[Diy_efi] Throttle body size

[Tlsalt at aol.com]

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

> 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.

Hello Adnan,

TB should be sized for the horse power of the engine.  Don't use the SU HD8 
as a guide (50mm X 2),  they are variable venturi carbs and a 3 liter six 
would never pull enough air get the pistons all the way up (2  HD8's=600cfm). 
A 3 liter engine only pulls 400 cfm at 7000 rpm (@ 100% VE). You are on the 
right track comparing OEM's sizing.  For example, Ford has a 345hp crate 
engine with a 65mm TB, so you know the TB is at least good enough for that 
output.  Jaguar used a 65mm TB on a 4.2 six, with a higher output than the 
same motor with 3 HD8's.  I would go with something in the 55mm range, unless 
you are talking really high rpm. The only problem with going too large is tip 
in response.  Take a closer look at the BMW sixes, I think the 2.5 and 2.8 
used a 51mm TB.

Paul

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi