Draw thru carb options?

[davecara]

I know CV’s are a no no and HSR42 is a yes yes but..

I’ve got a 1 1/2” SU carb sitting on the shelf, would that be any use? 

[Gixer1460]

Its will certainly work as 'back in the day' a 2" SU off a Jag was 'de rigeur' carb on supercharged installations. A 1 1/2" is 38mm and maybe a bit small, especially as its a 'CV' carb. An 1 3/4" may be a better overall bet - like the HiF44 from a Metro Turbo. Hardest bit will be tuning as can't imagine alternative needles are as common as they used to be?

[vizman]

I had a 1/34 on my non Suzuki draw through, binned it for a super e/b, as the hardleydangerouse stuff was easier to obtain.

the su worked ok, leaked a bit, but getting needles and bits started getting harder to chase....that was before I joined the internet.

 

there is a place on Instagram called chopswopuk (or similar) lots of super e/g/b come up for sale mega cheap...even the odd hsr 

[davecara]

Ta gents, I'll hold out for an HSR or S&S provided I can find one for reasonable ££ 

[dupersunc]

2" SU was good for 400bhp on a 2ltr Ford cosworth draw through set-up.  1"/34 should be fine. 

Burlen Services in Salisbury can supply just about anything for SU carbs.

[Samikoo]

On 3/23/2021 at 11:00 PM, davecara said:

I know CV’s are a no no and HSR42 is a yes yes but..

I’ve got a 1 1/2” SU carb sitting on the shelf, would that be any use? 

So what exactly is wrong with a cv carb on a draw through desing?

[davecara]

32 minutes ago, Samikoo said:

So what exactly is wrong with a cv carb on a draw through desing?

I dont know to be 100% honest but i've never seen one used so there must be a reason!

[no class]

how about a throttle body and each intake port gets an injector ?

[vizman]

2 hours ago, Samikoo said:

So what exactly is wrong with a cv carb on a draw through desing?

I’ve no real idea,  lack of accelerator pump....erm no push pull throttle.... once the boost is greater than vacuume... no idea

[MeanBean49]

5 hours ago, Samikoo said:

So what exactly is wrong with a cv carb on a draw through desing?

Iirc its because of the pressure difference between ambient and the outlet side, it stops the slide opening/closing properly, or can make it open/close too quickly depending on what the turbo/engine is doing

[davecara]

4 hours ago, no class said:

how about a throttle body and each intake port gets an injector ?

That’s much later on, I’ve got to get the blow thru bike done first...

[Duckndive]

Theres ting tong copies of HSRs and Super Es on ebog 

apparently the parts out of the super e copies fit straight into the real ones and work out cheaper than a rebuild kit 

 

[davecara]

1 hour ago, Duckndive said:

Theres ting tong copies of HSRs and Super Es on ebog 

apparently the parts out of the super e copies fit straight into the real ones and work out cheaper than a rebuild kit 

 

There’s a chinglish hsr on a certain site for £65 new and I’m sorely tempted!

[Samikoo]

On 3/29/2021 at 7:23 PM, vizman said:

I’ve no real idea,  lack of accelerator pump....erm no push pull throttle.... once the boost is greater than vacuume... no idea

But some have accelerator pumps and push-pull throttle. I've seen them on Hurley Durley aggricultural machines.

And the only thing a carb sees is sucky sucky, right? When the turbo starts making boost, it just sucks more air through the carb. I'm having a real hard time understanding how the carb could sense that it is feeding an air-fuel mixture in to a turbocharger instead of a naturally aspirated engine. I guess I'll have to bite the bullet and try one out myself.

[vizman]

No one is saying a carb ‘senses’ anything, I’m certainly not.

Edited April 2, 2021 by vizman

[MeanBean49]

1 hour ago, Samikoo said:

But some have accelerator pumps and push-pull throttle. I've seen them on Hurley Durley aggricultural machines.

And the only thing a carb sees is sucky sucky, right? When the turbo starts making boost, it just sucks more air through the carb. I'm having a real hard time understanding how the carb could sense that it is feeding an air-fuel mixture in to a turbocharger instead of a naturally aspirated engine. I guess I'll have to bite the bullet and try one out myself.

See my response above.

A CV doesnt just rely on the airflow through the body, it uses pressure differential to open and close the slide. On a blow through system the pressure differential between inlet and outlet of the carb is the same as in N/A use.

In draw thru the differential is much much greater due to their being way more vaccum at the outlet side than the inlet. This means the slide cant work properly

Edited April 2, 2021 by MeanBean49

[Samikoo]

4 hours ago, MeanBean49 said:

See my response above.

A CV doesnt just rely on the airflow through the body, it uses pressure differential to open and close the slide. On a blow through system the pressure differential between inlet and outlet of the carb is the same as in N/A use.

In draw thru the differential is much much greater due to their being way more vaccum at the outlet side than the inlet. This means the slide cant work properly

So how come an su carb works on a draw through setup? It's function is also based on the vacuum created by the engine. I'm not trying to argue here, it's just that I still can't figure out how, let's say a 1300cc na engine or a 750cc turbocharged engine, would make the carb act any different, if the cfm through the carb would be the same?

[MeanBean49]

2 hours ago, Samikoo said:

So how come an su carb works on a draw through setup? It's function is also based on the vacuum created by the engine. I'm not trying to argue here, it's just that I still can't figure out how, let's say a 1300cc na engine or a 750cc turbocharged engine, would make the carb act any different, if the cfm through the carb would be the same?

An SU carb only relies on vaccum only.

A CV carb relies on a pressure difference between inlet side and outle side. Has nothing to do with cfm.

[Samikoo]

1 hour ago, MeanBean49 said:

An SU carb only relies on vaccum only.

A CV carb relies on a pressure difference between inlet side and outle side. Has nothing to do with cfm.

I'm still not buying this. A cv carb functions solely on vacuum, and an su carb is a cv carb. Ea constant velocity.

The reason I mentioned cfm was to clarify that a carb only understands the air sucked through it, or pushed if it's a blow through system. How on earth could a cv carb work on a big bore twin if it could not handle a huge vacuum? The carb does not know if the air is being sucked through it by a na engine or a turbocharger.

Bear with me here, I'm an automotive engineer so I overcomplicate things.

[vizman]

Definitely over complicating things, a carb cannot possibly understand anything.

[vizman]

Forced induction, a lot more than the motor naturally will draw in irrelevant of cubic capacity?

[vizman]

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

[Samikoo]

9 minutes ago, vizman said:

Forced induction, a lot more than the motor naturally will draw in irrelevant of cubic capacity?

1300cc 6000rpm na and 750cc 9500rpm plus 5 lbs of boost equals roughly the same cfm.

[MeanBean49]

13 minutes ago, Samikoo said:

I'm still not buying this. A cv carb functions solely on vacuum, and an su carb is a cv carb. Ea constant velocity.

The reason I mentioned cfm was to clarify that a carb only understands the air sucked through it, or pushed if it's a blow through system. How on earth could a cv carb work on a big bore twin if it could not handle a huge vacuum? The carb does not know if the air is being sucked through it by a na engine or a turbocharger.

Bear with me here, I'm an automotive engineer so I overcomplicate things.

Whether you buy it or not thats the reason.

A CV carb does not function on vaccume.

It functions on the pressure difference between inlet and outlet

When you increase an engines capacity you increase the volume drawn in, but you do not change the pressure differential between inlet and outlet.

When you have a blow through system you increact the volume through the carb but you do not change the pressure differential

When you have a draw through you massively change the pressure differential and the slide will not work properly.

Im a degree qualified aeronautical airframe and jet propulsion specialist, and its not complicated at all. 

[Samikoo]

7 minutes ago, MeanBean49 said:

Whether you buy it or not thats the reason.

A CV carb does not function on vaccume.

It functions on the pressure difference between inlet and outlet

When you increase an engines capacity you increase the volume drawn in, but you do not change the pressure differential between inlet and outlet.

When you have a blow through system you increact the volume through the carb but you do not change the pressure differential

When you have a draw through you massively change the pressure differential and the slide will not work properly.

Im a degree qualified aeronautical airframe and jet propulsion specialist, and its not complicated at all. 

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?

Edited April 2, 2021 by Samikoo
Further rambling