- In the aftermarket tuning world, when it comes to load measurement, typically we're
relying on the input from a manifold absolute pressure or MAP sensor which reads the
pressure in the inlet manifold post throttle body.
That works really well and it gives us a nice reliable load signal for the vertical axis
of our fuel and our ignition tables.
And this works on the speed density principle where the ECU is using that manifold
absolute pressure signal to calculate the load or airflow into the engine rather than directly
measuring it.
On the other hand, in the OE tuning world, while speed density does exist, still by
far and away the most common option is to use a mass airflow sensor to directly measure
the amount of air flowing into the engine.
Now the speed density principle using manifold absolute pressure works pretty well in 99%
of circumstances.
Where it doesn't work that well though is with an engine that runs ITBs or individual
throttle bodies.
Now there's a couple of reasons for this.
First of all, with ITBs it's quite difficult to get a reliable vacuum source or manifold
pressure source.
We don't really have a plenum volume so to speak and if for example we took
the manifold pressure signal, directly off one inlet runner post throttle body,
what we're going to end up finding is that the manifold pressure signal is going to
oscillate or fluctuate wildly as the intake valves open and close on that particular
cylinder.
The single plenum and throttle body arrangement does a really good job of damping down the
individual pulsations with the valves opening and closing just due to the large volume of
air that's contained in the plenum.
The other option there of course is that we can tap into each of the inlet runners
and run these into a vacuum balance tube or something of that nature and then take
our MAP signal from there.
That does fix the problem of getting a consistent manifold pressure signal
but it still doesn't do a great job of replicating or giving us a load indication
like we have on a conventional engine.
And what this means is that while yes we can tune using manifold pressure as our
load signal, we're going to have some oddities in terms of controlling the
air/fuel ratio as we move through the full range of throttle movement from closed
through to wide open.
So what do we do here?
Well the option is to use what's referred to as alpha-N.
Simply put, this is using throttle position as the load axis for our fuel and our
ignition tables.
That works really well.
There are a couple of considerations we need to make if we want to use alpha-N
though and it's all about understanding the airflow through a throttle butterfly or
throttle plate.
Now if we could measure the airflow relative to throttle movement, what we're going
to find is that we see a large increase in the airflow over the first perhaps 5-15%
of throttle opening.
Once we get past maybe 50 or 60% throttle though, we tend to see that there
isn't actually much of a variation in the airflow for that last 20 or 30% of the
throttle opening.
And we need to keep this in mind because obviously this means that the relationship
between throttle position and airflow is really non linear.
And we need to apply this when we're choosing brake points for our load axis.
What I mean here is instead of just breaking the load axis down into 10% increments,
it's actually not really going to work well with throttle position.
We want to have really tightly focused break points down near closed throttle,
maybe 0, 1, 2, 4, 6, 8, 10 and then maybe 12.5, 15 then 20, right down near closed
throttle we've got tightly spaced break points.
Once we get a little bit higher in the throttle opening, maybe past about 20% we can move
to 10% increments and then once we get past 60 or 80% throttle, we might move up in
20% increments.
So it's just about matching those break points to the airflow.
So this works well on a naturally aspirated engine but what happens when we add
boost pressure into the mix here such as with Nissan's RB26?
This was one of the few engines out there that rolled off the showroom floor with
individual throttle bodies and turbochargers right from brand new.
Nissan's argument here was to improve throttle response and I'm not here to argue
whether or not that did work but it does give us some considerations when it comes
to aftermarket tuning.
First of all, Nissan got around this with the factory installation simply because
they ignored manifold pressure and instead used a pair of mass airflow meters.
So the mass airflow meters don't really care what's going on in terms of manifold
pressure or throttle position, they are just directly reporting on the mass of air
flowing through them and that ultimately is what the ECU needs to use.
In the aftermarket however, while it is possible to retain the mass airflow sensors,
they actually produce their own set of problems so it's much more common to
ditch the mass airflow sensors and instead go speed density.
Now of course we've just gone through the alpha-N tuning strategy that's common on
naturally aspirated engines but the problem when we add turbochargers into the mix
is that alpha-N on its own when we're only looking at throttle position isn't going to
work that well.
That's because when we immediately snap to wide open throttle at low RPM we may
have little to no positive boost pressure and the air/fuel ratio that we'll need at that
sort of load is very different to once the turbochargers have spooled up and we're
seeing 15 or maybe 20 psi of boost.
So we need now to have a way of varying our air/fuel ratio based on not only
throttle position but also on our manifold pressure.
The ECU that we've got in this R34 GTR is an Emtron KV8.
Let's jump into the software and we'll have a look at how Emtron deal with this.
While the ECU here is the Emtron KV8 and it might look a little bit different to other
forms of ECU, essentially the technique that we're looking at here is exactly the same.
So let's dive into the software and see broadly what we're dealing with.
So the table we've got here is our volumetric efficiency table so we can see it over here
represented numerically on the left hand side and then on the right hand side we've
got it represented graphically and we can pull this around to sort of get a better
view of the specifics of that table.
Now the first point to look at here, the vertical axis here, and it's called efficiency
calculation but essentially we can define what that efficiency calculation is coming
from in the Emtron software.
Here it is solely throttle position so we can see exactly what I was talking about here.
Down near closed throttle, up to let's say in this case 15% throttle we've got
quite tight zoning of those brake points, 0, 1, 2.5, 5% so that's what I mentioned there,
just dealing with the large inflow or increase in the airflow through those throttle
butterflies as we first crack the throttle from completely closed and right up the top
here we can see that we've got 60, 70, 80, so 10% increments, from 80 to 100% we
have jumped in one hit so a 20% jump there.
And what we can see is if we look at the numbers here, particularly down low at
our volumetric efficiency, we aren't really seeing a dramatic increase from 80%
throttle to 100% which really just explains why we can get away with that.
So that's our main table, now it is important to mention here, this ECU is working on the
volumetric efficiency or VE based tuning principle.
So this table isn't a fuel table if you like, it's actually telling us in simple terms,
or telling the ECU in simple terms how completely the engine is able to fill its
cylinders with air at any combination of RPM and throttle position.
Now as we mentioned though, it's not just the throttle position that's important.
We'll get into that in a moment but before we deal with that, let's just quickly dive
over and have a look at our ignition timing.
It's pretty easy because it's all dealt with relatively conventionally.
So we're looking at our main ignition table here and we can see that the load axis here
is just like any speed density system we're probably used to seeing.
We're just using manifold absolute pressure here and that's because with a turbocharged
engine in particular, we do need to vary our ignition timing based on not just engine
RPM but also manifold pressure so just using throttle position for this load axis wouldn't
cut it because the igntion timing we'd need at let's say 100% throttle, 4000 RPM and
10 psi of boost, is going to be very different to what we'd need at the same RPM and
throttle position but 20 psi of boost so we can see as we move from low load which
in this case is the top of the table, down towards the bottom of the table there,
where we're higher load, higher airflow, we can see that our timing numbers retard,
that's exactly what we'd see with any conventional turbocharged engine.
Alright so at this point we've looked at our main volumetric efficiency table, we've had
a quick look at our ignition table, let's get back to the fueling though because that's
the trickier part when we're looking at an ITB turbo setup.
So we need to vary our air/fuel ratio target based on our manifold pressure.
So that's the key here, how do we do that?
Well let's head back over to our fuel tables and we'll take a look.
What we want to do here is come over to our fuel drop down menu and this time we're
going to come down to our lambda target table and we'll look at lambda table one.
So this is the other key aspect with a VE based fuel model.
We've got our volumetric efficiency table which tells the ECU essentially how much air,
what mass of air is entering the engine and we also need another set of tables that tell
the ECU what our target air/fuel ratio is.
The way this works is that if the ECU knows what mass of air is entering the engine,
there'd be another set of parameters that also define what size injectors are fitted
or in other words, what mass of fuel will be delivered for a given pulse width
delivered to the injectors.
It becomes very easy for the ECU to then calculate what pulse width to deliver in
order to get to our target air/fuel ratio.
So the workflow when we're tuning any VE based ECU regardless whether it's a
single throttle and plenum or multi throttle turbo like this, is that we start by setting
our desired target air/fuel ratios.
Once we've done that, we then simply tune the volumetric efficiency table until the
ECU is delivering our target air/fuel ratios and if everything has been set up correctly
in the background, this means that our VE numbers should now be accurate.
We don't need to touch that VE table again unless we make a change that
will actually affect the airflow or volumetric efficiency of the engine.
So here if we then wanted to make a change to our target air/fuel ratios,
we do it directly in this table so let's take a look at this table.
With our target lambda table target air/fuel ratio table, depending what units
you want to work in, the vertical axis we've got here is manifold absolute pressure so
now we're encompassing our manifold pressure so we can now vary our target air/fuel ratio
based on our boost pressure so that's the key point.
Obviously we've got RPM as the horizontal axis on this table and simply put, we enter our
desired air/fuel ratio targets in here, pretty simple really.
We can see here for our cruise areas in particular, we've got our target lambda set
to lambda one, stoichiometric air/fuel ratio 14.7:1 there for pump fuel.
Up in the higher RPM, higher load regions I should say, we're going to be targeting
250 kPa or thereabouts, around about 22 psi.
We can see that we're targeting around about 0.81, 0.80 down in the lower RPM regions and
when we're right up in the high RPM, high load, we're at 0.78 lambda so we just set these
to what we want.
So the way this works as well is we do have a background calculation that the
ECU is doing and this works on the principle that if we double the manifold pressure,
then to maintain a consistent air/fuel ratio, we also need to double the amount of fuel
supplied and this background calculation works in the background with a lot of aftermarket
ECUs, most of the time we don't even know it's there and it's there because this way the
main fuel table has better resolution, we don't need to account for manifold pressure
changes in our fuel table.
Instead we are only really accounting for VE changes.
So that's happening here as well.
Now there is one problem with this system, it works really well up to a point but there is
one problem with this in that once we really start pushing the turbochargers harder,
we're going to find that our air/fuel ratio will start to taper a little bit richer than
our target.
This is because at high boost, high RPM, the turbos will start to become a choke point.
The exhaust manifold back pressure's going to start going through he roof,
we also may be finding that we're operating outside of the peak efficiency of the compressor.
So this principle here really relies on the fact that once we're at 100% throttle that the
volumetric efficiency doesn't change regardless whether we're at 0 psi or 25 psi
and obviously that's simply not going to be accurate.
So the tuning method works really well, how we do this is we start with our minimum
wastegate spring pressure, let's head back over to our main VE table and what we're going
to do is tune this VE table on the dyno right up to 100% throttle.
So of course once we're at 100% throttle we're operating right up here in this top
100% throttle row of our VE table.
So we do this at our minimum wastegate spring pressure and I've heard people talk
about tuning these engines by removing the wastegates and wiring them open, we don't
need to do anything like that and in fact that'd be counter productive, it's actually
going to give us inaccurate results anyway so we just set the wastegate to their minimum
boost, we tune the 100% row in our VE table and then what we're going to find is
once we start increasing the boost, at least initially, the air/fuel ratio simply and almost
as if like magic, tracks our target.
Of course there is no magic here, it's just the physics of what's happening behind the scenes.
so that will work really nicely as we start increasing the boost.
In the case of our R34, wastegate spring pressure around about 10 psi, that worked
really nicely.
Once we get up to around about 1 bar, 15 psi of boost we start to see that at
higher RPM, the air/fuel ratio tapers a little bit richer just like I said.
It gets worse the higher the boost pressure and the harder we are pushing the turbo.
So how do we deal with this?
Of course if we dealt with this in the volumetric efficiency table, that's going
to then undo all of our work and if we drop back down to lower boost pressure,
the air/fuel ratio's going to be lean so we don't want to do that.
Here we're going to use a bit of a fudge table or a bandaid here which is
a compensation table called our secondary load table.
So let's take a look at how that works.
With our secondary load table here, this is a table that is set up with our manifold
pressure on the vertical axis, RPM on the horizontal axis and we can see that we've
got some numbers in here that are just trim values to our final fuelling and we
can see that those only really start around 200 kPa or 220 kPa actually in this case,
around about the 18 psi mark.
And as we move up there, particularly once we're up around 250 kPa, 22 psi,
we can see that we're pulling around about 3.5% fuel.
So this is how we're going to address the area once we start to find that the turbo's
become a restriction.
As long as we know how to apply this tuning strategy, it really is faultless and once
we've dialled it in we're going to find that the engine is incredibly responsive and
irrespective of our throttle position and our boost pressure, the air/fuel ratio is simply
going to track our target rock solid and is really nice and stable and gives great
drivability.
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