Vacuum advance general guide

[Philip Lochner]

Is there a general tendency as to how timing should be advanced as you come off the throttle with MAP dropping to lower values?

Say I'm at 3000rpm with 30deg advance at full throttle (100Kpa). Should I advance
1) linearly with, say, 1 deg per 10Kpa or
2) exponentially with lots of advance initially but less as MAP drops away or
3) Logarithmically with very little advance intially but increasing rapridly towards lower MAP values.

This is most likely best set on the dyno but it would be nice to know this as a first guess.

[ronsonol]

Philip,

have you read through this post yet? http://www.msefi.com/viewtopic.php?t=99 ... sc&start=0

Lance has made some good suggestions for advance timing under vacuum conditions. Its worth the read.

We Jag lovers gotta look out for each other =)

[Philip Lochner]

Thanks ronsonol!!

Darn, this is embarrassing. Lesson learnt: , then read ALL stickys, then search, then post.

I saw the sticky title but simply ASSUMED it would not cover vacuum advance.

0.3deg/kpa is a little more aggressive than I've ended up with (0.26/kpa) through "seat of pants" tuning

In the 4.6 Rover, I could not advance more than 42Deg as the spark was jumping off the rotor (rotor tip too narrow), in the jag it does not seem to be an issue although 42 seems enough. Will be tuning this on the dyno to get exact map before I take control of fuelling.

[krisr]

Hey Phil.. I've setup manifold & vacuum advance in my spark table because people said it could be done .

Here's how I setup my 400, but remember i'm running high compression (10.5:1) and use the highest octane fuel I can find!

Be very careful though if you do try it. Fuel mixture makes a MASSIVE difference and my AFR/fuel table is relative to the spark table.

[ronsonol]

Philip,

the 1980 has the pre-HE engine does it not? I am interested in your table should I ever get around to doing my v12, but it is the HE engine. I know the compression on the HE engine is 11:1.

Many thanks.