Calculating cylinder head velocity
Calculating cylinder head velocity We have all read that velocity of the air coming out of the cylinder head into the chamber is what creates torque. So, let’s talk about measuring cylinder head flow and velocity....
I read somewhere that cylinder head velocity can be measured by dividing the flow number by the curtain area of the valve (valve size x Pi x lift).
So, a stock LS1 head at .2 lift would have a curtain area of 1.2566. If the head flows 146 at .2, then the velocity would be 116.186. .5 lift of 275 would result in 218.845.
However, I have also read that larger intake runners create less velocity.
If velocity is measured by curtain area, then the intake runner size doesn't matter.
I read somewhere that cylinder head velocity can be measured by dividing the flow number by the curtain area of the valve (valve size x Pi x lift).
So, a stock LS1 head at .2 lift would have a curtain area of 1.2566. If the head flows 146 at .2, then the velocity would be 116.186. .5 lift of 275 would result in 218.845.
However, I have also read that larger intake runners create less velocity.
If velocity is measured by curtain area, then the intake runner size doesn't matter.
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Velocity depends on (among other things) the cross-section area where you measure the velocity. Think about water pouring out of a garden hose (low velocity) vs what happens if you put your thumb over the end of the hose to reduce the cross-section... The velocity at the end of the hose gets much higher because you've reduced the cross-section - but the velocity back at the spigot probably goes down a bit, because you've also created a restriction.
A formula based on curtain area will give you the velocity at the valve curtain. That is interesting, it's just not not the entire story. It would also be interesting to measure the flow rate at roughly the middle of the intake runner (assuming the runner isn't too weirdly shaped) or maybe at the head-manifold interface (just because it's easier to measure than some location buried in the middle of the manifold).
Also, I suspect it would be more useful to combine that area with the actual flow rate (based on the MAF sensor or speed-density and VE), rather than what the head flowed on a bench.
A formula based on curtain area will give you the velocity at the valve curtain. That is interesting, it's just not not the entire story. It would also be interesting to measure the flow rate at roughly the middle of the intake runner (assuming the runner isn't too weirdly shaped) or maybe at the head-manifold interface (just because it's easier to measure than some location buried in the middle of the manifold).
Also, I suspect it would be more useful to combine that area with the actual flow rate (based on the MAF sensor or speed-density and VE), rather than what the head flowed on a bench.
Also flow is not uniform. Flow at the center is faster than the edges. And laminar flow is faster than turbulent flow.
What I find is a good way of figuring which heads flow with highest velocity is to compare port efficiency. Port Flow divided by port volume.
Two heads flow 330 cfm. One has a 220cc port, other has a 260cc port. The 220 has higher velocity.
330/220 is 1.5, which is stellar port efficiency.
What I find is a good way of figuring which heads flow with highest velocity is to compare port efficiency. Port Flow divided by port volume.
Two heads flow 330 cfm. One has a 220cc port, other has a 260cc port. The 220 has higher velocity.
330/220 is 1.5, which is stellar port efficiency.
We have all read that velocity of the air coming out of the cylinder head into the chamber is what creates torque. So, let’s talk about measuring cylinder head flow and velocity....
I read somewhere that cylinder head velocity can be measured by dividing the flow number by the curtain area of the valve (valve size x Pi x lift).
So, a stock LS1 head at .2 lift would have a curtain area of 1.2566. If the head flows 146 at .2, then the velocity would be 116.186. .5 lift of 275 would result in 218.845.
However, I have also read that larger intake runners create less velocity.
If velocity is measured by curtain area, then the intake runner size doesn't matter.
I read somewhere that cylinder head velocity can be measured by dividing the flow number by the curtain area of the valve (valve size x Pi x lift).
So, a stock LS1 head at .2 lift would have a curtain area of 1.2566. If the head flows 146 at .2, then the velocity would be 116.186. .5 lift of 275 would result in 218.845.
However, I have also read that larger intake runners create less velocity.
If velocity is measured by curtain area, then the intake runner size doesn't matter.
Something else to consider is that Curtain Area is really only relevant for the opening and closing of the valves. After a certain point, the Curtain Area is greater than the port area and will no longer the restriction. This is why most consider the MCSA to calculate air speed because at the end of the day, it is the limiting factor is how much air makes it through the port.
Also flow is not uniform. Flow at the center is faster than the edges. And laminar flow is faster than turbulent flow.
What I find is a good way of figuring which heads flow with highest velocity is to compare port efficiency. Port Flow divided by port volume.
Two heads flow 330 cfm. One has a 220cc port, other has a 260cc port. The 220 has higher velocity.
330/220 is 1.5, which is stellar port efficiency.
What I find is a good way of figuring which heads flow with highest velocity is to compare port efficiency. Port Flow divided by port volume.
Two heads flow 330 cfm. One has a 220cc port, other has a 260cc port. The 220 has higher velocity.
330/220 is 1.5, which is stellar port efficiency.
Originally Posted by FCar2000TA
That is what I started off looking at. It is starting to sound like that is better than using curtain area. I will have to find that spreadsheet.
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If the ECU/PCM will let you log MAF (grams per second) or load (grams per intake stroke), those seem like better numbers to base the rest of the math on. I mean, it's already measuring air flow, and if the AFR and injector characteristics are right then you can assume it's doing a pretty good job of it.
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Valve size also matters for the coefficient of flow. A head with a 2.165" valve that flows 280cfm at .400" is not as efficient as one with a 2.08" valve that flows the same. Regardless of port volume.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
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Verdad Gallardo Valve size also matters for the coefficient of flow. A head with a 2.165" valve that flows 280cfm at .400" is not as efficient as one with a 2.08" valve that flows the same. Regardless of port volume.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
It's one reason I did the heads I did... they have a 264cc port and a 2.125" valve... and flow 375cfm. Yes, a 2.165" valve with 400cfm is going to flow more. But it won't be more efficient. In fact, this head has the option of the 2.165" valve that flows 400cfm... and guess what, it makes less power. Even on a 4.125"+ bore.
Based on Coefficient of Discharge and Port Efficiency, TEA Stage 2 heads are the best from .400 to .600 lift. AI 226, AI 232, TEA Stage 1 and TEA Stage 2 are all nearly identical at .300.
The TEA Stage 2 are even on par with the TFS as cast 220s, which are probably the only after market heads that I would be ok with the price of.
The TEA Stage 2 are even on par with the TFS as cast 220s, which are probably the only after market heads that I would be ok with the price of.
I would like to be able to run low 11s with a cam that will work in a true daily driver, with AC on, in 120 degree summers.
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Valve size also matters for the coefficient of flow. A head with a 2.165" valve that flows 280cfm at .400" is not as efficient as one with a 2.08" valve that flows the same. Regardless of port volume.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift.
Curtain area has a maximum. Once it surpasses valve surface area minus stem area you hit rapidly diminishing returns, only allowing you to see gains past the maximum from flow angle and deshrouding.
Originally Posted by JoeNova
Curtain area has a maximum. Once it surpasses valve surface area minus stem area you hit rapidly diminishing returns, only allowing you to see gains past the maximum from flow angle and deshrouding.
Curtain area calculation is valve diameter x .98 x pi x lift.
To perform the calculation correctly you can’t go by the valve diameter itself. You have to go by the flow diameter, which is where the actual valve seat begins. This is usually about 0.040-inch smaller than the average valve diameter. The valve seat will always be a “smidge” smaller than the valve, hence the .98 in the formula. Splitting hairs I know, but that’s the correct way to find actual CA.
To perform the calculation correctly you can’t go by the valve diameter itself. You have to go by the flow diameter, which is where the actual valve seat begins. This is usually about 0.040-inch smaller than the average valve diameter. The valve seat will always be a “smidge” smaller than the valve, hence the .98 in the formula. Splitting hairs I know, but that’s the correct way to find actual CA.
Another important thing to consider is the saturation point of the port with regard to valve curtain area versus port cross-sectional area. It’s typically somewhere in the mid-lift range (roughly 0.300 to 0.400 inch) for most applications. Beyond this point, the valve curtain area becomes larger than the port cross-sectional area (c/s) and the port itself becomes the restriction. You can determine this point with the following formula:
Valve curtain vs. port saturation lift point = valve lift x port c/s ÷ valve curtain area Example: for a 2.02-inch valve at 0.400 lift and a port cross-sectional area of 2.15 square inches measured at the hump in the port wall adjacent to the pushrod.
Valve Curtain Area = 2.02 x 0.98 x 3.14 x 0.400 = 2.486 square inches
Port c/s = 1.87 x 1.15 = 2.15 square inches Saturation Point = (0.400 x 2.15) ÷ 2.486 = 0.346-inch lift
So 0.346 inch lift is the point where the valve curtain area exactly equals the port cross-sectional area. Above this valve lift the port cross section becomes the controlling factor in flow capacity.