[Diy_efi] Throttle body size

[Eric Byrd]

The SDS folks I just got done reading from their site say this about
throttle bodies:  if the vacuum taken off the MAP sensor is higher at
full throttle than at idle, your throttle body is too small.

And here is a rule of thumb.  Since flow rate is (density) x (flow area)
x (flow speed), and since flow resistance in a given kind of obstruction
depends strongly on flow speed through it, if you know a certain engine
has a given TB size, and the engine you are looking at is, for example,
twice as large, then in general, its TB should be larger by a multiple of
(root)2.  No doubt there are lots of other factors, but I don't know any
more than that.

On Thu, 12 Dec 2002 12:56:49 -0500 "Merchant, Adnan"
<Adnan.Merchant at TycoHealthcare.com> writes:
> Hi,
> 
> This is a subject that's been covered before so my apologies in 
> advance.
> 
> How does one determine the correct size for a throttle body for an 
> engine?
> Flow rate is the most logical parameter but the few data points I 
> have from
> production engines only serve to confuse.   Here's what I've found 
> so far:
> 
> Honda 2.0 non-VTEC: 55 mm
> BMW 3.5 (M30): 65 mm (nominal, since it's tapered)
> Mustang 5.0: 65 mm, 70 mm
> Mustang 4.6: 70 mm
> 
> My application is a 3.0 straight six, normally aspirated, plenum 
> style
> intake.
> 
> Any suggestions on how to figure out the right size?   The original 
> carbs
> were twin SU, 2" throats.
> 
> Thanks,
> Adnan
> 
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