James G. Posted July 17, 2011 Share Posted July 17, 2011 Hello guys. I have a 72 Monte, BB454 car and am looking to upgrade the rear end in it. I currently have the 8.5 stock 2.73 open rear end (not posi), and want to get much better pull and quicker response with a different gear set up. I have a manual Borg Warner Super T10 4-speed tranny also. So, I was thinking of going to 3.42 or 3.73 gears. I used to have 3.73 gears in my 88 Monte SS and really liked them, but my 454 is built to 425 hp, compared to my SBC 355 in the 88 Monte so it's kind of apples and oranges, and I want it basically for a good, strong street setup in my 72. I don't care too much about MPG, but want a good mix of power without completely dumping fuel mileage. So, should I change out the guts of this open 10 bolt rear end and build this one with the gears I want, or, look to buy a completely new posi rear end with the gears I want in it. I thought about finding a Chevelle 8.5 10 bolt that has the 3.42 gears in it and trying to bolt it in, but wasn't sure on what I had to change for the conversion. Any help and suggestions on the best route to go with this would be greatly appreciated, thank you! -James Quote Link to comment Share on other sites More sharing options...
robs70monte Posted July 17, 2011 Share Posted July 17, 2011 3:73 will be great for what your doing i have 3:73s in my big block chevelle and it runs great at the track and on the street my Monte has 3:31s in it and it still doesn't take off the way i want it to its a bit sluggish Quote Link to comment Share on other sites More sharing options...
James G. Posted July 17, 2011 Author Share Posted July 17, 2011 Rob, thank you for the advice. Do you think I should go out and get a posi rear end with 3.73's already in it, or try and upgrade the one I have to 3.73 gears? I am wondering which way is A) more cost effective, and going to handle the punishment of the motor and tranny the best. I have no issues with the rear end in it now, it runs great, but just looking to go down this road only once if you know what I mean. -James Quote Link to comment Share on other sites More sharing options...
zmanabba Posted July 17, 2011 Share Posted July 17, 2011 Well James maybe I can muddy the waters a bit lol. Would probably need a little more info about your combo. What is the first gear in your trans for starters? The lower the first gear in the trans the higher gear in the rear end you will be able to get away with. Also are you goung to be doing a lot of highway driving? 4000 rpm at 60mph can stress some people out. what is the rest of your engine combo. What rpm is the hp and torque peaks? Two ways that I personaly lean for a 454/4speed combo are a 12 bolt rear with a mid 3 series gear for the street. 12 bolt for the weight of the car and the torque the axles will be receiving The mild gear for the torque of the engine. For example if you have a 2.88 first gear in the trans and 3.42 in the rear that would give you a first gear launch of 9.85 to 1. A M22 with a 2.20 first gear and a 4.10 rear would give you 9.02 to 1. And you could drive yours on the on the highway. Something to think about. P.s. I favor the 3.31 but they are only found used. I have been told you will fell no difference with the 3.42 gears. Well I stirred up enough lol David Quote Link to comment Share on other sites More sharing options...
James G. Posted July 17, 2011 Author Share Posted July 17, 2011 David, thank you for the reply, let me give you more info on my car: Super T-10 Transmission ratios: 1st - 2.64 2nd - 2.10 3rd - 1.46 4th - 1.00 454 Engine Power Info: 425 hp @ 4800 rpm 470 tq. @ 3250 rpm I hope that info helps you (and others) give me a good direction to head with my rear end question. Also, I don't do too much highway driving with the car, which is why I was thinking 3.73 originally, but probably more practical of a combo would be the 3.42's. It sounds like that is a good all around gear from what you, and others, have said. So, it would probably be better to go out and buy a 12 bolt rear end rather than find a 10 bolt replacement or try and put a set of 3.42's in my current rear end? Sorry for the questions, I just don't have a lot of experience swapping out rear ends, lol. Also, what would I have to change if I went from my current 10 bolt to a 12 bolt rear end? Thanks! -James Quote Link to comment Share on other sites More sharing options...
zmanabba Posted July 18, 2011 Share Posted July 18, 2011 With that 2.64 gear I still wouldn't go any higher than the 3.73. Those two gears combine to a 9.85 to 1. As far as racing the magic number seems to be 10 to 1. So that set up would be perfect. I don't have the formula at hand to see what you would top out at for speed-not sure if that is important. But as far as street with that torquey set up you have you will be able to smoke the tires as long as you want in 1st. 3.42 gears puts you at 9 to 1 so you would probably have the same "problem" with 1st with a little more top end. As far as 12 vs 10 bolt the factory would put a 12 bolt behind a 4speed/454 combo-the bean counters wouldn't let the engineers put a more expensive unit back there unless there was a reason. If it were me I would drive the car with the 10 bolt (driving is the most important lol) but not put any money into it. Except for brakes-safety first and they will change over to the 12 bolt. Enjoy the car and save up in the mean time for the 12 bolt. The only thing that might be needed to change is the u-joint and possible shorten the drive shaft. I got lucky when I upgraded my 67 Firebird and had to do neither. My 70 Monte already had a 12 bolt so I got lucky there also. You could beef up your 10 bolt to take the punishment but the cost would be within hundreds of dollars of what you would spend for a Moser or a Strange unit and would be even stronger than a factory unit. Or you could wait and shop for used parts-the only thing I haven't seen for sale used over the years is the 3 series posi unit. Good hunting lol David Quote Link to comment Share on other sites More sharing options...
robs70monte Posted July 18, 2011 Share Posted July 18, 2011 If i was you i would look around the junk yards for a 8.5 10 bolt posi rear end all you need is a posi unit out of it then you can pull out the summit catalog and ordered a ring gear kit This will work for you http://www.summitracing.com/parts/SUM-741001/ and a install kit http://www.summitracing.com/parts/RAT-360K/ Quote Link to comment Share on other sites More sharing options...
James G. Posted July 22, 2011 Author Share Posted July 22, 2011 Thanks for the suggestions guys. I think I am going to save the coin for a good 12 bolt unit and drive this one in the meantime. Unless, of course, one of you has a used 8.5 10-bolt unit with 3.42 or 3.73 gears close to Maine you want to get rid of for the right price, I can come pick it up from you. Quote Link to comment Share on other sites More sharing options...
Big Jim Posted July 23, 2011 Share Posted July 23, 2011 I found this add on craigslist in NC. charlotte craigslist > for sale / wanted > auto parts please flag with care: [?] miscategorized prohibited spam/overpost best of craigslist 12 bolt chevy rear end - $200 (dallas) Date: 2011-07-17, 8:54PM EDT Reply to: sale-auafj-2499532993@craigslist.org [Errors when replying to ads?] out of a 1971 monte carlo 350 2 speed car 704-953-0894 Location: dallas it's NOT ok to contact this poster with services or other commercial interests PostingID: 2499532993 Copyright © 2011 craigslist, inc. terms of use privacy policy feedback forum Quote Link to comment Share on other sites More sharing options...
James G. Posted July 29, 2011 Author Share Posted July 29, 2011 Thanks Jim, I will check it out! -James Quote Link to comment Share on other sites More sharing options...
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