Ported manifold pressure vs Absolute - how do they compare?

[Quagmire]

Hi guys,

I have Megajolted my P6 which is my daily drive - see here:

http://www.classicroverforum.net/viewto ... &start=105

I am running on an old map I used in the Landrover before, but it never really was 100% optimised.

I am trying to build a map based on the Rover figures, however the vacuum advance comes in from a ported connection and I am now using a true manifold pressure measurement.

Has anyone ever measured how the two compare to each other?

Rover figures state that for my dizzy 16 degrees of advance was added by vacuum starting at 3.5inHg and finishing at 15inHg. Google says this is between 10 and 50 kPa.

Obviously this had no effect at idle with the throttle closed, so do I just apply the 16 degrees additional in a linear fashion to the mech figures across the rev range and sort of blend it out around idle?

Cheers

Jamie

Edit! Just realised I put this in wrong section!

[Quagmire]

Just to follow up on this one:

I built a map using the rover figures for my dizzy (35D8 stamped 41573) - figures taken from rover classics below:

Models with Lucas No. 41573 stamped on distributor body
Ignition timing
Crankshaft Angle (6 degrees BTDC) Engine rev/min
26-30 degrees 4200
23-27 degrees 3500
18-22 degrees 2600
12-16 degrees 1500
8-12 degrees 1200
6-7 degrees 800
6 degrees Below 600
Vaccum advance
Starts 90 mm (3.50 in) Hg
finishes 381 mm (15 in) Hg

Put the quoted advance figures for each given rpm along the 100kPa row, then at 50-60kPa I started to add the advance that the vaccum would have been providing. I did what I mentioned and just blended out the vacuum advance additions around idle, with the exception that i left a bit more than the figure above and its runnind around 10 (it seems happy enough there).

Car is running the best it's ever been, and gave a fairly respectable 24.5mpg on the last tank. Not bad for a 40yr old engine on carbs!