747 timing

[rr]

If I'm correct on what you are asking, it's a 3-way, 16-point
interpolation between the table values.

Think of the point of 53kpa map & 1700 rpm as being someplace inside
of a box. The four corners of that box, are the four table values
that surround the defined point. The distance from that point, to each
of the four walls is estimated, with the final result based on those
four values.

Works something like this...

Part of main SA table:

map>   50    60
rpm
1600   20    24
1800   22    26

If the map is 53 and the rpm is 1700, first lookup gets the 20 & 24
degree values, and interpolates between them. The difference between
these two are divided into 16 points:

24deg - 20deg = 4deg, 4deg / 16points = .25 deg per point

60map - 50map = 10map, 10map / 16points = .625 kpa per point.

53 map @ 1600rpm => 20deg + (3map / .625) * .25 = 21.2 deg.

So, 21.2 deg is the 16 point interp. between 50map & 60map @ 1600 rpm.

(Note: on a 2D table, we would be done, and this value returned).
(Note: The 3D lookup, just calls the 2D routine three times with
different values to go between).

The routine then advances a full row to interp. between the
22 deg and 26 deg values found at 1800rpm.

So, using the same math as above, 23.2 deg is the 16 point interp.
between 50map & 60map @ 1800 rpm.


The third and final interp. is done between the answers of the
first two lookups. 21.2 deg & 23.2 degrees.

The rpm is divided into 16 points the same. For this example,
it's right in the middle of table values 1600rpm and 1800rpm,
with 1700 actual.

So, 21.2 deg & 23.2 deg => 22.2 deg as the actual returned,
looked up interpolated value.

BobR.

I almost confused myself with this one <g>


CSH-HQ wrote:
> 
> Some weeks ago, I think RR shared the way a 747 looks up timing.  Would
> someone mind explain what it does when the MAP value is say 53 K/Pa?.
> Thanks
> Bruce