http://67-72chevytrucks.com/vboard/showthread.php?p=3895102
http://www.customclassictrucks.com/techarticles/0905cct_a883_four_speed_overdrive_transmission/index.html
MY-6 / New Process 833 four-speed transmission.
First Gear: 3.09
Second Gear: 1.76
Third Gear: 1.00
Fourth Gear: 0.73
the HAMB also has some cool info...
Just do a search. Ratios for the MY6 should be 3.090 1.670 1.000 0.730. Almost identical to a 4L60E/700r4... very wide, but shouldn't be terrible in a light car with lots of torque like your idea, but it will be an old-school feeling transmission.
Light car + V8 torque = lots of options. Big heavy vehicles like 18 wheelers need 17 forward speeds because they have a very narrow RPM band and they need to keep the engine in the sweet spot. That means a lot of closely spaced ratios to do that and still have far enough to go for highway speeds. Light vehicles with lots of torque can get by with fewer, wider ratios since there is enough torque all over the RPM range to get you moving. I'm not arguing for a 4-speed, but in your application it wouldn't be a bad thing.
Here is how I choose a transmission/rear ratio. First gear final drive ratio should be vaguely in the 9:1 to 12:1 range, higher numerically for heavy cars with low torque, lower numerically for light cars with lots of torque and varying shades of grey in between depending on tire size, etc. Highway RPMs for a stock LS-type engine should be around 1800-2000 ish. That's not lugging it down nor is it too high that it kills MPG. So, your top final drive ratio should support that target.
Here were my calculations on my Impala SS stock with 4L60E and 3.08 rear:
1st final 3.06 x 3.08 = 9.42
2nd final 1.70 x 3.08 = 5.24
3rd final 1.00 x 3.08 = 3.08
OD final 0.70 x 3.08 = 2.16
Since I liked my highway RPMs, which planned to switch to T56, I just found a ratio to keep the highway RPMs (4.10) and then verified that first gear final drive was acceptable:
1st final 2.66 x 4.10 = 10.91
OD final 0.50 x 4.10 = 2.05
So, the T56/4.10 gives me a slightly deeper first (which is good since I'm swapping cams and it will slightly reduce low RPM torque) with a slightly higher OD, narrower shift ratios in between, and everybody's happy.
The bottom line is, choose a trans/rear that offers the right first and high final drive ratios, then choose how many gears in between based on the car's weight and the torque you have. Since you have a lighter car, you can get away with slightly fewer gears and a wider ratio box... if you want to.
http://www.vibratesoftware.com/html_help/html/GM/gm_transmissions.htm
What Curtis said. The number of speeds doesn't mean much for a light car with a broad torque curve. Heck, the pickup was pretty good before the MY6 swap with the three speed even.
He's smart, listen to him about everything else, too.
:blush:
curtis73 wrote: :blush:
Well, you are bringing sexy back, right?
cxhb... what is the rear axle ratio?
That's yet to be determined... If we go the IRS Ford 8.8 route we can change it to whatever we want. If we go Nissan R200 it should be 3.79 IIRC
Either way the MB differential has to go. No LSD and we aren't going to weld it.
I just filled the well-used T5 in my Mustang with Royal Purple Syncromax and this box is no longer notchy. I'm really curious what a new Hurst shifter would feel like on this thing...
cxhb wrote: That's yet to be determined... If we go the IRS Ford 8.8 route we can change it to whatever we want. If we go Nissan R200 it should be 3.79 IIRC
3.79 is squarely in T56 territory, but its also within the reach of any of the common 5-speed ODs with a top ratio of around 0.75. If you did a 4 speed, you would probably want more like 3.23 or 3.08 to keep highway RPMs in check... and your first gear would still be OK since you have the torque to handle it.
The problem with most 5 speeds is that they are (in general) weaker. Most of the 4-speeds come from the muscle car years so they're strong. Most of the 6 speeds come from the last 20 years so they're strong. Most of the 5 speeds came from 1975-1995 back when engine outputs were super wimpy. T5 can be built to take modern V8 torque, but for the money you spend you might as well just get a stock T56 and be done with it.
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