Draw thru carb options?

[vizman]

Loopie just told me I could use a cv carb on a draw through, but there’s better options (of carbs) out there....

[MeanBean49]

37 minutes ago, Samikoo said:

How does the pressure differentiate when in a draw through system compared to a na engine?

Edit: And why does an su carb function in a draw through design?

Do I have to tell you the same thing again!

On a draw through, your only affecting one side of the carb. So the pressure is way different on one side of the carb which makes the differential way more than under normal n/a operation or in blow through operation where you have the same increase or decrease on both sides of the carb.

This means the slide will not function properly.

Lets put some fictional numbers on it to help.

N/A inlet side 1 bar, outlet side 0.5 bar pressure differential carb is designed to operate with 0.5 bar

Blow through, system inlet 3 bar, outlet side 2.5 bar, pressure differential 0.5 bar, carb functions as normal

Draw through, inlet side 1bar, outlet side 0.25bar, pressure differential 0.75 bar, carb cant function as normal, slide will not work properly and float chamber cant function right either

 

You proably can mess about and get one to function somthing like at different pressure differential but I doubt it would be easy, and would be trying to get it to work at a different velocity as what was designed, and likely to still have issues on transitions from WOT to closed and vice versa

Edited April 2, 2021 by MeanBean49

[clivegto]

1 hour ago, vizman said:

I have no idea, I’m guessing, I’m going to ask loopie tomorrow.

[Samikoo]

1 hour ago, MeanBean49 said:

Do I have to tell you the same thing again!

On a draw through, your only affecting one side of the carb. So the pressure is way different on one side of the carb which makes the differential way more than under normal n/a operation or in blow through operation where you have the same increase or decrease on both sides of the carb.

This means the slide will not function properly.

Lets put some fictional numbers on it to help.

N/A inlet side 1 bar, outlet side 0.5 bar pressure differential carb is designed to operate with 0.5 bar

Blow through, system inlet 3 bar, outlet side 2.5 bar, pressure differential 0.5 bar, carb functions as normal

Draw through, inlet side 1bar, outlet side 0.25bar, pressure differential 0.75 bar, carb cant function as normal, slide will not work properly and float chamber cant function right either

 

You proably can mess about and get one to function somthing like at different pressure differential but I doubt it would be easy, and would be trying to get it to work at a different velocity as what was designed, and likely to still have issues on transitions from WOT to closed and vice versa

Yes you do, because saying the same thing over and over does not clarify it.

So you assume the draw through has a greater vacuum than a na engine? Why? And no, I'm not trying to wind you up!

I understand the reasoning with barometric pressure vs. carb outlet pressure. But how come the draw through carb outlet pressure is less than that of the na engine? And I'm not comparing two engines with the same displacement. Please sir, you are an expert on all things flowing air. Explain, I do want to learn.

[MeanBean49]

31 minutes ago, Samikoo said:

Yes you do, because saying the same thing over and over does not clarify it.

So you assume the draw through has a greater vacuum than a na engine? Why? And no, I'm not trying to wind you up!

I understand the reasoning with barometric pressure vs. carb outlet pressure. But how come the draw through carb outlet pressure is less than that of the na engine? And I'm not comparing two engines with the same displacement. Please sir, you are an expert on all things flowing air. Explain, I do want to learn.

It less because the turbo is creating a greater vaccum than an N/A engine is. 

Its not positive pressure until after the turbo.

The problem is being able to get the slide to react properly.

If your at closed throttle at quite high rpm, the turbo is spinning creating quite a lot of vaccum, you open the throttle a little bit, but you get a massive initial airflow because of the big vaccum, pressure differential is huge, and you get fully open slide and more float bowl pressure, and then massive excess of fuel dumped into the motor at once.

Its transitions in throttle position that are the problem, not max flow, or constant throttle, or max cfm stuff.

If CV's worked or were easy to get to work then tgats what people would use instead of spending a fortune on expensive carbs

[Samikoo]

20 minutes ago, MeanBean49 said:

It less because the turbo is creating a greater vaccum than an N/A engine is. 

Its not positive pressure until after the turbo.

The problem is being able to get the slide to react properly.

If your at closed throttle at quite high rpm, the turbo is spinning creating quite a lot of vaccum, you open the throttle a little bit, but you get a massive initial airflow because of the big vaccum, pressure differential is huge, and you get fully open slide and more float bowl pressure, and then massive excess of fuel dumped into the motor at once.

Its transitions in throttle position that are the problem, not max flow, or constant throttle, or max cfm stuff.

If CV's worked or were easy to get to work then tgats what people would use instead of spending a fortune on expensive carbs

Yes, that sounds right. I think I'll have to try it for myself when I get my build up to that point.