Estimating airflow turbulence

[sciguyjim]

Hello,

I've been reading about how reducing turbulence in the air intake tubing, MAF sensor, etc. helps increase airspeed into the engine. I was wondering if there is a way to estimate how much improvement (smoothing of the airflow) I can expect if I do something to smooth out the "ribbed" portions of the intake tubing between my air filter and the throttle body. I can measure the inlet diameter, estimate the depth of each rib and calc the cfm needeed by the engine. I'm wondering if this is worth doing. Any thoughts on this?

 

[GregLocock]

I don't know if it is worth doing, but an easy experiment is to measure the relative pressure across the length of your intake using a U tube manometer. This should allow you to decide whether an improvement is worthwhile, and even find the high resistance parts. Performing engineering measurements at full power is always good fun! Cheers

Greg Locock

 

[sciguyjim]

You wouldn't happen to have an extra manometer I could borrow, would you? I estimate I can make the necessary alterations in 1 hour and for about $3. Much faster than taking measurements. I think the best results would be found at high RPMs, but I don't drive at those speeds.

 

[GregLocock]

A piece of clear fuel tubing with water in it? I think it would be quicker to make one than find one.

If you aren't driving at full power the benefits will be slight. You can work this out by imagining you are a manufacturer: cost of adding 1 hp to engine via development program 3 million dollars,
hp penalty per psi drop in manifold pressure: 6%

As soon as you close the throttle slightly then you are just replacing throttle with friction in the intake - both are equally efficaceous.

In my old job we always had a MAP gauge, I can't remember ever seeing more than 0.6 psi at WOT, and on most cars it would be say half that. I think you are beating a dead horse, to be honest.
Cheers

Greg Locock

 

[sciguyjim]

You're right. It was just an idea. Thanks for the input.

 

[parima]

this is interesting.

1. corroguted piping /ducting has at least 10 times
more flow resistance than smooth wall.

2. Map sensor calibration effects.

If the air needs to be mixed prior to reading, sensing,
than the corrugated pipe would give a better service.

But why would the air need to be mixed?

Supposedly you want more o2 under load and in traffic under
idle conditions. where the air is exhaust from the car in front of you.


is this correct?

 

[sciguyjim]

Parima,

I'm surprised there is so much more resistance in corrugated ducting.

The MAF sensor can be close to being out of calibration if the screens on both ends are removed. Keeping the screens in place evens out the airflow across the diameter of the intake tube rather than letting it get more laminar with a high flow in the center and less near the walls.

Since I don't plan to race, I really don't need all the modifications I could do. My airflow will be well within the capabilities of the stock setup. It's fun to try the experiments though.

 

[buzz41]

parima -
Supposedly you want more o2 under load and in traffic under
idle conditions. where the air is exhaust from the car in front of you

This caused me to start "cipherin'", as someone called it. If one is behind another vehicle which is burning fuel at the rate of 20 miles per gallon, the exhaust level in a 10 X 5 foot wake will be ~240 parts per million. Hardly enough to make one strangle frome lack of oxygen - or confuse the closed loop engine management system.

OTOH, being stuck in traffic behind a recklessly modified anything can be unpleasant, that's for sure.

 

[sciguyjim]

Buzz41, "...being stuck in traffic behind a recklessly modified anything can be unpleasant, that's for sure."

As far as I'm concerned, being behind even a well tuned diesel vehicle is bad because I don't like breathing the soot they produce. But that's another story (& a pet peeve of mine.)

 

[eRAM]

there are formulas for the effects of bends in the air flow passages. remember, you dont get more speed when you clean up the air ways, you get less of a pressure drop across them. The throttle is the largest pressure drop across the intake. It goes from almost full close, with the exception of the idle air opening, to WOT. ribbing can cause a pretty large pressure drop equivilant to about reducing the diameter by about 50%. IN fact, we have flow benched intake tubes. bell mouthed inlets on tubes of 2.5" diameter, can flow better than 3.35" diameter tubes. Put some ribbing on the 3.3" tubing, and it may only flow equal to a 2" tubing with a nice inlet.

Most of the theory behind air flow dynamics is counter intuitive. Dont try and guess it out, ask the questions!!
Mark

 

[sciguyjim]

Wow! Ribbing can reduce the effective diameter by about 50%! That's amazing. I may change my mind again and smooth out the ID of the intake. My air intake tubing is a good 3' long with 2 ribbed sections for flexibility.

I recently added an airfoil to the space between the butterfly valves to smooth out air going in there. I had the screens off my MAF sensor for a few years but I've recently come across info that explains why they're there so I put the screens back.

I've been trying to route some tubing to give me a cold air intake but I just decided today that it can't be done. At least not with my lack of tools or desire to cut holes in my car. I just don't have enough room to rearrange large objects like the air filter can. I used to have an open air filter but people convinced me that I'd be better off with even a restrictive cold air intake rather than using hot under-hood air. I just can't find a good way to get the cold air into the under-hood area. I don't race the car so I don't need the absolute maximum airflow. This is the only thing in my favor.

The only other modification I have right now, since I have more than sufficient airflow capability, is I added an oiled foam air filter in series with the existing paper filter. I found it removes a -LOT- of fine dust that was getting through the paper.

Thanks, eram.
Jim.

 

[sciguyjim]

Mark (eram),
I've been thinking about what you said about ribbing decreasing the effective diameter of a tube by as much as 50%. If I have a ribbed tube 4" in diam with ribs .25" wide and .25" deep, then is it accurate (even a little) to estimate that the eddies causing the drag extend at least 1" from the walls (about 50% of tube has major eddies)?
Could you estimate, how much would the eddies (and drag) be reduced if I reduced the wall ribs to 1/20 the original depth? Assume I have a tube about 4" I.D. and an airflow of about 500 CFM.
I'm trying to form a mental image of the amount of turbulence and laminar flow in the tube, and how much the drag could be reduced by smoothing the ID of the tubing. It's just an interest I've been obsessed with recently. Thanks.