Air Velocity (searched, couldn't find)

[iammike]

I was just trying to find out how fast the air is moving through our intake plenium and manifold on the average system with a CAI....searched but couldn't find any numbers. Thanks

[RigsPGT]

Once, i came up reading the same topic here in PT and somehow that 45mph was the key word, but im not sure. Maybe another search would light up the situation. It was under CAIs (sorry to tired to find it myself)

[PGT Mac]

The total air injested by an ideal (meaning no losses) engine is:
Displacement in inches*RPM/(2*1728). That gives a number in cubic feet per minute at some specific RPM. Then you can divide by the area to get a speed.

so from 1000 to 7000 rpm an ideal Probe engine would consume 44 to 308 CFM. Divide that by the area of the 3" CAI air inlet tube and you'll see speeds in the CAI from 10 to 70 mph.

Remember, if you are calculating flow into one cylinder only you will need to correct the formula accordingly.

[Adrag10]

Keep in mind also that the formula assumes 100% volumetric efficiency. At low rpms the VE will be around 75% and will go over 100% at higher rpms. So at idle, the actual speed will be slightly less than 10mph and at high rpm, it will go higher than 70mph.

Alin

[Rick_96PGT]

Also keep in mind you calculated from 3", the TB ID is 60mm (2.36"). This will affect the speed.

[spada]

Originally posted by Adrag10
Keep in mind also that the formula assumes 100% volumetric efficiency. At low rpms the VE will be around 75% and will go over 100% at higher rpms.
Alin

it is IMPOSSIBLE to exceed or even hit 100% VE for any cycle in the entire universe. our engine would have SWEET n/a power if it was 100% efficient. ZE's are better with efficiency because of their higher compression ratio. i am not sure of the numbers at all, so i will not mention any numbers. use your best judgement when calcuating it.

[PGT Mac]

Originally posted by Rick_96PGT
Also keep in mind you calculated from 3", the TB ID is 60mm (2.36"). This will affect the speed.

Yes, correct. You'll need to check the speed anywhere the diameter changes, if knowing the speed is important. Thus my comment "Then you can divide by the area to get a speed."

Perhaps the next question to ask, "Is knowing the speed important?"

[iammike]

I was only curious as far as if the air is moving so fast, the aluminum down pipe I have shouldn't matter for the cold air intake.

[Adrag10]

Originally posted by spada
it is IMPOSSIBLE to exceed or even hit 100% VE for any cycle in the entire universe. our engine would have SWEET n/a power if it was 100% efficient. ZE's are better with efficiency because of their higher compression ratio. i am not sure of the numbers at all, so i will not mention any numbers. use your best judgement when calcuating it.

Come on Spada, read a little more carefully before you post. What you are referring to is THERMODYNAMIC EFFICIENCY. I know that no cycle in the universe can achieve 100% TE. What I was talking about was VOLUMETRIC EFFICIENCY or VE. VE is simply the volume of air the engine ingests based upon how large it's displacement is. So if our 2.5 liter engine pulls in 2 liters of air during a cycle then it has a VE of 80%. If you were to somehow stuff 5 liters worth of air into the 2.5 liter engine, then it would have a VE of 200%(very possible with supercharging).

The VE will fluctuate depending on rpm but it starts off pretty low at idle, usually no better than 75%. Depending on intake/exhaust/cam tunning, the VE can shoot to over 100% at the torque peak. For more info on the subject, search for "intake and exhaust tech(very long)". Or go visit your Formula SAE team. I assume Mich State has an engine dyno in which case, the dyno will plot VE throughout the rpm band. Ask the dyno operator what the VE is at the torque peak. You will find that it's somewhere around 120-130%.

Alin

[PGT Mac]

Originally posted by iammike
I was only curious as far as if the air is moving so fast, the aluminum down pipe I have shouldn't matter for the cold air intake.

Personally, I think you are correct. Further, the air immediately next to the metal won't be moving at all, and will function somewhat as insulation. You know, if you open up an oil cooler, you'll find twisted metal strips designed to mix up the fluid to insure maximum cooling. You won't find that inside your cold air intake.

[spada]

Originally posted by Adrag10
Or go visit your Formula SAE team. I assume Mich State has an engine dyno in which case, the dyno will plot VE throughout the rpm band. Ask the dyno operator what the VE is at the torque peak. You will find that it's somewhere around 120-130%.

Alin

wow, i always thought that in order to achieve over 100% VE, it had to be under boost. my bad. i might get in touch with an SAE member, or possibly just head to the library to read up on it. i like to be one of the guys sitting in the aisles of the library reading about random shit.

[hudnut]

who can figure out how much the temp of incoming air increaces through the length of say a hotshot intake at 3K RPM (highway speed)? This is the real debate of the cold air intake and phenos, isn't it?

[iammike]

All I can say is go driving for awhile then park the car, pop the hood and feel your intake, my downpipe is always cool to the touch unless I've been in very bad stop and go traffic. The way I look at it, if the pipe is cool, it's pretty obvious to me that the cool air inside is winning. Now don't get me wrong, if your just sitting there at an idle, the result would be much different over time.

[pgt95]

These debates make me laught, while interesting we are maybe talking about 2 CHP difference.... he is some decent info though that is pertinant to this topic:
CAIs are typically made of thin-wall steel tubing (e.g. Hotshot) or plastics like ABS or even PVC. Metals are good conductors of heat and can allow the cool air drawn in to be heated in transit to the engine. Plastics aren't as bad but they also tend to hold on to heat more than the metal so they may heat the air for a longer period of time, though probably to not as high a temperature. If an engine like the KL03 is running, say, 80% volumetric efficiency at its 5500RPM power peak, it's moving something like 194.8 CFM of air. The HS pipe is 3" in diameter and runs, say, 2 feet long (it's actually shorter than this, but for the sake of discussion...) so it's internal volume is 0.098 cubit feet. This volume of air is evacuated by the engine in about 30.2 milli-seconds. How much heating of the air occurs in 30.2-mS is a mystery, but I bet it's small. Heating effects occur more under part throttle conditions, right after the throttle is cracked after spending 10-minutes stuck in traffic in August.
Speaking of airflow, a few notes about this. That air moving at 194.9-CFM through the pipe will have a velocity of 3967.4 ft/min (about 45.1 MPH). Compressible gas flow in a pipe may be turbulent or it may be laminar. Which it is depends on the so-called Reynolds number, symbolized by Re and which is basically a measure of the quality of airflow. This value describes the expected nature of the flow in the tube; if it is a high value, the flow will be chaotic & turbulent. If it is low, it's considered laminar. In our case, the Re turns out to be about 103,893. This might be interpreted as meaning that the airflow in that pipe is terribly turbulent in nature and that any means by which it might be smoothed can only help.
Pipe diameter has a direct effect on several important CAI characteristics. First, it affects the velocity in the pipe. The 194.8 CFM flow rate mentioned earlier worked out to 3967.4 ft/min through a 3" pipe whereas with a 2.5" pipe, the velocity increases to 5714.5 ft/min. This might generally be considered good since high airflow rates mean high inertia and added ram-effect. However, the Reynolds number Re for this pipe, using the simplified equation used above, works out to 124,672. Thus, the air is theoretically moving faster but it's also much more turbulent still than the 3" pipe. Not enough empirical evidence exists, that I know of, to know what the optimal size is for these intakes. However, this is food for thought.


Ben

[iammike]

I was just curious, I'm not trying to be scientific about it or do anything about it. I was basically just curious of how fast air flows through our cold air intake systems.

[hudnut]

That air moving at 194.9-CFM through the pipe will have a velocity of 3967.4 ft/min (about 45.1 MPH). I just don't see how keeping the actual CAI tube or intake manifold cool could make that much of a difference.

[iammike]

Here is my logic,

Go drive your car around for awhile and make sure the engine bay is really good and heated up, pull over somewhere and pop your hood.

Put your hands on different parts of the intake, if it's cool too the touch, my logic tells me the air inside is "winning" against the heat outside but if it's so hot it burns you (like an intake manifold) well, I think the cool air is loosing and it most likely picking up a fair ammount of heat.

Gotta remember, the intake downpipe is not directly connected to the engine in any heat transfer kinda way but the intake manifold pulls heat not only from the engine block itself but the throttle body as well. The air flowing through it is not even close to enough to cool the manifold back down.

I probably made no sence at all but hopefully someone get's my logic application here.