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 Post subject: Increase in flow
 Post Posted: Sat Jun 02, 2012 12:40 am 

Joined: Sat Jan 07, 2012 5:22 pm
Posts: 63
How does increasing flow on a given size venturi affect carb operation?

If you improve airflow past the throttle plates by machining down the shaft and using smaller low profile screws and to a certain extent booster changes how does that affect the carb and metering?
Are they designed to work within an airflow window as such or would going to a bigger sized venturi be a better idea say going from a 750 -850 holley to get more airflow instead of increasing efficiency?

I noticed on the Weber and Carter carbs they seem to have really thick throttle shafts that the throttle plates slide into instead of the Holley design.


 Post subject: Re: Increase in flow
 Post Posted: Sat Jun 02, 2012 1:07 am 
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Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
Improving flow in the throttle plate area reduces the vacuum signal at the T-slot at say WOT. If the carby is correctly tuned with the right relationships of main jet, IFR, T-slot etc the main jet will be the controller and nothing will change. The ideal situation is the T-slot to flow a small amount and the booster to flow a lot at WOT. Technically with an emulsion carby if the throttle is slightly closed from WOT the AFR shouldn't vary. The throttling plate flow has to interact with the main venturi flow that's why the throttle plates are sized like they are. You could build a carby with a 3" throttle and a 1" venturi if you wanted to but it would be difficult to get a consistent tune across the rev range at the different throttle openings a driver may choose. The worse thing to do is go too big on the main venturi compared to the butterfly size so working on flow in the throttling area is a safer option.

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