"Some did and some didn't", maybe has something to do with the quality and repeatability of the Filter Minder. I wish Dodge had left the Filter Minder off and put rear fender liners on instead.

 

Sorry, Rattle, did miss your post here. We addressed your question as to why the filter minder is pulling down, now I'll see if I can hack out an answer as to how to make a filter minder work with the Scotty II

A filter minder is an attempt to measure a reduction in flow caused by a clogged filter. Since putting a flow meter on every truck is a financial impossibility, and since the pressure drop across the stock paper element and stock airbox as it gets dirty is a known quantity, reading the pressure differential between the atmosphere and the turbo inlet hose is a pretty good indicator.

Notice where I say that the pressure drop is known quantity and the other comment about outside atmospheres? Thats why the factory filter minder won't work properly on a Scotty II, even without the venturi effect. First, at the inlet of the filter, you don't have atmospheric pressure anymore. It will be anywhere between .5 and 1 PSI above atmospheric. And the pressure drop across the oil gauze filter is different than a stock paper element, when the oiled filter is considered dirty. So your filter minder would need a new spring calibrated on the higher inlet pressure (since we can't mount the filter minder body INSIDE the air unit or in the cowl) and it would also need to be calibrated to read the lower pressure drop across the filter (an oil gauze is considered dirty with less pressure drop than a paper element).

Now lets add another variable to the equation. As your horsepower level goes up, airflow requirements go up. Thats an obvious. So lets say the Scotty unit (or another air system, it doesn't matter for the sake of this discussion) is capable of flowing 1200 CFM. Your truck at 300 HP needs, say, 600 cfm. Before a pressure drop will be read in the turbo inlet hose, your filter will need to be dirty enough to restrict 600 cfm. So you'd need to calibrate the spring in the filter minder to measure whatever pressure drop equates to a reduction of 600 cfm. Now, lets say you boost HP to 500 HP. Suddenly your air requirements shoot to 1000 CFM. So your filter is now "dirty" when you have a reduction in flow of 200 cfm across the filter. The pressure drop WILL be different in this circumstance, and as such, the spring will need to be recalibrated to read the new pressure drop.

See where that cheap little minder just won't work? What happens if you turn the TST box down to level 1? Your airflow requirements have changed, and so has your pressure drop across the filter, so the filter that was dirty at level 9 is no longer truly "dirty".

Use your EGT gauge as a guide to whether or not your filter is plugged, if you don't want to pull it out. Its the only reasonably accurate indicator of a plugged filter, other than an airflow gauge.

Rod

 

Rod, what about the vacuum which is what a filter minder measures?
If your engine needs lets say 600 cfm and the filter is only flowing 400 a vacuum will be created on the clean side, the minder will move.
It doesn't matter what the cfm requirement is, the filter minder will only move when the filter doesn't flow what the engine requires and a vacuum is created.
The spring will only change the Hg range of vacuum the minder measures, like going from a 30 lb gauge to a 60. As long as the range isn't so low that the minder instantly pegs or so high that it never moves it will work.
Not considering any possible venturi effect here.

 

I think we're missing one another here, so I'll try another example, with some hypothetical numbers. I've got some real ones around here somewhere from manometer tests, but don't have the time to dig them up. We also won't bother with cfm requirements, as this should illustrate at the very least, why a stock filter minder and a ram air system won't work together.

Lets say you have outside atmosphere at 14 PSI and a stocker airbox. You pile on the throttle and you'll read about 13.5 PSI in the turbo inlet tube. Pressure in the turbo inlet hose with a stock airbox, or any other air system that is not ram air, will always be lower than outside air pressure due to the resistance that the filter element puts up against flow. Your filter minder will not move a micron, because the spring is calibrated to prevent movement at this point. The .5 PSI pressure drop, which creates a vacuum action against the filter minder, is expected.

As the filter slowly clogs over time, the pressure inside the turbo inlet hose is going to drop as resistance to air flow is increased. Eventually, you'll reach a point where your pressure inside the turbo inlet is low enough to overcome the spring, and the minder begins to move. Again, hypothetically, we'll say by the time the pressure inside the turbo inlet hose has reached 10 PSI, the filter minder has been pulled down completely. You have 4 PSI of low pressure acting on the minder, ie: against the spring.

Now, lets take a Scotty II, Ram Air III or any other ram air system. You need to throw another variable into this equation, and thats the cowl induction air pressure or the pressure inside the actual housing.

With the same engine as above, same 14 PSI outside air pressure, you peg the throttle. Pressure inside the airbox will actually be about 15.5 PSI, higher than outside, of course now also depending now on your vehicle speed. Pressure in the turbo inlet hose is going to be about 15 PSI. In order to convince the filter minder to pull down to its lowest level, you still need to see 10 PSI in the turbo inlet hose. So if you're seeing a 5 PSI pressure drop from pressure inside the air box housing, to the pressure inside the turbo inlet hose, that filter on the Scotty II is more than clogged by this point in time. Its probably dirty enough that dirt particles are working their way through the gauze material and into the air stream. And that doesn't include low speed maneuvers at fairly high boost where air flow demands are different. At low speed, high boost, the pressure inside the ram air housing may be equal to outside atmospheric (14 PSI), and the pressure inside the turbo inlet hose may be 13 PSI, even with a completely clean filter. And it won't take as much dirt to lower turbo inlet hose pressure to 10 PSI, which is where the filter minder is now showing fully clogged.

As Markus put it, the objective of an air system is provide enough air for the engine to operate efficiently. Thats why EGTs are your best guide to whether your air filter is clogged.

Rod