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# Air Intake

## Air Intake

(OP)
Remembering what I have learned in physics, that air flowing through a pipe or tube can increase or decrease in speed and pressure based on size changes in the tube.  According the calculations and live tests this is true, but what about turbocharging engines?

I am working on turbocharging on engine right now, and if I were to say have a 3" pipe come off the turbocharger and into the intercooler, then have it decrease in size coming out of the intercooler to say 2.5" and gradually decrease in size again to 2.25" or even 2" at the intake manifold, would there actually be an increase in speed here and thus more power?

According to my calculations there would be an increase in flow, by almost 300cfms, and a pressure drop of .01 psi.

Pressure from a turbocharger though comes from restrictions in the path of the air from the turbo to the combustion chamber, would the decreasing pipe size be such a restriction that boost pressure would increase and ultimatly hurt the reliability of my engine vs increasing power by increasing air flow by basic physics?
Replies continue below

### RE: Air Intake

Then your calculations are wrong. Bernouilli says that 1/2*rho*v^2+P is constant. Mass continuity says that A*rho*v is constant. Adiabatic expansion says P*(1/rho)^1.4 is constant.

Use those equations, and consistent units. The adiabatic assumption is a big ask, but will do for rough calculations.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

### RE: Air Intake

As you increase the speed, you also decrease the cross sectional area. These offset each other to an extent as can be calculated in a manner Greg suggests.

Also an increase in pressure will result in an increase in temperature which will have a negative effect on potential maximum power output

Regards

eng-tips, by professional engineers for professional engineers
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### RE: Air Intake

(OP)
I am running stock internals right now, so pressure is something that I want to keep down yet maximise power.  I did use Bernouilli's equation in figuring these numbers.  My turbo flows 678CFMs, and I calculated air flow over 3 feet of 2.5" diameter pipe and 5ft of 2" diameter pipe which gave me a total of 1061.7CFM increase, using Si units.  I plan on running 7psi, and the pressure drop over the length of pipe I got was 6.99psi.

I am not an engineer but am going to school to become one, and am a very serious engine builder.  If I should hold off until I am finished with school and have some experience under my belt before posting on this forum let me know and I will not trouble you all any more!

### RE: Air Intake

#### Quote (Lq194):

"My turbo flows 678CFMs, and I calculated air flow over 3 feet of 2.5" diameter pipe and 5ft of 2" diameter pipe which gave me a total of 1061.7CFM increase, using Si units."

Yes, you should wait till you've graduated, and understand the conservation of mass, before posting on this or any of the Eng-Tips forums.

Also if the turbo can only flow 678ft^3 of air (unit time)(at 7psi) how do you expect to have available 1061ft^3 (?at 6.99psi?) at the other end of a very long tube. Where is the extra 383ft^3 of air going to come from? The pressure will definately drop more than .01psi. (That might be the pressure drop due to the tubing, the volume increase is going to be a bit more though.) From Pv=nRT you cna get: (ooops looks like my typing was way ahead of my calculator on that one) 4.47psi (I may have done that wrong. I am very out of practice with gas law thermo... Sorry Dr. Lifer)

Nick
I love materials science!

### RE: Air Intake

You cant learn if you dont ask questions. Take the suggestions these gentleman offered, try them out and provide some feed back on numbers. This way you will learn how to apply the formulas. This is a must in the engineering profession. All the while you will also gain knowledge in what could and could not work.

Good luck!!

Quote: "Its not what you know, its who you know" - anybody trying to find a decent job

### RE: Air Intake

LQ-
thats what these forums are for, finding answers.

I heard from my engineering profs that "the difference in a smart man versus an intellegent man is the intellegent man knows where to get the answers"

Keep plucking away, some of lifes greatest finds have been on accident.

Alex

### RE: Air Intake

Also Patprimmer is right, increase in pressure is increase in temputure and then what is the point because you would have to back down the pressure to aviod detenation? I know its spelled wrong.

Unless of course you plan on have a water injection system.  There is a book out there called TurboCharging, and I remember it has a twin turbo Banks motor on the cover.  Awesome book for learning about turbos and tweaking them.  I actually read the book twice from cover to cover, loved it.

Alex

### RE: Air Intake

LQ,
Another way around the problem of added head is to utilize methanol injection. Does wonders for cooling the intake charge. As for you calculations, think of things in a logical progression. Decreasing the tube diameter will increase the flow velocity(as well as pressure and temp.) but the overall quantity of air in the system does not increase. You would want to maximize the flow numbers while minimizing flow losses and heat buildup. Keep researching and reading and you will pick these things up. And don't get discraged as there are always people who can't act professionally when asked questions as you will learn more when you work in the industry!!

### RE: Air Intake

Try a book called "turbo charged" by Corky Bell.  I'm an electrical engineer, but thinking of making the switch (and eeekkk...going back to school) to automotive.

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