Lets say you have a hypothetical engine that makes 375 hp. This is enough engine to go 108 in the qtr.pulling 3675# This is a nice easy 360 build.
The peak power might come in at 5600, and with good heads can rev out to say 6200 before choking.
ComboA) Let's say you had a regular 904 with ratios of 2.45-1.45-1.00. The splits are .59-.69. So the rpm drops will be to 3558,and 4278,for powerband requirements of 2642/1722, or about 2182 average. So you have to put the right bolt-ons to make that happen.
To trap well,say at 5800 plus slip, you would need about 4.30s. These gears will get you a starter gear of 10.54, and 65=3472 plus slip.That's what it will take with the 904.
Combo B) Now, let's work it out with the 4L80
The ratios are 3.06-1.63-1.00-.70 and splits of .53-.61-.70od. Let's say same 6200 rpm shifts; the drops are to;3180 and 3660, for a powerband requirement of 2820/2540 or about 2680 average.
To trap at 5800 plus slip ,will require 4.30s.
The starter gear is 13.29, and 65=2430plus slip.
ComboC) Finally lets put a GVOD behind an A999, and split gears.
The ratios are 2.74-2.14-1.54-1.20-1.00-.78od, and splits of .78-.72-.78-.83-.78 Same 6200 shifts; the drops are to 4836-4464-4836-5146-4836 The powereband requirement will be about 1488 rpm
To trap as before, but now in second-over (ratio 1.20) will require a rear gear of 3.55s.
The starter gear will be 9.73, and 65=2236
Now compare the numbers and
select a tranny based on your useage.
Obviously a street/strip car would like either of the second two options rather than Combo A).
But which one?
Well Combo B) looks pretty good. But I can tell you two things. 1) the 13.29 is a ridiculous street starter gear. and 2) the powerband requirement of 2680 is still pretty wide.
Combo C) cures the starter gear very nicely, and with a 2236 cruiser gear, that's nice also. But best of all is the powerband requirement of just 1488rpm.
http://www.hughesengines.com/Upload/TechArticles/land_dyno.jpg
Take a look at a typical dyno graph. On this one the average hp over 1488 rpm, centered on the peak, is about 420hp. Now look at it again with a 2680 average PB; does it look like 345 hp average?
Which tranny do you think will be faster, and which one are you liking.
And BTW, you could gear combo C) up 20% to 4.30s like the others and take advantage of the short 5thgear. The new starter would be 11.78, and 65=2708. This would tighten up the powerband yet further. . Furthermore, the very small difference between the peak power and the average power, would allow you to detune the engine one cam-size for sure, maybe more.
The point is do not underestimate the transmission. The right one can turn your mild streeter into a dragon-slayer;the wrong one into a turd.
If you have a different engine, than noted above, you will have to work the numbers out for your combo. There may come a point that the tranny choice changes. For instance; if your combo requires a 4.10 to trap well, then the 4L80 is back in the game with a 12.67 starter gear, and 65=2318rpm
And if you have a much bigger engine, that pulls like a freight-train, 5 gears is kindof overkill.
Splitting gears works well on small engines wanting to work the top of the curve , and they will run a higher mph than expected, on account of the higher average horsepower put down during the run.
I'm not trying to steer you towards a GV. A GV may not be the answer for everybody. But I tell you what; it made my 367 with a 223* cam really come alive. And there was a time I had the combo geared like a 1.97 finaldrive, and 65mph was under 1600 rpm, and she was a fuel sipper.
By the same token, I think the 4L80 has a very nice ratio spread for a bigger engine. You just kindof have to build the engine to take advantage of them, so you don't have to run big gears like 4.30s with 27/28 inch tires.The 13.29 starter-gear is just too deep, unless you have taller tires, like 31/32s ,soooooo it could make a great drag-race tranny.
