Aren't those the same gear ratios as the A500/42rh ?
HYUP; the A500 is a 904 with an overdrive strapped onto the back. and
the A518 is a 727 with the same od bolted on.
The overdrive ratio is a .69 . If you can figure out how to engage it manually, I bet you could use it as a splitter at least between 2.74 and 1.54. That .69 od ratio is a pretty sweet number, as overdrives go, but as a splitter, not so good, and you will soon see why
That would give you ratios of
2.74-1.89-1.54- (1.06/1.00-.69), and splits of .69-.815-.69-.943-.69 In this array you can spot the splitter inadequacy, which in the 1-2 split is still better than the A904 with splits of
.59/.69
If you gear that for 60 mph @ the power peak in Second-over;
Say you had a 262* cam, and a smogger 8/1-Scr 318/273. That 262* peaks at around 4800 and you can stretch that to 5100 easy enough, and 60mph requires a no-slip ratio of 6.70,
so; 5100@10% slip it would be 6.00,
and 6.00/1.89=a 3.20 ratio,rounds to 3.23s
for a final rpm of around 5110 with 3.23s in second-over; about perfect.
Your starter gear would be 2.74 x3.23=8.85, which you can adjust with stall ......... because you have an overdrive. That makes a two gear runner. and 65=1840
But, with the overdrive;
You could make it a 3-gear runner tire-frying runner,with a short 2-3 split of
.815. This would require a rear gear of 3.91s for a final rpm of 5040@10% slip. BadaBoom!.
Your starter gear is now 3.91 x2.74=
10.71/dynomite. and you can still cruise at
65=2220 in loc-up! The lo-po 318/273 will spool thru those first THREE ratios rather quickly, and feel like a much bigger engine.
Lest you think otherwise, the new second road gear is ;
3.91 x2.74 x.69=
7.39 , compared to;
3.91 x 1.45=
5.67, with a regular 904, which is a full gear lower (at 5.67/7.39=.767). For a 904 to have a 7.39 second gear road gear, it would need a 7.39/1.45=
5.10 rear gears. Imagine your 318 slamming out with 5.10s; the starter is then 5.1 x2.45=12.50!
But the really big deal is the .815 "2-3" split.
Hitting 5100 at the top of First-over, the R's will fall to 4160 (5100 x.815), which is well up on the power curve, instead of 3020 (5100 x 1.45/2.45) with a regular A904. And I probably don't need to tell you what 3020 with an 8/1, 318/273 sporting a 262* cam feels like. Using this combo, you could even use the stock TC, cuz with a starter of 10.71, your A-body will be moving out briskly, even with the lo-po-318/273.
The full deal with splitting is;
2.74-
1.89-1.54-
(1.06-1.00)-
.69od splits in red. As you can see; Second-over(1.06) and Drive(1.00) are same enough to just pick one. So what you have here is a 5-speed automatic, with a loc-up, that is worth at least another half gear, so I would call it a 5.5.
But it gets better.
Inside the TC, the engine torque gets multiplied hydraulically. This multiplier changes with input torque and with torque differential/mph. At zero mph the multiplier is largest at about 1.8. The faster you go, eventually you will hit the minimum multiplier ratio of about 1.1 or a lil less. But lets go back to Zero mph. When you hit it, it has the same effect as multiplying your rear gear by that 1.8, so your 3.91s, for a small space of time, turn into 7.04s. Then immediately, as the car moves out, that 1.8 ratio ramps down towards the low end, but in First gear you will hit shift rpm before the TC spools down to much lower than say 1.35 . In other words, compared to a manual trans, that TC is an automatic infinitely variable gearbox with a ratio change of about 1.8 to 1.35... in first gear, which is 1.35/1.8=.75 split..., saweeet!
So then, the TC is in fact, a 6th gear; for a total of 6.5 ratios, not counting second-over!
Let's line them up, first as roadgears, using 3.91s;
10.71-6.02-3.91-2.70 ... with splits of
.. .562--.649--.69
Then lets put the TC in there; And
I have to take some liberty here with the multiplier cuz I have no idea how it is gonna play out in real life; so
First gear of; 19.27@(1.8) diminishing to say 14.46(@1.35), Then
first-over at say an average of 1.25 in the TC, equals 9.24, then
Second of an average of 1.20, then 7.22, then,
Drive at say 1.15, so 4.50 , then
Overdrive of 2.70 x 1.10= 2.97, and finally
loc-up at 2.70
Lets line them up;
19.27/14.56-9.24-7.22
/4.50-2.97-2.70; there you go; splits of
... .76 --- .63 - .78
/.62 - .69 - .91 equals 6.5 gears. Notice how close 2.97 and 2.70 are to the factory gears of 2.94 and 2.76. Hyup, the loc-up is actually closer to a full gear than to a half-gear. So then we have a grand total of SEVEN gear ratios. You can't get much better than that, at the buy-in price of 3.91s plus the A500.
Now all you gotta do is figure out how to engage overdrive at the top of first gear.
In this story, I referenced the 318/273 engine, because, at their power levels, tirespin will be at a MINIMUM, so you can actually measure the ET difference, from a standing start to 60mph. From my experience, a stout 360 on street tires,will just spin the tires the whole way, with or without the splitter, even with 3.55s. So I don't see splitting for a 360 as an advantage,........ unless maybe you built it for fuel economy. So then you would lose one ratio, with SIX still remaining, namely; 2 in first gear with the TC, and 3 more in the box, plus the loc-up. But..... notice that at FullPower, you will rarely use more than 2 gears,3 ratios. Also notice that without splitting, first and second are quite far apart at .562split, so then your lo-compression 273/318 would struggle thru that a bit with late-closing intake valve.
If I was 20 years younger, knowing what I now know, I would put this together in a heart beat.I would never have dinked around with the 4-speed and GVod.
And I have this old 273 in the shed, that, with "Purple-Horny glass-pacs,would sound pretty wicked screaming thru 3 gears to 60mph,lol. Or maybe, a 340 with a 318 top-end/cam........ again. That was pretty wicked in 1976, in the 65V100 wagon,that I then had.
I wonder how much mpg I could get out of that lil 273honey at 65=2220 in loc-up? Hmmmmmmmm
I did a lot of editing in an effort to make it understandable, so I hope it worked; I just get really excited when I get to do math,lol.
