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#1 S1 vacuum for 123 Ignition

Posted: Mon Jan 12, 2026 12:49 pm
by 40GT
I would like to use manifold vacuum for my 123+, but searching here and far ( google ) I can't find a conclusive thread that directs to the ideal location of a tapping for the vacuum take off on an S1 4.2 manifold.

I know the ported system on the HD8 at idle can be ignored on the 123 by using the " Ignore MAP below rpm '' setting, but once the throttle is open, is the vacuum generated a good source for genuine useful manifold pressure?, if it is then there seems little point in using pure manifold vacuum.

Be interesting to hear thoughts, I may not be that good at searching, but I have found some really long threads on the subject.

#2 Re: S1 vacuum for 123 Ignition

Posted: Mon Jan 12, 2026 4:14 pm
by Gfhug
Have you searched for ‘123’ then within the results for ‘vacuum’. I found a lot of good results that could well help answer your question.

Geoff

#3 Re: S1 vacuum for 123 Ignition

Posted: Mon Jan 12, 2026 4:44 pm
by mgcjag
Hi Sean....although manifold vacume has been discussed a lot I don't think many use it....I havent seen a post showing a take off point fitted to a S1 manifold that I recall.....may be worth a post on JagLovers ....Steve

#4 Re: S1 vacuum for 123 Ignition

Posted: Mon Jan 12, 2026 5:03 pm
by bitsobrits
If you have a triple SU car, then you have the ported vacuum source you need at the front carb. Why do you want to pursue full manifold vacuum?