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HUBBA.HUBBA

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Just wondered, when the trottle is opened and compressed/charged air comes through is the air after the throttle still a vacuum. Cause how I understood forced induction is that the air/fuel mixture is forced into the cylinders, so that would be a positive pressure, but if that is the case how does the fuel get drawn through? Doesn't that rely on vacuum to draw fuel through the jet ? Or is it still the case that compressed air/ fuel mixture is still being sucked/drawn into the engine?
Cheers in advance.


JT

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Pretty sure the carb sees boost as the turbo carbs need to be sealed differently.

My build thread..

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=542985


HUBBA.HUBBA

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Thats what I dont understand, yes the carb sees boost, but hif44 carbs rely on vacuum to draw fuel through the jet.


wil_h

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Simples. The float bowl is pressureised around 3psi above the venturi. So fuel is drawn through owing to the pressure differance.

The difference in pressure is is created by the restrictor in the plenum. This reduces the pressure in the venturi but the float bowl is pressurised through a tube that is fed from an unrestriced part of the plenum.

The fuel pressure regulator increases fuel pressure to compensate for the float bowl being pressurised, otehrewise you'd run out of fuel in the carb.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


Sprocket

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Preston On The Brook




On 7th Feb, 2016 HUBBA.HUBBA said:
Thats what I dont understand, yes the carb sees boost, but hif44 carbs rely on vacuum to draw fuel through the jet.


Vacuum is a 'measure' of lower than atmospheric pressure. The SU carb principle is that it works on pressure difference and venturi effect, rather than the requirement to have a vacuum.

In a boosted application, the restrictor ring in the plenum under boost creates slight pressure drop, which allows the float chamber to have a slightly higher pressure than the main choke of the carb where the mouth of the jet is situated, so under boost fuel is 'pushed' out of the jet, not just relying on venturi effect alone, to compensate for the increase in density of the air.

As more air mass flows through any given restriction the pressure drop increases. Ergo, increase the boost (more mass air flow), the restrictor in the plenum creates a higher pressure drop, thus pushes more fuel out the jet.

On 26th Oct, 2004 TurboDave16v said:
Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...


On 26th Oct, 2004 Tom Fenton said:
Yep I agree with TD........


mossy2a

389 Posts
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Derbyshire

The bernoulli principle.

increase the velocity across the venturi profile, and reduce the local pressure at the jet (fuel inlet) creating entrainment.

increase velocity, increase fuel entrainment.

The fuel is not pushed out of the jet.

The positive pressure in the float bowl is to give the same delta p (pressure drop) over the inlet and suction.


HUBBA.HUBBA

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Thats grest. Cheers all


Sprocket

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On 8th Feb, 2016 mossy2a said:


The fuel is not pushed out of the jet.

The positive pressure in the float bowl is to give the same delta p (pressure drop) over the inlet and suction.



What's the restrictor for then? If not to increase enrichment with increase in air density (boost)

Float chamber pressure will be higher under boost than in the main choke, ergo the Delta P will increase with boost.

On 26th Oct, 2004 TurboDave16v said:
Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...


On 26th Oct, 2004 Tom Fenton said:
Yep I agree with TD........


mossy2a

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Derbyshire

I design venturis for multi phase streams for a living

The fuel is not under higher pressure than the upstream motive pressure. Half your comments I will agree with.

Have a look at this. Hopefully it will help
http://transvac.co.uk/howanejectorworks.php


wil_h

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On 8th Feb, 2016 mossy2a said:


The fuel is not under higher pressure than the upstream motive pressure. Half your comments I will agree with.



What's the 'upstream motive pressure'? I'm pretty sure the fuel is at a higher pressure in the float bowl than through the carb in a turbo application. Much like it is when in NA (it's all relative of course, no reference pressure, but fuel must go from high to low pressure).

The pressure in the carb in the turbo application is not just about the flow through the carb, it's also affected by the plenum restrictor.

You don't need the restrictor. You can just pressurise the float bowl with a signal from a higher pressure point in the intake system; such as before the the IC. But you'll need a greater pressure difference

Edited by wil_h on 8th Feb, 2016.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


Brett

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The fuel will be higher than the plenum pressure pre restrictor by what ever you set it to at idle

The fuel pressure regulator is referenced from this point and will go up by the same amount.

Fuel pressure in the float bowl will not be the same as the pressure of the fuel in the feed line post regulator
The pressure in the float bowl is created by the restrictor only and to my knowledge and testing should be around atmospheric at idle and then increasing with boost
But importantly the fuel pressure in the float bowl will be imo the same as the boost pressure pre restrictor

This is all assuming the lean off pipe is blocked

If this is used the pressure in the float bowl does some funky stuff at part throttle

*happy*

Yes i moved to the darkside *happy*

Instagram @jdm_brett


mossy2a

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Derbyshire

upstream motive pressure = air inlet in this case. the main energy source in the venturi calculation.

The fuel cannot be at a higher pressure than the inlet or back flow will occur.

on balance, in a dynamic application, using multiphase, the pressures will have to be Inlet = high pressure, suction (fuel) = low pressure (at least lower than the inlet), discharge (mixture or air and fuel) medium pressure.

The technology is getting on a bit now and this could be designed a lot better in terms of efficiencies and mixing capabilities.

It would be a good project to create individual port venturis, which in theory should work. You could even have a second stage to add some water into the mix.


On 8th Feb, 2016 wil_h said:
On 8th Feb, 2016 mossy2a said:


The fuel is not under higher pressure than the upstream motive pressure. Half your comments I will agree with.



What's the 'upstream motive pressure'? I'm pretty sure the fuel is at a higher pressure in the float bowl than through the carb in a turbo application. Much like it is when in NA (it's all relative of course, no reference pressure, but fuel must go from high to low pressure).

The pressure in the carb in the turbo application is not just about the flow through the carb, it's also affected by the plenum restrictor.

You don't need the restrictor. You can just pressurise the float bowl with a signal from a higher pressure point in the intake system; such as before the the IC. But you'll need a greater pressure difference


wil_h

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Back flow? The normal flow of fuel is from the float bowl to the carb throat. Back flow would occur of the inlet was a higher pressure than the float bowl; surely?

Remember, it will only be pressurised when boost is made and fuel is required. The amount of fuel is metered by the needle profile.

How does fuel get into the carb if it's not at a higher pressure than the inlet? I can assure you is the restrictor is removed, there is insufficient pressure difference and the fuelling goes lean.

We must be talking cross purposes here.

On 8th Feb, 2016 mossy2a said:

The fuel cannot be at a higher pressure than the inlet or back flow will occur.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

On 2nd Jan, 2013 fastcarl said:

the design shows a distinct lack of imagination,
talk about starting off with a clean sheet of paper, then not bothering to fucking draw on it,lol

On 20th Apr, 2012 Paul S said:
I'm mainly concerned about swirl in the runners caused by the tangential entry.


Sprocket

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Preston On The Brook

So if the restrictor, or for that matter a pitot tube, does nothing to increase the float chamber pressure, why have them? You would simply connect the float chamber to the 'inlet' air pressure at the throat of the carb.

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=402578

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=272999

On 26th Oct, 2004 TurboDave16v said:
Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...


On 26th Oct, 2004 Tom Fenton said:
Yep I agree with TD........


Carlzilla

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Quarry Bonk

A turbo carb works along with a plenum and turbo fuel pressure regulator to create a pressure differential, ensuring that the fuel is at a slightly higher pressure than boost pressure at all times. It's very very basic stuff. A pitot tube instead of a plenum mainly uses pressure waves instead of just a restricion. Used on a lot of turbocharged bikes. Don't try and overcomplicate it with fancy stuff, it is what it is, and it simply works, and works well!

On 26th Jan, 2012 Tom Fenton said:
ring problems are down to wear or abuse but although annoying it isn't a show stopper

On 5th Aug, 2014 madmk1 said:
Shit the bed! I had snapped the end of my shaft off!!

17.213 @ 71mph, 64bhp n/a (Old Engine)


metroturbo

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North Yorkshire

The fuel pressure regulator serves one purpose - to ensure fuel is at a high enough pressure to be able to enter the float chamber. It has no influence over float chamber pressure, and does not help fuel to be drawn up the jet.


Carlzilla

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Quarry Bonk

If it didn't reference pressure from the plenum, then it wouldn't be able to provide sufficient pressure to fill the float bowl with fuel, so its part of the system as a package.

On 26th Jan, 2012 Tom Fenton said:
ring problems are down to wear or abuse but although annoying it isn't a show stopper

On 5th Aug, 2014 madmk1 said:
Shit the bed! I had snapped the end of my shaft off!!

17.213 @ 71mph, 64bhp n/a (Old Engine)

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