Fast becoming one the must have modifications when upgrading to a larger turbo, the fitment of the larger air flow meter from the Z32 300zx with VG30DETT engine offers multiple advantages. This page will seek to explain them, along with comparing them with the SR20 unit as well as the RB20/25 afm.

Factory SR20 AFM Internal Diameter Restriction

The SR20 afm has two main problems. Firstly it’s internal diameter is very small at the narrowest point, which can’t really been seen from the outside. Although the largest external diameter is 84mm, the smallest internal passage is closer to 45mm. Many argue that it is a restriction, and gains can be made from removing this restriction the same way an exhaust pipe is upgraded in diameter. While I have never seen any conclusive testing to prove a huge loss, logic does say that the smallest diameter is much smaller than the rest of the piping. The diameter changes within the afm can’t be good from a flow point of view either.

Factory SR20 AFM Flow Mapping Restrictions

This loss can be negated but something that can’t is the voltage vs. air flow mapping. Each afm has a set range in air mass it can read from 0-whatever kg/minute, etc of air. The afm provides a 0-5V feedback to the ecu that directly corresponds to the mass of air travelling through it. This acts as the primary sensor of the ecu to calculate the fueling needs for the engine to obtain the desired air/fuel ratio. After performing multiple modifications the amount of air the engine can ingest may exceed the amount of air the afm can read, and as such the maximum ~5V (more like 4.8V) reading occurs.

The Dreaded ‘Boost Cut’

When this maximum voltage output condition happens the ecu will see it as a big problem and cut the fueling and timing to stop the engine. Known commonly and incorrectly as the ‘boost cut’, this is what fuel cut defenders aim to override. The condition is called the ‘boost cut’ because people often discover it when raising the boost. The ecu does not know what boost the car is running however, the addition airflow of the new boost level causing the afm max instead. This means that the cut does not occur at the same boost level on every car. People might say that the ‘boost cut’ is at 14psi, but this is wrong. The maxing out of the afm will depend on the overall mass of air into the engine, which is obviously decided by a lot more than just the boost pressure a turbo is running.

Fuel Cut Defenders

As mentioned the fuel cut defender aims to stop the cut occurring. It does this by intercepting the afm signal to the ecu and modifying it to suit. If the fuel cut occurs at 4.8V, then the fuel cut defender will always feed a maximum voltage of say 4.6V to the ecu. This does stop the cut, but at what consequence? As mentioned the afm is the primary source of information that the ecu uses to run the engine. If this signal is incorrect then the ecu will be running the engine either incorrectly or guessing the required equations. Not good at all.

Solution 1: MAP Sensor

One way around the problem is to eliminate the afm all together. The alternate sensor to the afm that is used by some ecus to run engines is the MAP sensor. This stands for Manifold Absolute Pressure, and measures vacuum/boost pressure instead of airflow. Many aftermarket ecus use these instead of an afm, and thus switching to one allows the factory afm to be banished.

It is possible to retain the factory ecu and run a MAP sensor, but this cannot be done by itself and the engine still run properly, changes need to be made. An aftermarket device like a HKS Vein pressure Converter converts the voltage signal of a map sensor to one that emulates the signal of an afm. Very rare in Australia at least, this option is not often taken. So most removing the afm opt for a MAP sensor based aftermarket ecu.

Solution 2: Larger Factory AFM

The most common solution is to fit a different afm from another Nissan engine. The SR20 afm is both smaller in internal diameter and restrictive in flow reading potential compared to many other Nissan air flow meters. The most common however are from the RB20/25DET, the RB26DETT, the VG30DETT, and the VH45DE. These engine are all larger than the SR20DET, and would probably use a bit more air, particularly being in heavier cars. It makes sense then that they might have both larger diameters as well as a larger potential to measure air flow. Upgrading to any one of these listed offers an improvement over the SR20 unit, but not all afms are created equal.

SR20DET vs RB20/25DET vs VG30DETT AFMs

The two popular upgrades come from the Skyline and 300zx engines. Although they look near identical the RB2X and Z32 afms differ significantly. The pictures below the physical differences between the three.

From left to right: SR20DET, RB20DET and VG30DETT. Please note that the RB unit has a GREEN sticker and the Z32 an ORANGE sticker. Also not a ~3inch pod adaptor is bolted to front of the SR20 unit in these shots. Some RB25 afms have a PINK sticker.

The internal diameters are seen here. Although not much smaller from the outside the SR20 afm pales in comparison where it counts. The inner shape dramatically narrows. The 76mm internal diameter of the other two is seen here as an obvious improvement. Note that both the black afms usually have a protective screen, only the front screen remains on the z32 afm, all the others are missing.

Now the plugs. Of course they are different. The SR20 uses a 4 pin plug, the RB20 a 5 pin, RB25 a 3 pin, and the Z32 a 6 pin plug.

What About the RB26DETT and VH45DE AFMs?

The RB26 (GTR engine) afm comes in at roughly 65mm but on that engine two are used. Obviously the flow readings would somewhat be halved compared to having just one. Possibly a worthwhile upgrade but possibly not. Remember that as great as the GTR is, not all of its parts are upgrades for other cars. Take for example the T28s it uses. These will be out performed from any of the stock SR20DET turbos, from T25 to BB T28. The GTR seems to maximise their potential at ~260 rwkw, which means 130 rwkw per turbo. As my car shows a humble T25g will easily outdo this.

The VH45DE afm is probably the most desirable of all but seems to be quite rare. Coming in at 90mm and existing in single form on a 4.5 litre V8, on paper they would seem to do the job very well on a smaller engine. But they are rare and hence expensive, and because of this not much seems to be publicly known about them. You could argue also that by the time an 80mm restriction was too large, that the car would be making enough power to warrant aftermarket management and do away with afms for good.

Flow Measuring Differences

In my experience I’ve never seen anyone test the various afms to see exactly what their flow measuring potential is. Therefore approximations and guesstimates will be used to illustrate the point.

The SR20DET afm is said to run out of puff quite easily, with many claiming to hit the ‘boost cut’ without extreme modifications. I personally have never experienced it, and I know what is inside my ecu. Go figure. Most claim that somewhere between 160-180rwkw will make the factory afm redundant, so we can ride with that.

The RB20DET is commonly quoted as good for a bit over 200rwkw, maybe as much as 220rwkw. Probably great for a mild upgrade, such as a T28 without cams.

The RB25DET is unknown to me at least to be any different from the RB20DET, with stories of people switching the two without consequence. The green diameter is the same at least, although it would make sense that the RB25DET afm be mapped differently with 500cc more displacement and a larger turbo. Maybe the 80mm RB20DET afm is just overkill? It would seen so compared to the tiny SR20DET afm, which is also on a 2 litre turbo running similar rwkw for similar mods. This uncertainty prevents a lot of people from choosing an RB2XDET series afm.

The Z32 afm from the VG30DETT is widely accepted as much greater in scaling then any of the others. This makes sense considering it feeds a 3 litre twin turbo six. People claim they can read up to 300rwkw, which is plenty for most SR20s. I certainly don’t expect to run into any problems maxing out the voltage of one.

Wiring up the Z32 AFM to the SR20DET Loom

As seen above the afms all have different plugs. When wiring up a Z32 afm to an SR20 it is best to buy a new plug and do some fresh wiring. I purchased a new bosch plug from Unigroup Engineering for $30. It features a nice retaining spring clip that is much easier to use than the factory clip setup.

The SR20 loom has only three wires leading to the afm, one black, one white and one one black/white. The Z32 plug is marked ABCDEF, from left to right. The wiring goes as follows:

A – nothing

B – white wire from SR20 afm wiring

C – ground (just a normal grounding connection to somewhere on the body) NOTE: Didn’t end up needing it but did it just to be safe.

D – black wire from SR20 afm wiring

E – black/white wire from SR20 afm wiring

F – nothing

I wired in the two plugs in parallel so that I could make a convenient change back to factory if need be. Simply strip back the insulation on the wiring, twist and solder on the new wiring and reinsulate. It should go something like this:

The finished plug:

Induction Piping Changes

More physical changes are needed to fit the larger afm. Firstly, the flange pattern is different on the air filter side, so a new POD filter adaptor is required. Secondly the induction pipe between the turbo and the air flow meter will be too small to fit over the new air flow meter. Perhaps it can be heated, stretched and forced over but this will be a huge pain every time it has to come off.

I had this pipe fabricated by S&R Pro in Penrith from mandrel bent aluminium. They quoted me between $200-$250 for the pipe as well as some other modifications to the existing FMIC piping. The final price was $300 (including my injectors being cleaned and flow tested) so without this (quoted $130) and the other welding I’d say a reasonable price to aim for would be $150.

The larger piece of jutting pipe is for the BOC return, with the smaller for the oil breather return. As can be seen the pipe matches the turbo inlet diameter and flares up to 80mm just before the afm. In place is a $40 POD filter adaptor from Autobarn.

Here a comparison between the standard inlet pipe and afm is seen with the new items:

First trial fit didn’t have the bov and bov return pipes aligning well:

But a bit of fiddling and twisting had them good enough. The oil breather return was perfect.

Here is a dummy fit with the z32 afm in place:

With a dodgy bit of hacked rubber pipe the standard afm is in place on the new pipe.

Cleaning the New AFM

This is the simple bit. Buy some carby cleaner, spray up carefully over the element, and place for the afm to drip dry so that drips do not touch the plastic. It’s been said that exposure to the cleaner will damage the plastic so be careful when you spray. This might be harder if you had both screens still attached. Perhaps one of the openings in the mesh could be stretched open temporarily to allow a spraying tube inside.

ECU Mapping Changes

Don’t expect the car to run properly with the new afm plugged in. Due to the differences in mapping for air mass vs voltage, the ecu needs to be recalibrated.

For example (and only an example, figures are fictitious) at 300rpm coming on boost the SR20 afm might have been reading 3V, but due to the massive headroom the Z32 afm might only read 1.6V in the same situation. Obviously this will play havoc with the fueling of the engine. The car will probably be undrivable and if it’s not then very dangerous to the health of the engine.

For Power FC owners a Z32 afm setting can be selected from the menus, but beware once again that this may only be safe enough to have the car running. Every report I’ve heard confirms that a complete retune is still required to optimise the fuel and ignition settings.

For people like myself choosing to retain the factory computer and have it remapped, things aren’t as simple. A base calibration must be made to adjust the ecus function for the new afm.