21 replies to this topic

[Ethel]

It's why the carb has to be sealed (that 'n to stop it spraying fuel round your engine bay)...

 

The boost pressure is also fed to the float chamber*, so the venturi still creates a pressure differential to push the fuel up the jet the same as on a N/A engine - just at a higher base pressure than atmospheric.

 

*turbo carbs have foam filled floats so they don't crush, and upgraded needle valves.

 

An extra factor is that the venturi really works off the velocity of the air going through it, so it can't compensate for the boosted air being denser. That's why there's a restrictor in the plenum chamber, the air going in to the throat of the carb has to go through it, while the communication passage to the float chamber is upstream. So, as the boost rises, the pressure seen in the float chamber rises an extra bit more and more fuel goes up the jet to keep the fuel/air ratio inline with the increased air density.

[Pigeonto]

"One other thing, anybody know how far up or down the piston/needle is likely to be at cruise speed."

Someone, such as AC Dodd, who uses a rolling road would be the best to answer that because it can be seen at all speeds/loads

However if I was to guess it would not be up very far at cruise, maybe 1/4 of it's travel

[Ethel]

The general consensus is to divide the length of the needle into 16 (for a swing needle) "stations" for comparing the width. The first at the thick end is for idle, around the next 4 or 5 will be used for cruising.

 

Fixed needles have 13 stations, swing needles need to always keep the end located in the jet.

[David128]

anyone had a camera looking down the carb when running?

[HUBBA.HUBBA]

The general consensus is to divide the length of the needle into 16 (for a swing needle) "stations" for comparing the width. The first at the thick end is for idle, around the next 4 or 5 will be used for cruising.
 
Fixed needles have 13 stations, swing needles need to always keep the end located in the jet.

So the second quarter ish!
I ask the question cause I'm getting really lean mixture at cruise (2500-3000rpm at slight load, but not acceleration) gets worse when the load is more, say going up hill. If I'm going flat out on boost its ok, only lean when tootling along.
Fyi on a rolling road its fine( but I guess there's less load on it)

[Ethel]

Is it a genuine Metty Turbo SU with part throttle lean off, that's the hose that goes in a loop from in front of the dashpot to near the fuel hose connection?

 

If it runs ok it may be doing what it should and helping your mpg.

[MRA]

Airflow in a pipe, in this case a carburettor ....  density of air only changes with heat, so if it maintains a constant temperature the density stays the same, 

 

In any given airflow we have density (Rho) r, pressure P, and velocity v, we know that the venturi is a restrictor ie smaller in cross sectional area (CSA) lets say a certain amount of air is passing down this "pipe" in to the engine, as it reaches this restrictor irrespective of the pressure it is currently under, several things happen, the air pressure reduces locally to the venturi and in direct proportionality the velocity increases, now this assumes the density stays the same, which for this application it does .   

 

However the main point here is that irrespective of the pressure there is still a localised area of reduced pressure, not vacuum, but reduced pressure which then allows petrol to fill the "gap"

 

Simple to prove, as you have neither gained nor lost any air flow, your volume of air is the same as when it first started its journey,