Lots of OEM turbos have multiple tap locations at various points around the scroll of the compressor housing (one down at the origin of the scroll, one in the middle, one near the outlet - or some combination of the three). Just out of curiosity, is there a rule of thumb estimate of the difference in pressure between these?

 

Why do you think there would be a difference?

 

Well what's your explanation? They just did it because they were bored? Or maybe the holes got where they are by magic?

#ad

 

Maybe certain applications need more than one dedicated boost signal for high signal strength? Or maybe one is used and the rest are plugged and they use the one thats most physically convenient per application. I don't see why there would be an appreciable delta pressure between any given point after the compressor wheel tips. But it has always made me wonder why some compressors have one at the furthest inward part of the scroll and then a master power will have one at the far end of the outlet.

Tom?

 

Wasnt trying to be a smart azz. In your oringinal post you assumed there was a difference. I was only asking, why you thought there was a difference. I couldn't come up with a reason for a difference. They are all perpendicular to the direction of flow and they are all outside the compressor. Maybe it was for packaging or to simplify manufacturing with only one casting for multiple applications. This would explain why, in your example pic, there is only one port drilled and tapped. The question is 'Is there a difference in pressure at different location around the scroll?'. This was the point i was trying to convay. sorry if it came across differently.

 

Haha, INTERNET! No prob.

So the compressor cover is a tapered pipe, coiled into a circle. Area increases as you go down the pipe. With air flowing in a tapered pipe I have to assume there will be pressure differences at different places along the tube, but I ain't got enough book learnin' to know whether pressure will be higher or lower when comparing two locations along a tapered pipe.

I want to know because pressure differentials make the world go 'round (well, OK, not literally, but...). You can do neat stuff with pressure differentials. Make fuel flow through a jet. Keep a BOV closed, until you need it open. Open a carb's power valve. If you know what amount of pressure differential you have to work with, you might find a new neat thing to do with it.

It's got to be more than just packaging concerns, otherwise the three locations wouldn't be so evenly spaced. How come there are no covers out there that have three holes all in a row right next to each other? I'm sure there is some application out there where that would allow for better packaging if 2 or 3 taps are needed, but it's never done like that.

I just grabbed that pic from ebay because I was lazy, here's 3 that live downstairs in my garage:
(how about the last one... FIVE!?lol)

#ad


#ad


#ad

 

In the simplest way of looking at it, the scroll housing is a divergent duct. So, the pressure at the inside port would be lower than the outside port. As the air travels through the housing, the area increases, the velocity decreases and the pressure increases. Thats my guess.

 

The last picture looks like ejector pin marks. In plastic casting, they have ejector pins that remove the part from the mold. The marks in the plastic look just like the last picture. It varies if the marks are male or female "in or out" from the normal surface of the part.

 

I agree with this.

 

Going to a larger area would decrease the pressure if the volume of air stayed the same.

 

Yes, on the little T04B there was casting flash around the center of the bosses so it was pretty clear it was something to do with the casting process, but they serve double duty, they don't need to be nearly that big unless there's a possibility they might need to be drilled & tapped depending on the application.

 

Hey man I didn't make the rules. I just know thats how it is.

#ad

 

In this case Volume would'nt be a constant. Air is entering around the scroll as it diverges.

 

Why doesnt someone with holes already drilled/tapped just put a couple of el-cheapo gauges on each port and report back?

 

This is true.

 

There might be a bit of (very slight) difference in the pressure on the gage as the air travels and expands around the snail. Same deal as: "If I add one tablespoon of water to Lake Michigan, did I make a change? ABSOLUTELY! Did it really make any real difference, no.

Typically if we read any pressure off the compressor cover, it is at the final boss location. My vote is the other bosses are for the casting process, as others have said but whether the boss is drilled for a reason may be due more to the ability to hang stuff (brackets) off the cover vs anything else.

Tom Vaught

 

There is sure to be a pressure difference. Will it even be enough to notice on a gage, probably not

 

Casting process and ability to mount some type of hardware/brackets.