HELP...Serious Vacuum Queston with PHOTO

[MNspiderman]

Well maybe.
Here's my situation.
I just got done porting my intake manifold for my twin 44IDF's. Each cylinder has an individual runner obviously. The front runner has the port for the brake booster. Keeping that in mind. The third intake runner has the port for the dizzy. keeping that in mind. Knowing that vacuum is created on the downstroke of the engine. HERE IS THE MAIN QUESTION: Wouldn't it be wiser to drill and put a port into each runner, hook them all up together in a single line and then still just run a single line to the brake booster? My thinking is this. When the engine is idling with multi carbs, the vacuum is "throbbing" because it only works on the downstroke of one cylinder of the engine. If I put a port into each intake runner, being they are all independent of each other that would give me better vacuum for my brakes. More like smooth even vacuum, not throbbing vacuum.

Next thought: Just an analogy or an easy logical way to put this. If my brake booster requires 15lbs minimum of vacuum to operate. and each cylinder creates 5lbs of vacuum. Then I am short 10lbs of vacuum for my brakes to operate easily. where as if i put a port to each cylinder, I'd have 20lbs of vacuum and my brakes would work correctly. Ya, Ya, I made the numbers up for easy calculations. But in theory is this correct? Remembering these aren't V8's and don't have 8 cylinders to create vacuum sufficently. Because now I lost my centrally located vacuum by converting to independent intake runners. Anyone following me here? Maybe i am overthinking this but maybe I am onto something. Give me some thought before i put my intake back on. And distributor vacuum is variable and comes from the carbz and thats a diff subject. I know.

[carl55]

You can use a vacuum canister with a check valve between the can and manifold.

Carl

[manoa matt]

The stock Fiat waffle manifolds for dual 40 IDF carbs have the brake booster vacuum source in the same location on the #1 intake runner...guys have been running these setups for years with no problems. Think about it this way: At idle the engine is ticking over about 1000rpms. 1000/4 = 250 revolutions per cyliner. It takes two engine revolutions to complete one combustion process so 250/2 = 125 vacuum strokes. The #1 cylinder makes 125 vacuum strokes per minute or roughly 2 vaccum strokes every second. The vacuum "pulse" would be so rapid it might not be detectable. If anything I think the booster provides too much brake assistance.

Don't forget the booster already has a vacuum check valve.

The single inlet track design for dual carbs will produce a more stable vacuum source than the single phlenum intake design for single carbs. The single phlenum intake suffers from wave interference between the runners. As one cylinder is on the intake stroke another is on the exhaust stroke. Depending on the valve overlap some of the exhaust from the one cyliner gets sucked into the the cylinder that is on the intake stroke. That is part of the reason individual throttle bodies and individual intake runners like those used for dual carbs are so efficient and produce more power.

The electronic distributor needs "ported vacuum" ie: a vacuum source above the throttle plates, which the IDF's do not have. That vacuum port downstream of the throttle plate will not work "properly" for the electronic distributor.

[MNspiderman]

You can use a vacuum canister with a check valve between the can and manifold.

Carl

Thanks, but that really wasn't the question at hand. Although I never thought of that. My 3 1969 Cougars had what your describing to operate the headlights.

[MNspiderman]

The stock Fiat waffle manifolds for dual 40 IDF carbs have the brake booster vacuum source in the same location on the #1 intake runner...guys have been running these setups for years with no problems. Think about it this way: At idle the engine is ticking over about 1000rpms. 1000/4 = 250 revolutions per cyliner. It takes two engine revolutions to complete one combustion process so 250/2 = 125 vacuum strokes. The #1 cylinder makes 125 vacuum strokes per minute or roughly 2 vaccum strokes every second. The vacuum "pulse" would be so rapid it might not be detectable. If anything I think the booster provides too much brake assistance.

Don't forget the booster already has a vacuum check valve.

The single inlet track design for dual carbs will produce a more stable vacuum source than the single phlenum intake design for single carbs. The single phlenum intake suffers from wave interference between the runners. As one cylinder is on the intake stroke another is on the exhaust stroke. Depending on the valve overlap some of the exhaust from the one cyliner gets sucked into the the cylinder that is on the intake stroke. That is part of the reason individual throttle bodies and individual intake runners like those used for dual carbs are so efficient and produce more power.

The electronic distributor needs "ported vacuum" ie: a vacuum source above the throttle plates, which the IDF's do not have. That vacuum port downstream of the throttle plate will not work "properly" for the electronic distributor.

Thanks, Thats the kind of answer I am looking for. As far as the IDF's There is actually a brass screw plug next to each air/fuel mixture which you could buy the brass port to hook up a variable vacuum. But I am going to go another route anyway, I think. It goes vacuum port, air/fuel mixture screw, air bipass screw

[So Cal Mark]

drawing that amount of vacuum from 1 cylinder is not ideal. You would be better off to link a hose to draw off all cylinders. That may affect how your intake manifold works though. I wouldn't draw from the bottom of the runners though

[manoa matt]

I'm pretty sure the plug you speak of may look like its above the throttle plate, but the actual port in the carb body casting may still come out below the throttle plate. If it were the case that the plug was ported vacuum then there would be no discussion regarding use of electronic distributors with dual carbs, (it's a hot topic).

You have a few distributor options with dual carbs.

1.stock points distributor
2.stock points distributor with electronic points elimenator kit (Pertronix MR-LS1, or Crane XR 700 or Crane XR 3000)
3.electronic distributor with vaccum advance removed, advance plate locked, recurved by Terry T.)

Those ports can be used to balance the carbs.

[So Cal Mark]

vacuum advance can certainly be used with manifold vacuum. With perf cams it works well to get a better idle and still be able to start the car

[MNspiderman]

Mark is right about how you can run your dizzy vacuum off of an intake port instead of ported vacuum. Ported vacuum was designed to make your engine run hot when idling in order to heat up the exhaust gasses in order to lower emissions coming out of your tail pipe. Thats a fact Jack. That is the only reason it exists. Old Old OLD cars never had a vacuum dizzy either.

[bradartigue]

Every so often the old vacuum distributor arguments resurface. There really are only two solutions if you can't get ported vacuum from above the throttle plates - get a Marelliplex or convert yours to one. Using a modified points distributor is a downgrade; using manifold vacuum produces a rough, pulsing, abusive bounce to your dizzy advance plate.

The emissions argument may be true but the cars accelerate poorly if the vacuum isn't hooked up and working correctly. Take any stock Spider and disconnect that vacuum line and it will accelerator like an 850.

the IDF70 series carburetors (as in 40IDF70) have the ports above the plates, you tie them all together and run them to the dizzy. Other IDFs don't have those ports but some have a PCV that opens with the throttle. Put some sort of restriction in it (like a carburetor air correction jet) and it might do the trick as well.

And hey everybody knows you solve your IDF problems with a good downdraft carburetor and 1800 manifold. If I'd listen to my own advice I wouldn't have so many carburetor issues...