Last week in Airflow Meter Bypass, Part 1 we introduced the idea of using an airflow meter
bypass to lift the flow and measurement capabilities of a normal airflow meter.
In short, by using a bypass around the airflow meter and the recalibrating the
meter’s output with the Silicon Chip
Digital Fuel Adjuster, we’re able to reduce the pressure drop through the intake
system and also increase the voltage headroom before the meter maxes out.
This week we’ll take a look at the installation process on a guinea pig
Nissan Maxima Turbo.
The Nissan
The 1988 Nissan Maxima runs the VG20ET turbo engine. In previous stories we've
have covered the fitting of an intercooler, intercooler water spray, an electronic blow-off
valve, a new exhaust and a new modified filter box. (For more on these stories,
do a site search under ‘Maxima’.)
The airbox mod involved cutting away the lid so that air could reach the
filter across its full area. A thick foam rubber strip then sealed this assembly
to a bonnet opening so that only cold air could be inhaled. Making this simple
modification dropped the total measured intake system peak load pressure drop
from 30 inches of water to 20 inches of water. (Using a sensitive pressure
gauge, the total system pressure drop was measured just prior to the turbo
intake. With pressure drop measurements, the lower the numbers, the less
restriction there is to flow.)
So why still the 20 inches of pressure drop? Following the airbox is a hotwire airflow meter that in turn connects to a large
resonant chamber (top-left in this pic). From the chamber a long duct connects
to the turbo intake. The pressure drop still being recorded comprises the flow
losses through the airflow meter, through the resonant chamber, and along the
intake pipe from the chamber to the turbo. (The pressure drop through the flat
panel filter? Not measurable – the pressure drop with the filter in place or
removed remained just the same, as is so often the case.)
Given that the resonant chamber is nicely built inside (the air has to do a
U-turn but the exit duct is equipped with a bellmouth and the insides just look good), and that the pipe between
this chamber and the turbo has no sharp bends, it seemed likely to me that
plenty of the restriction was in the airflow meter.
At this stage I had never actually removed the airflow meter (it’s fairly
hard to get to) but since the engine power in standard form is only about 110kW,
I figured that it was probably a small one. I sourced a new airflow meter (from
an Australian VL Commodore, a car equipped with a 3-litre Nissan six cylinder
engine and up to 150kW of engine power) and then used a workshop manual to work
out which pin on the new airflow meter did what. (As mentioned last week, even
for an airflow meter from a known car, this often isn’t as simple as it first
appears.)
I then pulled out the Maxima’s airflow meter – only to find that it was the
same diameter as the one I’d bought to replace it! And furthermore, the
replacement airflow meter looked as if it may well flow less, as it has many
alloy heatsink fins projecting into the airstream.
Time for a rethink. Getting hold of a larger hotwire meter was out of the
question – the budget would have been blown out of the water and plus, I
couldn’t afford the time to go from wrecker to wrecker trying to find a larger
meter for which I could get the pin-outs.
That’s when the bypass approach was adopted.
The Layout
The most time-consuming part of the mechanical changes was working out
exactly how to do it. With tight space to work with, and with a typical home
do-it-yourselfer’s dislike of getting stuff welded (it always takes so long when
you’ve gotta tape metal plumbing together and then take it somewhere to be
welded – only to find that some of the parts moved in transit and so nothing now
lines up!), it was a juggling act to get the right financial and flow
outcomes.
My first approach was to use two off-the-shelf alloy boxes to form manifolds
at each end of the airflow meter. As shown here, a new pod-type airflow meter
would be bolted to one of the boxes, which would transfer air to both the
airflow meter inlet and also the bypass inlet. On the other side of the airflow
meter another alloy box would be used to join the flows so that this combined
air could be sent on to the engine. The use of readily available flanges would
allow easy connection of the pod filter to the box and the engine to the
assembly.
However, the extra length that the two boxes gave to the assembly meant that
it was difficult to fit it all under the bonnet. Time for a rethink. Looking at
the standard underbonnet architecture I realised that I may be able to attach
another flange to the metal resonant chamber into which the standard airflow
meter directs air. I’d need two pod filters – one for the airflow meter and
another for the bypass. The advantages of taking this approach was that no
welding would be needed, two filters would flow better than one – and most
importantly, it looked as if everything could be fitted in!
I bought two oiled cotton pod filters (AUD$25 each on special) and two
~75mm plastic flanges (AUD$20 each). I then used a
holesaw to cut a hole in a relatively flat wall of the box, then doing a little
panel beating to give a surface that the flange could bolt up against.
To make sure that the seal was good, I cut a gasket out of sheet rubber and
sandwiched this between the flange and the box (yellow arrow). The bolts holding
the flange in place were inserted from within the box and then doubled nuts
(tightened against one) another were used to lower the likelihood of a bolt
coming loose and then floating through the turbo compressor. These nuts are
shown by the red arrow.
With the box back in place, the approach can be more clearly seen. The top
opening is the new one created for the bypass while the lower opening is the
original that connects to the airflow meter.
Some short lengths of thick-walled plastic pipe were then used to join the
flanges to the pod filters.
Tuning
As you would expect with such a large bypass, the car wouldn’t start or run
without the Digital Fuel Adjuster in place. The DFA was wired-in at the ECU,
with the wire coming from the airflow meter connecting to the ‘in’ terminal and
the wire from the ECU connecting to the ‘out’ terminal. With power and earth
supplied, the electrical installation of the DFA was then finished! At this
stage the oxygen sensor was also disconnected so the tuning wasn’t confused by
ECU’s learning behaviour. The DFA was also set so that it always intercepts, ie
the car doesn’t have to first start before it starts work.
Cranking of the engine showed on the DFA hand controller the load sites being
outputted by the airflow meter. I increased the correction at these and all
surrounding load sites - but I initially increased these outputs by far too
much. I had thought that a massive correction would be needed at all loads but
as I subsequently found out, only very small corrections were needed near idle.
In fact, with the DFA set to its normal coarse mode of operation, a +6
correction was all that was needed to have the car idling happily. (I’d started
at +60!)
(For those who are interested in more numbers, measurement showed that the
ECU needed a 2.8V signal from the ECU to run properly at idle, and the actual
bypassed airflow meter output was 2.4V. The +6 correction brought the airflow
meter voltage up from 2.4 to 2.8V.)
With the car idling happily, I plugged in the same +6 correction at higher
load sites – that is, I worked ahead as much as possible, putting in figures
based on the corrections being used at the lower value load sites. At this stage
I also attached a MoTeC air/fuel ratio meter to the exhaust so that I could see
exactly what I was doing.
(As we have covered in another story - Real World Air/Fuel Ratio Tuning - much tuning can be carried out using just a multimeter and a normal
narrow-band probe – and certainly getting the car running at idle and light
loads would be no problem at all using just this type of sensor. However, for
full-load tuning, an accurate air/fuel ratio meter must be used. Tuning the DFA
is so easy that even 20 minutes on a dyno will be enough to finish the tuning if
the light loads have already been set.)
At this stage the DFA map looked something like this – the car idled at load
site #30 but I put in the correction for Load Sites 25-29 for better starting
and idling behaviour.
|
Load Site |
Adjustment |
|
25 |
5 |
|
26 |
5 |
|
27 |
5 |
|
28 |
5 |
|
29 |
6 |
|
30 |
6 |
|
31 |
6 |
|
32 |
7 |
With this much tuning done we decided to hit the road. My driveway is very
steep and it took us three goes to get up it – the air/fuel ratio was varying
between 10:1 and 18:1! However, some more load site adjustment of sites between
33 and 38 (shown below) gave good light load driveability. At these loads I was
aiming at an air/fuel ratio of around the mid-fourteens (eg 14.3 – 14.9:1), the
near-stoichiometric ratio used for best emissions performance.
|
Load Site |
Adjustment |
|
33 |
7 |
|
34 |
7 |
|
35 |
7 |
|
36 |
8 |
|
37 |
9 |
|
38 |
9 |
The next loads involved going lightly into boost, where I set the DFA to give
air/fuel ratios in the mid-thirteens. Note how the amount of correction needed
to the airflow meter output is increasing with load as more and more intake air
takes the bypass route (table below).
|
Load Site |
Adjustment |
|
39 |
9 |
|
40 |
9 |
|
41 |
10 |
|
42 |
10 |
|
43 |
10 |
|
44 |
11 |
Above Load Site 45 (see below) the engine was on substantial boost, with the
peak load site being 58. (Always put in numbers above the max load site in case
the engine is in a situation where it develops more power than when being tuned
– eg on a very cold day.) At these loads I set the DFA to provide an air/fuel
ratio going into the mid-twelves and then progressing into the high-elevens at
absolute peak load.
|
Load Site |
Adjustment |
|
45 |
12 |
|
46 |
12 |
|
47 |
12 |
|
48 |
12 |
|
49 |
13 |
|
50 |
15 |
|
51 |
15 |
|
52 |
15 |
|
53 |
16 |
|
54 |
16 |
|
55 |
16 |
|
56 |
16 |
|
57 |
16 |
|
58 |
16 |
|
59 |
16 |
|
60 |
16 |
|
61 |
16 |
|
62 |
16 |
Note that during this tuning process the DFA was set in its coarse mode of
adjustment, that is, each up/down increment moves the output voltage much
further than if fine mode is selected. This was done to allow speedy tuning, and
in fact the accuracy with which the air/fuel ratio is maintained – and so the
excellent driveability – means that the DFA can be left set in this mode.
Testing
The bypass and DFA were fitted in order to reduce intake flow restriction –
so how well did it all work?
Testing needed to take into account that there have been two major
changes here – the replacement of the stock airfilter box with the two pod
filters, and the use of the bypass. To (mostly) separate the effects of these
two changes, two tests were undertaken.
Firstly, the bypass was completely blocked and the DFA tune returned to
normal. (Just removing power to the DFA does this – it’s then bypassed.) This
test was designed to show the gains made by the removal of the airbox and the
fitting of the pod filter to the end of the airflow meter. However, full load
testing showed that these changes had made no difference at all to the intake
system flow restriction! It still remained at a measured 20 inches of
water.
The bypass was then re-opened, the DFA switched back on and the testing done
again. This time, the total intake system restriction had dropped to just 10
inches of water – the use of the airflow meter bypass and extra pod filter had
halved the total intake restriction! Furthermore, the maximum output voltage of
the airflow meter is now well down from its ceiling voltage – this airflow meter
is now probably a 350hp design...
Remember that the total measured restriction includes that caused by the
U-turn the air needs to take through the resonant box and the flow down the
resonant box to turbo feed pipe. It’s likely that the restriction of just the
combined filters, bypass and airflow meter is now only 2-3 inches of water.
Conclusion
The use of a bypass in conjunction with the Silicon Chip Digital Fuel Adjuster is a
cheap and very effective way of upgrading the airflow meter capability and at
the same time giving control over air/fuel ratios. In this case I chose to use a
very large bypass but a smaller one could instead be used, which would allow the
airflow meter to work across a broader proportion of its original range.
Either way, we can’t think of any reason why you’d now want to swap an
airflow meter for a large unit - not when it’s this easy to improve the power
capability of the one you’ve already got...
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