Carb sizing

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Plastermaster
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Joined: Sun Sep 30, 2001 2:01 am

Carb sizing

Post by Plastermaster » Sun Aug 27, 2006 7:02 pm

The Idea of mixing and matching carbs with venturies other than what came in the box has been discussed before. Usually in reference to trying to make big carbs work on small engines or visa-versa rather than buying the standard size for the engine. Often it is usually in the context of high performance as in a quick quater miler. After reading the thread on smaller plenums and also considering how in carb tuning, the rule is to remember we spend 95% of out time driving around on the idle circuit, I really wonder if the body of the carb in relation to the engine shouldn't be considered more carefully. Usually people choose the size of the carb based on the size of the vents it comes with. I'd like to know more how the size of the TB (if thats what its called in a carb) effects performance where we drive the most. With my 2270 the torque is sufficient that for normal driving I never need to give it more gas to accellarate then what is needed for the target speed. In other words If I want to go 10 or 15 mph faster I don't need to give it alot of throttle and then back off again when I am up to speed. The only time I am fully in the main circuit and making the most of the main vents is when I want to have some fun with it.
So, what really happens if you put 40mm carbs with 36mm vents on a 2.2L engine, be it T1 or T4? It has been suggested that you will be in the main circuit at a lower rpm, but that it can be tuned to compensate for that by jetting and float level. Maybe so, but I think that overlooks the fact that on a smaller carb the plate has to open wider to allow the same air flow as a larger carb. This will put the plate past the progression holes at a lower rpm.
Any thoughts or 1st hand experience with this?

Thanks
Ron

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Racer Chris
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Post by Racer Chris » Mon Aug 28, 2006 6:19 am

Beyond a certain size the opening in the top plate becomes more restrictive than the venturi and must be opened up for the carb to continue working effectively. With 40IDFs this is necessary for anything over 34mm.
You can actually run with no venturis and get really good top end power but the bottom and midrange will suffer badly.
The upper end of performance for a 40IDF carbed engine without top plate mods is about 140hp IMO.

MASSIVE TYPE IV
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Post by MASSIVE TYPE IV » Mon Aug 28, 2006 6:22 am

On larger engines it's very difficult to keep smaller carbs from running rich, but then again thats impacted by everything else including exhaust.

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