Thanks AJ. Good info here to plan from. I think my ideal would be a 3.55 rear end and overdrive.. as I want to keep a comfortable cruise RPM. Have not, until now considered the results a different torque converter would bring..
The TC is an incredible gizmo. It varies the output torqur to input torque depending on how hard the engine is working and the speed difference going in to going out..... and it's all automatic.
It has an internal hydraulic multiplier that can can be as high as 2.0 but more often is in the range of 1.6 to 1.8.
This Multiplier is greatest at zero mph, when the engine spools up to it's stall speed. And it is smallest when just loafing along.
As road speed increases, with a fixed throttle position, the ratio continuously diminishes.
What does this all mean to you?
Well lets take your assumed to be 2400TC.
Letssay your 318LA puts out
100hp at 2400; that would be ~220 ftlbs.
When you mash the gas from a dead-stop, that 220 is gonna go into the TC and the TC is gonna multiply it hydraulically to 220 x 1.6 say to equal 352 ftlbs instantaneously. But this ratio will rapidly diminish as the roadspeed goes up, so that by 5000/47mph (depends on your other gears) the ratio could be as low as 1.1 or perhaps up to 1.2 .. if your engine made 150hp at 5000, that would be ~160ftlbs, so 160 x 1.2say equals 192 out the back. That takes care of First Gear. So remember the numbers;
352 off the line, and 192 at the top of first gear.
Now lets do the same with a 2800.
At 2800 your engine might be up to
124hp or 233ftlbs.
The 2800TC might have a ratio of 1.7 at zero mph, so we get;
233 x 1.7=396ftlbs off the line. Again at 5000, the ratio might be just a tad better so say 1.25, so we get 160 x 1.25=200ftlbs at the top of second. To recap;
that is 393 to 200..
Now lets compare them; the 2800 is 11.6% stronger at 0 mph, and 4.17% stronger at 5000. And you never did a thing yet, to the engine!
Notice the above numbers in blue; 100hp versus 124hp that is a 24% boost in hp at zero mph, at 2800rpm. There is no bolt-on you can install, to compete with that, short of forced induction.
Ok now recall that the TC is continuously varying it's output ratio from ~1.8 @2800WOT and zero mph) to perhaps 1.1 at the top of second gear, still at WOT.This translates to every gear having two ratios, the one at low-rpm with lotsa torque input, to the other at max rpm with less torque input. This makes your 3speed, act like a 6 speed.
Lessee how that works out, with an A904, with ratios of 2.45-1.45-1.00.
Off the line your chassis will be seeing.... 1.8 x 2.45@zero mph =4.41
At the top of first the TC may be down to 1.4, so...... 1.4 x 2.45=3.43.
Into second, the TC may fall to 1.3, so.................... 1.3 x 1.45=1.89
By the top of second,the TC might be down to 1.2, so 1.2 x 1.45=1.74
Going into third; the Tc might be down to 1.15, so... 1.15 x 1.00=1.15
At the top of third,the TC will be down to it's minimum so 1.1 x 1=1.10
As you can see;
in third gear the TC has settled down to be nearly the same throughout the run.
In second, the variance is a small but noticeable 8.6%. This is similar to one rear gear change.
But get a load of first! Here we have a difference of 28.6%, which is the equivalent of 3 rear gear sizes!
OK so lets average second and third, then line them up; and add the loc-up, so....
4.41-3.43-1.82-1.125-1.00 Ok count the ratios; I get 5, granted loc-up is only a half gear, and you will never race in it... but then with 2.2/2.45s you never will in "drive" either.
Lets see what your engine sees, which is the splits;
.78-.53-.62-.89
So that's how the TC works.
Now lets add some rear gears; I choose 2.94s to keep your buzzzzz down to 2400. So now we multiply every one of those numbers just above, by the 2.94s and I get
12.97-10.08-5.35-3.31-2.94 That's sweet!Compare to my 8.33/GVod
......10.97-6.82-4.97-3.55-2.77, my ratios.
In your Imperial, you only care about the first three, because top of 5.35 gear with 28" tires is 71 mph@5000
Now, none of these ratios are written in stone, as the TC is continuosly varying it's ratio, depending on how much torque is coming in to the roadspeed and the selected gear. To me this is fascinating stuff.
Lets compare what could be to what is.
Let's say you still have an A904Loc-up, a 2400TC(1.6ratio),2.45 rear gears, and 28" tires. Your gears might look like ;
....... 10.80-8.40-4.62-2.82-2.45 and splits of .78-.55-.61-.87 compare
12.97-10.08-5.35-3.31-2.94 splits of .. .78-.53-.62-.89 2800/2.94s
As you can see, the 2800/2.94s is trading away one gear at the top (2.45) to get one (12.97) at the bottom.
Lets see what that means in terms of torque to the road, first the current, then the 2800/2.94s
Again let's say at 2400, your 318LA is putting out 100hp/220ftlbs, so at zero mph this looks like 220 x 10.8=2376 into the axles.
At 2800, as before,124hp/233ftlbs x 12.97=3022 into the axles.
This is 3022/2376=plus 27%, and you don't even have a 4bbl yet!
Ok lets convert 3022# into the axles to on the road ftlbs and I get 2590ftlbs. This is enough to squeal the tires a fair bit, and when the 4bbl opens up, and if the exhaust gets away, you could have a pseudo-race-car there,lol. At least for a little while.
Ok now the disclaimer;
Notice all the assumptions I made. And how often I use definiteness. This is because I have no way of knowing anything about your combo, cuz it hasn't yet been revealed. For all I know your engine is sick, or you might have an A999, or 2.20 gears,or a 2100 stall; I just don't know.
But the whole point of this exercise is to make you aware of the tremendous potential your car has, to respond to gearing and stall, exactly because it is,now, so far away from performance. You got no other place to go but better.