Rearend choice

by Hs1973 Sat Jun 20, 2020 12:38 pm

Hi guys.

I could use some advise. I have a 3:23 8.5 10 bolt rear with a limited slip dif from a 77 trans am in my chevelle. I would like to chance it to get faster out the hole. I have 350 with a 700r4 trans and it feels a little slow off the Line. I thought about putting a 3:90 in it. Can the Stock slip diff handle it or should i upgrade that too. And if so what diff is best for street use?
And what about the 28 spline aksels. Are they ok or do i need bigger? I have about 300hp now but will eventually get more, but keep it at 400 maybe 450. Im looking into getting new topend kit at some point.

I am looking for just a little better umpf, as now it seems it has alot off rumble but not so much go until i hit second then it gets up and gets going. Any input is welcome.

by Joe73 Sun Jun 21, 2020 7:42 am

Everyone I know running an overdrive trans loves the 3.73 ratio. It matches up real well for acceleration as well as highway manners.

If you do the math you will see the results to give you an idea. A number of "9" is the most all around fun to drive. Going much higher than 9 and you start heading into the unstreetable range.

Your original turbo 350 had a 2.52 first gear ratio. Multiply your 3.23 x 2.52 = 8.13
A 700r4 has a 3.06 first gear ratio. 3.23 x 3.06 = 9.88 (Fun, but you can do better and still maintain highway manners.)

Now, 3.06 x 3.73 = 11.41 first gear ratio. That will launch the car nicely!!

Another thing to remember, your original Turbo 350 took 35 horsepower to operate it. Your 700r4 takes 70 horsepower to operate it. But I think your trans choice was worth it. http://www.oldengine.org/unfaq/leadfoot/trans.htm

The 8.5 diff can handle some decent power, even with the 28 spline axles. Are you definitely sure its an 8.5? In 1977 Gm started switching to the 7.5 diff. I'd leave the 8.5 in there if the bearings and posi are in good condition. Also, when you change the ring and pinion, first, the bolts are left hand thread on the ring gear. Second, examine the ring gear bolts carefully. There have been issues with the head size and diameter of the shoulder just below the head. Some companies when to a 5/8" head as opposed the GM/Ford 11/16" head. Yes we can use Ford bolts.

Heres some reading.
https://nastyz28.com/threads/clunk-clunk-and-there-went-my-rearend.244801/

https://www.camaros.net/threads/warning-10-bolt-ring-gear-bolts-can-come-loose.131993/

Good luck with your choices.

by Hs1973 Sun Jun 21, 2020 10:16 am

I knew someone in here was abel to give me the advise i needed. I for one did not know of the Left hand bolts. That is wery good to know. I have some experience in putting in a rear and put in the trans am diff myself.

I Think im going to go with the 3:73 then. The diff itself works fine and makes 2 Black stripes and no noise at all. So i will just inspect everything for problems. However im told it is a problem finding new cluthes for it. Im sure its a 8.5 10 bolt with 28 spline aksels. It was verified 2 years ago when i put the 3:23 in it. It is quite a job setting it up so il wait till winter. Thanks alot for the advise.

by Hs1973 Sun Jun 21, 2020 10:41 am

Oh and Thank you for the link to nastyz28 that is some scary Reading. This is wery useful information, and for anyone thinking of trying rebuilding a rearend. It is not that hard however it takes alot of patience and the right tools. If you skip a step or dont do the research there is a good chance it will be a noise mess when done.

by Iggy Sun Jun 21, 2020 8:48 pm

Friend of mine races a 1978 Camaro on the 1/8 mile track. 427 big block with a TH400 trans, 8.5" 10-bolt rear end with a spool and stock 28-spline axles. His bracket in 6.5 seconds and has no issues with the strength of the 8.5" 10-bolt. He is running more than 500 HP normally asperated without Nitrous Oxide. You should have no issues with the 10-bolt, and I also recommend 3.73 gears for what you are doing.

by Hs1973 Mon Jun 22, 2020 12:09 pm

Then 3:73 it is then. I must say i like the 700r4. But i only used it because i Got it for free. I always planned on using a 200r4. But i Got this from a friend that upgraded his camaro bigtime, and im pretty happy with it as is. However i have this idea in mynhead as to how i want it to behave and ride, and im not quite there yet. Of course if i had the funds i would have just done it kindigit style with a new chassis and new ls and so on. But i sadly am on a wery tight budget. So all summer i plan on what to do with it over winter, and save up the money. And some times i must compromise as wife and kid plus house wants there share.
Im hoping this upgrade will give it a little more go. Thanks for poiting me in the right direction.

by bracketchev1221 Mon Jun 22, 2020 8:39 pm

Gears and looser stall converter are the easiest ways to boost acceleration. But they come at the expense of gas mileage.

by jerry46765 Mon Jun 22, 2020 10:08 pm

Going from 3.23 to 3.73 will only increase engine speed by 300 rpm. It will not feel like a big seat of the pants change, as say going from 2.73 to 3.73. I am not sure going with a shorter gear 4.10 or 4.30 is a good idea with the 700r first gear.

If money is tight, maybe consider another area of improvement like more power or a converter. But as someone said any of these changes will decrease fuel mileage and increase cost to operate.

Thanks -

by bigredlaguna Tue Jun 23, 2020 9:20 am

What are the specs on your engine? I have a vortec headed 350 with a cam that has only 206 @.050 duration.

I started with the stock 2.73 gears with open differential. The engine had enough torque off idle to spin the one tire for blocks if I wanted. I changed the gears to a 3.42 and posi carrier along with better tires. I also tuned the carburetor better for higher rpm performance.

The car definitely accelerated better with those changes, but with the stock stall convertor it still had a small bog at off idle WOT performance. I then bought a cheap stall convertor from jegs and that made all the difference for me. This is with a th350 so I am not sure if I would have felt the same way with the different gear ratio in first if I had a 700r4. I probably would have been wondering why it seemed to fall off so bad in the upper rpm range. I am wondering if that is what your symptom is, or if it is the off-idle performance.

If it is falling on its face in the upper rpm range, you might look at timing and carburetor tuning first, then depending on the cam specs, you may need to adjust your 1-2 shift point to fit the power curve of the engine.

by Joe73 Tue Jun 23, 2020 9:37 am

If its falling on its face at higher rpms, Id have a question. What valve springs are on the engine? Do they match the rpm capability of the cam? You dont get much rpm out of stock springs especially if theyre tired. More valve spring tension, the more rpm potential. And I wouldnt go more than about .475 lift on stock springs.

by Hs1973 Tue Jun 23, 2020 12:47 pm

I just installed a brand new gm performance engine 290 horse 350. I havent tried all she Got yet as i only have 500 miles on it so far. I just feel like it could feel alot better with better gears as it has a slow Feeling about it even if i beat on it alittle. If im doing a kickdown at speed it feel alot faster and better. Also it runs very low Rpms at highway speed, and it almost Logs cruising with lockup on.almost like a 5 speed manuel going up Hill i to High a gear. If you just Cruise around it feel ok and a Nice cruiser. Im just looking for a little more pep. Im sure when i get it adjustet proberly too it will also help.

But i have always wanted a little more gear even before the new engine as it was the same with the old. Maybe my stall converter is also to tight. I have a 2000 Rpm stall in it. That Came with the trans. As funds did not alow for a new at the time i just went with it.

by Hs1973 Tue Jun 23, 2020 1:44 pm

Dont get me Wrong im ok with how she performs now, she is a really Nice cruiser as is and drives beautiful just cruising around town. and maybe im overthinking it, but i cant help but feel a tiny teeny disapointed cause i was expecting a little more umpf from a 300 horse engine. Since my old was maybe 220 ish i expected a little better from the new one.

I know off course that i havent been giving it all as it is new and so it May surprice me when i get it over 2500 Rpm. But still i feel like the acceleration dosent quite match the Rpm. i expected a little seat Feeling even at 2500 Rpm i first gear. But of course 4500 would be better. Or am i just expecting to much from this engine, or do i just need to floor it. And when can i safely do so without braking anything?

I have been told that a topend kit can really Wake this particuler engine up as the cam is to Big for the heads. So i hope to get cash together for a topend kit next year. But im stil sure that a little gear would also help and make it feel better also on the highway.

by Iggy Tue Jun 23, 2020 2:16 pm

You should have about a 2200 RPM stall converter with that setup. Top End Kit that brings the compression up will help a lot, 8:1 compression is a little low for that cam (basically the old Corvette L82 cam). A 64cc chamber head would bring it up to around 9.3:1 which the cam would like much better, Vortec heads with the stock springs would work as the cam does not go beyond the lift the Vortec heads can handle with stock springs and no machining.

by Joe73 Tue Jun 23, 2020 3:27 pm

Id like to see a bit more compression. Specs on that engine show it at 8:1 with 76cc heads. Putting on some 64cc heads will boost you about 1.5:1 so you will be at 9.5:1. But I wouldnt be too quick to open up a brand new engine.

Gears are your best bang for the buck. And I recommend them either way.

But also, hows your timing? Are you getting 32-36 degrees total? What rpm is it "all in" at? Does that rpm match the cam? If your timing is lagging, that will hurt performance big time.

by 73ss Tue Jun 23, 2020 7:49 pm

To the best of my knowledge, GM didn't install anything less than a 3.42 in a OD equipped car/truck. This was to avoid the lugging you described with the lock-up engaged. RPM's are just too low with less than a 3.42.

The biggest gripe I hear with a 700r is the wide ratio spread between 1st & 2nd, almost a 50%rpm drop when the 1~2 shift hits. The 3.06 first was to help the heavy low powered cars of the day get moving. Most of these cars had a small V-8, V-6.

200r has a tighter ratio spread and a tad higher OD gear, .67 vs .70 in the 700.

FWIW I think You'll like a 3.73. I'm running that gear with a 200r. 70 MPH is 2200 rpm's with the lock-up engaged.

by Iggy Wed Jun 24, 2020 9:01 am

When they first came out with the TH7000-R4 the automobiles used 2.73:1 gears, the Corvette used 2.87:1 and trucks received 3.08:1 gears. The 3.42 gear ratio was not standard until the TBI trucks of the late 1980's. In the mid 1980's you could special order 3.42 gears on an F-Body as the "Performance Gear" otherwise they had a 3.08's or 2.73's depending on the engine and vehicle.

Those vehicles had very poor throttle response due to the poor gearing and lugged really bad - at 45 mph your car would shift into OD and RPM's would drop to 1200 or less! That was a huge complaint from new car owners back then. Because of the way the EPA calculated fuel mileage back then, the awful gear ratios increased the Corporate Average Fuel Economy (CAFE) Ratings - had nothing to do with real world fuel economy.

I had a 1983 Caprice with 2.73:1 gears and TH700-R4 and it actually got better mileage in Drive than it did in OverDrive. My 1982 Corvette could not drive around town in OD, had to put it in Drive or it would just "hunt and lug". 3.73 gears brings the RPM's up enough that it eliminates the "hunt and lug".

The 1980's were "dark days" for those things! My local Chevy Dealer back in 1985 looked at me very strangely with I ordered a light duty 3/4-ton pickup with OD and 3.73 gears, but they ordered it anyway. That set up put me right below the peak torque at highway speeds and I would get 19 MPG from a 1985 K20 with a 350! My current 2009 Sierra does not do that well.

by Hs1973 Wed Jun 24, 2020 9:59 am

Thank you guys. Of course i am not itching to tak a brand new engine apart. So i will see how far i can get it with a new gear setup and off course it needs to be adjustet again. I am sure we can find some more performance and make it run even better if we use some time fine tuning carb and timing. I just Got some mixed info as some guys around me insisted a 3:92 would be the right choice. But i always leaned more Towards 3:73 myself. So now im sure. thanks for all advise. Regards Henrik.

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