Standard Ignition - mapping.

[DaveG]

Updated formulea: 5/8/15
For those who are interested I have set out below the spark Advance calculations for the Standard Lucas Distributor.
I have been swapping between 14CUX and Megasquirt trying to fix a 'shunting" problem (See topic below "Shunting") and will commence a bottoms up re-tune starting with the ignition. Since the car performs well with the dizzy, I will use this to Map the spark curve.
The key measurements from a Lucas plot are as follows:
Centrifugal:
Rpm Advance
800. . . .0 degrees
2000. . . .10
2700. . . .16
Vacuum:
Hg Advance
5". . . .0
17". . . .20
In the case of vacuum I am assuming linearity. From these points I have derived the following formulea (simple y=mx+c)
Std Distributor:-
WOT Advance = (X BTDC) + 16(centrifugal)
eg. 10 + 16 = 26 degrees

Centrifugal Advance = [(8.42 x rpm)/1000] - 6.74

Vacuum Advance = (1.67 x "Hg) - 8.33
where "Hg = inches of Mercury
Vac Advance = (0.505 x Kpa) - 8.33

E.g.
Assume Cruise Advance occurs at 1750rpm, @ 15"Hg(50Kpa) and BTDC = 10 degrees
Cruise Advance = 10 + [(8.42 x 1750)/1000]-6.74 + [(1.67 x 15)-8.33]
= 10 + 8 + 13.3
= 31 degrees (+/-?)

I will need to run my Vac guage under CUX and note the ported and manifold pressures (Should be the same with throttle open).
I can then compare these with my MS datalogs showing the Vacuum (MAP-Kpa)at various load/rpm's points? Noteing that the Kpa as a vacuum is the inverse %. ? (ie. WOT = 100Kpa and atmosphere (29.92 "Hg)is 0Kpa.
It is interesting to note that under heavy load it is essentially just BTDC + (0-16)Degrees and rpm based to 2700 rpm. Ie WOT = 26 degrees after 2700.

Edited by DaveG on Tuesday 4th August 06:27

Edited by DaveG on Wednesday 5th August 14:38

Edited by DaveG on Wednesday 5th August 14:41

Edited by DaveG on Thursday 6th August 13:31

[DaveG]

Hi Mark. The guy that gave me the curve plots was a sales/service/repairer of all distributors. He charged me $20 and took 10 mins or so to test mine, which is standard for the 500. He could not look up TVR in his manual (a document about 3 ins thick), but he had all the Rover / Lucas stats. Assuming your's is a 3.9 Rover and you have the dizzy ID you should be able to get what you want from a specialist?

[DaveG]

Mark is right. The ported take-off completely shuts off the vacuum. I did a few tests today on CUX, looking at both ported and manifold (plenum). I was surprised at just how quickly the vacuum comes in on take-off. It rises quite quickly to 15" and sits happily between 15" and 18" normal driving and light throttle. Remember that 17" gives 20 degrees advance. This implies normal driving and cruising requires around 34 degree of advance (at 1500rpm and 10 BTDC) according to the calcs!! Or 38 at 2000rpm with 16" of vac. Even light acceleration does not drop the vac below 15", you have to hit the throttle hard to get the vac off.
My experimental remap under MS will look a bit different to what I have been using.