# TBI question...

• posted on December 20, 2006, 6:02 pm
Do the injectors on the throttle body fire simultaneously as one unit or separately? Seems like an awful lot of work for each to fire 4 times for each rotation of the crank.
~jp
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 20, 2006, 6:17 pm

Since you didn't bother to mention what vehicle you're asking about, the older TBI is only timed while cranking. Once the engine starts they just sort of free run.
Al
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 20, 2006, 6:37 pm
Well, I don't have a TBI-equipped vehicle...so sorry about that.
I mean...like say the TBI on a 1991 V6... take your pick on the model.
The whole "free run" thing doesn't make sense to me, as I would imagine the injectors are either in a state of open or closed, not halfway as the metering rods on a carb would be.
I mean, it obviously has a way of metering the fuel and maintaining the A/F ratio via input from the O2 sensor. So I'd think it would do that through varying the length of the pulses? No??
~jp
Big Al wrote:

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 20, 2006, 7:13 pm
Thottle body injection runs on a pulse width system, meaning they calculate how long the fuel should spray per a given time, say 1 second. So if the computer calculates that you're running at 10% of the maxiumum horsepower, it will spray the injectors for 1/10th of a second, every second. Since the fuel and air are traveling down the intake manifold they don't need to be timed with the intake valves, the trip thru the manifold takes longer than the opening and closing event.
That being said, most port injection systems fire in a left bank right bank sequence. Most sequential injection systems only do so at lower rpms. After a certain speed the injector takes longer to spray fuel than the valve is open. But at higher speeds the momentum of the air traveling thru the manifold keeps the fuel in suspension until the valve opens again.
Tinker

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 20, 2006, 9:38 pm
Ok... that makes sense. Is this firing of a left or right bank what I've heard referred to as "batch firing"?
~jp
Zopilote wrote:

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 20, 2006, 9:43 pm
That is another name for it.
Tinker

calculate
horsepower,
the
be
than
bank
After
is
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on December 21, 2006, 12:00 am
Batch firing is normally a term used with port injection. Like on a 89 5.0 Mustang, the individual injectors are fired one at a time in the firing order. SEFI, sequential electronic fuel injection. The 89 5.0 T-Bird fires 4 injectors at a time. That's called batch firing.
Al