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Intake Tuning Guide for Multiple Throttles

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:

 

Intake length calculator:

Matching an intake to a 20V engine:

 

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.