Bernoulli Principle and Turbos

[tobsta]

Hi guys,
Im pretty new to the entire car modding scene but recently my mate bought an MKIII so I've been getting into it quite a lot. The question i have is in regards to turbo piping and vacuum tubes. For instance a turbo guage will get its reading from a vacuum hose connected to the intake manifold right? Surely the contraction in the nipple from the intake manifold would mean that the pressure would decresase and the veolicty increase. This is according to the bernoulli principle which is demonstrated here

Can somebody please explain to me how this isn't the case or if this does actually occur but its negligible.

Regards
Toby

---------- Post added at 08:07 PM ---------- Previous post was at 07:56 PM ----------

Is it anything to do with the fact that the gas in not escaping, possibly related to pascal's principle?

 

[bioskyline]

the Bernoulli Principle applies to fluid which doesnt compress like air does. the Bernoulli Principle would apply to hydrolic systems, but a turbo car is the same as an air comprssor. dont matter if its a 3 inch hose, or a 1/8 hose. 120 psi is 120 psi, the CFM rate will vary, plut pressure will be the same.

 

[IJ.]

the Bernoulli Principle applies to fluid which doesnt compress like air does. the Bernoulli Principle would apply to hydrolic systems, but a turbo car is the same as an air comprssor. dont matter if its a 3 inch hose, or a 1/8 hose. 120 psi is 120 psi, the CFM rate will vary, plut pressure will be the same.

Air is a fluid....

Better tell all the Aviation guys the Bernoulli Principle doesn't apply to aerofoils before their planes fall from the sky... :nono:

 

[tobsta]

hmmm, slightly confused, and I've even studied fluid mechanics at uni. Only just passed though

---------- Post added at 11:28 PM ---------- Previous post was at 11:14 PM ----------

Apparently at low mach numbers bernoulli principle also applies to compressible flows such as air. Anyone got any idea of what mac the air would be travelling in a turbo?

 

[tobsta]

EDIT: I think it may have something to do with the ideal gas law. Can anybody confirm this? Cheers

 

[suprarx7nut]

I have no idea how fast the air moves through the intake system, but it will vary pending the location.

You're talking about a boost gauge sensor. This sensor does not flow. Air doesn't flow through that small nipple out to anywhere. I think of it as a static portion of the intake. Sure, air circulates and moves in and out, but it doesn't flow out to anywhere.

I hated my fluid dynamics class in college, which is unfortunate. I should have payed more attention and really dug into the material more.

Bernoulli principle is based on flow though. I don't know that it applies here since there is no effective flow through the gauge nipple.

Furthermore, there is no effective 'velocity' through that gauge nipple.

Interesting topic.

Sent from my ADR6400L using Tapatalk

 

[IJ.]

You need to size the IC piping to keep the air under mach .4 (304mph) above this friction becomes a factor heating the charge air.

re the original question, there's no "flow" through the nipple being closed at it's end by the sensor/gauge so no speed.

 

[tobsta]

ok, so if we look at in terms of an air compressor. If i were to attach a nozzle to the end of the the guage pipe would the air coming out the end be at the same pressure as the guage or would the restrictive nozzle change the pressure?

 

[IJ.]

Pressure should remain constant, the smaller orifice will change the speed that any change in pressure is shown on the gauge.

 

[crisp]

You need to size the IC piping to keep the air under mach .4 (304mph) above this friction becomes a factor heating the charge air.

re the original question, there's no "flow" through the nipple being closed at it's end by the sensor/gauge so no speed.

How do you calculate this value, Ian? I'm pretty sure I'm well "under IC'd" at the moment!



-crisp

 

[IJ.]

How do you calculate this value, Ian? I'm pretty sure I'm well "under IC'd" at the moment!



-crisp

You need a flow map for your Turbo to work out the CFM @ a given pressure ratio then I have the data for up to 3" pipes below.

2" piping
1.57 x 2 = 3.14 sq in
300 cfm = 156 mph = 0.20 mach
400 cfm = 208 mph = 0.27 mach
500 cfm = 261 mph = 0.34 mach
585 cfm max = 304 mph = 0.40 mach


2.25" piping
3.9740625 sq in = 1.98703125 x 2
300 cfm = 123 mph = 0.16 mach
400 cfm = 164 mph = 0.21 mach
500 cfm = 205 mph = 0.26 mach
600 cfm = 247 mph = 0.32 mach
700 cfm = 288 mph = 0.37 mach
740 cfm max = 304 mph = 0.40 mach


2.5" piping
4.90625 sq in = 2.453125 x 2
300 cfm = 100 mph = 0.13 mach
400 cfm = 133 mph = 0.17 mach
500 cfm = 166 mph = 0.21 mach
600 cfm = 200 mph = 0.26 mach
700 cfm = 233 mph = 0.30 mach
800 cfm = 266 mph = 0.34 mach
900 cfm = 300 mph = 0.39 mach
913 cfm max = 304 mph = 0.40 mach


2.75" piping
5.9365625 sq in = 2.96828125 x 2
300 cfm = 82 mph = 0.10 mach
400 cfm = 110 mph = 0.14 mach
500 cfm = 137 mph = 0.17 mach
600 cfm = 165 mph = 0.21 mach
700 cfm = 192 mph = 0.25 mach
800 cfm = 220 mph = 0.28 mach
900 cfm = 248 mph = 0.32 mach
1000 cfm = 275 mph = 0.36 mach
1100 cfm max = 303 mph = 0.40 mach


3.0" piping
7.065 sq in = 3.5325 x 2
300 cfm = 69 mph = 0.09 mach
400 cfm = 92 mph = 0.12 mach
500 cfm = 115 mph = 0.15 mach
600 cfm = 138 mph = 0.18 mach
700 cfm = 162 mph = 0.21 mach
800 cfm = 185 mph = 0.24 mach
900 cfm = 208 mph = 0.27 mach
1000 cfm = 231 mph = 0.30 mach
1100 cfm = 254 cfm = 0.33 mach
1200 cfm = 277 mph = 0.36 mach
1300 cfm max= 301 mph = 0.39 mach

 

[crisp]

Awesome! I think you posted the T04z map once before in the past? I might have even pulled it down. I'll look around, then see if I can figure this out as an exercise. Thanks for the data, Ian! (I copy/pasted it to a word file in my Turbo folder. Good to have on hand!)


-crisp

 

[IJ.]

No probs Chris, 3" works for your turbo right up to 28psi from memory.