# Air flow in pipe, diameter change, affect performance?

1. Sep 26, 2009

### 550gts

I am attempting to build an intake setup for an engine I'm working on. I currently have a 3" diameter aluminum pipe on the throttle body that just curves off to the side. I would like to try and get some cooler air by routing it down into the bumper. I have access to 2.5" diameter piping. I was thinking of just expanding the 3" pipe with the 2.5" pipe into the bumper. This would mean that the air coming in would be into the smaller pipe and about half way to the TB, it would expand into the larger pipe. I know if the diameter increases, the velocity of the air will decrease, but would this affect performance of the engine at all? Any input on this would be greatly appreciated.

2. Sep 27, 2009

### Ranger Mike

see post Feb14-09, 04:36 AM #1Mgt3

read it for background on CFM

3. Sep 27, 2009

### 550gts

Thanks for that, A LOT of great information in that thread. I don't think that quite answers my question though.

4. Sep 27, 2009

### Bob S

Here is an online pressure drop calculator for air in tubes:
http://www.pipeflowcalculations.com/airflow/index.htm [Broken]
For a 5-liter 4-cycle engine running at 5000 RPM, I calculate 12,500 liters per minute. For this flowrate in a 75 mm tube, the pressure drop is 0.018 atm per meter; for a 63.5 mm dia, it is 0.037 atm per meter; for a 50 mm dia it is 0.11 atm per meter.
Bob S

Last edited by a moderator: May 4, 2017
5. Sep 27, 2009

### 550gts

Ok, so that means the 3" pipe is about 2x as efficient at getting the air through than the 2.5" pipe. Does that mean its bad to have the smaller pipe before the larger pipe in the intake?

6. Sep 28, 2009

### Ranger Mike

as far as your application...since you are not messing with the ports, intake runners etc..any improvement to cold air flow to the throttle body is a plus, and smoother is better (transition between pipe sizes..)
go for it

remember this..
internal combustion engines need a lot of CFM at Wide Open Throttle (WOT)
you can really mess up a pretty good set up by over sizing the intake...in many cases a smaller bore intake, runners, ports etc...will be more responsive to engine requirements...i have ported out a 4 cylinder head on the intake size to where the flow actually stalled due to too large of a port size. we had to replace the head with a stock one that was ported much less and the performance really took off. calculate for the RPM range you will use 90 percent of the time...it is all about compromise.

7. Sep 28, 2009

### hhn2002

general rule of thumb:
1)the more volume in your intake track the the more sluggish the throttle response for sudden opening of the throttle but the better the top end
2)the intake track is only as strong as its bottle neck, generally speaking. so a 4ft long intake tube thats half 2.5in diameter and the other half 3in is alot like a straight 2.5in diameter tube as far as maximum efficient airflow.

8. Sep 29, 2009

### 550gts

Thanks for all the info, guys. I think I'm going to try it. It's no smaller than the stock intake that was on it. It's mainly just to bring in cooler air. The intake I have on it now gets pretty hot sometimes. I can always take it off later if it doesn't work out for me. Thanks again!

9. Sep 30, 2009

### Ranger Mike

one major factor is the air filter..look up K&N air filters...stock paper elements are restrictive..K&N is the hot set up

10. Sep 30, 2009

### 550gts

I wouldn't use anything else

11. Feb 9, 2010

### tchuck

Sorry for the thread resurrection, but I am also designing an intake tube and I don't think that OP's question was quite answered. He was asking about the relationship between intake tube diameter and engine performance. Either way I have some new questions that pertain to the same topic. For the sake of simplicity, lets assume the absence of an air filter and no modifications to the cylinder head (valves, ports, etc...).

1) Is this correct? (below)

Doesn't the flow potential of a tube depend on both it's diameter and its length? Ergo, a 4'x2.5"dia tube would be more restrictive than a 2'x2.5"dia tube connected to a 2'x3"dia tube? Further, doesn't the nature of the transition between the two diameters have an impact as well?

2) What is the relationship between tube length and internal diameter in regard to its flow potential? For instance, If I have a 3" and a 6" straight tube how do I calculate their lengths in order to achieve equal air velocity and restrictive characteristics?

3)Given a constant air flow, say for instance 432.36CFM, through a tube via an infinitely powerful source of suction, a smaller diameter tube will facilitate a higher air velocity and conversely a larger diameter tube will have a lower air velocity. Obviously a naturally aspirated engine is far from an infinitely powerful anything so at what point does decreasing the diameter (or increasing tube length) stop increasing the air velocity and start restricting flow?

4) How do I determine the effects of bends in the tubes?

Thanks in advance for any insights you may have. I just joined this forum last night and so far I have read some extremely interesting things. (Hilsche vortex tube! WTF??!!??!) Keep up the good work!