Trevor :
I think you're quibbling over terms here.
> It is not the intake vacuum that pulls fuel through
> the jet. The intake vacuum pulls air over the top of the
> jet, and the venturi effect pulls fuel in from the
> jet. (Bernouilli's principle)
But the Bernoulli venturi effect creates a depression, which is how it pulls
the fuel ...
> The intake manifold depression is created by the
> pistons and held between them and the throttle
> butterfly. The jet is on the OUTSIDE of this
> area, before the throttle. Other than the restriction
> caused by the air filter and air trunking, the
> air pressure at the jet is atmospheric.
You forgot the restriction of the venturi itself, which is hopefully much
larger than that of the filter and ducting.
> Therefore, the "vacuum" at the jet is only there
> because of the air flowing over it, not from any
> vacuum caused by the mechanical action of the engine
> itself.
But the air flow is caused by the vacuum caused by the mechanical action of
the engine ...
> Case in point, the intake manifold depression is
> highest at idle, and lowest at full throttle. Obviously
> the fuel flow is LOWEST at idle and highest at full
> power, which would not be the case if fuel was pulled
> by this vacuum.
But Dave carefully did not say he was talking about intake manifold
depression (aka manifold vacuum), since he wasn't. Since the depression
above the jet is basically constant, the fuel flow is governed by the needle
position, which is at it's lowest (smallest opening) at idle.
> Fuel flow is kept roughly proportional to air
> flow by the venturi, with two exceptions. The oil
> in the dashpot piston causes the venturi to stay
> smaller-than-usual during acceleration, which raises
> the venturi effect and increases fuel flow.
Actually, with the exception of the damper effect, the velocity through the
venturi stays essentially constant, and hence the depression that sucks fuel
stays relatively constant. Thus the name "Constant Depression".
> As well, the taper of the needle compensates for
> non-linearities in the venturi effect over different
> airflow rates by effectively increasing the jet size
> for higher airflow rates.
The needle taper is actually the main device for making fuel flow
proportional to air flow. A quick look at a needle chart bears this out,
the open jet area (for a TR3, which is the chart I have handy) goes from
.000157 square inches at station 1 (idle) to .003436 square inches at
station 12. Roughly a 20:1 change.
Randall
/// triumphs@autox.team.net mailing list
/// To unsubscribe send a plain text message to majordomo@autox.team.net
/// with nothing in it but
///
/// unsubscribe triumphs
///
/// or try http://www.team.net/cgi-bin/majorcool
|