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SU Carbs have strange vacuum fitting

Geo Hahn

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I would think that arrangement would have some strange effects on piston rise -- can't imagine that was ever an original fitting on an SU.
 

trrdster2000

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This could get interesting!! What in the world could that purpose accomplish?

Wayne
 

DNK

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wp_000850.jpg
 

TR3driver

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My guess is that someone was trying to get some visibility into what the piston was doing while driving down the road. Not particularly effective I would think, but the vacuum inside the chamber should vary a bit with position, due to the action of the spring.
 

Roger

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Maybe someone's idea of a setup / tuning device? If both carbs have the same fitting, maybe you drop a couple of bits of straight wire of equal length into these and see how the pistons rise?
 

Keith_M

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The fitting looks like it's made to accept a small diameter hose tube. I wonder if someone was trying to control the amount by which the pistons rose, either by adding vacuum or adding a controlled leak. I suppose you could dynamically control the mixture that way.
 
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Bruce100

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I got thinking about this, and old physics/science memories came back. Someone geeky enough to do this probably was geeky enough to make a manometer and added these fittings to use it to sync the carbs.

manometer.jpg
 

Andrew Mace

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The fitting looks like it's made to accept a small diameter hose tube. I wonder if someone was trying to control the amount by which the pistons rose....
...or, now that I think about it, hooked up a vacuum gauge either temporarily for tuning or permanently for viewing somewhere on or under the dash?
 

TR3driver

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...or, now that I think about it, hooked up a vacuum gauge either temporarily for tuning or permanently for viewing somewhere on or under the dash?
That was my point. Thing is, it wouldn't show manifold vacuum at all. As long as the engine is running, the vacuum in the chamber is just enough to support the weight of the piston plus whatever force the spring supplies. Obviously the piston remains the same, and the spring is pretty light. I doubt you'd get even 1 psi difference between idle and full throttle.
 

Keith_M

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That was my point. Thing is, it wouldn't show manifold vacuum at all. As long as the engine is running, the vacuum in the chamber is just enough to support the weight of the piston plus whatever force the spring supplies. Obviously the piston remains the same, and the spring is pretty light. I doubt you'd get even 1 psi difference between idle and full throttle.

The vacuum is created by the venturi effect from the air passing through the carburetor. As far as I understand it, it has nothing to do with manifold vacuum.

Here's another thought: I wonder if the DPO hooked these two connectors together with a tube in a (I think) misguided attempt to auto-balance the two carbs. It certainly would make the pistons rise to the same height, but that height is supposed to be determined by how fast air is rushing through the carb. I'm not sure how effective it would be.
 

poolboy

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You got it wrong, Keith.
Putting it simply, if it weren't for a depression ("vacuum") in the manifold there wouldn't be any air movement over the jet. The same depression draws the air valve upward.
 

Andrew Mace

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That was my point. Thing is, it wouldn't show manifold vacuum at all. As long as the engine is running, the vacuum in the chamber is just enough to support the weight of the piston plus whatever force the spring supplies. Obviously the piston remains the same, and the spring is pretty light. I doubt you'd get even 1 psi difference between idle and full throttle.
Hey, I never said it was a good idea. :glee:
 

Keith_M

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You got it wrong, Keith.
Putting it simply, if it weren't for a depression ("vacuum") in the manifold there wouldn't be any air movement over the jet. The same depression draws the air valve upward.

Agreed. The point I meant to make is that the two values are not necessarily correlated. For example, under hard acceleration there is actually very little vacuum in the manifold, but there is high flow and high vacuum in the top of carb. Conversely, at idle, the vacuum in the manifold is actually fairly high (because the butterfly valve is restricting flow), but the flow is low and vacuum in the carb is low.

I think this is right, but as has been proven before in this forum, I can occasionally be wrong (but don't tell my wife). :smile:
 

TR3driver

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You got it wrong, Keith.
Putting it simply, if it weren't for a depression ("vacuum") in the manifold there wouldn't be any air movement over the jet. The same depression draws the air valve upward.

Nope, not the same depression. The chamber over the piston is open to the venturi, so it "sees" roughly the same depression as the venturi, which is more or less constant. (That's the constant depression in "constant depression carburetor".) The manifold vacuum is on the other side of the throttle plate.

If you put manifold vacuum on top of the piston, it would simply slam to the top as soon as the engine started. Normal idle is around 18" Hg, roughly 9 psi below ambient. The piston in an H6 is about 3" in diameter, so has an area of about 7 sq in. 9 psi times 7 sq in would be 63 pounds of force pushing the piston up.

Think of it like washing your hands. The pressure inside the water pipes is what pushes the water out, sure, but it is much higher than the pressure at your hands.
 

poolboy

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Perhaps I should have said the "source" of the vacuum was from the manifold, for the sake of simplicity if not technically precise either.
When I said "same" I did not intend it to be interpreted as equivalent or to expand into the whole 4 stoke cycle or Bernoulli principal.
I understand your desire expand the concept, though, so I've still left plenty of room to do so, Randall or anyone else.
 
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