This is a really great thread
I have been down this road, running every 4 speed tranny Mopar makes, with almost every rear gear you can imagine, and finally adding the GVOD, and swapping even bigger chunks. I only ever went to the track two times.
I have an old "mobile dynamometer" called a DYNORAD. It Takes speed and Gforce readings, and generates horsepower data which you can plot on a graph, and see the shape of the engine output. If you enter your vehicle weight it will spit out mph numbers for the qtr. I used this device to ascertain an approximation of what my engines horsepower delivery might look like. From this I could see the trap rpm zone I should be in. Knowing that, I, like you, had the same choices, except using a 4 speed, my choices were 4th or third.
Well if you do the math, coming off the line at peak torque in first, and 4.88s, it is easily possible to generate torque into the rear axles in excess of 5000 ftlbs. And predictably, with street suspension, this is pure tirespin. Even with 325/50-15 DRs heated up and and at 12 psi. What I found was that even coming off the line at 2500 and at part throttle, and at slipping the clutch, the shift into second would blow the tires away.
So the point is this if you are going for ET, the 60ft is the principal thing and who cares what the Rs are thru the traps, almost. To get the 60ft working in those skinny wheelhouses is gonna take some special engineering on your part, and it all starts by not using those short skinny rollers.
But if trap speed is your goal, as it was mine, then it is more important to run all your horsepower, all of the time to generate the most average power thru the run.And that starts with knowing the shape of your horsepower output.
If your power peaks at 5200 and by 5400 it has dropped into the basement, there is no point to continue the revs 600 past the peak. On the other hand if she peaks at 5200. and by 5800 has fallen to just 95% of peak, perhaps you should be wringing out even higher. If you don't you could be leaving mph under the table.
Then there is the small matter of rpm drop during the shifts.
With a 904, the ratios are 2.45-1.45-1.00 and the 1-2 drop is to 59.2 %. So shifting at 6000, the drop will be to 3550. Your powerband will thus need to be from 3550 to 6000 or 2450rpm. That's a lot.
With an A999 the ratios are 2.74-1.54-1.00, and the 1-2 drop is to 56.2%. So again shifting at 6000, the Rs will drop to 3372. Your powerband will thus need to be from 3372 to 6000, or 2628. That is 178 rpm more than a lot. So even tho the A999 gives you a greater torque multiplication on the line, it steals it from the rest of the run.
Then there is the small matter of stall. The stall is used to adjust the engines torque at the starting line to deliver just enough torque thru the gear ratios to maximize take off with just the right amount of tirespin. Too much spin and the 60ft suffers. Too little, and the 60 ft suffers.
The stall can also be used to reduce the powerband requirement. Say you had a sub 3300 stall TC and your rpm drops to 3372 at the 1-2 shift. Well, your TC will be locked up and is not involved. But say you had a 4000TC. Now the TC is involved.Your Rs will not drop to 3372, where your engine has very little power, but rather, it will drop to about 4000, where your engine will have a lot of power. This how to generate more average power during the run. And more average power,means more MPH.
So with a mild 318 and just two gears, you are gonna have to pick one or the other, or just run it for the fun of it.
But If it was me I would do three things;
1) get some real traction back there, be it tires or traction aides or whatever, cuz 2.2 60fts plus, are embarrasing. And
2) I would get a handle on the powercurve so I could make a more educated shift change, and gear selection.And
3) I would throw the stock TC as far as I could. It will do nothing for either your 60ft nor your ET, nor your trap speed.
And BTW
To solve the age old dilemma, I too broke the bank by purchasing a GVOD. Now I have many gears to chose from. I use it as a splitter. I run 3.55s, 27inch tires, and a 3.09 low gear . I am geared for 4 ratios to 110mph at 6850rpm. The road ratios are; 10.97-8.56-6.43-5.01 . The street ratios are as in my sig and work out to 10.97-6.78-4.93-3.85-2.77, and 65=2236
Gone are the days of swapping chunks.
I have to agree with whoever said just GO! Run what you brung. Then think it over.
