For people with multiple
throttles the intake system length is somewhat easily changed in length
via the trumpets/air-box, this opens up the options of tuning the
intake to suit the engine and driver. While this can also be done on a
single throttle engine it is considerably harder and this guide is
written with the focus on tuning multiple throttle engines only,
however it can be applied to both.
Basic principles of harmonic resonance in the intake:
When the intake valve of an engine opens a low pressure wave travels up
the intake until it hits the end of the trumpet/plenum entry and this
then reflects a low pressure wave back down the pipe. During which time
the engine has gone though its combustion. If the pressure pulse has
taken the right amount of time it will hit the intake valve just as it
is opening sending a positive pressure wave into the intake of the
engine. This however will only occur at its greatest at one point in
the rev range (variable cam timing exempted). If an engine is at the
right rev range for this to happen it is having a harmonic resonance in
the intake. This means that in theory the power compared to standard
will slowly pick up through the rev range till it reaches the tuned
point and then diminish. Also note that the tuned rpm point of the
intake is not necessarily where the maximum power is, for example the
intake can be tuned for 4000RPM but the engine could still make maximum
power at 7000RPM.
Adjusting the intake length to suit:
To adjust the rev range at which the resonance will happen the length
of the intake can be changed, for owners of 4a engines with quad
throttles this is relatively easy. Basic theory is that the longer the
trumpet the longer it will take the pulse to reach the end and come
back, this will work for a lower rev range because the engine at lower
revs will take longer to go through its 4 strokes and hence the time
delay needs to be longer to get the right resonant frequency. However
this is only a rough rule as despite the theory sometimes engines act
in a way that is unexpected and 4age engines are a good example of
this. One report is that 100mm length trumpets on a 20V engine gave
overall good power and the best top end despite the shorter lengths
theoretically optimising the top end power better.
One possible option is to have an easily altered setup using on the end
of the throttle bodies is the flange with a short section of pipe
welded on it, a set of trumpets with no flange at all and a selection
of rubber hoses. By bolting the flange on, clamping the rubber hose to
the short section and then clamping it to the trumpet it is possible to
change the length by chopping down the rubber pipe. While highly
un-recommended for anything more than basic testing it is ideal for
tuning work on a dyno or on the move on the road.
Determining the theoretical ideal length:
It is possible to work out the length at which the intake will
theoretically resonate at, to work this out the angle at which the
intake valve closes and the length of the inlet tract is needed. An
excellent guide on working this out is available at the website of
“Mr Acoustic”, it is located here:
However as mentioned previously the ideal theoretical length may be quite different than the real world ideal length.
Tuning with variable valve timing (VVT):
The 4age 20V is fitted with a very simple form of variable valve timing
that allows for two different inlet cam timing positions depending on
the RPM. This alters when the valve closes and therefore there is two
points in the rev range where the intake resonates.
Integral adjustable lengths:
Originally pioneered by formula one technicians, a variable length set
of intakes that worked in proportion to revs were fitted to their
engines. This system worked by having the trumpets slide into the
runner to change the effective length. Car manufactures over time also
incorporated variable systems that were/are primitive compared to the
formula one technology but was still quite effective. Most use/used
butterflies to select a shorter or longer intake path for the air to
travel down. Although there are no known reports, it is theoretically
possible for a home builder to make a custom manifold to incorporate
such a system. For this a system could be adapted off an engine that
has it fitted as original equipment. A rev based output on an EMS or a
simple rev based switch could be used to easily control the changeover
point.
Harmonic resonance tuning and forced induction:
The 4age 20V is fitted with a very simple form of variable valve
timing that allows for two different inlet cam timing positions
depending on the RPM. This alters when the valve closes and therefore
there is two points in the rev range where the intake resonates.
Tuning with variable valve timing (VVT):
The 4age 20V is fitted with a very simple form of variable valve
timing that allows for two different inlet cam timing positions
depending on the RPM. This alters when the valve closes and therefore
there is two points in the rev range where the intake resonates.
Removing VVT:
Having VVT in place means that the intake cam is not readily adjustable
via a system like those on adjustable cam gears. The VVT pulley can be
removed by using something along the lines of Toda's "VVT canceling
gear" which is an adjustable cam gear that suits the shape of the 20v
cam but is just a normal adjustable cam gear. The second option is a
much simpler and cheaper one; some aftermarket cam gears are shaped
such that they suit a normal 16V intake gear and hence any readily
available/home made adjustable gear can be used.
These
two gears are made by Toda Japan for the 20V engine. It can be seen in
the middle of the bottom gear how the center has been made so its
raised to suit the shape of the 20V camshaft.
Aftermarket ECU control:
How the VVT is controlled is determined by the options of the ECU in
its auxiliary controls. If the aftermarket ECU being used has the
option of turning an output on at a set revs early in the rev
range and then off again at a higher revs then a single relay can be
used. The relay would ground the
second side of the VVT solenoid and would be triggered directly.
For an ECU that can only switch the output once at a certain set revs
then two outputs are needed. Each of the outputs would turn on a relay
each, where the first relay would activate the VVT solenoid and the
second cut the power to the first.
Aftermarket cams and VVT:
Beyond a certain duration/lift of the intake cam the VVT cant be used
as it either will cause the intake valves to hit the pistons or retard
the valve timing so that it is of no benefit. The exact figures that
are the safe limit are open to some debate but 272 degrees (measured at
zero lift) with 9.5mm of lift is a proven safe and worthwhile option.
There is also one report of the use of a 280 degree duration camshaft
but only with 8mm of lift. So therefore is the cam profile is too
aggressive then the tuner would be forced to not run VVT. Please see
the article '20v camshaft choices' for further information on how to
make the most of VVT with various cam choices.
Tuning:
To determine the right points to turn the VVT solenoid on and off two
dyno runs are needed; one with the solenoid locked on and vice-versa.
The with both power charts overlapped on top of each other the highest
peaks are chosen of each of the two curves. As a guide the VVT will be
turned on early in the rev range off at approximately 6000RPM. These
figures are almost always different between engines due to their
different combination of parts.
