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Brake Booster Plumbing 

 

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Posted: Wed Feb 14, 2007 1:23 pm 
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Read on:

http://en.wikipedia.org/wiki/Manifold_vacuum

 

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Posted: Wed Feb 14, 2007 1:25 pm 
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4.9 EF Futura wrote:

It's the throttle body which creates vaccum in the manifold. Engine is an air pump, drawing air. Throttle body closes and severly restricts the flow of air entering the manifold. The only air entering the manifold is coming through the ISC - a minute opening compared to the TB. Vaccum condition prevails.

When the throttle body is open on a nat/asp car, manifold pressure nears atmospheric (restriction removed between manifold and air intake), the engine is free to draw as much air as it wants. On a boosted car, it goes one step further and you get positive presure in the manifold...


I dont see how adding a turbo makes it different.
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Posted: Wed Feb 14, 2007 1:31 pm 
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Its the fact that the engine can theoretically pump 4L of air through it per revolution and the throttle/isc only lets through a relatively tiny amount and that creates the vacuum. / pressure variation..

If you had the vacuum hose on the intake side of the turbo there isnt a throttle on it and wont create that pressure variation.... While theres a huge amont of airflow there wont be that vacuum.

The pressure will drop below atmospheric (the faster a fluid flows the lower the pressure) but not enough to pump (or suck?) up the booster.
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Posted: Wed Feb 14, 2007 1:38 pm 
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Ahh I see.

/Light bulb switches on

That makes sense now.

So there will be little to no vacuum at idle but how about on boost? Does the same rule apply where there needs to be a big restriction or would the intake pipe be enough of a restriction if that much air is being sucked in?

I guess it wouldnt matter regardless as the brakes would be crap when not on boost.
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Posted: Wed Feb 14, 2007 1:46 pm 
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Just add a vacuum tank into the line to the booster as I had to with the enormous cam I had in the GTHO :shock: :D :D :D

 

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Posted: Wed Feb 14, 2007 2:18 pm 
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glenneaux wrote:
If you had the vacuum hose on the intake side of the turbo there isnt a throttle on it and wont create that pressure variation.... While theres a huge amont of airflow there wont be that vacuum.

The pressure will drop below atmospheric (the faster a fluid flows the lower the pressure) but not enough to pump (or suck?) up the booster.


This post by glenneuax is the key Dan.

Yes, there would be a drop in pressure inside of the intake tube, but only a marginal one. The restriction of the air filter and the intake plumbing itself is not enough to create the 20odd inches of mercury vacuum which one finds in the intake manifold.

It would be interesting to plumb a vac gauge into an air inlet to get some readings and demonstrate the matter at hand.

 

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Posted: Wed Feb 14, 2007 2:46 pm 
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In between the filter and the turbo you will have very little vacuum, maybe 5kpa (the s**t the filter the more vacuum you will get). Between the turbo and the throttle body you will have a vacuum or pressure depending if you are on boost or not. After the throttle body you will have lots of vacuum when the throttle is closed (80-90kpa), plenty at idle (50-70kpa), and pressure when on boost. Even when cruising around you will have 40-50kpa of vacuum in the manifold.

 

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Posted: Thu Feb 15, 2007 1:27 pm 
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The one way valve on booster MAY not be holding boost back? Just fit a V.L / Skyline brake booster inline valve.. If you pump brakes too much you will loose efficiancy ... But generally you don't have boost and brake at same time, so you "should" have vacuum... The old school of "heal & toe" braking is not the best way of braking on converted turbo cars.. As there is little or no vauum between gear changes... I try not to do this when driving in anger as pedal pressure increases due to little vacuum...Fittg one way valves to accessary [heater/ air vents etc] helps alittle too...

 

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