what im saying is if the engine demands 200cfm, the head needs to flow more than 200cfm at full lift or it will be losing considerable power.

im not even sure why there is a question here, the equation is right and the math appears right, so im sure that is right

Edit: obviously its not going to show those numbers because in the real world because pressures change, castings are different, etc. but it should be within a couple %

 

I know what you're saying, and it is not correct. I don't know how many head you've flowed, but I have done enough to know that if you make a head flow more than the engine needs it will not make power. There is a balance between flow and velocity, and when you find that you make power. You don't just make something flow as much as possible. The engine needs a certain amount of flow, but MORE IMPORTANTLY, it needs a certain amount of velocity. Too much flow, and you kill velocity. I don't know how to explain it better than that.

 

There is a balance between velocity and pressure. Greater air FLOW IS better - pretty sure you're getting some concepts confused. An engine is not going to lose power by increasing flow at any given pressure differential. You also need to understand that the original post has nothing to do with bench testing head airflow when that is done at significantly higher pressures than atmospheric pressure (1 bar) as I mentioned above.

 

I'm not getting anything confused. Perhaps trying to dumb it down sounds like it. Greater air flow is better IF you can sustain the same velocity. Going back to what I was saying earlier, if you can flow a greater amount of air, but at a lower velocity, you have gained nothing and most likely, lost something. Here's an analogy. Imagine you have a tube 2" inner diameter that can flow enough cfm for the given application. Then you replace it with a tube that's 4" inner diameter. The 4" tube will obviously have more AIRFLOW, but what have you lost? Velocity. If you think that it's more important to have airflow than velocity then that's your business. Velocity makes torque, which makes horsepower.

After more consideration I understand that flow benching the head may not be the same thing as what the OP is describing. However, I am no engineer and am trying to relate that information to something I am familiar with. BTW, flowbenching a head is not done a a pressure higher than atmospheric. Industry standard is to flow a head at 28 inches of water. That translates roughly to 1psi. 1 Bar is roughly 14.7psi so actually it is done a less than atmospheric pressure.

 

1 PSI guage pressure=/ 1 PSI actual pressure

 

What are you talking about? That has nothing to do with what I posted.

*Edit: I see what you mean. At sea level 14.7 PSIA=0 PSIG. True. I am not using psi other than in conversion though. You kind of neglected the first part of my post.

 

How can you possibly decrease velocity while increasing airflow or visa versa? You can't given the same pressure and temperature. The measurement of airflow is a "rate", just like velocity is a "rate" and they will be proportional to each other given the same pressure.

I misread your post as 28 psi - oops sorry. 28 inches of water is around 1 psi but as you've deduced flow through a port is different that flow through a tube.

 

Flow is flow, through a port, through a tube, doesn't matter. CFM is CFM, and velocity is velocity. Physics don't change. What is a port, but a shaped tube? If you don't understand how increasing airflow can decrease velocity, I obviously can't explain it over the internet. I know what I know, and you know what you know. Let's just leave it at that.

 

Flow = Velocity x Cross Sectional Area

TK

 

True, however, just because flow increases, doesn't necessarily mean the velocity stays the same and CSA increases.

Velocity could decrease, with the CSA increasing enough to compensate for it and increase flow, despite the fact the velocity decreased.

 

This is so wrong and misleading. Sorry man. You're ignoring several basic concepts in physics, like say the venturi effect through a port (orifice), and the fact that an intake system (tube) has a 3rd dimension (length) where a port does not, not to mention turbulent vs laminar airflow. But as you say, let's just agree to disagree.

Anyway to get back on topic, it is possible to see gains from a tuned intake system if the airflow increases for a given pressure differential. The capacity for an intake to flow xxx cfm is not as relevant as the ability to flow higher cfm for the same pressure. Keep in mind that on a flow bench as you increase pressure, the same pipe will flow more.

 

I have a whole off-topic response but I suppose I'll save it for another thread or time.

On topic, I agree with the second paragraph you wrote. This is exactly why you have to take head flow numbers with a grain of salt unless you know what pressure they are flowed at. I can make a head flow whatever I want (within limits of the flowbench) just by raising the pressure.

Like I said, I am not engineer. rcdash seems to have a good knowledge about some of this in a numbers and paper aspect, whereas I have a good knowledge in a real world applicable aspect. Some parts the same, some parts different. It would be fun to sit down a pick each other's brains.

Either way, commendable to even think about things this way in the first place. Most people never do their own thinking.

 

I think I'm understanding your logic here (trying to, really) but I really think you're discussing a different concept altogether. You can decrease velocity by say using a larger diameter pipe or by increasing port size, but still increase air flow. It was the reverse argument you were arguing though. A few posts up, you said air flow wasn't imporant, velocity was. The bottom line is the mass of air getting in to the cylinder is what determines power. Air flow and the density of air determines the mass air entering the cylinder. I think we can all agree to that, I hope!

 

Agreed, however at RPM where piston speeds cannot compensate for lack of velocity, mass air flow IMHO and from results I have experienced first hand, is less important than velocity of air.

With enough velocity, less dense/less flowing air/ports can be overcome by ''overfilling'' the cylinders with the high velocity of air entering achieving close to or higher than 100% VE.

Anyway, it's getting late here and I have to be at work at 7am tomorrow so I'll call it a night.

I will say that it's been enjoyable discussing something intelligent with someone competent rather than reading about whose body kit is the hotness.

Good night to all, maybe this thread has been informative or at the very least inspiring for others to read some more on these subjects. I know I plan to.

Cheers

 

I got a ques. Even tho you through these facts on a stock car.. Wouldn't it be beneficial to have the aftermarket intakes on a car with bigger exhaust piping to get more air into the car?? Or the stock intakes would provide the same amount of airflow??

 

what were basically saying is that the stock intakes will flow just about as much as the engine can possibly breathe in, even at 100% efficiency. so stock intakes dont really help because there isnt much room for improvement

 

So I can do say a fuel true dual exhaust. Replace everything except for a headers.. And it still wouldn't matter if I use the stock intakes?? What about a drop in filter?? Would that help at all??

 

drop in filter might give you a little, but it would be like any other intake

 

I may not have all the math correct here, but I think I'm beginning to grasp the large discrepancy between head flow CFM, and required CFM for the engine to fill all cylinders.

As a whole, at the intake filter, air flow is kept pretty constant, since theres multiple cylinders, the closing of a set of valves for 1 cylinder, doesn't have an effect on overall flow, since there are others that are open and drawing in air. Also at the filter, at a given RPM, since theres always an amount of air being drawn in by some cylinders, the air isn't accelerating. This isn't taking into consideration the increase in air velocity as diameter decreases, for all practical purposes we'll say the intake pipe is constant diameter. Air flow is constant, non-accelerating, and doesn't need any more flow than the the volume of the cylinders. For the VQ, @ 7500rpm @ 100% VE, ended up being around 460 CFM.

Where it started to get really hard to understand for me, was when looking at the smaller picture of a single port, or single port/runner. When theres only 1 cylinder involved, when the intake valves close, forward motion of the air essentially stops. When the intake valve opens again, the air now needs to accelerate from near zero velocity and cover the distance from the beginning of the runner/port, to the bottom of the cylinder. At a given RPM, in this case 7500rpm, the opening of the valve is near instantaneous. Assuming the valve is open for exactly half a revolution, it ends up being something like .008 seconds, or 8 milliseconds. I used a random number, of 10in, as an assumed distance between the front of the runner, and the bottom of the cylinder. To cover that distance, in that short of a time, the acceleration needs to be 7937.5 m/sec^2, or 312,500 in/sec^2.

That acceleration applied to the time, of 8 milliseconds, the velocity of the front of the column of air reaches around 2500 in/sec at the bottom of the cylinder, or something around to Mach 2.

Since Flow = Velocity X CSA(which is fairly constant), I used 2in^2 as an arbitrary constant just to see end numbers.

2500 in/sec X 2 in^2 = 5000 in^3/sec = ~173.6 ft^3/min or CFM.

A lot of these numbers were just made up for the sake of calculations, but the theory is sound, I think. All the calculations assumed a straight shot from the front of the runner to the bottom of the cylinder. Since a real engine doesn't follow these theoretical flow paths, it makes sense that a CFM higher than 173 is much more likely for the flow rate of a real head.

173 CFM represents how much flow would be needed to move a column of air for an instantaneous amount of time, at the huge velocity required to cover the distance. Also its a lot more aligned with your flow bench data. My calculations need that much CFM per bank, due to the relative constant velocity of air, and relative zero acceleration. Turned out to be an entirely different story when isolated to a single port, and the closing and opening of valves got involved.

All this was done based on my limited knowledge of physics, and a lot of help from google, so I don't know how right the calculations are. Even if all my math and equations are wrong, I'm almost positive it has to do with the nature of the air accelerating and stopping in the runnner/ports. Sorry for the long winded posts, but this has been racking my mind for half the night.

TK

Edit: Even if I'm wrong with all the theoretical data, I know I'm on the right track.

Re-Edit: After some more careful looking over my work, it just occured to me that the cylinder fill time is 4ms, and not 8ms, so the ending theoretical 173 CFM is a very conservative figure, the new answer using the corrected time of 4ms, would result in a value thats closer to 347 CFM.

 

If thats the case then it proves other intakes give gains over stock. Or was that the point of this and I didn't know cuz it blew over my head.. And atleast it's cheaper then aftermarkets....