Once one of our dealers was tuning what’s
commonly called a semi peripheral 13B engine. For those not
familiar with this porting arrangement, this is a standard 13B
2-rotor engine, where the two side intake ports are retained,
but an additional peripheral intake port is added. So each rotor
has 3 intake ports.
If
you’ve looked at a standard FD intake manifold, you’d have seen
that each of the side inlet ports has its own runner, so there
are 4 runners in total. On a semi peripheral intake manifold,
the peripheral port gets its air from the original runner for
the secondary port.
This
engine had a Borg Warner turbocharger so it was making about 5
PSI of boost around 2000 RPM, even at about 30% throttle. The
engine could be tuned correctly for this condition, but then
when the throttle was opened to 100%, it was lean, and needed
about another 20% more fuel.
Why
would this be? If the pressure in the inlet manifold is the
same, why would the engine need different amounts of fuel? This
doesn’t usually happen. Originally I thought there might be
something funny with the car, or there was fuel pressure
pulsation or something strange.
But
within a few minutes of trying it on the dyno myself and looking
at the engine, I worked it out. The FD has staged throttles; the
first 30% or so of travel only opens the primary throttle, which
provides air to the primary ports only. The remainder of the
travel opens up the secondary throttles, which provide air to
the secondary ports. Therefore, the port timing depends on
throttle position.
And
we all know what that means, when you have a turbocharged engine
where the VE changes with TPS independently from boost, it means
the engine has to be tuned by MAP x TPS. (a) Go
to
Tuning Fuel. (b) and (c) Click on Fuel Map 1 Settings. (d)
Set the operator to MAPxTPS (e) You will notice Fuel map for
MAP will appear together with Fuel map for TPS.
Now,
there are a few ways you can do this. The way I chose to do it
was to tune the MAP based on the primary throttle, and the TPS
map tells the ECU how much better the engine breathes as the
secondary throttles are opened.
The
way to do this is to lock the secondary throttles so that they
don’t open. This can either be done by manually closing the
second throttles which are normally under ECU control, or
removing the linkage that opens the second throttles. Then set
the TPS map to 100% everywhere, and tune it up on MAP as per
usual, up to wastegate boost pressure.
Then
re-enable the secondary throttles, and then adjust the TPS map
to get the correct mixtures, again up to wastegate pressure.
Then as you increase the boost, make corrections on the pressure
based map.
This
tuned up excellently on several of these that I’ve helped tune.
Depending on the exact porting though, it might be possible that
drivability is not going to be great a certain low RPM / high
throttle openings, due to intake reversion. In this case, the
options are to either drive around it, or better is to retain
the ECU control over the secondary throttles, using a vacuum
tank, check valve and a solenoid valve as on the factory system.
Then when the solenoid is activated and vacuum is applied to the
diaphragm, the secondary throttles close. So the ECU can be
configured to hold this output low when the RPM is below say
3000 or wherever the problem area finishes.
We
would like to thank these people; Ric Shaw, Aaron
Parker and Alex Rodriguez for giving us visuals on some
parts of their car.
Thank
you and happy learning!