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SVX inlet system design. Pt.2. Resonance.
SVX inlet system design. Pt.2 Resonance.
Since I first wrote about the IRIS back in August 2001, most has been about the Inertia end, as it is uniquely effective in this engine, and the first that I had come across, of this type. The high speed end of the system, I have only mentioned briefly. So I thought I might as well delve into the resonate system, and how it works, as it does come up from time to time. The Inertia system operates at the low speed end of the engines rev range, from just above idle, to 4000 rpms. It uses the pressure created by the flowing air column, in each side of the manifold, to create a positive pressure at the start and the end of the inlet stroke. This pressure provides a very high filling of the cylinders, to about 98% at about 3500 rpms. The mass of air that produces the inertia, moves too slow for high speed operation. So we use the much faster sound waves. The resonate system has to carry on this cylinder filling process, but instead of using a positive pressure in the inlet tract, it will use a negative pressure in the cylinder, at the start and end of the inlet stroke, to achieve the same results. This is done by using the pressure change that is produced by the sound pressure wave traveling the inlet and exhaust tracts. Without going into the theory of resonance sound waves, too far, it is sufficient to know that when a sound is created at the end of a pipe, the sound wave travels to the other end of the pipe, to be reflected back again, to the start. When this sound wave reaches the end of a open pipe, it is reflected back as the opposite sign, above atmospheric pressure or below i. If the end of the pipe is closed the sound wave will be reflected back as the same sign. This sound pressure wave moves through the air in the pipe, changing the pressure. It does not move the air along the pipe, though the pressure change does influence the air movement. The resonant system starts when the IRIS valve opens, to join the two inlet tracts together, to form a large volume plenum chamber, fed by the two throttle bodes, that each cylinder can draw its air from,. The wave starts when the inlet valve opens, to draw its air, it travels up the inlet tract till it reaches the plenum, where it is replaced by a positive pressure wave. This travels down the tract, through the inlet valve, down the cylinder till it meets the piston. This forms a closed end to the pipe, to reflect the same sign, positive wave back up the tract, to the plenum where it is replaced by a opposite negative pressure wave, that travels back down to create a negative pressure in the cylinder as the inlet valve is about to close. Creating the negative pressure in the cylinder, as the valve is about to close increases the flow of air into the cylinder, just as the Inertia system did with a positive pressure outside the cylinder. The Inertia system, also produced a positive pressure at the start of the inlet opening, so we need the wave energy to do this also. This one starts when the exhaust valve opens, a positive pressure sound wave is started. This wave travels down the exhaust header pipe till it reaches a large volume chamber/resonator, that allows the sound wave to expand, the same way it would, if it was the end of the pipe. This changes the pressure wave sign, to a negative pressure wave that travels back up the exhaust pipe, till it enters the cylinder again just as the inlet valve is about to open, thus the inlet tract is connected to the exhaust system, on valve overlap, till the exhaust valve closes. This then provides the negative pressure that will start the air flowing through the inlet valve, to increase the cylinder filling, even though the piston is stopped at the op. Both the inlet and the exhaust tract lengths are chosen to have the waves travel the tract and arrive at the cylinder at a particular engine speed, usually the peak torque speed. They both have to work together to allow the engine to breath at high speed The inlet tracts on the SVX have about a 2"/50mm difference in lengths, as the exhaust pipe lengths are different also, this is so that the resonant peak is spread over about 750 rpms to make the torque peak wider. It is why I always say to leave the front end of the exhaust the same till after the resonator, as it is imperative for the engine to develop high speed power. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#2
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in other words...don't get caught in a null point!
Nice discussion, Harvey! -Bill (who's been working with this same behavior on the optical side lately- interferometric telescopes!)
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Retired NASA Rocket Scientist Most famous NASA "Child" - OSIRIS-REx delivered samples from asteroid BENNU to Earth in Sept. 2023 Center Network Member #989 '92 Fully caged, 5 speed, waiting for its fully built EG33 '92 "Test Mule", 4:44 Auto, JDM 4:44 Rear Diff with Mech LSD, Tuned headers, Full one-off suspension '92(?) Laguna, 6 spd and other stuff (still at OT's place) My Locker |
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Harvey,
On the serious side, have you had the time to calculate how the temperature of the intake charge and exhaust charge influence the effective length of the "pipe"? I've kept the location of the secondary cat constant (i.e. in it's stock location) and have replaced both primary cats with 1 5/8 OD pipe that connect to a preformed Y feeding into the secondary cat. While I haven't had time to isolate the exact source of the vibration I'm getting right around 3,000 rpms, I think it might be resonance in the exhaust system. -Bill
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Retired NASA Rocket Scientist Most famous NASA "Child" - OSIRIS-REx delivered samples from asteroid BENNU to Earth in Sept. 2023 Center Network Member #989 '92 Fully caged, 5 speed, waiting for its fully built EG33 '92 "Test Mule", 4:44 Auto, JDM 4:44 Rear Diff with Mech LSD, Tuned headers, Full one-off suspension '92(?) Laguna, 6 spd and other stuff (still at OT's place) My Locker |
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Quote:
The Y pipe is not the go, a two in, is a must. The resonate wave will split at the Y, so that only half the energy will get back to the cylinder. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. Last edited by oab_au; 10-25-2005 at 07:54 PM. |
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How does pipe diameter effect this sound wave assitance?
I'm specifically interested in the exhaust side of things, because I'd like to replace my exhaust from the headers back with 2.25" pipes, two high-flow cats, a 2 into 1 resinator, split back out to two mufflers, and out through the stock tips. If I change my exhaust diameter, and/or the location/flow of the cats, how do I need to move the resinator to keep the flow assistance designed into the stock exhaust? And back to the intake side of things, how does forced induction change the IRIS's effectiveness? I'm assuming that the change in the length of the intake pipe resulting from tossing a turbo in there will effect at least the resonance side of the IRIS. 'Course, with forced induction, you would think that the IRIS system becomes useless, and that intake manifold is less important, but I read a quote from the guy that designed the intake manifold on the WRX/STI and he said that's what he assumed when he started working on the manifold, but it turned out the opposite was true, and that it was very important to design the mainifold to match the turbo application. I wonder if going to forced induction and short ram intakes, etc, is really just hobbling the motor and undoing a lot of creative engineering on Subaru's part.
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3 turbos, 28 cylinders, 96 valves, 18 cams, 1400hp/1600ftlbs: '09 Outback XT / '06 Outback Sedan / '02 WRX / '94 SVX / '01 F-250 SuperDuty |
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sperry How does pipe diameter effect this sound wave assitance?
I'm specifically interested in the exhaust side of things, because I'd like to replace my exhaust from the headers back with 2.25" pipes, two high-flow cats, a 2 into 1 resinator, split back out to two mufflers, and out through the stock tips. If I change my exhaust diameter, and/or the location/flow of the cats, how do I need to move the resinator to keep the flow assistance designed into the stock exhaust? And back to the intake side of things, how does forced induction change the IRIS's effectiveness? I'm assuming that the change in the length of the intake pipe resulting from tossing a turbo in there will effect at least the resonance side of the IRIS. 'Course, with forced induction, you would think that the IRIS system becomes useless, and that intake manifold is less important, but I read a quote from the guy that designed the intake manifold on the WRX/STI and he said that's what he assumed when he started working on the manifold, but it turned out the opposite was true, and that it was very important to design the mainifold to match the turbo application. I wonder if going to forced induction and short ram intakes, etc, is really just hobbling the motor and undoing a lot of creative engineering on Subaru's part. Quote. 1. The dia. affects the amplitude of the wave, the larger the pipe dia, the lower the energy delivered to the cylinder. 2. You are really duplicating the standard set-up, with larger header pipes. The problem is that the tuned length that it has now, is the results of all of the componets that are in the system, they all affect the final tuned length. So it is hard to just build the same length and hope it will resonate at the same rpms. 3. The IRIS will still operate, with a turbo, or a positive displacment blower. The added length for the turbo won't affect it. The tuned length for the resonate rpms, ends at the plenum, about 15" from the inlet valve. Past that is not in the resonate length. 4. Yes the inlet design for a turbo, is mainly centered around the low end of the rpm range, when the boost is not there. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
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Quote:
Harvey, Gid'ay back at you! Wouldn't the reflected wave energy coming back out of a "two in" be a function of the internal design of the Cat? I agree with your "two in" vs "Y" in comment. -Bill
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Retired NASA Rocket Scientist Most famous NASA "Child" - OSIRIS-REx delivered samples from asteroid BENNU to Earth in Sept. 2023 Center Network Member #989 '92 Fully caged, 5 speed, waiting for its fully built EG33 '92 "Test Mule", 4:44 Auto, JDM 4:44 Rear Diff with Mech LSD, Tuned headers, Full one-off suspension '92(?) Laguna, 6 spd and other stuff (still at OT's place) My Locker |
#8
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Quote:
When there's a decent sized volume change from the pipe to the resinator, there's a pressure drop. It's this pressure drop that will prevent the sound wave from one pipe traveling back out the other pipe. Essentially, it's the location of the resinator that determines the "tuned length" of the exhaust. Someone correct me if I'm wrong, I'm new to this topic!
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3 turbos, 28 cylinders, 96 valves, 18 cams, 1400hp/1600ftlbs: '09 Outback XT / '06 Outback Sedan / '02 WRX / '94 SVX / '01 F-250 SuperDuty |
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