Try this on for size;
your current combo is 25" tires/A904/3.55s/ and 70=3340@zero-slip; could be
3500 on the tach@5% slip.
Your starter gear is 3.55x2.45=8.70, and corrected to a 12" radius is; 8.70x24/25=
8.35 that is a good number for a street-automatic.
Lets say you installed an A999 lock-up, with 3.23s and 28s;
70 would be
2710 in loc-up.
Your starter gear is; 3.23 x2.74 x24/28=
7.59
that is a loss of 9% TM(Torque-Multiplication)
You didn't mention your current stall rpm, but since the A999 has a lock-up, you can run about anything you want, so the 9% shortfall can easily be made up for by increasing it. Easily. Unless you are running something already crazy..... which your cruise-rpm does not point to.
So here you have a low-cost simple and effective way to lower your cruise-rpm AND simultaneously increase your take off power.
How's that? you might ask.
Well say your current TC stalls at
2200, and your engine makes 120 hp/280ftlbs there; I mean just let's say.
To the road at WOT this might translates to
3830 ftlbs @ a TC ratio of 1.8 @ zero mph.
>So,how about the A999/3.23@
2800TC
Lets say the HP is now 155/290, and that might translate to
3960 . This is now an increase of
3.4%..
>lets try@
3200TC
Lets say the HP is now 180hp/300 ftlbs. And at zero-mph this might translate to
4100. Ok now this calculates to an increase of
7% torque to the road.
Don't look at the numbers; instead look at the percent increases.
That's how you do it.
But there is no free-lunch;
with the current combo, 60mph=about
4775rpm in second@WOT..
With the A999/3.23/28s; 60 in second at WOT,will be about
4120; a loss of 14%. Most of this is in the tires. With the 25s, the Rs might be 4615, a loss of only 3.4%.
To understand this;
a 25" tire has a roll-out of 78.54 inches, whereas the 28 has 87.96; a difference of exactly 12%
Knowing that;
swapping 28s into the current combo, will drop your cruise rpm, the same 12%, or from 3500 on the tach to 3100, with no other changes. This has the same effect as decreasing your gearing from 3.55s to 3.12s.
I thinkit was @RustyRatRod , who first mentioned the A999, And I agree with him.
The slip at 70 is likely to be in the range of 5 to 8% ; so 175 to 280
So leaving the 3.55s in there with the A999, and swapping to the 28s will get you 70=2980 in LU. That is a drop of 520rpm or nearly 15% This is a pretty cheap solution. And
no cutting required, no engineering, and the driveshaft even swaps over.
So to recap;
the 28s are worth 12% rpm reduction, over 25s
the LU is worth 5% to 8%, rpm reduction
and 3.23s are ~9% rpm reduction
And the A999 low gear,is an INCrease~11.8%, makes up for the drop in gears, to 3.23s
And the stall will make up the difference in first gear, to at least maintain the current performance.
The ratios are;
2.45-1.45-1.00 for the A904
2.74-1.54-1.00 and LU for the A999
Be careful; AFAIK; the A998 could have either ratio, and comes with or without LU! depending on application.
HappyHotRodding
