Distributor Vacuum Advance
Discussion
I just realised that the plenum I have refurbed for Wendy doesn't have a vacuum take off for the distrubutor.
Rather than tap the plenum (which maybe redundant in a few months) could I add a T piece to the FPR vacuum and use that?
Alternatively, the plenum base has a spare take off on the front right. Could I reduce this and use it?
Rather than tap the plenum (which maybe redundant in a few months) could I add a T piece to the FPR vacuum and use that?
Alternatively, the plenum base has a spare take off on the front right. Could I reduce this and use it?
My V8 plenum has a spare one in the middle.
It's the same thread as the vacuum for the brakes.
I wanted to have a boost/vacuum pressure meter so I found a spare brake take-off adapter and used that. However the brake adapter has a one-way valve so I trapped a stout piece of wire in the outer hose union to keep that valve permanently open.
It's a neat job - loos like it was "meant to be".
It's the same thread as the vacuum for the brakes.
I wanted to have a boost/vacuum pressure meter so I found a spare brake take-off adapter and used that. However the brake adapter has a one-way valve so I trapped a stout piece of wire in the outer hose union to keep that valve permanently open.
It's a neat job - loos like it was "meant to be".
mk1fan said:
So no point in the simpler option of teeing in to the FPR vacuum?
Yeah that would work too.batman400 said:
Doesn't the proximity to the throttle vane have some affect on the vac advance signal ?
Quite right it's there for a reason, when you open/close the throttle the vacuum changes quicker than at the other side of the plenum, so the dizzy responds slightly faster (speed of sound over a foot? maybe a millisecond)It's above the throttle plate for a reason; the plate passes over it when the throttle is opened. The idea is to retard the ignition at idle, mostly for emissions, but you mess with it at your peril as it will have other effects... Mr Benoulli's theorum - pressure is inversely proportional to velocity and the velocity is very high at the edge of the throttle plate.
There's plenty of lively discussion on the web about ported vs manifold vacuum.
I had my MAP tee-d into the FPR line with megasquirt initially, but I found the pressure in the line fluctuates depending on what the FPR is doing especially at idle; so if you do decide to go for manifold vacuum then put it on a separate tapping into the plenum.
There's plenty of lively discussion on the web about ported vs manifold vacuum.
I had my MAP tee-d into the FPR line with megasquirt initially, but I found the pressure in the line fluctuates depending on what the FPR is doing especially at idle; so if you do decide to go for manifold vacuum then put it on a separate tapping into the plenum.
Afternoon Martin,
Just the person I wanted to advise.
If I drill and tap a barb next to the butterfly would this be suitable for a MAP take off too?
I have a spare take off at the front of the plenum base so could use this - just needs stepping down.
Just the person I wanted to advise.
If I drill and tap a barb next to the butterfly would this be suitable for a MAP take off too?
I have a spare take off at the front of the plenum base so could use this - just needs stepping down.
Edited by mk1fan on Friday 2nd June 16:12
Don't know about the Rover lump, but all (classic) Ford engines were set up to work via the port by the throttle plate.
This gives vac advance only at small throttle openings and possibly at idle, depending upon exactly where the hole is drilled.
So officially, it will NOT work correctly when connected to the brake hose, or to plenum/mfold.
However, as the plenum/manifold vacuum follows the throttle plate opening angle anyway, more or less,
vac adv could be calibrated to follow manifold vacuum, and it might not be that much different.
It very much depends on where that hole is drilled by throttle.
Plus, some engines then had 'delay and hold' valves to mess with the vac advance response, so it can get very complex.
This gives vac advance only at small throttle openings and possibly at idle, depending upon exactly where the hole is drilled.
So officially, it will NOT work correctly when connected to the brake hose, or to plenum/mfold.
However, as the plenum/manifold vacuum follows the throttle plate opening angle anyway, more or less,
vac adv could be calibrated to follow manifold vacuum, and it might not be that much different.
It very much depends on where that hole is drilled by throttle.
Plus, some engines then had 'delay and hold' valves to mess with the vac advance response, so it can get very complex.
Edited by RCK974X on Friday 2nd June 21:43
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