If fun is your only goal, then clutching might be it.
But;
If you don't also change the rear gear, you will lose a lil snap off the line. This is because inside the TC there is a hydraulic torque multiplier, that is at the very least equal to one gear size, but off the line is nearly double.
Once you get past stall-speed, the 4 gear has tighter splits, and, geared the same, outpaces the auto.
However, if you happen to have the typical 2.76 rear gears, then second gear will get you 65=~4700, with 27" tires, so not a lot of shifting going on at WOT.
But it can still be a lot of fun short-shifting at say 3400, which will get you ~65 at the top of third gear, so that's two shifts ....., then into cruise mode. But it won't be any quicker than the automatic.
If it's performance that you are after, your best mod is a higher than stock stall TC, which is also way cheaper. This will snap you off the line, quicker. (see note 1)
And the next best is racier rear gears. This will reduce your roadspeed at every shift, thus putting down more average power during a measured-distance run.
The third mod could be more gears for less rpm drop in each gear, so now putting down even more average power over distance.
Note 1
short description at the last paragraph.
The standard A833 4-speed ratios are; 2.66-1.92-1.40-1.00
The 904 ratios are; 2.45-1.45-1.00. . But this does not reflect the Convertor, with it's internal hydraulic TM(Torque-Multiplier). This could be as high as 1.8 at zero MPH, diminishing to 1.1 with speed.
So then, your A904 first gear is infinitely and automatically variable from about 1.8 x2.45, to 1.1 x 2.45, which equals 4.41 to 2.70; which is the simple math. Second and third will also be affected, but not to the same degree. The simple math has them at times 1.1, so second will be 1.60 and third will be 1.1.. Lining them up you get;
4.41/2.70-1.60-1.10; Hyup that is 4 gears already . Lets add in the 2.76s....
12.17/7.45-4.42-3.04........ , these are the numbers to beat, and I will post the 904 numbers in blue from now on. With the 4-gear/2.76 I get
.........7.34-5.30-3.86-2.76
So as you can see, the 4-gear is severely handicapped on take-off. But then has a better ratio during the next two ratios, and then finishing with a nice hiway gear. To get that same 12.17 starter gear, the 4-gear would need to run 12.17/2.76=4.41s..... but that would kill cruising....
If you average first gear, you get 9.80, and for the 4-gear this would take 3.69 rear gears.
But there is another A833 that on paper looks to be better fit for you, and it will kill the regular A833, and hurt the auto bad.. This would be the Commando 4-speed.
The ratios are 3.09-1.92-1.40-1.00,and with 2.76s
.... 8.53-5.30-3.86-2.76, compared to the A904 @
avg 9.80-4.42-3.04
Bear in mind that with 2.76s only the first two gears are meaningful, cuz third will be speeding or close to it.
And I got one more,the trans most of us shun, the A833od The ratios are 3.09-1.67-1.00-.73od. Adding the 2.76s we get
.... 9.98-4.61-2.76-2.01.. compared to the auto;
avg 9.80-4.42-3.04, so that looks like a good match, as to the first two gears. But I doubt your slanty can pull a 2.01 at 65 mph. Lets try a lil deeper rear gear, and match second gear to 50 mph at shift-rpm of 4400. I get 4.30 rear gear, for 4400= 49.3mph. So 4.30s gets you
. 13.29-7.18-4.30-3.14, compared to the A904/2.76s
12.17/7.45-4.42-3.04 deryago. Looks spot on.....
There is one minor fly in the ointment with the od box; namely the 1-2 split is way out there, meaning at normal cruising you might have to rev it a lil higher than "normal" to have some acceleration in Second gear, so the engine doesn't seem bogged down. As you can see tho, by the numbers, it ain't much worse than the auto.
Here's an example;
At 32mph in second gear, with the od-box and 4.30s, the RPM is calculated to be 2855. That is a good rpm for the slanty to pull from. But to get to 32mph in first, requires the engine to rev to 5280 in first!, see what I mean about second being out there?
A comfortable rpm for a slanty to be granny-shifted is 2800. Shifting the standard box there, the rpms will drop in at ~2000. Acceleration from 2000 is gonna be mundane, but I just want to show you what happens with the od-box.
Again shifting at 2800 with the od-box now, the Rs in second will drop to 1513, ouch! If you want it to drop into second at 2000, you will have to rev first to 3700
It is for this reason that I rarely recommend the od-box for a small engine. I know the factory put them into certain F-bodies, but who ever bragged on how quick his car was with it? My stock 1973 318 didn't like it nor did my hi-torque 367.
I included the math, only to satisfy your curiosity.
In conclusion then, the box of choice is the Commando, but I wouldn't handicap the slanty with 2.76 gears!
Again; looking to 4400@50mph, in second, I get a rear of 3.69s, rounds to 3.55 for hiway compatibility, or 3.73s for city , and the cruise rpms for those are 65=2870(3.55s) and 3010(3.73)
Say you chose 3.55s. the roadgears then with the Commando, are;
10.97-6.82-4.97-3.55 compared to the A904/2.76 of
12.17/7.45-4.42-3.04
As you can see, in the first two ratios, the auto is winning, with third being only just a tic behind. and the auto leads the cruise rpm at 65=2230@zero-slip, maybe 100 to 150 more depending on circumstances.
I can't imagine this being a whole lotta fun............
But it is what I run.lol. albeit with a 367 cuber ..........
Whatever you choose,
Happy HotRodding.
