It Howls with small throttle bodies

[Stephen Lamb]

>On Wed, 30 Oct 1996, tom cloud wrote:
>
>> Now I'm confused....  With my limited knowledge and cranial ability,
>> I thought that throttle size had little effect on FI.  I can see that
>> a larger TB gives less velocity and therefore less momentum and
>> mixture of the fuel/air, but can anyone 'splain to this ole boy
>> how to determine a TB size (talking normally aspirated).
>
>Ok, to my understanding a "stock" type car has throttle bodies optimized
>for nice city driving (ie good power down low). Now if you increase this
>size heaps, the velocity of air in the throttle bodies is very low at low
>rpm because of the venturi? effect. But at high rpm's the car should have
>an advantage (along with other mods) of being able to suck in more
>air/fuel because of the greater tb size (thats what I was saying). I didnt
>put the 45mm's on, but I want to choke them down a bit to increase low rpm
>velocity and there fore low speed drivablity. Please correct my if I am
>wrong. If there is a formula to calculate the size, please say so (the car
>used to run carbies).

Jeremy,

I seem to remember seeing such a formula in the Haynes Weber carby manual.
Unfortunately I don't have it here in front of me, but will check tonight
at home and get back.  Of course, this information is for carby's, but
would still be applicable (right ??...............waits for the avalanche
of contradictory e-mail)

BTW, my experience is with Datsun L-series 6's and the general consensus
with these engines is that the 45's really only begin to work properly on a
race engine operating mostly high in the rev band.  If you want flexibility
- and for a street car this is more important (IMHO) - then 40's are the
way to go.

Just my 2c (sorry to be a little of topic)

Cheers

Stephen Lamb
Dept. of Defence DSTO, AMRL
506 Lorimer Street
Fishermans Bend  VIC  3207 Australia
Tel: +61 3 9626 7525
Fax: +61 3 9626 7089

IZCC #180