Intake Manifold runner length
09-07-2008, 02:29 PM
#22
Most stock manifolds are used to further reduce turbo lag by dramatically increasing low R.P.M. torque giving you that feeling that your car is fast, without it actually being so. As for the type of flow in the pipes, there should be very little flow that can be considered laminar as the Reynolds number is from 55000 to 100000. The cutoff for laminar is 2300. Its also pulsed as that’s another indication the flow in runners is entirely turbulent. That’s the reason the S2000 head flows so well despite all its jagged edges. It takes advantage of the turbulent and non-Newtonian characteristics to setup fake intake port walls that vary depending on the volumetric efficiency and speed of the engine. Very detailed science there...
6 inches should be a solid starting place. 4 Inches should give maximum top end performance. Don't go below 4 inches as total induction length should be too short to benefit the R.P.M. range. It is not true that inertial tuning isn't important. It’s just as useful at any intake manifold pressure. Whether you are on Mars, in Death Valley or on Mount Everest the intake manifold length matters when it comes to output. This is because regardless of how much pressure there is in the plenum there is only so much air that can get down that pipe, and that amount of air is based on when the plenum feels the intake pulse from the cylinder. As the pulse comes up the pipe it signals the air in the plenum to start moving. Since the speed of sound is the fastest information can travel in a gas, there is a period of time when the valve is open and the pipe is flowing, but the air around the intake pipe is completely stagnant.
If that’s hard to understand think of it like a stoplight. If you’re the 5th car back behind 4 tall semis. You have no idea that the light is green and you can go until the semi in front of you starts to move. Then you can move. If you setup the intake system right you can line it up so you can get a lot of cars through at once. To get that to happen think about the pulse waves as the semis, and there must be so many semis to cars. The cars are the good part of the pulse, the fresh air. If the R.P.M. is such that a semi is moving and a pack of cars gets through that would be a tuned system. If the system is such that the semi can’t get started fast enough and gets stopped by the stop light before he can get through and let the cars behind him through then that’s an R.P.M. where the waves are setup incorrectly. An adjustable runner system makes sure that there is always just the right amount of semis. Having a turbocharger is like increasing the amount of cars in between each semi. Short runners makes sure there is always a correct amount of semis for High R.P.M. and long runners make sure there is a correct amount for low R.P.M.
I hope that helped clear it up...
6 inches should be a solid starting place. 4 Inches should give maximum top end performance. Don't go below 4 inches as total induction length should be too short to benefit the R.P.M. range. It is not true that inertial tuning isn't important. It’s just as useful at any intake manifold pressure. Whether you are on Mars, in Death Valley or on Mount Everest the intake manifold length matters when it comes to output. This is because regardless of how much pressure there is in the plenum there is only so much air that can get down that pipe, and that amount of air is based on when the plenum feels the intake pulse from the cylinder. As the pulse comes up the pipe it signals the air in the plenum to start moving. Since the speed of sound is the fastest information can travel in a gas, there is a period of time when the valve is open and the pipe is flowing, but the air around the intake pipe is completely stagnant.
If that’s hard to understand think of it like a stoplight. If you’re the 5th car back behind 4 tall semis. You have no idea that the light is green and you can go until the semi in front of you starts to move. Then you can move. If you setup the intake system right you can line it up so you can get a lot of cars through at once. To get that to happen think about the pulse waves as the semis, and there must be so many semis to cars. The cars are the good part of the pulse, the fresh air. If the R.P.M. is such that a semi is moving and a pack of cars gets through that would be a tuned system. If the system is such that the semi can’t get started fast enough and gets stopped by the stop light before he can get through and let the cars behind him through then that’s an R.P.M. where the waves are setup incorrectly. An adjustable runner system makes sure that there is always just the right amount of semis. Having a turbocharger is like increasing the amount of cars in between each semi. Short runners makes sure there is always a correct amount of semis for High R.P.M. and long runners make sure there is a correct amount for low R.P.M.
I hope that helped clear it up...
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09-10-2008, 05:58 PM
#24
www.not2fast.com/gasflow/Lecture08.ppt
Check there, that system takes advantage of an entirely different way of looking at it. Interesting enough the values which take advantage of intertial tuning usaully don't give good results in helmholtz and vice versa. Having said that I've never used this system, although I'm planning on doing a N/A manifold on the Miata that uses it. Its so much more complex to work with. Its all planted in Diff EQ. Inertial tuning can be done with algebra. I've always heard that helmholtz is very R.P.M. specific so I would have to imagine that they really played a good game to get it to respond over a broad range.
Check there, that system takes advantage of an entirely different way of looking at it. Interesting enough the values which take advantage of intertial tuning usaully don't give good results in helmholtz and vice versa. Having said that I've never used this system, although I'm planning on doing a N/A manifold on the Miata that uses it. Its so much more complex to work with. Its all planted in Diff EQ. Inertial tuning can be done with algebra. I've always heard that helmholtz is very R.P.M. specific so I would have to imagine that they really played a good game to get it to respond over a broad range.
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