I'm strongly considering a DIY electronic ignition system (distributorless) on my car, which currently has Weber IDFs on it. Not planning on going to EFI yet, as that's a bigger deal, and I need ignition sooner. Thing is, these are older carbs w/o the ported vacuum fitting, so I can't make the ignition be load sensitive.

I *could* just use a throttle position sensor (cobbled up to connect to the linkage) and guess load based on throttle position. I can also just say to hell with load sensing altogether and just use engine speed by itself. A manifold pressure signal, however, would really be ideal.

I know newer IDFs have this port on them for vacuum advance distributors, and I'm wondering if one can be added to these older carb bodies. Anyone tried this? Any clues at all? Anyone have one of the newer carbs they can take a picture of?

I added vacuum ports to a /6 manifold a few years back. Its not necessary to drill the carb body, pick a spot on the manifold that will allow a clean install of the lines, and drill there

I used 1/8in brass tubing, and epoxied it into place. The set up worked well.

For the most accurate set up of aftermarket EFI, you should use BOTH manifold pressure and throttle position. The more info you can supply to the ECU the better. Engine speed, Intake air temp, TPS, manifold vacuum, can all be used to tailor the engines power and driveability

If you have some wild cams your vacuum signal may be erratic, so it can help to make a vacuum collector ( like the big block boys use to operate the brakes when they have wild cams)
this will show accurate vacuum, and evens the pulses.

PS we sell Haltech

shameless plug

Consider a port on the maniflods or a sandwhich plate with a port between the carb and manifold.

I'm sure CB Performance has something.

Thanks, Tim, but right now, this is only for ignition, not full EFI. And I won't be using Haltech or even Megasquirt, this is all my own project (circuit, processor programming, etc). Sheer bloody-mindedness on my part, combined with extreme cheapness (Haltech ECUs cost too much).

It was my understanding that the correct place for the MAP pressure signal wasn't the manifold, but for so-called "ported vacuum", which apparently coincides with the throttle plate. This is part of where my confusion lies, as I've never heard a good explanation of what "ported vacuum" means. Perhaps what I need to do is simply tap the manifold and see what pressure signal I get as throttle position and load changes. Mike's idea of a sandwich plate is a good one. I'll have to see what CB has, as that's a bit past my meagre machining skills and tools.

The vacuum is developed when the piston travels down the cylinder (on the intake stroke, ie intake valve open), vacuum is pulled between the throttle butterfly and the piston... you can pick up manifold vacuum anywhere between the intake valve, and the throttle butterfly!

On some turbocharged engines we have built we use two MAP senors, one a 3 bar that is plumbed to the manifold, the other is an Absolute pressure sensor that reads ambient pressures..

Many "modern" cars, use a MAP sensor that picks up its signal in a common area of the intake manifold, this way cam overlap etc is damped....Individual throttle bodies cloud things a bit, but not much

Lemme go shoot a pic of TWM's on my 911

Here is a pic of where TWM pics up vacuum on their TBs, the castings are drilled so signal from each barrel is used.. Note the pick up for vacuum is bellow the butterfly, and above the intake valve...

12 coils! Whatcha got going on under thar?

Twin-plug 911 engine.

I'm the same color green as your cooling tin