3rd member help

[sixty9cobra]

I'm looking to upgrade the rear on my 69 mach 1. Right now I have 3.89 28 spline equilock or clutch type posi. I rebuilt it long ago 1980 ish and added a few extra shims. I went to the Fe Reunion and was doing the one wheel peel in the burnout box. I have a TKO 600 with .82 overdrive and a 1350 yoke. Car ran 12.38 and 12.46 103mph ans 113 mph. I had a 6200 chip and hit it twice on the 12.46 run.I will upgrade to 31 spline Axel's, carrier and nodular case. Should I stay with 3.89 or bump up to 4.11? I have a 275/60/15" MT ET street drag radials. Car is a weekend driver roughly 500 hp.

[jayb]

On my Mach 1 in 2005 I went from 3.89s to 4.11s and picked up three tenths, Harry.  If you can run the engine to 6500 or so I'd definitely move up to the 4.11s.

[sixty9cobra]

I can run it to 6500 it has a 294s Comp Cam it just stopped making power on the chassis dyno around 5800 I I have made a few changes since then. 3" exhaust, bigger carb got rid of the flex fan and went electric. Besides I can always use 5th gear.

[RICK LAKE]

sixty9cobra going to a 31 spline is going to be stronger and take more abuse. The questions I have is what is causing the motor to go flat at 5,800 rpms?? If its the valve springs, they need to be changed. What is the 60' times? A little suspension work with adjustable shocks  would help. Battery in truck of car? What are the air pressures on the tires? What clutch are you running? Video your car racing and look at the hookup. Front end lift and rearend squat? If you have to make another shift in the 1/4 mile, it will cost you more time that going through redline. Ask Jay but I think that Jay motor was a 6,700 rpm limit motor and still making power. 500 rpm over the max HP rpm is all you need to goto. The motor will spin higher but you are wasteing time and motor over time. 3" pipes may also be too big for your motor. We need some back pressure to help the motor make max power. 1-2 hg. Electric cut outs with short pipes may also be an idea for your racing. Over the years, I have seen may guys goto higher gearing myself too and the problem is blowing the tires up on launch and high 60' times. Jays car has alot of testing done to get max power to the track. Was it 2005 before you raced with the blower? Believe he was running a 511 with WORK. Good luck Rick L.

[sixty9cobra]

The motor doesn't go flat at 5800 the power curve just starts to roll off there. 60 foot was terrible I don't think the posi is to healthy. I don't want to get sidetracked on the 1/4 mile times, I'm trying to decide what posi unit I should get. I hear lockers are clunky on the street from some people and others say they are fine.
Here is a video of the 12.38 pass.   http://www.youtube.com/watch?v=1rQV4EZhVtc

[afret]

Did you see what RPM was going though the lights?  Going 113 MPH I think you need 4.56 gears or at least 4.30 gears with your 28" tall tires.  At that speed with 4.56 gears and no slip, your engine should be doing about 6100-6200 RPM.  About 5800 with 4.30 gears.  If you can get your MPH to 120 it will be around 6500 and 6200 respectively.  I would think with your overdrive, either gear would be tolerable on the street.

If you're going to get serious and also plan to switch to rear discs (if you haven't already), you can get the big bore 3.25 nodular case and go to 35 spline axles.  You can get a 35 spline detroit locker.  You should switch the rear housing ends to the big Ford ends or the later Torino ends for this.  They do make bearings that will let you use these axles with the small Ford housing ends (so you can keep the original brakes) but the axles bearings probably won't hold up to a lot of street use since the bearings have to be made "thinner" to fit the bigger axles.

[cjshaker]

It really depends on what you want your cars main use for. It sounds more like a driver, so I wouldn't concentrate too much on 1/4 mile stuff either.

I have a '69 Mach with 3.89 gears and 28" tires also, but no overdrive like you have. 3.89 is as big as I want to go with my 4spd so that I'm not winding the engine out on a highway. That puts me at about 2800-2900 rpms @60mph. With your overdrive, that would put you about 2300 @60mph. 4.11s would put you at a little over 2400. 4.30s at 2540. Still pretty reasonable. I would definitely step up to either 4.11s or 4.30s depending on where you want to hold your highway cruise RPMs at. It would really make a big difference with either.

As for the rear choice, the later Detroit Lockers that I have used have no "clunkiness" to them. The old styles were somewhat notorious for that when going around turns, and a lot of guys can't get that impression out of their minds. I just haven't seen it with the newer Lockers. I love them and wouldn't use anything else for a solid performance car. They're bullet proof, trouble free, NO clutches to wear out or hassle with. They're basically a lifetime trouble free investment for anything short of a drag car. The only drawback is having to watch any sudden throttle gooses around corners, which usually isn't a good idea on any car unless you like getting friendly with curbs and parked cars.

The 31 spline axles and N case or similar stronger aftermarket case is also a wise choice.

[sixty9cobra]

Afret I didn't notice the rpms on either run but I had a 6200 chip in and didnt hit it. I was so glad I didn't miss 3rd on both runs. Since I put in the TKO 3rd has been a problem for me. Pushing the shifter strait up instead of up and to the right has been a hard task to learn. I was happy with crossing the finish line even though I let off before the finish line on the first run. I haven't been to the track in 30 yrs so I had a big smile just being there. I don't think I will go disc or change the housing, there isn't a track within 100 miles of my house. Also I am on a limited budget college for my daughter has been hirting my wallet. A little street racing is all it usually sees. I'm looking at Strange or Moser packages looks like 1500-1600 for a nodular case and a 31 spline locker. I didn't think the larger axles and small housing would be a big problem unless I put on slicks. I didn't consider 4.30s at all that would be a very short first gear with a 2.87 first gear.

[cjshaker]

I had the same problem getting used to hitting 3rd when I got my '09 GT. 3rd in the TR-3650 is straight up. Took me a while to get used to that. But once I did, it makes for some very nice quick shifts that are darn near impossible to miss. A good shifter makes a big difference, just line any manual. I used an MGW shifter on my Tremec and it made a huge difference over the soft factory shifter.

And yes, the 4.30s would make for a pretty short 1st. If it were mine, I'd probably stick with the 4.11s and try to keep the best of both worlds for a street driven car. I think you will be just fine with the small housing.

[sixty9cobra]

what are your thoughts on used nascar rears? Rousch has used nodular cases with lockers shipped for $850!!


http://www.roushyatesparts.com/product-p/ryp-119-430.htm

[mike7570]

sixty9cobra, you should be able to put a 3rd member together for less than the 1500-1600. I pieced this together off class racer forum and racing junk. The 3rd member is a strange large case with MW 35 spline spool and ball bearing support and was about $950 assembled, housing with new large ends about $480, axles and brakes about $900.