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Old 01-16-2013, 10:34 PM
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1961 Chevy truck, what rear end & gears for application?

I have a '61 chevy short wide that is my daily driver right now but fairly soon will be driven on the street mostly and will see a few runs down the strip several times a year during the summer months. I live in Oregon so that should give me about 4 months a year to play around, so I want it to be streetable and good fun down the strip too.

It has a 12 bolt truck open rear end with 3.08 gears, a TH350 with stage 2 shift kit and 2300 stall. I just bought my boss' old race motor and this is where it gets fun. It's a 350 bored .030 over, flat top 4 relief pistons, forged crank, h-beam rods, 2.02/1.60 valves, 1.6:1 high tensil stamped rockers, Weiand Team G single plane air gap manifold, hei dizzy with MSD module, high output coil, no vacuum advance and the lightest springs available on the centrifigul adv. so it gets full adv. right off idle, I think he said total timing was 30deg. at 3500rpm? 10.5:1 CR and the cam is a summit k1108 (specs below)


RPM Range3,400-7,000
Duration at 050 inch Lift: 244 int./254 exh
Valve Lift with Factory Rocker Arm Ratio: 0.508 int./0.533 exh.
Lobe Separation (degrees): 114
Camshaft Manufacturers Description: Rough idle, excellent high-end horsepower. Needs 10.5:1 and higher compression, 3,500+ stall, and gearing. 350+ c.i.d., bracket race cam.

He says this motor ran 450HP at 6200RPM on the Dyno with some kind of carb style fuel injection system which did not come with the motor so I'm going cabureted. I need to figure out what rear end will best for my application (described above), my low budget and the parts I already have.

I have a corporate 8.5" 10 bolt out of a 79 camaro with 2.41 posi that I was thinking of putting under the truck but I don't think it would hold up with that gear ratio and nobody makes a 3.08 or 3.73 gear set for the series 2 carrier. I know they make a spacer for putting higher (numericaly) gears on 2 series carriers but I've heard some horror storries that I don't want to live through. That being said, I don't know if I should get a series 3 carrier and gear set for the 10 bolt rear end or for the truck 12 bolt, and will the 3.08 12 bolt open rear get me by until I save up the money for the propper rear end solution?

Thanks in advance for all help. You guys are a great wealth of knowlege and I'm somewhat new to the technical aspects of hot rodding though I do have a basic grasp on what will generally make things go faster or slower.

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Old 01-17-2013, 05:55 AM
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I recommend a 4 series Eaton Posi and a 3.73 gear for the truck 12 bolt. It's a stronger rear end than the 8.5 and it already fits your truck. Don't forget to get a good installation kit with the Eaton and gears.
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Old 01-17-2013, 07:14 AM
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Forgot to mention that you will probably need a set of axles for that rear end. I think those had the weak 17 or 19 spline axles and you need to upgrade it to 30 spline axles.
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Old 01-17-2013, 08:03 AM
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What size are the tires on the truck? If you have really tall tires a little more gear.Decide how much high way driving you will be using the truck for and the RPM that you can cruise at with out going crazy. 3.73 is a good place to start your calculations. 62 mph =2700 RPM with 29 inch tall tires,,,62 MPH=3200 RPM with 25 inch tall tires
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Old 01-17-2013, 11:59 AM
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Originally Posted by vinniekq2 View Post
What size are the tires on the truck? If you have really tall tires a little more gear.Decide how much high way driving you will be using the truck for and the RPM that you can cruise at with out going crazy. 3.73 is a good place to start your calculations. 62 mph =2700 RPM with 29 inch tall tires,,,62 MPH=3200 RPM with 25 inch tall tires
Well, ideally I would like to run a 23000- 2500 stall and 28-1/2" to 29-1/2" by 10" wide cheaters but my cam wants a 3500+ stall so it sounds like I'll have to change my cam to use the stall and tires I want and keep the truck under 3000rpm at 65mph. The truck probably won't see a lot of highway driving but it will see some, mostly street and strip, so I could handle maybe 3000rpm for an hour if I had to. I like the way my truck drives the way it is, as far as highway rpm's but I'd like more power overall and a tad quicker off the line. I'm trying to find out if I will absolutely have to change my rear gears or if more hp is enough to do the trick so that everything else can stay the same (with the exception of adding posi). I'm just trying to make it as mean of a summertime street machine as I can that will also be fun at the track, not trying to break any records.
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Old 01-17-2013, 12:03 PM
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Calculators
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Old 01-17-2013, 04:31 PM
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The 12 bolt usually showed up around 64/65. Maybe you already have a newer 12 bolt?

Might be a good idea to pull a axle and get a spline count and length?

As BGH said the early ones have course spline axles.
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Old 01-17-2013, 04:57 PM
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Yeah it's a 73 truck 12 bolt, forgot to mention that. I converted the truck to 5 lug all the way around.
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Old 01-17-2013, 05:06 PM
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Originally Posted by CHEVILLAC View Post
Yeah it's a 73 truck 12 bolt, forgot to mention that. I converted the truck to 5 lug all the way around.
Hurry up and get that thing running... There's a couple good cruise Inns here in salem.
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Old 01-17-2013, 05:22 PM
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Originally Posted by prostreet6t9 View Post
Hurry up and get that thing running... There's a couple good cruise Inns here in salem.
That's why I'm here asking questions. I'm looking forward to bringing this thing to some events but I don't want to have to worry about breaking stuff and getting stranded. If I take it easy on the pedal, will I get by with my 3.08 12 bolt open rear or would the 8.5" 10 bolt 2.41 posi be a better choice until I can afford a 3 series posi unit for one or the other?
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Old 01-17-2013, 07:25 PM
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A good way to launch is steeper gears. The 700r4 over drive auto has a lower first gear for better launch.A longer stroke engine can make more torque at lower RPM for a better launch.
You just need to do the math of what you have and what you need to make the performance goals you need.
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Old 01-17-2013, 07:39 PM
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Keep the 12 bolt. The 8.5 has 28 spline axles and the gears will completely suck. If you want to keep the 3.08 gear then you will need a 3 series posi for it, but I don't think you will like the 3.08.
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Old 01-17-2013, 07:43 PM
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I'd use the 12 bolt if it was me. 342 or 373 ratio with a good posi.

Instead of thinking of a 700 trans.. I'd save up the money and just do your diff with good parts. 308's will be a little doggy but it will get ya by while you save up.
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Old 01-17-2013, 09:17 PM
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Awesome, that's the info I was looking for! Thanks again for all your help guys.
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Old 01-25-2013, 12:29 PM
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Ok, I've been doing a ton of reading and have come to the conclusion that a '79 8.5" 10 bolt with 28 spline axles is stronger than a '73 8.75" 12 bolt truck with 28 spline axles because the pinion shaft diameter on the 10 bolt is bigger than the 12 bolt. The only other difference I can find between the two besides ring gear diameter and track width is parts availability and interchangability. Parts for the 10 bolt are more interchangable as well as cheaper and easier to find, especially in a posi application. So when I can save up some money and get a 3 series posi carrier and 3.73 gear set for the 10 bolt, I'll adapt it to fit under my truck. I have trailing arm/airbag rear suspension so getting the 10 bolt to mount up will be a breeze. I don't mind the narrower track width since I'm going to run cheater slicks, that means I can get wider tires. In the meantime I just have to hope the 3.08 open rear will hold up. You guys think it will hold up to 400hp or so, or should I go easy on the pedal (which won't be easy)?
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