short intake restriction calcs?
Discussion
The critical fact most people seem to be missing is pressure recovery, and the two separate mechanisms for losses of energy in a flow stream.
1) A "restriction" results in the air stream accelerating, and conservation of momentum requires means that the energy to accelerate that air must come from the air stream itself, resulting in a pressure drop. However, when the air slows back down again, exactly the same amount of energy is available to be recovered, and depending on the geometry of the flow stream, large amounts can indeed be recovered.
2) The higher the velocity of the air, the higher the drag forces generated where that air impinges on a static wall, and the higher the parasitic drag. Tangential Pressure differentials (caused by velocity gradients between free stream flow and the stalled stream in direct contact with the wall) result in the flow stream vector diverging from the centreline of the duct, and hence also create a pressure loss in conjunction with local turbulence.
Unlike 1), The losses associated with 2) cannot be recovered, and for this reason, limiting the velocity within intake systems to something less than 50m/s, is, in general, regarded as a rule of thumb. A second rule, that is widely used for OEM intake design is to ensure the dynamic pressure doesn't exceed 1kPa, which really is the same limit, but with reference to air at 1bar and 25degC
If course, some restrictions are deliberate, for example the throttle plate! Here, the flow has to be choked (ie supersonic) for the throttle to actually throttle anything! Once a certain pressure differential exists, the flow becomes supersonic at the throat, and (outside of certain deliberate geometry forms such as convergent-divergent nozzles) cannot be accelerated any more. Here, the parasitic losses match the pressure diferential.
One thing you will notice is that all forms of motorsport twhich mandate air restrictors, closely control the geometry of the duct DOWNSTREAM of the restrictor (and not necessarily upstream of it!) Take a WRC 32mm dia restrictor, basic physics helps us calculate the pressure ratio of a critical (choked) flow stream, 0.52 (assuming a gamma (Cp/Cv) of 1.4) so with an upstream pressure of 1bar the pressure at the throat is 0.52 bar. And we know the flow is doing M1.0, so, with some correction for temperature and hence air density, we know that restrictor can flow, at an absolute maximum 0.193kg/s.
What happens next depends upon the pressure RECOVERY of the downstrema duct. Ideally we would have a nice, characteristically long divergent nozzle, that has an included expansion angle of less than about 6deg. This allows the air to decelerate nice and slowly, and hence we can recover a lot of the pressure lost to velocity, in fact, typically, we can get discharge co-coefficients up around 98% or even more. So, despite the low pressure and M1.0 stream at the throat, we can get back to say 98kPa in a static volume downstream of that choke point. If the expansion geometery is compromised, the pressure recovery is correspondingly poor. WRC rules mandate the restrictor throat must be within 50mm of the front face of the compressor wheel, meaning we cannot leverage high pressure recovery, and our compressor is forced to "suck" on nasty, low pressure, turbulent high velocity air, which further reduces the compressor efficiency (and hence further limits engine power) Despite that, WRC engines, with what is a pretty small restrictor (just 32mm dia remember!) make (well) over 300bhp, even with the heavily compromised compressor efficiency. An N/A engine with a 32mm restrictor and a full pressure recovery convergent duct can make 400bhp.
A 2.0 F3 engine with a tiny 28mm restrictor puts out something like 230bhp, or 115bhp/litre despite that very small hole! If you had an intake/combustion system as well designed as an F3 one, your 500bhp engine could suck through a 41.2mm diameter hole and still make that 500bhp!
But, your intake isn't as well designed as an F3 one of course, which is why you want to go to a bigger intake diameter to reduce the flow velocity, to ensure that you have less to loose through non-optimum design. But as the area of a hole scales with the SQUARE of it's radius, despite what most aftermarket tuners will tell you, you simply don't need a massive duct to make power.
Puma, sorry, Kia, points out correctly that a big volume plenum relative to the swept cylinder capacity (called a low "gulp" factor intake) results in a nice even flow through any restriction, and on an firing event basis this is correct, but you would need a truely massive intake to be able to negate any steady state flow loses in the upstream ductwork (your 500bhp engine is probably swallowing something like 300 litres/second of air (that's a ~1.5 standard sized oil drums full, every second)). And of course, eventually, even that huge volume must be evacuated down to the pressure that is required to fill it across what ever pressure loss is upstream! (of course, if there is no pressure difference there is no flow, regardless of how big, or small, any "Hole" might be......)
The OP can work out some basic velocity numbers for their Y piece, and that will show, as i have suggested that it is unlikely to be a restriction.......
1) A "restriction" results in the air stream accelerating, and conservation of momentum requires means that the energy to accelerate that air must come from the air stream itself, resulting in a pressure drop. However, when the air slows back down again, exactly the same amount of energy is available to be recovered, and depending on the geometry of the flow stream, large amounts can indeed be recovered.
2) The higher the velocity of the air, the higher the drag forces generated where that air impinges on a static wall, and the higher the parasitic drag. Tangential Pressure differentials (caused by velocity gradients between free stream flow and the stalled stream in direct contact with the wall) result in the flow stream vector diverging from the centreline of the duct, and hence also create a pressure loss in conjunction with local turbulence.
Unlike 1), The losses associated with 2) cannot be recovered, and for this reason, limiting the velocity within intake systems to something less than 50m/s, is, in general, regarded as a rule of thumb. A second rule, that is widely used for OEM intake design is to ensure the dynamic pressure doesn't exceed 1kPa, which really is the same limit, but with reference to air at 1bar and 25degC
If course, some restrictions are deliberate, for example the throttle plate! Here, the flow has to be choked (ie supersonic) for the throttle to actually throttle anything! Once a certain pressure differential exists, the flow becomes supersonic at the throat, and (outside of certain deliberate geometry forms such as convergent-divergent nozzles) cannot be accelerated any more. Here, the parasitic losses match the pressure diferential.
One thing you will notice is that all forms of motorsport twhich mandate air restrictors, closely control the geometry of the duct DOWNSTREAM of the restrictor (and not necessarily upstream of it!) Take a WRC 32mm dia restrictor, basic physics helps us calculate the pressure ratio of a critical (choked) flow stream, 0.52 (assuming a gamma (Cp/Cv) of 1.4) so with an upstream pressure of 1bar the pressure at the throat is 0.52 bar. And we know the flow is doing M1.0, so, with some correction for temperature and hence air density, we know that restrictor can flow, at an absolute maximum 0.193kg/s.
What happens next depends upon the pressure RECOVERY of the downstrema duct. Ideally we would have a nice, characteristically long divergent nozzle, that has an included expansion angle of less than about 6deg. This allows the air to decelerate nice and slowly, and hence we can recover a lot of the pressure lost to velocity, in fact, typically, we can get discharge co-coefficients up around 98% or even more. So, despite the low pressure and M1.0 stream at the throat, we can get back to say 98kPa in a static volume downstream of that choke point. If the expansion geometery is compromised, the pressure recovery is correspondingly poor. WRC rules mandate the restrictor throat must be within 50mm of the front face of the compressor wheel, meaning we cannot leverage high pressure recovery, and our compressor is forced to "suck" on nasty, low pressure, turbulent high velocity air, which further reduces the compressor efficiency (and hence further limits engine power) Despite that, WRC engines, with what is a pretty small restrictor (just 32mm dia remember!) make (well) over 300bhp, even with the heavily compromised compressor efficiency. An N/A engine with a 32mm restrictor and a full pressure recovery convergent duct can make 400bhp.
A 2.0 F3 engine with a tiny 28mm restrictor puts out something like 230bhp, or 115bhp/litre despite that very small hole! If you had an intake/combustion system as well designed as an F3 one, your 500bhp engine could suck through a 41.2mm diameter hole and still make that 500bhp!
But, your intake isn't as well designed as an F3 one of course, which is why you want to go to a bigger intake diameter to reduce the flow velocity, to ensure that you have less to loose through non-optimum design. But as the area of a hole scales with the SQUARE of it's radius, despite what most aftermarket tuners will tell you, you simply don't need a massive duct to make power.
Puma, sorry, Kia, points out correctly that a big volume plenum relative to the swept cylinder capacity (called a low "gulp" factor intake) results in a nice even flow through any restriction, and on an firing event basis this is correct, but you would need a truely massive intake to be able to negate any steady state flow loses in the upstream ductwork (your 500bhp engine is probably swallowing something like 300 litres/second of air (that's a ~1.5 standard sized oil drums full, every second)). And of course, eventually, even that huge volume must be evacuated down to the pressure that is required to fill it across what ever pressure loss is upstream! (of course, if there is no pressure difference there is no flow, regardless of how big, or small, any "Hole" might be......)
The OP can work out some basic velocity numbers for their Y piece, and that will show, as i have suggested that it is unlikely to be a restriction.......
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