I think this is the best place to ask this question... Please dont give me a hard time because it is a serious question.

I am a firm believer of "every little thing counts", because it all adds up. If I can do something for my car and see a gain in performance somehow, i will do it.

I have an 88 iroc with the 350tpi and 3:73 gears (i believe, i didnt install them but I know they arent the stock gears tho. Car does 4k rpms at about 60 mph) I love the pull of the tpi and the gears make it even better! I have the stock 700r4 but hope to switch to T5 or T56 in the future.

Heres my question: Does going from a 3rd gen rear to 4th gen (keeping my rear gears) hurt performance at all? I know the rear differential is longer/wider on the 4th gens, thats why 3rd gen rims need a backspacing of 2" (i believe) and i know that you want the rear diff to be as "short" as possible, you lose less power thru it that way. So my question is will i see any, even if its very slight difference, loss of power to the wheels if i switched to a 4th gen rear end?

Im planning on upgrading my front brakes soon and im debating on LS1 swap or 1LE, but I know the ls1 is better. I already have some ZL1-ZR1 17's off a 4th gen i could use, but my problem is i cant do ls1 brakes on front with stock rear end because the backspacing for the rims will be different. I know I could use adaptors in the rear, but ive heard that they can break under heavy loads, and when my car is done itll have ~400lbs of torque. So I dont want to worry about breaking an adaptor!

Also, if i swap rears, i could put ls brakes in the rear.

Again, sorry if it seems like a dumb question but im wanting the best for my car and dont like to do things twice. I want to make an informed decision. I dont want to lose any power to the wheels by switching rears if i dont have to. Thanks again!

 

The 4th gen rear housing is about 2 inches wider per side than the 3rd gen one. That fact alone dictates that it will take more energy to turn the wheels using a 4th gen rear vs a 3rd gen one because you are spinning an extra 2 inches worth of axle on each side. The difference would be so small it could probably not be measured on a chassis dyno and your butt dyno would definitely not notice. I wouldn't (and didn't) let that stop me from getting a 4th gen rear.

 

Keeping the same ratio I would say no noticeable change in performance.

I wouldn't even think you could accurately measure it on a dyno. Some could make argue over the ABS sensor and things but really for a normal person on the street it's not noticeable. Most of us aren't looking to chase that last 5 hp or less out of combo as someone bracket racing or something.

That being said if you plan on putting down serious power it's definitely a good idea to strengthen a 10 bolt with a girdle cover at the least.

 

Darn you beat me to it.

 

It just helps the OP more now since he has two people saying the same thing.

 

Even if you lose 1 hp because the length is more, it would still be worth it to get the 4th gen rear end because the disk brakes would be much better than the stock drums you have.

 

Ok, thats what i was expecting (and hoping ) to hear. Even if its a few hp loss to the wheels, the performance gained in braking makes up for it and then some.

Now, I could use a V6 rear out of a 4th gen and put my rear gears in it, right?

Whats the problem about the abs sensor though? 3rd gens dont have abs anyways right? lol

Are the wheel adaptors any good? Can they be trusted to hold 400lbs of torque reliably?

 

Unless you know how to setup the gears and have the ability and tools to do so (usually need a shop press, pinion depth tool, and impact) you'd be better off just using what ever gears are in it. By the time you pay most shops they will charge $200-$700 in labor alone for a gear swap. Then you figure another $100-$150 for needed install kit with new seals and things.

3rd gens don't have ABS so no worries there. You'll just have sensor ring that you're not using.

As for spacer I don't really use them so I can't say there.

 

You wont need spacers on the rear. You only need them on the front, if you swap to 4th gen wheels.

 

My father has a transmission shop, so he would be able to do it there. If I swap to 4th gen rear ill put my current gears in it, but ill be swapping to ls1 brakes on the front too, so i wouldnt need adaptors anywhere. But if i keep the 3rd gen rear, ill be using 1le brakes up front. Id rather go ls1 tho, of course.

 

What brakes you run on the front has nothing to do with needing adapters or not. LS1 front brakes + 4th gen wheels on the front = adapters needed. Also why would you use 1LE brakes on the front if you kept a 3rd gen rear in the car? Unless you have some desire to keep only 3rd gen parts on the car I would go with the LS1 setup up front. It will probably be a lot easier finding LS1 brake parts than 1LE brake parts.

 

Don't know if you know this or not, but changing the gears is not just a bolt in job. You have to set the pinion depth, bearing preload and backlash when swapping gears from one rear end to another, or installing new gears in a rear end. You also need to know how to read the contact pattern to know what adjustments need to be made to the pinion depth and backlash. I recommend doing a pattern check and a backlash check on the gears before removing them from the current housing so that you can set them up the same way in the new housing.

 

Adapters is a misleading name for them/ Spacers is more accurate because the patterns are the same but the backspacing between a 3rd & 4th gen wheel are so different that they require a spacer.

So yeah.....You need spacers on the front, regardless of what brakes you have. And the only way to not need spacers on the back is to use a 4th gen wheel, or stay with the 3rd gen rear.

 

When I said id put my current rear gears in the 4th gen rear i didnt literally mean me, i was saying i would have had it swapped over at my fathers transmission shop.

I didnt realize that with ls1 brakes up front i would still need adaptors/spacers, i thought since 4th gens came with ls1 brakes and 4th gen wheels and did not have spacers, since mine would be converted over i wouldnt need them either. Ive read the threads about it since so now i know why you still need spacers