Blow through carbs... I don\'t get it!

[RemarkLima]

Hey all,

Just thinking about carbs and stuff, and one thing that confuses me:

Normal carbs on NA applications I can understand, there\'s a vacuum working on the needle valve which sucks the fuel into the engine, all good and well.

The same thing on forced induction doesn\'t work as there\'s no vacuum, so I\'ve read about blow through carbs where you use the turbo pressure to pressurise the carb...

...But this is what confuses me, say I make 8psi of boost, and this also pressurises the bowl of the carb, but that will have 8psi as will the inlet manifold so the pressure difference will be 0... So surely the only fuel that\'ll get drawn through will be a dribble at best???

What am I missing here?

Cheers,
Mark

[antnicuk]

carbs work on velocity of air not pressure,  so by equaling the pressure the carb works normally.  Dont forget we (humans) always under about a bar pressure anyway.    the air still travels in the same direction.

N/a the carb is under 1 bar pressure (atmosperic pressure) and so is the float/bowl, when turbo\'d the carb goes up by 8psi but so does the float so it knows no difference.

It works,  I turbo\'d my m/bike,  180 bhp at the back wheel isnt funny!

[Mrs M@r]{]

Tony .. I think you should organise a \'practical\' demo day! Teach me how it all works!! :Thumbsup! I mean .. I get the basics but that is all Jap to me!! lol
 

carbs work on velocity of air not pressure, so by equaling the pressure the carb works normally. Dont forget we (humans) always under about a bar pressure anyway. the air still travels in the same direction.
 
N/a the carb is under 1 bar pressure (atmosperic pressure) and so is the float/bowl, when turbo\'d the carb goes up by 8psi but so does the float so it knows no difference.
 
It works, I turbo\'d my m/bike, 180 bhp at the back wheel isnt funny!

[RemarkLima]

Thanks Tony, I think I see now, so you still have a venturi to accellerate the air through?

And thinking back to my physics days, a fast moving gas is technically less pressure (blow over a sheet of paper and it\'ll lift up, same principle used for flight!), so even though the static pressure is the same, the moving air in the throttle is actually less and so will draw air through the same... Am I thinking on the right lines?

Oh, and turbo\'ing a motorbike is mental Supercharging I can see would be a bit easier to handle, but when it comes on boost that must have been an "adreline is brown" moment

[antnicuk]

it was brown, especially when it spat me off when th front wheel landed!

a few broken bones later and i know a little better!

I\'m not sure of the technicalities of it all, but think of a car water system,  its under 1 bar of pressure when hot but still flows around the engine.  

The air is still moving at a fast rate,  its just under more pressure than normal.  I learnt this when the bike used to come on boost and cut out, I hadnt put a boost line to the float chamber so fuel was pushed back up the fuel lines.  

Cheryl, what do you want to know?

oh,  our last guinea pig pegged it Christmas eve,  at last, the little sod was 6 years old!  New dining table on order

[Mrs M@r]{]

It\'s no use just telling me I need bits and pieces to play with to get to grips with it all!! For example one of the garage guys took me into his engine building room (no euphamisms here all perfectly innocent Mark was there) and showed me the basics on building an engine as I was asking hin all sorts of questions (much more effective than talk!). So i know what it all looks like, the rotors, springs, tips etc and where it goes!!! Which I am quite proud of. Now when I hear blitzboy at Santa Pod next year saying he\'s \'blown a tip\' I know where it is and what has happened etc. Nothing ground breaking but I want to learn!! I also want to turbo the MX5 this year so I need to be able to talk the talk! Reading what you guys talk about on here all helps! :Hammer
 
I\'m sorry that the last pig is dead.... that\'s an awful christmas present. I am impressed that he was 6 though!! You will miss him really!!
 

Cheryl, what do you want to know?
 
oh, our last guinea pig pegged it Christmas eve, at last, the little sod was 6 years old! New dining table on order

[Red and Single]

{\']I also want to turbo the MX5 this year

Now you\'re talking :D:D

Have you done any research ??

HKS do a good basic bolt on kit, as do Greddy.

Flyin Miata in the states do a more comprehensive setup...if you fancy worrying the crap out of Mark (You\'ll keep up with his rex !!)

I put a HKS kit on my old Mk2 Roadster and it transformed the car.... it ran at 6psi and gave around 190hp.

The FM kits run upto 12psi :burnout