- Individual throttle bodies have long been a popular tuning modification, particularly
for naturally aspirated engines.
Compared to a single plenum and throttle body arrangement which we
see most common in a production engine, moving to a set of well developed and
designed individual throttle bodies does give the potential for some tuning advantages.
First of all, it's very easy with individual throttle bodies to swap to different inlet
trumpet lengths and this can provide a tuning effect.
Essentially what we're doing here, or looking to do is to optimise the
tuning effect of the pulsations that occur in the inlet track between the end of the
trumpet and the inlet valve.
And by adjusting the length of those trumpets we can affect this so that we get a ram effect
at a certain RPM and this can essentially help improve the volumetric efficiency of the
engine.
When it comes to this tuning effect though, we do need to consider that what we gain
in one RPM range, we'll generally lose in another.
While there's no strict numbers here, and we need to find on the dyno what works,
generally a shorter inlet trumpet is going to favour high RPM performance while a longer
inlet trumpet is going to favour low RPM performance.
The other tuning tool with individual throttle bodies is the actual throttle body diameter.
And here it's not a case of bigger is always better.
While yes a bigger throttle body may gain airflow, what we also need to consider is
the air velocity.
And particularly if we go too big on the throttle bodies, this will kill air velocity
and this can really hamper low RPM performance so on this note, just switching
to a set of individual throttle bodies that haven't been well thought out and
developed with a specific engine in mind, can actually easily end up with you going
backwards in terms of engine performance.
Fitting individual throttle bodies also throws a little bit of a spanner in the works when
it comes to the tuning process.
With most aftermarket standalone ECUs we tune on what's referred to as the speed
density principle where the main load axis for our fuel and ignition tables is manifold
pressure.
It's really easy with a single throttle and plenum chamber to get a nice consistent
manifold pressure signal straight from the plenum chamber.
That's a little bit trickier with individual throttle bodies.
While yes, we can do this by taking a feed off each of the individual runners
and running this to a vacuum balance bar and taking our MAP signal off there,
even when we do this, we find that the MAP signal is no longer a really good indicator
of engine load so instead we favour using what's referred to as Alpha-N.
Simply put, this uses throttle position as the main load axis for the fuel and
the ignition tables.
And as long as we understand this and we use throttle position, the tuning is
absolutely seamless, no different to tuning using manifold pressure.
One other little trick when we are using Alpha-N though is understanding the
relationship between airflow through the throttle bodies and throttle position.
This is very non linear and what I mean by this is that we see quite a large increase
in airflow from a closed throttle for just the first few percent of throttle opening.
Once we get up to 60 or 70% throttle opening, we don't tend to see much in the
way of an airflow increase over the last 20 or 30% throttle opening.
So we can factor this in with the break points that we choose for our throttle
position table and we want quite a lot of tightly spaced break points down from
closed throttle so we might choose zero, two, four, six, eight, 10%, up to that first
10% which is really critical, then we can start spreading things out.
Once we get up to 80% we might move in 20% increments because
again the airflow change for our throttle position increase just simply isn't so great.
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