VFAD the myth

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VFAD the myth

Post by Lazar » Thu Jun 04, 2015 4:04 am

Firstly apologies if this has been covered before I couldnt find anything definative on the matter.

Ive almost bought a racing beat revi intake and duct a few times now when I get new parts withdraw symptoms and had to stop myself by convincing myself for £500 its not worth it.

What put me off originally was after doing some research into RX8 intakes (which theres no shortage of threads on) most of what I found was people stating that the VFAD Variable Fresh Air Duct intake system fitted stock to the 231 engine 8's was just a system fitted to limit intake noise at low rpm.
With no end of people bunging the vacuum pipe to disable it or removing it all together and fitting a CAI. The general consensus seemed to be that all it did was restrict air flow at low rpm by only having the longer intake horn open to limit intake noise for day to day driving until you got higher in the revs to open the secondary intake a thus double the potential air available.

After reading all this i didnt see a down side to going straight out and buying the revi intake and duct, until i found another article on the VFAD system that sounded a lot more plausible but ive not been able to find the article since.

This said that the VFAD system is a tuned variable length intake, and the longer intake horn at low revs increased low down torque and the secondary intake opens at higher revs to deal with the increased air demands of the engine at high revs.
And the system is also design to work in conjunction with the variable intake manifold.
I wont go into the science of it but this sounds a lot more likely than, the system is just there to limit a bit of intake noise.

But then again i dont know for sure, maybe it is just to limit noise.

Just looking to see if anyones got definative information on the subject.
Last edited by Lazar on Tue Nov 24, 2015 10:21 pm, edited 2 times in total.

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Re: VFAD the myth

Post by SteveD » Thu Jun 04, 2015 5:26 am

So why did they design the Mazdaspeed one then?
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Re: VFAD the myth

Post by Will66 » Thu Jun 04, 2015 6:58 am

The VFAD is mostly about noise rather than performance.
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Re: VFAD the myth

Post by Phil Bate » Thu Jun 04, 2015 9:25 am

Having spent many hours testing and data logging I can quite confidently agree with the above. The tract before the VFAD valve does a lot to reduce intake noise, particularly at cruising speeds (3-4K) where the deep tone can resonate with the exhaust noise and significantly increase droning.

I could find no change in mass air flow below 5250RPM with the VFAD disabled, however throttle response in that range appears ever so slightly snappier.
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Re: VFAD the myth

Post by Elv73 » Thu Jun 04, 2015 11:39 am

Ive had mine disabled for a few months now and to be honest cant notice any difference in either throttle response or noise from when it was working.
My main reason for removing was so I could remove the vacuum chamber behind the airbox so I had room to mount my catch can :thumright:

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Re: VFAD the myth

Post by warpc0il » Thu Jun 04, 2015 12:31 pm

I have a copy of a Mazda technical document that states that the intake hose ahead of the airbox is indeed a tuned length/volume designed to prevent the reversal of airflow under conditions where the throttle valve is suddenly closed, at low/medium engine speeds, in conjunction with the resonance chamber.

On the 4-port engine the tube is longer and thinner compared to the shorter wider tube on the 6-port; though both are about the same volume.

Even the short and wide tube on the 6-port proved to be a limiting factor on the 6-port , at higher revs and load demands, hence the VFAD to bypass the tube in these conditions.

In the artificial environment of a dyno-test, the revs and load can be applied at a greater rate than on the road (due to reduction in inertia and lack of air resistance) this is why test graphs usually show a dip just before the VFAD opens.

On the road the VFAD should open in time with the demand, without any dip in power, though there is hysteresis (backlash) in the algorithm to prevent hunting if the engine was held at or near the trigger point.

I have come across two cars where the VFAD was "sticky" and sometimes didn't open at all or only opened after a longer (variable) delay. In one case it was the valve itself and on the other it was the operating solenoid.

The addition of the intake tubes has the added advantage, from a type approval perspective, of reducing intake noise, and this is further enhanced by the use of non-resonating materials.

Based on all the above, reversing the one-way vacuum valve (or any other any means) to keep the VFAD open should have the following impact;
- an increase in intake noise below the engine revs/load at which the valve would normally open.
- a perceived increase in power, as most equate engine noise with power.
- a reproducible smoothing of the dip in power shown on dyno test results, which would seem to "prove" that power has been increased.
- no impact on high rev/load power delivery in real world conditions, unless the system was previously faulty.
- a slight hesitation in on/off power delivery at low revs, under conditions that can cause reversal of airbox airflow. This may be difficult to reproduce and the kick after such hesitation may even be perceived as an increase in power; smooth power delivery is less impressible than a step change; such as a turbo kicking-in.

Replacing the VFAD and airbox with a well-engineered aftermarket system could add a small percentage to the power above mid-revs and slightly increase maximum power, to the detriment of power delivery at low revs. Note that both the increase in noise and the less smooth delivery would both give the impression of an increase in power at low revs.

Replacing the VFAD and hose with a RB Revi Ram intake (standard airbox or RB intake) also increases the intake noise but only very slightly.

The ram air effect should prevent airflow reversal, as long as there is forward airspeed, it also provides a positive pressure within the airbox which significantly reduces pumping losses and therefore increases available power and reduces fuel consumption. This is a free implementation of light-weight forced induction.

Vehicle manufacturers usually avoid using ram air effect because it introduces variables that might not be acceptable to its customer base, mainly related to impacts of reduced pressure under different road conditions and the complexity of reproducing the forward airspeed when testing on a static rolling road. Current type approval testing, including power output and fuel consumption are all performed under static rolling road conditions with only sufficient airflow to provide engine cooling.

The impact of loss of forward airspeed can be theoretically anticipated and demonstrated under real world driving conditions, for example;
- in a headwind there is a further increase in available power, especially felt at low road speeds, but this becomes undermined at higher air (wind/road) speed as the air resistance increases by the square of the airspeed.
- in a tailwind the engine is slightly down on power (no more than it would be as standard) until the road speed becomes greater than the wind. However the tailwind also reduces air resistance, so the overall impact is still positive.
- in crosswinds the power delivery can be slightly variable, especially when passing larger vehicles, where eddies can cause local variations in forward airspeed.
- when approaching the rear of a (large) vehicle and picking-up the slipstream effect, the drop in local air pressure reduces the FI effect, just as your car is pulled along by the slipstream so less power is required to maintain the road speed.

All of the above effects are quite subtle and can be masked by all the other variables such as inclines etc.

While I said most manufactures avoid using ram air for a free increase in power, that's not across the board for motorcycles, where some sports bikes used ram air to great effect.

However, even this isn't common these days as bikes also have to conform with emissions regulations and the testing is all done on static rolling roads, where the ram benefits are totally lost.

In conclusion you are all (nearly) totally correct, it's just that nothing is ever quite that simple...
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Re: VFAD the myth

Post by kopite72 » Thu Jun 04, 2015 12:51 pm

Now that is how you explain something!! :shock: =D>

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Re: VFAD the myth

Post by Aido 8 » Thu Jun 04, 2015 1:10 pm

The man is a class act =D> . Posts like this makes this forum what it is, awesome :thumleft:
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