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Location:
Melbourne, Australia
Registered:
March 2005
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Quad throttle throat sizing questions
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Fri, 26 August 2005 14:43
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Hi,
I know in carburetted applications throat/choke sizes must be selected such that airflow velocity at low engine speeds is high enough and stable enough to promote good fuel atomisation, while not unnecessarily restricting top end airflow.
Given my difficulty in finding appropriate carbs for my motor I'm starting to look into quad throttles and fuel injection a la EFIhardware, injection perfection etc but it's a new science to me.
What are the important detirminants of best throat sizes for EFI applications? I presume the compromises are the same, ie. drivability v. max potential power?
(Not looking for a spoon feed really, just guidance for initial research... thanks)
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Quad throttle throat sizing questions
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Fri, 26 August 2005 14:46
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the vague idea is that you want a certain maximum air velocity in the throats.. iirc, it's something like 300ft/sec ????
i guess if anythig you'd want them the same area, or slightly bigger than the ports.. ideall with an X-degrees angle from th trupmet to the valve, to accelerate the air all the way....
or something like that 
also depends if you want a wide torque range, or a top end mini-powerband
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Location:
Melbourne, Australia
Registered:
March 2005
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Re: Quad throttle throat sizing questions
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Fri, 26 August 2005 14:51
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Cheers for the input. I think Ed cross-posted a relevant thread both here and on Performance Forums (btw. how downhill has PF gon now!)
Off to search I go.
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Location:
Adelaide
Registered:
May 2003
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Re: Quad throttle throat sizing questions
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Sat, 27 August 2005 00:42
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Heh, PF hasnt really gone downhill, more just become very very odd, as is the way with many online communities.
The thread which Ed posted is a good one, and if you are interested in the physics behind it all i suggest you speak to Nero on PF and do a backorder of the previous 2 issues of Race magazine, they have a very very very good write up on the physics of porting, which logically extends through to your throttles.
As for the throttles, the size is mainly determined by, as Stew has said, the inlet air speed which is required at the engine RPM which you are tuning for. So for example in my car, the tuned RPM is 7500rpm, and i cant remember exactly the inlet air speed, but it roughly equals 43.5mm throttle chokes, and im running 44mm (bored ST20v).
Lots of maths involved in that one though.
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