Hey new tonight, hope this works. I am starting to dive into the Mach 1 I recovered from Wyo and have a question about the rearend. It is a 9" and was wondering what HP it will safely handle, and what constant speed the 3.25 standard axle ratio will be ok with if I put an overdrive tranny in it later. Thanks John
Don't think you should have a problem with the 9" holding... if your worried about THAT breaking up... I want to see your engine! hahaha I'm running 3.00 : 1 in my diff. and with a C-4, I run somewhere in 1800-1900 doing 55-60 down the road 3.25 with an overdrive and you'll just be putting along...should make for a good road car. if you wanted a little more get up and go, i think you may be able to get away with putting in 3.50 : 1 if you've got OD. good luck, oh and .... welcome to :SNSign:
I'd say 3:70 gears are a minimum if you have OD. I'm thinking about putting 3:89 or 4:11s in mine. I need to be going over 60 to get the engine up to about 2000 rpm in 5th. Most of us aren't going to be routinely driving cross country with these old cars, so I'd set the gears up for more responsive driving around town or on a track. All 9" rears are pretty tough, but some are tougher than others. Unless you're planning to hit the track hard, you can pass on a nodular 3rd member and/or the 31 spline axles. There are 2 pinion and 4 pinion Traction-Locs, and obviously the 4 pinion is stronger. If you're going to build it up you might as well get the better one. Even better than that though is the Detroit Tru-Track. Welcome to the forum, post some progress pics, and check out the 69 stang forum also: 1969 Mustang Supersite Web Forums - Powered by vBulletin
i have a 3.25 in my stang and i love that gear becuase it's a great all around gear for combination city and highway driving. i was planning on re-using my 3.25 at one time with an AOD tranny and if i were going to do that i would get the wide ratio low gearset for the aod and the car will act like it has a lower gear like a 3.70 or 3.89 but still be able to cruise at 70mph turning less than 2000rpm. i recently acquired a vintage hone-o-drive unit which is basically the 70's version of a gear vendors unit except the one i have bolts up to the 9" diff instead of bolting up to the transmission like a gear vendors. with the hone unit i will probably go to a 3.70 or 4.11 gear because the hone unit has a deeper OD ratio than an AOD or T5. i may still end up using the 3.25 with the hone depending on which OD ratio it has, there 2 different ratios but the most common was the one i described above. i can't remember the ratios off the top of my head but they were like 30% and 40% overdriven IIRC, they were originally designed for motor homes and tow truck and that kind of thing so that those kinds of trucks could still get decent mileage with their 4.11, 4.30, 4.56 and 4.88 etc type gears. they were very common on these types of vehicle during the gas crunches of the 70's and became popular for muscle cars when Joel Rosen of Motion performance started using them in his Motion supercar camaros, vettes and chevelles in the early 70's
If you are making good power and are running a manual overdrive transmission, I would avoid the higher gear ratio's. I have 3.73's and I find they are too high. Shifting 1/2 way through a left turn sucks, and the gearing is so low that it's impossible to use due to traction issues. I'll live with it for now since it's all brand new, but if I was to do it over again I'd definately go with the 3.25's or 3.50's.
I guess part of the equation is which OD tranny you plan to use. The T5 has a super steep 3.3 or so 1st gear, so when you multiply that times a 3.73 rear gear, you're gonna be revving it up pretty fast. That's a lot of why I converted my TR3550 into a TKO, so I could use the 2.8 1st gear. It still winds out fairly quickly, but it's certainly not annoying.
New story, now that it's open. Well this is the new guy again with a new story. Now that I have the rear end open, much to my surprise someone has been there before me. What I found was a 37 ring gear and 9 pinion. A 4.11 set up. It is a 28 spline and looks really good inside. I am installing a Detroit Truetrac. Any comments if I will like it and how is the installation if you have done it. Thanks John, Love this site.
Third member color Looking for the Color code for the 9" third member on my 69 mach 1. The third member is a red and the axle is black. What are the official colors and finish. Thanks John
