Feedback on a headlight intake, airbox & ducting

[heinzboehmer]

Just keep in mind that if you go the manoneter route, you'll have to correct for temp if you want to find volume of air. IAT should be as close as possible to the probes. Might even be worth sticking separate temp probes next to the pressure probes.

[Darbshaw]

Yes. I do it caveman styler and use my in-car gopro to record the readings which is OK. I need to figure out something better. I'm not an electrical genius so would be great if someone could figure out a way. With that said, I can overlay the speed data and see the manometer reading as long as the lighting it right. I have gotten enough clear video to be useful.

I've only taken readings to test out the rig. I will do a baseline and then measure changes over the summer...possibly.

It does give useful data. For example, I do know that the gurney flaps in front of the hood vents adds about .3 in pressure differential at 120-ish MPH.

Unfortunately I'm as caveman as they come. I'll likely not be the one to figure that functionality out.

What would I be looking for with the pressure differential? My intuition says that the smaller the drop the better, making the lesser drop the greater airflow.

[bigjae46]

Just keep in mind that if you go the manoneter route, you'll have to correct for temp if you want to find volume of air. IAT should be as close as possible to the probes. Might even be worth sticking separate temp probes next to the pressure probes.

I did not know that but now I do. Thx!

[bigjae46]

Unfortunately I'm as caveman as they come. I'll likely not be the one to figure that functionality out.

What would I be looking for with the pressure differential? My intuition says that the smaller the drop the better, making the lesser drop the greater airflow.

I'd think you want lower pressure behind the headlight duct so you get more airflow through the duct.

[heinzboehmer]

I did not know that but now I do. Thx!

The run is short, so I don't think the temp difference will be huge (although that aluminum tube might make things interesting). But yeah, if you want volume, you need to know both pressure AND temp: https://en.wikipedia.org/wiki/Ideal_gas_law

Unfortunately I'm as caveman as they come. I'll likely not be the one to figure that functionality out.

What would I be looking for with the pressure differential? My intuition says that the smaller the drop the better, making the lesser drop the greater airflow.

Ideally you want to measure air mass (not volume) to quantify how much air is making it into the engine, hence my MAF suggestion. MAF wikipedia article does a decent job of explaining why that's the better bit of telemetry: https://en.wikipedia.org/wiki/Mass_flow_sensor

However, Jae's suggestion should give you a good indication of what's going on. Bigger pressure differential means more air wants to flow between the two points (https://en.wikipedia.org/wiki/Bernoulli%27s_principle). This all depends on where your measurement points are of course, but that's the general idea.

[Darbshaw]

Thanks for the feedback guys. This has been helpful.

I do have some concerns with the aluminum tube and extra heat soak. The tube itself is simple to recreate and print. I'm just a bit concerned running it as a part post filter. If it melts or fails, airbox or the engine are the next in line to catch.

I still have my old maf in the parts bin, so I can rig one simply enough. Even more so if I replace the aluminum with plastic.

Without having to switch the ecu back to reading the maf. Couldn't I read the voltage output from the maf? Then use this to interpret the airflow reading. It wouldn't be as scientific as the ecu output, but should provide some insights.


In regard with the tube material, I'm printing ABS so temp shouldn't be a failure point, but it is a fear. I can always goto fiber reinforced materials down the line

[heinzboehmer]

Without having to switch the ecu back to reading the maf. Couldn't I read the voltage output from the maf? Then use this to interpret the airflow reading. It wouldn't be as scientific as the ecu output, but should provide some insights.

Yes, but with caveats.

Ideally, you want the air mass reading from the DME, as the MAF sensor itself doesn't measure air mass directly. In reality, the voltage coming from the sensor is a reading of how effective the air that is rushing past it is at cooling the hot wire. This usually relates well to air mass, but is also affected by the density of the air (which is, in turn, dictated by air temp and pressure). The DME takes this output voltage and derives air mass from it by taking into account all the other variables from all its other sensors. You could do that math yourself, but that would involve setting up more sensors.

(Worth noting that you could cheat the math and do your tests back to back, once everything has reached thermal equilibrium. That way, air density will be roughly constant between tests and thus air mass will be proportional to sensor voltage. But this is hacky and relies on that big assumption of "at equilibrium".)

There might be some standalone measurement device that does all of this for you and then just spits out an air mass value, but haven't looked into that. I still think the path of least resistance is to hack up a stock filter housing and let the DME do all the heavy lifting. The other nice part about this approach is that the geometry of the stock tube + trumpet have been designed specifically for the MAF + engine combo by the engineers at BMW. This is likely gonna result in a more efficient system and more accurate measurements than some random air mass meter that sorta matches the geometry of what's on the car.

[bigjae46]

I plastidip’d mine and it lowered IATs by 15 drgrees.

[Darbshaw]

Yes, but with caveats.

Ideally, you want the air mass reading from the DME, as the MAF sensor itself doesn't measure air mass directly. In reality, the voltage coming from the sensor is a reading of how effective the air that is rushing past it is at cooling the hot wire. This usually relates well to air mass, but is also affected by the density of the air (which is, in turn, dictated by air temp and pressure). The DME takes this output voltage and derives air mass from it by taking into account all the other variables from all its other sensors. You could do that math yourself, but that would involve setting up more sensors.

(Worth noting that you could cheat the math and do your tests back to back, once everything has reached thermal equilibrium. That way, air density will be roughly constant between tests and thus air mass will be proportional to sensor voltage. But this is hacky and relies on that big assumption of "at equilibrium".)

There might be some standalone measurement device that does all of this for you and then just spits out an air mass value, but haven't looked into that. I still think the path of least resistance is to hack up a stock filter housing and let the DME do all the heavy lifting. The other nice part about this approach is that the geometry of the stock tube + trumpet have been designed specifically for the MAF + engine combo by the engineers at BMW. This is likely gonna result in a more efficient system and more accurate measurements than some random air mass meter that sorta matches the geometry of what's on the car.

It's been a while since I've done the MAP/IAT swap. I don't believe it required any permanent modification so hopefully I should be able to swap back to the MAF fairly easy, I'm just trying to be lazy and avoiding the retune. Even if it is temporarily.

Do you still have the stock airbox lid? I figure the depth of the MAF into the airstream is important. If you have one, are you able to get the measurements off it? As much as I'm open to hacking up the stock one, if I can recreate this section for $10 instead of $50 that's a win. Especially so with using the MAF just as a test bed.

[Darbshaw]

I plastidip’d mine and it lowered IATs by 15 drgrees.

Plastidip has a purpose again! Never thought I would reunite with it since I had my VW in highschool

[heinzboehmer]

It's been a while since I've done the MAP/IAT swap. I don't believe it required any permanent modification so hopefully I should be able to swap back to the MAF fairly easy, I'm just trying to be lazy and avoiding the retune. Even if it is temporarily.

Do you still have the stock airbox lid? I figure the depth of the MAF into the airstream is important. If you have one, are you able to get the measurements off it? As much as I'm open to hacking up the stock one, if I can recreate this section for $10 instead of $50 that's a win. Especially so with using the MAF just as a test bed.

Mine is buried somewhere not very easy to get to. Might be easier if someone else on here can measure theirs.