red90rs

Is this a good idea?? I've heard the 3.73 will help about the most for their money. I found a full rear axle with 81k from a trans am should that work. It had discs also and I have drumms will that swap be hard??

CaliCamaroRS

You should get the proportioning valve from the disc brake car.....but other than that it should bolt right in.

KED85

ALSO THE MASTER CYLINDER!
Get as much brake line stuff as you can from the front. Brackets, tubing to proportioning valve, etc.
All ya need is a set of flared brake line wrenches from Sears, $20. YOU MAY NEED METRIC, too!
Good score!
How's your tranny?

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

red90rs

My tranny is in great condition. I have 130k but I bought it from my mechanic so it was kept up great. If I could find one with drums would the conversion be easier?? am, I gonna notice that much better braking??/

red90rs

My tranny is in great condition. I have 130k on mine but I bought it from my mechanic so it was kept up great. If I could find one with drums would the conversion be easier?? Am I gonna notice that much better during braking??/ also I have a thermo master chip and a 160* thermo and a k&n. these mods allowed me to peel a little as to where I couldnt before. What 1/4 times would I be looking at with 3.73 and my few mods?? any Ideas??

CHVYPWR

You'll notice better braking if you take the time to go through the calipers and clean the slides. The early rear disc set-up likes to stick and not function properly. Anti- seize the caliper slides real well after cleaning them, that should keep them freed up.

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Ya can't have too many fast Chevys!

TomP

It's not the slides that cause the rear calipers to stop gripping; it's the design of the piston itself. The pistons "screw" outward to grip the rotor... and the return spring inside the piston (aka "actuator") that GM used was too weak. Consequently the pistons stay "locked", and they won't grip the rotor when the e-brake's on.

GM sells the recall parts... they want over $100 for each piston, and after that, all you need is a regular 'ol caliper rebuild kit. But new slides & mounting bolts is definately a great idea.

I did this swap last summer, the part that hung me up for a week was the damn master cylinder. The 82-88 4-wheel-disc option code was "J65", the disc/drum braking system code is J50. I couldn't bleed the rebuild J65 m/c for the life of me! Turned out the thing was bad; I returned it, got another, and it bled much easier. I still have a problem somewhere tho; if I try a powerbrake, my car inches forward, the pedal sinks, and the "BRAKE" warning light comes on.

I also restored the axle while I was at it; it got new axleshaft bearings (aka "rear wheel bearings") and seals, and a coating of POR-15. I rebuilt one caliper, and replaced the other with a Pep Boys one (the other one came from rust city- too rusted to rebuild). I had to bend a new driver's side rear brake line since the old one snapped on me. I also put on new rear hoses and new rear parking brake cables.

If you go with this axle and need part #'s, let me know, I'll get 'em for you. I ordered all my stuff through carparts.com ... I found a "$50 off $100 order" coupon on the internet. The only thing I got locally was the J65 master cylinder... good thing, otherwise, that would've been hell to return.

Oh- someone on the chassis board discovered that GM had a technical service bulletin out for the master cylinders! I almost forgot bout this.... seems that some owners of brand-new rear-disc f-bodies complained the brakes were too hard to push. The solution was to install the J50 drum/disc master cylinder. The TSB said that the J65 m/c was designed for increased pedal effort, since performance drivers usually like a more responsive pedal to "feel" aggressive driving conditions. So it looks like you don't really Have to swap your master cylinder out if you don't want to... I wish I knew that before I did the swap. It took maybe an hour to put the new axle underneath and hook up the brake lines; and 7 days to fight that bad master cylinder.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

TomP

Ah- almost forgot- the 3.73/posi/disc is a rare axle. Avoid an '82-'83 axle if you can; they used SAE (standard double flare) brake lines. '84-2001 f-bodies use ISO (metric bubble flare) brake line fittings. You'd have to buy adapters to convert your ISO flare to an SAE flare, or flare your own adapter lines. Careful; an SAE flare tool is about $20, an ISO flare tool is $80 (eastwood company . com) If you find an early trans-am or Z28 with a manual transmission and engine code "G" (think "G" for "gold"), you've found the axle.

The '84 Camaro H.O. (says Camaro HO on rear left bumper) came with a 3.73/open/drum rear axle.

The 2.5l I-4 f-bodies (4 cylinders were from '82-'85) with manual transmissions sometimes had 3.73/open/drum axles.

No matter what axle you get, always pull off the rear cover (put a drain pan under it first!) and check the gears out. Make sure they've got the correct ratio (count the teeth on the ring & pinion gears, then divide, a 3.73 = 11:41), and that the gears aren't chopped into little pieces.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

red90rs

thanks tom, what mods do you have on your 2.8??? idi you notice the acceleration alot?? so do you think I should be able to do the swap in a day..if not then a weekend?? let me know, thanks alot

KED85

The swap will definetly take more than a weekend.
THE BEST IDEA IS TO FIND EXACLTY WHAT YA HAVE AND FOLLOW MOST OF Tom's suggestions.
Check the fittings, etc.
I still have to do a disc brake swap on my Wifes 1968. I have the right parts. BUT A SWAP IS CUSTOM. Everything is/MAY be slightly different!
It's fun that surprise, it's 6 PM Sunday, no parts stores and you're frantically phoning for a ride for work!
Plan ahead, check everything and then have the fun!
For my swap I need two more adaptor fittings. Just gotta find what I have to adapt to what I have to adapt!
Part of the fun customizing car!

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

Brian K

I am planning on doing a 4.10 rear and a locker.... As soon as cash comes and I get it done I will let you know.. I am going 4.10 cause this car will never hit the free way again or long trips....

red90rs

OK guys what if i just did one without discs, is that gonna be hard.. My braking is really pretty good, and its not like I'm autocrossing or anything. I know of a few more places with rearends, if i just idi 3.73 would that be a shortly timed swap?? Also how hard is it to convert to posi...is it all in the rear end or will it have to be the whole axle?? Sorry for all the questions...I'm new to rear ends and such

KED85

Study Jeniffer Lopez, Real Hard
Study Brittany Spears, Real Hard
Study Sarah Jessica Parker, Real Hard
Study every Cheerleader you can meet, Real Hard
Figured out rears, yet?
Everything is inside that center section.
Posi, gears, etc.
It's the wisest to get the best ya can for the money invested.
A milion and one things can be done to rebuild, set up an rear axle assembly.
If ya pay someone, you pay alot. Can go over $1,000, easy! That excludes the cost for the axle assembly, too!
IF ya buy it working and with everything ya can (disc/posi) buy it. It is a wise investment. Always inspect it before you spend the labor installing any component. OR you pay someone to install it fo you.
When you do get this yard axle, get the brake line fittings at the end, both male and female.
Get the fittings matched up, on your car, before you try installing the axle.

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

CHVYPWR

Hey Tom, thanks for clearifying that . It's been about 5 years since I last messed with 3rd gen rear discs! I guess I'm a little rusty LOL!!

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Ya can't have too many fast Chevys!

TomP

Mods, lets see...

Engine's got 217,000 miles on it, a Cloyes True dual-roller timing chain set, a removed air conditioning system, Hypertech 160 degree thermostat, TB bypass, AP mandrel-bent y-pipe, Products for Performance high-flow cat, Dynomax super turbo cat-back (#17493). Intake is a homemade cold-air/ram-air with a big K&N conical. Ignition is a rebuilt distributor, MSD 6Al box, MSD Blaster SS coil, MSD Super Conductor 8.5mm wires, and Splitfire SF2C plugs. I'm on my third rebuilt transmission, with a B&M Shift Improver Kit (stage 2), B&M 4-3 Kickdown Kit, B&M Megashifter, B&M Supercooler, and Summit remote trans filter, with some Valvoline semi-synthetic fluid inside. I have poly bushings for the torque arm, trans mount, and anti-sway-bar end-links... the rest of the suspension's untouched.

There was a noticable improvement in acceleration- the car definately woke up. Now, I'm running large tires on the back... the large tires combined with the 3.73 rear gears raise my "effective" (aka "it feels like") gear ratio to a 3.46. Basically you could say I put the effective ratio "back to stock", since it's stock rear gear ratio (with stock P215/65r15 tires) was 3.42. So you should notice a big improvement going from your 90's 3.23's to 3.73's. I'm toying with buying shorter rear tires for the back to get closer to an "effective" 3.73 ratio...

I disagree; you could do the swap in a weekend if you're not diving into the axle, or swapping brake systems. As long as all of the old bolts/nuts break free and aren't rusted together, you should have the original axle out and the junkyard axle in a couple of hours. Hooking up drum brakes will also be quick, and you'd just have to bleed the back brakes since you'd keep your existing m/c.

Now, keep in mind the price! I spent over $600 to put that axle under my car. For $600, I could've bought a $200 set of 3.73 gears, the SLP take-off posi for $100, and paid someone $200 to install it all. And I still could've bought & installed new rear axle bearings & seals, AND painted the thing with POR-15. Originally I thought I was saving money, but one thing led to another ("since it's apart, I might as well do this, and then do that, and might as well do this, too"). Yeah I got disc brakes out of the deal, but I still need to buy the +$200 rear caliper service parts to get them to work at 100%!

It was a great experience, and I'm proud of the job, but it probably wasn't the best way to get a 3.73/posi rear under there. Plus, I'm not sure what I'm going to do with my original 3.42/open/drum axle... it's in my backyard.

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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

[This message has been edited by TomP (edited July 17, 2001).]