Building a New Airflow Meter, Part 1

Got a late model car that uses a voltage-output airflow meter with a detachable sensing element? Well, if the airflow meter is causing a restriction or it’s reached its voltage ceiling, the solution is easy. Just build a new bigger airflow meter and install the factory sensor in it. That’s what we did, using only low cost bits and hand tools. And then we tuned the mixtures with the incredibly cheap Digital Fuel Adjuster kit!

This week we build the new airflow meter and then next week we do the tuning.

The Starting Point

Here was the starting point – a Toyota airflow meter which is integrated into the airfilter box.

In this design, the sensing element can be removed by undoing two screws. Note the rubber O-ring (arrowed) that is used to seal the assembly into its mounting tube.

An accurate measurement of the internal diameter of the airflow meter shows it’s 45.2mm, giving a cross-sectional area of 1600 square mm.

Buying

The next step is to head to the hardware store.  A mixture of plumbing types was used in this design: (A): A thick-walled pressure plumbing joining section (this will form the main body of the new airflow meter); (B): 50mm storm water bend (the flared ‘socket’ ends are cut off and used to sleeve down the main body); (C): a second thick-walled pressure plumbing joining section (used to form the mounting flange); (D): 26 x 25mm thick-walled T-piece (used to form the airflow meter sensing element support tube). You’ll also need PVC pipe glue, a can of black spray-paint and some sandpaper of different grades. The way these bits and pieces come together will become clearer in a moment!

Sensing Element Housing

The 26 x 25mm T-piece (piece ‘D’ in the above pic) is cut with a hacksaw along the red line. A similar cut is made symmetrically on the other side of this section of tube to form an open U, so the lower section of the T-piece can sit on top of another tube.

The cut T-piece is heated with a heat gun until it becomes pliable and is then placed on top of the thick-walled joiner section (‘A’). The two pieces are pushed together in a vice so that the lower, cut, section of the T-piece opens-out to match the external diameter of the joiner piece. A socket is placed in the vertical arm of the T-piece keeps its diameter intact while the plastic is soft.

That process was a bit hard to see in the above pic, but this one better shows what you want the result to look like. Actually, what’s being done here is the T-piece (‘D’) is being rubbed on some fine sandpaper wrapped around the joiner piece of pipe (‘A’) to give the T-piece mating section its final shape, allowing it to nestle perfectly on the larger pipe.

To provide a mounting flange for the sensing element, a section of the wall of the spare joiner pipe (‘C’) is cut out, heated and then flattened by being compressed between two pieces of wood. A hole is cut in the middle of this section and then it’s glued (using the PVC pipe adhesive) to the top arm of the T-piece. A hammer was placed on the assembly to hold it in place while the glue set. When shaping this mounting plate, remember to leave clearance to the wiring plug.

Main Body

The next step is to prepare the main body of the airflow meter. The joiner pipe (‘A’) is shortened at each end with a hacksaw - if electrical tape is wrapped around the pipe to form a guide, it makes it a lot easier to cut it ‘square’. (It’s shortened at each end rather than one end because in the middle of the inside diameter of the joiner piece there’s a small flange against which pipes butt up. Shortening the pipe at each end allows this flange to remain in the middle of the assembly.)

Rubbing the ends in a circular motion on smooth concrete will allow you to perfectly square them up. Once they’re square, the rough edge can be smoothed with sandpaper.

Because the joiner pipe (fast becoming the main body of the airflow meter!) is larger in diameter than the ~50mm internal diameter wanted in this application, it needed to be sleeved down. When cut off, the socket ends of standard 50mm stormwater fittings are a tight squeeze inside the pressure pipe fitting – these were cut off ‘B’, sanded down a little in diameter, then pushed home, one from each side. Once in place, they can be cut off flush.

The next step is to cut a hole in the side of the main body of the airflow meter – this is where the sensor element will protrude through into the airstream. This cutting can be done with a hole-saw and then final adjustments made with a round file.

Here’s the main body of the airflow meter. Two new sections of standard 50mm stormwater tube have been pushed into each end of the assembly to provide the plumbing connections to the airflow meter.

In close-up, the cut-off socket-ends from the storm water pipe fitting can be seen sleeving down the internal diameter (red arrow) and the pushed-in sections of 50mm stormwater pipe can be seen providing the plumbing connections (green arrow). These pipe sections are a very tight fit, so pipe glue is unnecessary.

A check of the internal diameter of the main body of the airflow meter shows a diameter of 51.8mm, giving a cross-sectional area of 2100 square mm, or an increase of 31 per cent over standard.

Joining the Two

The next step is to place the T-piece over the hole that’s been cut in the main body and then glue them together with more PVC pipe adhesive. When doing this, place the airflow meter sensing element in the housing to help align the two sections. Hold the assembly in a vice while the glue is setting (give it 24 hours).

Remember the O-ring that sealed the sensing assembly in place on the original airflow meter? Well, that can be used again to perform the same function. If it’s not an ideal fit in the upper arm of the T-piece, thicker O-rings are available from hardware stores. If the fit is a little tight, the inside diameter of the mount (arrowed) can be filed or sanded away. The airflow meter sensing element is held in place with screws and nuts.

Finishing

Once the build of the new airflow meter is finished, it can be sanded smooth, using wet-and-dry paper used wet. An excellent finish is possible if you’re patient and do lots of sanding.

The airflow meter body can then be painted using a spray-can.

That’s the airflow meter finished – next week we calibrate its output with the Digital Fuel Adjuster

Temperatures.... Plastic plumbing of the sort shown here is made from a thermoplastic called PVC (polyvinyl chloride).  Its maximum continuous operating temperature is 80 degrees C. In an underbonnet environment, temperatures can exceed this level. This is especially the case near exhaust pipes and turbos, directly behind the radiator and close to the engine block. Therefore, the approach shown in this article should not be used where the airflow meter is located in a hot area of the engine bay. Furthermore, heat-shielding of the airflow meter is a good idea (as it is in all applications where the airflow meter would otherwise be exposed to excessive heat). If in the application the airflow meter is to be subjected to very high temperatures, it should be made from metal. A partly completed airflow meter adaptor designed to be used on steel tube is shown here. However, that said, the airflow meter assembly is continuously cooled by the intake air flowing through it, and the thick-wall construction of the PVC unit gives it much more strength than normal thin-wall stormwater pipe fittings. In fact, most of the pictured airflow meter body is 10mm thick, and the section around the sensing element mount is no less than 15mm thick!  If placed appropriately in the engine bay (and in higher heat areas protected by a heat shield), we don’t think you’ll have any problems.

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