is what grenades gears. Wheel hop is detrimental to the differential. Whatever you do, there has to be a weak point in your driveline. If your driveline isnt up the the task of hard launches and sticky tires, the weakest point in that equation is whats going to break. Usually the rear gears grenade especially if you use a numerically higher gear such as 4.11. Yes the 3.91 gear is stronger on the 8.6" rear end, because the teeth are larger and can handle a higher torque load, but 4.10 / 4.11 gears can work just fine if you dont launch the car hard with slicks or drag radials. Tires should be your weak point, so that they break loose instead of breaking driveline components, up until you beef up the rest of your driveline. Everything you do to your car performance wise, had a trade off in some way. Being aware of the trade offs and negating those trade off are what makes the car.

[acammer]

Guys, you can launch reasonably hard on 4.10s. It's all about managing dynamic shock loads. Think about it like this - you're building a camp fire and you have a moderately sized branch you need to break in half to put in the fire. You try to break it in half by hand, but it's too strong, so you slam it down over your knee and it breaks right apart.

The same thing happens when you dump the clutch on a drivetrain that you've not preloaded. There is slack in every bushing, cv joint, gear tooth, axle spline, clutch, etc. That slack gives everything a chance to start moving and then really slam once it takes up the slack. I truly believe that you need a little preload to significantly drop those dynamic loads, and then use a little clutch slip rather than a harsh release, to keep these rear ends alive.

It's pretty easy to do, when I launch I roll into prestage, stop the car, and then grab the ebrake and pull it on strongly. Then I start to ease up on the clutch till I feel it start to grab just the faintest bit- you don't need much. Then, when I'm ready to take fully staged I just ease the brake down till the car creeps, and then pull it back on firmly when it lights the full stage bulb. My clutch foot never moves from that lightly preloaded position. When the other lane is in full stage I get up on the 2step, and when it's time to go I drop the ebrake and make a very quick but smooth release on the clutch. Car's done a lot of 1.7x 60' times on sticky tires with 4.10s and never had a problem like this.

An added bonus is that it helps prevent wheelhop - you don't have the violent wind up and release of the cradle bushings and other suspension parts when you add some preload.

LPW cover is a smart investment IMO, you add rigidity to the case, and you provide support on the ring gear caps. It definitely transfers more gear noise into the cabin (even more if you have hard diff bushings and solid subframe bushings - depends on how hardcore you want to go.

[LCPLPunk]

Have 3.91's in my L99 also with a 3200 converter and it ROCKS!

[acammer]

Just keep in mind the L99/A6 guys have a higher gear ratio in their transmission, so they don't need as much in the rear end to get the same effect.

[EJSS]

Originally Posted by acammer

Just keep in mind the L99/A6 guys have a higher gear ratio in their transmission, so they don't need as much in the rear end to get the same effect.

Yeah I think the consensus for l99 is 3.7 gears. I'm wondering if I should get them myself. Hmmmmmm

[CFD]

Originally Posted by G8One2

Too much traction is what grenades gears. Wheel hop is detrimental to the differential. Whatever you do, there has to be a weak point in your driveline. If your driveline isnt up the the task of hard launches and sticky tires, the weakest point in that equation is whats going to break. Usually the rear gears grenade especially if you use a numerically higher gear such as 4.11. Yes the 3.91 gear is stronger on the 8.6" rear end, because the teeth are larger and can handle a higher torque load, but 4.10 / 4.11 gears can work just fine if you dont launch the car hard with slicks or drag radials. Tires should be your weak point, so that they break loose instead of breaking driveline components, up until you beef up the rest of your driveline. Everything you do to your car performance wise, had a trade off in some way. Being aware of the trade offs and negating those trade off are what makes the car.

Improper installation grenades gears much easier and more often than hard launches!!!!

[LCPLPunk]

Consensus for L99 is 3.91s!!!


UNLESS you are going FI (non-Centri)

[judkins75]

Thank you all for your input, but I think 3.91 suits my needs for daily driving.

[acammer]

Originally Posted by judkins75

Thank you all for your input, but I think 3.91 suits my needs for daily driving.

I'm curious - what makes the 3.91 a better daily driver choice for you?

[judkins75]

[QUOTE=acammer;10634956]I'm curious - what makes the 3.91 a better daily driver choice

Hours in research on the 3.91 and its daily driver stories on YouTube and this forum, plus gm supplies the part; so no issues with my local Chevy dealership doing the work at a decent price.. I don't track the car or race it on the track. It's a manual, so Im looking to get this big boy moving, when needed...

[judkins75]

Originally Posted by camguynj

That’s a very good price for a dealer install. But two things before you commit, will the dealer give you a warranty (if not walk away) and ask if the tech has experience in installing them, if they say yes demand to speak to the tech. Also get your old gears back so you can sell them.

Didn't ask about warranty, but they do gears a lot there with experienced techs! Been dealing with these guys since owing the car; so I developed trust and a good repore with these guys.

[camguynj]

I got mine installed at dealer and got 1,000 mile warranty. Also make sure they apply the speedo recalibrate update.