Filter Minders for Dummies

[davidbrady]

Folks, a friend asked me about the Filter Minder and it's suitability as an intake plenum diagnostic tool. One such use would be in determining whether the LXi's DD Series-60 has an overly restrictive fresh air intake system.

The Filter Minder (FM) is a device that measures intake restriction and potentially indicates the need for air filter replacement.

The FM has 5 predetermined click off points. The FM is graduated on the side of it's clear plexiglass chamber with 5 hash marks. The hash marks are available in a variety of calibrations to suit many applications. A quick google led me to these common calibrations:

All values in inches of water vacuum pressure:
a) 4, 8, 12, 16, 20
b) 7, 10, 15, 20, 25
c) 8, 11, 15, 22, 25
d) 14, 16, 18, 22, 25
e) 15, 18, 22, 27, 30

As the FM detects vacuum a yellow cylinder advances in the plexiglass chamber. If the vacuum exceeds one of the five click off points then the cylinder is latched at that point. In effect the FM locks a peak vacuum reading value, and one of five values can be locked with the last value considered the "red zone" where air filter replacement is suggested.

There are some issues with using the FM outside of it's intended design use as a filter replacement indicator.

1) The resolution of the FM is the distance between the hash marks. Choosing (a) above, for example, if the intake plenum has an actual vacuum of 18 inches of H2O, then the filter minder will log it as 16 inches of H2O. Here the resolution is 4 since any value between 16 and 20 will be logged as a 16.

2) The accuracy of the FM is +- 10%. Again, choosing (a) above, if the "true value" of vacuum in the plenum is 20 inches of H2O, then the FM can read this anywhere between 18 and 22 inches of H2O and because of it's resolution some FM's may lock the reading as 16 while others lock it as 20 inches of H2O.

3) The FM, as installed in our LXi's, is a fast response sensor. This is true because BB installed the device directly into a 1/8" NPT bung tapped into the 7" intake plenum. As a fast response sensor, the FM is susceptible to measuring transient conditions. The Detroit Diesel Series-60 Installation Manual specs the following intake restriction tolerances:

All tests should be performed with the engine operating at maximum rated speed and wide open throttle (full fuel):

The maximum permitted inlet restriction for a system with a clean air cleaner is 12 in. H2O (3 kPa).
The maximum permitted inlet restriction for a system with a dirty air cleaner is 20 in. H2O (5 kPa).

What DD is measuring here is steady state intake restriction, not transients. The steady state condition specified is continuous operation at maximum rated engine speed and continuous full fuel. What it's not measuring is transients such as downshifts, on the fuel off the fuel, cruise mode immediately followed by full throttle full load. The FM can't distinguish between transient and steady state so is apt to report vacuum conditions created by a turbo boost surge or by slow or stuck wastegates. To make an accurate determination of intake plenum restriction we want readings at steady state max rpm, max fuel, max load conditions as specified by Detroit Diesel.

4) The FM is susceptible to it's mounting location. Detroit Diesel is very clear in where a vacuum probe should be tapped into the intake plenum. DD specifies 5 inches before the turbo and after the last bend. Installing an FM at an elbow, whether at the inside radius or outside radius, can distort the readings. Installing the FM at tapered reduction or expansion diffuser can distort the readings. The FM is also susceptible to dirt and grime. Any clogging of the port or of the FM can distort the readings.

Before I'd recommend cutting metal, we need a precise continuous real-time vacuum measuring probe. We need an analog sensor that has high accuracy, low resolution, high repeatability, and low response time, with the ability to measure and log steady state vacuum conditions. We also need a continuous real-time dash mounted display. We also need to tap the vacuum probe into the intake plenum according to the manufacturer's (Detroit Diesel) specifications. The Filter Minder doesn't meet these needs.

Only when these requirements are met can we be sure that the data we're collecting is meaningful enough to allow us to make thoughtful modifications to our intake systems. Until we measure with high quality measuring devices and collect accurate and precise data, we can't say that we know anything about the system. Anything less is hacking, not engineering.

Here's a link to pdf's of a high quality analog vacuum sensor and a display offered by Engineered Products Co that suits our needs:

http://wanderlodgegurus.com/Thread-Intak...61#pid1061

Here's a pic of your typical Filter Minder: