Hello all-- this is my first post so be gentle! I have a '92 base Firebird which I've had work done to. It now has a Formula hood and spoiler and a freshly built 355 motor with a posi 2.73 rear end. Finally, today, the mechanics got it all tweaked out, timing, carb adjustment, etc.
So my question is this: I think I need a higher rear end, just to get more pep off the line. I won't be racing or anything, just rather have the power on the line if I needed it. What would you guys recommend as a better rearend for my purposes? This car has a rebuilt 700R4 tranny. Also, I live in northern Michigan and will be looking for said new rearend. Thanks for any advice!!! Greg

 

Can you give us the specs on your motor? The cam, heads, intake, exhaust. The gears need to be matched to the specs of your engine to get the most performance out of your set up.

 

Hi and thanks for responding. Sorry, I had nothing to do with the mechanics part, just the money! But here is the info I got back from your statement: rformer dual plane, exhaust is Flowmaster full 3" cat back. Cam is .450" lift int/exh, heads are 76 cc with 1.94/1.50 valves, intake is edelbrock pe.
Hope that makes sense, it's greek to me! Thanks for any info, Greg

 

Those heads don't flow very well and they are large for a 355 cubic inch motor. The stock cast iron L98 TPI heads '081' castings are 64cc's. The 76cc heads that you have are most likely those cylinder heads that were used on a lot of the low compression smog motors from the late 1970's. They may decent torque for the low compression, low output motors they were attached to but they are not something to write home about in terms of performance.

There are some cheap vortec iron cylinder heads available with that same valve size (1.94 inch diameter intake / 1.50 inch diameter exhaust) that would flow a lot better for that cam. Those vortech heads flow better than the stock L98 iron heads. Although if you found some cast iron L98 heads at a junk yard those would be a cheap way to step up your performance as well. Another wise move would be to consider getting some aluminum heads with 62cc-65cc and 2.02 inch diamter intake / 1.60 inch diameter exhaust valves. Any of those cylinder head options (as well as others) would perform a lot better than what you have now.

The cam that you have is a good mild performance cam that should give you a operating range of idle-5,500rpm and 5,500rpm would be the upper limit I would put on that combination.

Also since you have the 700r4 trans you should also look in to getting a high stall torque converter. Especially since you said the trans was rebuilt and they probably upgraded some of the internal parts like the servo and maybe a shift kit. High stall converters really help the engine get in to the powerband (and keep the engine there) while you are accelerating. They make a night and day difference in an automatic equipped car. It just needs to be matched to your engine components and what rear end gearing that you decide to run.

The stock stall on our cars is around 1,600rpm and thats really low. Most combinations that are not on the high-reving side (the engine is going to spend most of its time below 5,500rpm-6,000rpm) work well with a stall that is between 2,200rpm-2,500rpm. I've got a 2,400rpm stall in my car and I really like it. You want to try and pick a converter that's roughly 500rpm-to maybe-700rpm below where your engine makes its max torque. That's a ballpark estimate as there is a lot of variance with that rpm range with respect to your specific engine and drivetrain combination.

With the 2,200rpm-2,500rpm stalls you aren't going to be slipping your motor all of the time like you would with a really high stall (5,000rpm stall) racing converter that is best suited to a car that is going to see a lot of drag racing and not a lot of street time. Also you won't be creating a lot of excessive heat that a really high stall converter would produce either. You should look in to a trans fluid cooler as well any time that you run a higher stall. Heat is the #1 enemy of an automatic transmission and a higher stall converter is going to add to that so a trans fluid cooler is a smart idea. Although with the 2,200rpm-2,500rpm stalls you can hold off on getting a fluid cooler right away as those don't raise the heat issue concern as much as the 3,000rpm and up stall converters do.

What kind of rear end are you running? I'm assuming you are running a thirdgen 10bolt rear end (7.5 inch or 7.625 inch ring gear diameter). Ha ha yeah many of us got rear ends with those kind of highway gears. The good news is that with a 10bolt as long as the housing is straight everything else can be purchased from the aftermarket. GM made so many of these rear ends and installed them in so many vehicles that parts are very easy to comeby.

For your set up I would look in to 3.23 or 3.42 gears. When you start looking at gears that are steeper (such as 3.73 or 4.10 gears) you usually pair them with an engine combination that has it's powerband higher in the rev range (6,000rpm and up) to get the best use out of those gears. While you have the rear end apart now would be a good time to upgrade the axles to 28 spline. I think the 1990-1992 cars got 10bolts with 28 spline axles but after all this time the best move would be to check the spline count yourself. There is also no need to look in to getting a 4th-gen 10bolt rear end. The thirdgen 10bolts with 28 spline axles have the same strength as the 4th-gen 10bolt rears. You may want to look in to upgrading your rear brakes to disks if you have drum brakes while everything is apart for the rear end rebuild. You said you already have a posi unit, check and see if it needs to be rebuilt or replaced as well. While you have everything apart now is the time to upgrade that kind of stuff. Believe me that is the kind of job that you want done right the first time so that you don't have to worry about it and comeback and change stuff later.

Also keep in mind that the 700r4 has a very steep 1st gear ratio of 3.06 so with steep rear axle gears you are going to have a hard time hooking up. In most street cars paired with the 700r4 I wouldn't want to go much deeper than 3.42.

Besides the traction issue that can come up you also will start dipping in to your highway fuel economy since the engine will be turning higher revs with steeper rear axle gears. If you have a car just dedicated to racing fuel milieage isn't that important. For most of us though we drive our cars and the street so fuel economy is important! Ha ha I sure as hell am not rich so I want a good paring of performance and fuel milieage.

The 7004's 0.70 overdrive ratio (30% gear reduction overdrive off of the 1.00 third gear) and lockup torque converter allow you to turn low rpms even with steeper gears like 3.42's.

Ha ha I know I wrote a book here but hopefully that helps out some.

 

Holy moly Yaj! Now thats what I call an answer!! The specs you talked about may not be perfect, but for my semi mild driving style, hopefully they will be sufficient--also, I spent enough! Ha...I think I will go with your recommendation of the 3.42, provided I can find one somewhat near. Thank you again for your awesome information!!!

 

Ha ha yeah sure no problem. One of the cheapest was to get a 3.42 geared 10bolt rear is find a 1991-1992 V6 powered thirdgen in a junk yard. They came from the factory with 3.42 gears. You would still have to check the wear pattern on the gears and make sure there wasn't any real problem with them.

 

Ok 3.42 it is, will check around, just tough up here in no mans land! Thanks for the direction Yaj, great site!

 

The automatic V6 cars had a 3.23 rear gear. The manual trans V6 cars had the 3.42 gears. If your posi unit is good the 3.42 or 3.23 gears will not work right as they would be for a series 3 carrier and not the series 2 posi carrier that you have. You may need to replace your posi unit to use these gears.

 

My car has a stock 5.0,TBI,700r4 that had a open 2.73 with 26 spline axles.I upgraded taking into consideration what I had read here and the recommendation of the shop I had doing the rebuild,called "The Rear End Shop" they have been in business for over 20 years. I wanted the best replacement parts for my street/dd he had to offer for my driving conditions and he accommodated me with a set up that could handle with street tires and automatic transmission around 400 to 450 hp should I need it. I went with the 28 spline Eaton posi., 28 spline axles and 3.42 ratio.and L.P.W. cover.This is not a used set up and I suggest trying to find a used one over the price I payed.( I have to say that because I was criticized for suggesting new parts without mentioning a used alternative).Would I do it differently as far as using new parts,no. Do I love the new Eaton posi.,3.42 ratio,cover,etc. Yessss.If you end up using the LSD and axles you have than having a shop install the gears (thick gears or you will need the new carrier/LSD as mentioned above)and all bearings and seals will run you around $620.00 Thats including labor (around $300.00) axle bearing and seals and gears.

 

Thanks Phoesnix, I was planning on a complete used rearend swap. I did get a lead on one near here (5 hours), for $150.
So Ron, how do you like the 3.42? Someone else suggested a 3.27 but that doesn't seem like enough of a difference to do the swap from a 2.73, but I just don't know. I'm prodding along, its hard not being a "car" guy.. Thanks for the great info guys!!

 

yaj15 has some really good stuff to say and I wont pretend to be in the know when it comes to engine tech. like him but Ive been on this site for quite a while now and have learned the basics.Most I have applied.yaj15 mentioned 3.42s and from my experience with the 2.73 to 3.42 upgrade it seems to be the perfect ratio.with the 700r4.I would not settle for anything else. You still get some first gear off the line ,its short but just enough to dig deep with the posi. off the line then I get a nice bark of the tires hitting 2nd and another chirp hitting 3rd. Even with the 5.0 I get pushed back pretty good.Seat belts lock on me everytime.Perfect gearing with the 700r4s OD on the highway.With the higher hp your running your going to have fun.The gear swap I did literally is a night and day diffrence.3.42s all the way

 

I was looking around for shops that do rear end work and I've heard good things about the 'rear end shops' from other people as well.

What rear end shop location did you use to get your rear end rebuilt?

I just got my new driveshaft in the mail yesterday and I've already got my BMR torque arm. I want to have all that stuff installed when I get the rear end rebuit at the same time. Thats the last drivetrain component that I haven't gone through on my car yet. Its still original to my car so it's got 196,000 miles on it now. Works ok but I want to change the gears from 2.77 to 3.27 and get another posi.

 

Ha ha thanks for the compliment ronusmc. I've learned a lot over the years from this website, books, magazines, working on my own car and friends.

Most of what I know is around centered around Gen 1 small block motors though. Thats what I have in my car and I'm going to keep it old school. I know a little about LS motors too but, I don't too much about big blocks either.

When thirdgens were new an LB9/T-5/3.42 (10bolt) geared or 3.45 (9bolt) geared car was just a few ticks slower than an L98/700r4/3.27 (9bolt) geared or 3.23 (10bolt) geared car.

That entire equation really changes once you start making modifications. The 350 cars make more torque than the 305 cars do and they don't need as much help from the torque multiplication from deeper gears. Thats part of the reason why (especially if you have a TPI induction set up like I do that dosen't breath as good as other intakes do on the top end) why steeper gears aren't necessarily required to get good performance. The carb and TBI motors breath better on the top end and probably would see a better benefit from steeper gears than 3.42's than a TPI car would depending upon the rest of the components that were used. With TPI having its main focus as torque production and midrange power 3.73's and that short 3.06 gear ratio of the 700r4 would make 1st gear useless and you would start turning high rpm's on the freeway which would start cutting in to your fuel milieage.

That being said 3.42 gears are a good all around performance gear for both the 305 and the 350 motors. I don't care if you are running a 305, 350, TPI injection, TBI injection, or a carb; the freeway gears that these cars were equipped with from the factory really zap the performance.

Most of our thirdgen cars will really wake up just like ronusmc said when he upgraded his rear. Ha ha I'll be glad to change my gears from the current 2.77's that I have now to 3.27's. That was the perfromance axle ratio from GM so I know the performance will increase compaired to what I'm running now. The best thing about the 2.77 gears that I'm running now is that they are good for fuel economy and cruising at high speed while turning low rpm's. I still want to have good fuel economy and high top speeds (when I want to ring it out) so I would only go as deep as 3.45's since I'm still going to keep my borg warner 9bolt rear.

With these freeway gears I have I can cruise at 70mph around 1,900rpm-2,000rpm. I drove back home on the highway last night and ran it up to around 90mph for a bit and the motor was only turning 2,500rpm in overdrive and wasn't working hard to do that.

It's just when you really want to accelerate fast and get through the gears, thats when I really notice the lack of torque multiplication that shorter gears would provide. Its a compromise though between fuel economy and performance.

 

I just happen to live in Santa Rosa CA where one of the shops resides. I made sure I had all the parts necessary including what they needed to have in stock and they had me in and out in around 3hrs.($300.00)labor.The only thing I did not buy from them is the differential cover and ended up with a 3 year warranty.(they advertise 1 year which is not bad considering if its not installed correctly you should know it way before then.)Privately owned by Dan and really nice knowledgeable guys.I would call and talk to him one on one and don't be afraid to mention me (Ron with the red Camaro) = http://rearendshop.com/differentialrearendservice.html
I don't know where exactly you live in Central valley but going to 3.45 with the 700r4 should not be a problem getting back home.My tech. does not work but going buy ear I can maintain a 65mph through 70mph with no problem.Be sure to mention any aftermarket (brand name parts)you may want to use.There house brand is Sierra Gear and Axle.

 

Thanks thats good to know. I have bought a lot of parts from 9bolt.com. I live in Tracy so the closest rear end shop to me is in Livermore wish is about 15-20 miles from me so it's not that far. Yeah I'll have to check over all my parts then give them a call.

Do they have the parts to correct your speedo too so it will read correctly after the swap?

 

Lol sorry Lefman didn't mean to hi-jack the thread hear.

Yeah, you will like the way your car performs after the mild gear swap that has been discussed here.

 

Haha! No this is great info listening to you two! Someone mentioned elsewhere a 3.73 but after listening to ron and you go on, I'm gonna stick with the 3.42. You guys keep it up, very informative!

 

The 3.73 seems to be popular with the manual trans.IMO,you want some first gear and with the 3.73 your not going to get it with the 700r4.The last thing you want to do is sit there and spin. The higher the gear ratio. the more prone you are to do that.Once you start to use sticky/slicks to prevent the spin you jump into the hard launch category that's so famous with the 10 bolt giving out.The support cover I chose is top of the line and is there to help prevent that .I still only run street tires that don't have that hard launch grab and the automatic trans also helps to prevent a hard hook at launch.Even that being said for a little stock/with bolt on's 5.0 engine it still gets it pretty good using the Eaton posi.with the new gears.Yours with more torque will do even better.
http://www.jegs.com/i/LPW/619/301-7.5G/10002/-1
lefman,
Did they use a Corvette servo.during the trans. rebuild ?If your not sure you may want to find out.Even without a shift kit the Corvette servo made a huge difference in how crisp the shifts are and it only cost me $23.00 and about 2 hours to install it.It can be done quicker but it was my first time.
http://www.sethirdgen.org/servo.htm

 

Hey Ron. First off, I didn't say this but the car is for recreational use only, no racing! Just for fun but I wanted the power there when I needed it. I like getting going better than I like the higher end speeds, etc. Not really sure about the servo, but I did buy a shift kit from Summit, not sure of the exact specs of that, if thats what you meant. This car probably really won't be ragged on, my wife will never ride with me then!
BTW--Love your Camaro!!

 

Thanks,
A Corvette servo can/is used in conjunction with shift kits and play a large roll in there performance.When its time to install the kit I would be sure to have one.Personally I don't like the idea of a mechanic(I have yet to find the right one)digging into my transmission to the extent necessary for the kit install.Thats why I installed the servo first,.All I wanted was crisp/hard shifts and it gave me that so afterwards I did not need a kit.
Anyway it sounds as if you have a really nice project going on and its coming along nicely.Please keep use informed as to how things go.

 

Ok will check into the servo--Thanks for the direction!

 

With that cam, heads, and intake, I dont see a need to go past 3.23. 3.42 wont hurt anything, though.

 

Yeah that Corvette servo should be a good addition to your car. When you get time to install a higher stall converter - that in combination with the 3.42's will really wake your car up.

People forget that while the stock 305's and 350's in these thirdgens aren't at LS9 power levels they still make decent power. From the factory these motors are tuned for low end torque and midrange power more than anything else. Obviously that can be changed.

Ha ha yeah a stock 305 will have a lot of trouble trying to get power to the ground with 3.73's and a 700r4 trans.

On the L69 305 high output cars you could get 3.73's but most of the time they were paired with the T-5 manual trans which has a 2.95 first gear ratio so the torque multiplication wouldn't be as high with respect to a 700r4 so wheel spin wouldn't be as much as a problem.

Having said that though the L69 was rated at 190hp and 240 pound feet of torque.

Stock the L03 305's were rated at 170hp and 255 pound feet of torque. Keep in mind when the LB9 305 TPI motor came out in 1985 it was rated at 215hp and 275 pound feet of torque. The LB9 cars for that year were often paired with 3.23 gears and 3.42 gears and the 700r4. For 1985 you couldn't get the LB9 with the T-5. You couldn't get the LB9/T-5 drivetrain combination until 1987. At that point the LB9 was rated at 215hp and 295 pound feet of torque thanks to the cam from the L98 350 TPI motor.

The LB9 those short 3.42 gears and, the steep first gear in the 700r4 made lighting up the tires an easy task. Look up Chazman's thread in the history/originally section. He has a yellow 1985 LB9 IROC with 3.42 gears and he has a problem hooking up. Partly because he dosen't have positraction but, wheel spin can be an issue with these torque biased motors and steeper axle ratio gears.

The L03 cars are a little down in power compared to the 1985 LB9 but they are only down 20 pound feet of torque. The highest output LB9's were rated at 230hp and 300 pound feet of torque from 1989-1992 but only with the T-5 trans and a G92 performance package that included 3.42 or 3.45 gears depending on if the 10bolt or 9bolt rear was used.

Once someone starts making mods to the L03 torque and top end power is really going to start coming up and tire spin will become more of an issue.

Your next move is going to go in the suspension area. Boxed lower control arms, track bar, and poly bushings won't flex on you like the stock parts do and will help put that power to the ground. Power is great to have but if you can't put it to the ground it's useless.

I'm not going to come on here and say that TBI or TPI is a better induction set up. Both systems have their pros and cons. The point here is that either system can be worked on and modified to perform better than the way it came from the factory.

Check out Franks TBI website. His car has a lot of smart mods done to it and he started of with a factory 1988 Camaro sport coupe L03/700r4 car and really made it perform well. He has a lot of good information on that site. I learned a lot about TBI cars when I went on that site as well.