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Old 10th May 2012, 18:54   #1
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Default Miata MX-5 VAF problem solved

We had a peculiar requirement recently.

There was this Mazda Miata 1.6 which came withh a Vane type Airflow meter (also called as the Variable Air Flow meter or VAF). This type of unit has a flap in the path of the airflow and generates a voltage depending on how much open it is. This is used by the ECU to adjust fuelling. The VAF unit also incorporates a sensor for intake air temperature and also a switch to operate the fuel pump when there is airflow.

The unit had the following problems:

1. The unit had worn out and was giving intermittent problems.
2. Since the vane was in the path of the airflow, it was causing a restriction and reducing the airflow to the engine.

In order to resolve this problem, we resorted to the following:

1. Fitted a MAP sensor to the manifold.
2. Removed the original intake air temperature sensor and fitted it directly.
3. Fitted a Unichip Q+
4. Connected the fuel pump relay to the Unichip.

Before removing the VAF, we fitted the MAp sensor and correlated the VAF voltage with the MAP voltage. Thereafter, using the Unichip's MAF-MAP conversion feature, we simulated the VAF voltage based on the MAP voltage. This voltage was fed to the ECU which continued to think that the voltage it is getting is from the VAF.

The Inlet air temperature sensor was directly connected to the ECU, so no modification to the ecu input there.

The Unichip was programmed to operate the fuel pump whenever the engine was cranked.

This way, we got to replacing the restrictive VAF sensor with a MAP sensor and still have the main ECU functioning normally.

Due to the lower restriction in the intake, the airflow improved.

Since had the Unichip in place, we also remapped the fuelling and timing.

End result - a more driveable and reliable car !
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Old 11th May 2012, 06:26   #2
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Default Re: Miata MX-5 VAF problem solved

What about transient response, ie, during shifting, off throttle, roll-on, on throttle etc? Has that changed because the ECU is not getting an instantaneous readout directly from the sensor but a converted reading, which takes just that bit more time? Is there a lag?
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Old 11th May 2012, 08:52   #3
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Default Re: Miata MX-5 VAF problem solved

Quote:
Originally Posted by 1self View Post
What about transient response, ie, during shifting, off throttle, roll-on, on throttle etc? Has that changed because the ECU is not getting an instantaneous readout directly from the sensor but a converted reading, which takes just that bit more time? Is there a lag?
The car is much more driveable with the exception of very slight cold start idling problem. I have attached a torque graph showing the before and after figures.


The cold start problem should be easy to sort out by richening the mixture for cold start. We just need to connect the engine temperature sensor output to the Unichip and increase fuelling below a particular temperature for this.
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Miata MX-5  VAF problem solved-1.jpeg  

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Old 11th May 2012, 09:13   #4
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Default Re: Miata MX-5 VAF problem solved

What about the A/F ratio? I think if you it lean out just that little bit you would get more top end. The mid-range & area under the graph look awesome!
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Old 11th May 2012, 23:13   #5
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Default Re: Miata MX-5 VAF problem solved

Quote:
Originally Posted by 1self View Post
What about the A/F ratio? I think if you it lean out just that little bit you would get more top end. The mid-range & area under the graph look awesome!
Hi

Thanks for the response !

We maintained the AFR around 12.8 right through except the top end where we dropped to 12.

I am also posting the bhp graph for your reference.

Yes, we coud have squeezed out a little more power, but we always believe in the maxim "better safe than sorry" !

We have done a turbo miata 1.6 where we achieved 220 bhp out of the same engine with stock internals - but that is another story !

Cheers
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Old 12th May 2012, 05:56   #6
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Default Re: Miata MX-5 VAF problem solved

Good work! So, have you advanced timing or is this gain in power just because of better air flow after the restrictive VAF was removed? How much have you advanced?

Since this is an naturally aspirated engine, I feel a timing RETARD at the top end (only one or 2 degrees) will help power.

What other mods does this Miata have?
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Old 12th May 2012, 10:51   #7
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Default Re: Miata MX-5 VAF problem solved

Quote:
Originally Posted by 1self View Post
Good work! So, have you advanced timing or is this gain in power just because of better air flow after the restrictive VAF was removed? How much have you advanced?

Since this is an naturally aspirated engine, I feel a timing RETARD at the top end (only one or 2 degrees) will help power.

What other mods does this Miata have?
We did not alter the timing - more or less stock timing - to enable using regular fuel.

The gains are mainly on account of better breathing due to less restriction in the inlet.

This car is bone stock except for this change !

Last edited by sapl : 12th May 2012 at 10:56. Reason: typos
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