[Diy_efi] Throttle body size

[Bernd Felsche]

On Thu, Dec 12, 2002 at 12:56:49PM -0500, Merchant, Adnan wrote:
> 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?

Air flow - and hence maximum power.

> 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.

One approach is to use ganged throttles in the same throttle body; a
small one (about 30mm) for small throttle openings and a large one
(>50mm) for mid-range to high loads. i.e. the larger one doesn't
start to open until the smaller one is almost fully open. That
provides good throttle control - nothing more.

Quite a common arrangement on VW-Audi, Porsche, etc. up until
throttle bodies become motorised.

-- 
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