Im replacing the rear and the tranny...

I have an 1985 Z28 w 355 quite a few mods..tranny and rear are stock.drum...except for auburn posi..i beleave it has 2.73s (correct me if im wrong)

Ive been reading the boards...and im thinking ..

4th gen rear end..im not sure which year...
(help me w/ that)

but i want 3.42 gears (i think thats right)
disc brakes
posi

tranny..

alot of people are saying 89-93 700r4

does that sound like a plan?

help me out before i make a big mistake and waste money..

the tranny is slipping real bad and it smells like burning :P

I want more acceleration out of the reara and less top end..

help..


Thanks

 

Well tranny wise, a well built TH700R4 is only good to about 450 ftlbs of torque so if your engine is really crazy, you may want a stronger unit like a TH400 or a 4L70 from Yank (expensive but well worth it from what I hear). You may also want to look at a stall converter if you don't already have one. Consult your cam manufacturer for a range of stall speeds for your engine combo (you can post your combo here and people can recommend some ranges as well).

As for the rear end, any 4th gen rear from a 6speed car that has 3.42s will work - year doesn't matter. Only 6speed cars had the option of having 3.42s though, so don't look at any Automatic cars for their rear axles if you want the 3.42 gear. If you are going to change the gear right away anyways, than a 3.23 gear rear will also work and you can reuse the carrier since it is a 3 series. The axles that are lower than 3.23 (ie 2.73) have a 2 series carrier and are nowhere near as durable as the 3.23 and higher axles. Than again, if you are going to replace the carrier AND the gears, any rear axle will do as you will be basically only buying it for the housing and brakes.

The 4th gen rear into 3rd gen car swap is nearly a direct one with the exception to the brakes lines (depending on the year) and the parking brake cables. Everything else is a direct swap.

The 3.42 ratio is also somewhat of a higher speed ratio and is a little bit of a compromise of acceleration. If your engine makes good power above 5500 rpm, you could likely use a 3.73 gear and get better 1/4 mile times. A 3.73 gear with a TH700R4 will still be manageable at highway speeds but will give you less mileage than the 3.42 gear. The gear you choose is really dependant on what you do with the car and what your power band is like on your engine. If the car is a street car first and a drag car second you will likely be happier with the 3.42 gear. If the car is driven only to the local street races and the strip, a 3.73 gear will give you better performance depending on whether or not your engine makes power in the higher rpms (ie if you have too much gear you will actually run slower since you will either be winding out your motor in third gear far beyond the point of it making any power or you will need to upshift into fourth gear, which is an overdriven gear and will kill your performance, before you get to the end of the 1/4 mile).

Some better info on your combo will make this an easier issue for people to give you input on. ie what kind of engine is this (cam, heads, intake, etc etc.)?

------------------
1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

The 3.42's are a good ratio compromise as the lower you go (higher number) you limit top end. I have never heard of a 4L70 but the 700/4L60 is good to 600 HP with the TransGo Reprogramming kit and proper build up.

 

ok here sthe engine...Ill try not to miss anything..


91 Z71 block ->355 ported polished, hookers,
3inch exaust, highflow cat, cold air, air foil,stock throttle body, intake n such,
has alot of interanl parts as well...

I recently bought it from my friend..Ill have to IM him for the internal parts.

ran a 14 flat spinning and smoking the tires the length of the track.

so im thinking switching the tranny and rear should help since the auburn posi hasnt much.

Id tell ya more but I really dont know off hand..

first performance car...learning as i go.

Now the rear...

look for a 4th gen rear from a six speed...it will have 3.42s (which i want) and posi?

disc brakes to...

I can get braidedbrake lines right? easier than trying to bend currnet lines or the 4th gen lines right?

Thanks for your help its much apreciated




[This message has been edited by PonyKillerZ (edited September 20, 2001).]

 

Leo - the 4L70 trans is Yank's version of a very beefed up TH2004R. Depending on componants, the 4L70 Yank trans is good to 850 ftlbs of torque . The 4L70 name is not a Hydramatic name it is just one that Yank slapped on this basically 'race-prepped 2004R'. I personally know the guy with the silver 98 Z28 on their home page (Yank's website) that runs solid 10s NA with that transmission and a 4400 Yank stall converter and his car is pretty much insane. He also has an 11 second GN and his experience with his GN lead him towards the 4L70 with it's better gear ratios and stronger overall internals than a TH700R4/4L60. I only wish I had the cash for one of those.... .

Anyways.... Yes, a 4th gen rear of any year from a 6 speed car will likely have a 3.42:1 ratio (I have only seen a few with 3.23:1 ratio but I think that the 3.42s actually were an option). It will also have a posi carrier (limited slip) and large rear disc brakes (11.6"). You will have to bend up some hardened metal lines for the rear end though. There is no way to run flexible stainless braided hoses from the hardened line that runs along the frame of your 3rd gen all the way to the rear calipers. Don't worry about it that much though, it is pretty easy. All you need is a little tubing bender from your local hardware store and you can do it yourself in a few minutes. The line (or lines if you end up needing to do both of them) are fairly short and don't do any really complicated turns. Just get some shelf stock brake line and bend them up to mirror the ones you took off the car. Just take the existing componants to your local auto parts store and make sure that you get the proper fittings on the ends of the lines so they fit and seal correctly.

As for your engine combo, it sounds pretty stout, but we really will need more information about the exact cam specs and what kind of head castings were used (ie valve size and port volume) so that we (and you) can make an estimate on the power band of the engine and what kind of gear the car will respond the best to.

Laterzzzzzzzzz
Matt

------------------
1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Im e-mailing my frined about internals...chip..other lil specs..

Ill get it posted asap

 

some cam specs( this will be choppy) I dont think he knew what he wsa talkin about either.

112 lobe seperation..

pistons are hypertectics 30 overs..

compression..9.3 - 9.5.1...

polished crank..

he says the cam is comparable to a

 

some cam specs( this will be choppy) I dont think he knew what he wsa talkin about either.

112 lobe seperation..

pistons are hypertectics 30 overs..

compression..9.3 - 9.5.1...

polished crank..

he says the cam is comparable to a ZZ9 from tuned port specialty...

cam powerband 2700-6000 rpms

ported and polished heads..

spring rate...375# of seat pressure...

thats all he gave me...as he typed it...

hope it helps...for my sake

Thanks

 

hmmmm

 

With a ZZ9ish cam and well flowing heads you should be making good power to about 5500rpm. At this point you are in need of a much better intake system. Your fairly stock intake system might limit your peak power to around 5200rpm. With a fully ported Long Tube Runner system (ie SLP runners, ported plenum, high flowing base) you should make power up to 5500rpm. If you went to a SuperRam or MiniRam you would make power past 6000rpm. With a useable powerband up to 5500 you could run the 3.42 gear with good results and still only turn around 2000rpm at 60mph with a TH700R4. If you went to a 3.73 ratio you can figure on turning about 3000rpm at 60mph. So as you can see, the 3.42 gear is a much more street friendly gear and it will still yield good 1/4 mile performance.

I would say the 3.42 gear would be the best for your current setup and would still work well if you got a better intake setup. You won't need, or really want, to step up to a 3.73 until you get your engine making more high rpm power or dedicate your car to only strip use.

Some ideas...... Good Luck....

------------------
1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

thanks

 

this is my new name (lacking posts)

would a miniram be way to much for my set up...


I here minirams are only good for strokers or big blocks.