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Old 08-09-2001, 09:44 PM
oab_au oab_au is offline
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SVX Inlet system design.

The Subaru SVX is a unique auto designed from a clean sheet of paper owing little to any other auto, with many unique designs, the inlet system being one of them. Subaru have looked at the other designs and built a better one!.

The modern inlet system has evolved into a highly tuned, resonate system that provides increased torque over a wide rev range. With the introduction of fuel injection, because all the injectors fired together, and each cylinder inducts one at a time, in turn. The inlet tracts were lengthened to prevent one cylinder from stealing all the fuel. This lengthening also allowed the tracts to be of a tuned length, so the pressure waves resonate to match the peak crankshaft torque revs.

The next inlet system to evolve is the two stage design, to suit high and low engine RPM. This is done by designing a long tract to resonate at low RPM , a short inlet tract for higher RPM. These are then switched between at the appropriate engine RPM. Most modern engines are fitted with this type of inlet system, in one form or another, Porsche use 3 and 4 stage tuned systems.

Ford in Aust. use a long tract that starts at the valve passes under the plenum chamber curves over the top to enter the chamber from the top, for low RPM. The short tract is provide by a butterfly valve that opens the tract to the bottom of the plenum, by-passing the long curl over the top.The long tract starts at the valve and runs to the plenum chamber, where the sudden increase in volume returns the negative wave as a positive pressure, this is used at low RPM, when the time for induction is longer, about 20mil/sec. This longer tract also allows the charge to develop enough inertia, to keep the inlet charge flowing into the cylinder, around BDC near the end of the inlet stroke. Even though the piston has stopped moving at the bottom of the stroke, before moving up on the start of the compression stroke. This assists cylinder filling and torque is increased at these low RPM. It must be noted that only one cylinder inducts at a time, 120degs apart and that the piston does stop, at the top and bottom of the stroke.

The short tract starts at the valve and ends at the plenum chamber, used at high RPM. because the time for induction is much shorter about 6.6mil/sec, too short to move the long column of air in the inlet tract, the inertia now starts to work against us by creating a lag between the maximum piston speed (at around 83degs. ATDC.) and the maximum air speed which may arrive about 20degs later. The short tract can now induct directly from the plenum which is refilled from the central throttle valve.

.Subaru have used a difference approach, take the covers off, look under the manifold !. Subaru uses two inlet tracts separated by a large butterfly valve so that when closed at low RPM there is a smoother flowing 3 branch inlet for each side, inducting 240degs apart, without a plenum in sight. Because of the long, and increasing diameter of the inlet tract, without any sudden changes in cross sectional volume, the tuned length can now extend right back to the throttle bodies, instead of being terminated at a plenum The shear weight of the moving charge causes a useful pressure to be present at the cylinder. As one valve closes on a fast moving column of air, the next cylinder opens its inlet valve to take the flow, even though it's piston is stopped at the top of the stroke. Because the main section of this inertia is developed before the individual cyl branches, this pressure can be passed on from one cyl. to another, thus the induction is assisted not only at the end, but also at the start of the inlet stroke. This is not possible when the low speed tracts end at a plenum chamber.
This is the type of Ram induction that was used successfully by Chrysler in the 60s, to increase the bottom end torque, unfortunately they didn't have a high speed tract to switch to so the top end was lacking. (not that they had much )

The engineers have made no attempt to keep the low speed tracts the same length, if fact allowing a 7in. variation in tuned length, there by spreading the resonate peak over about 1000 RPM. This effect is carried over to the high speed tracts, which have about a 2in variation in length to spread the torque peak over about 750 RPM .

At. around 4500 RPM the large butterfly valve between the inlet tracts opens, connecting both tracts together, this then forms the large cross sectional volume which terminates the high speed tract at the now formed plenum, that functions as a Helmholts resonator, to allow each cylinder to induct its 550cc of air from the chamber, which is now supplied by two throttle bodies.

What Subaru designers have achieved is a two stage, tuned length, inlet system that spreads the torque from about from 2000 RPM to 5400 RPM, with a very effective ram charging system that operates from idle right up to when the high speed butterfly valve opens to form the plenum. This is where the engine gets it wide spread of torque and strong low speed response from.

Harvey. (sorry this is so long.)
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  #2  
Old 08-09-2001, 10:15 PM
Ron Mummert Ron Mummert is offline
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Hey eddycat, you writin' all this down?
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Old 08-10-2001, 02:27 AM
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Hey eddycat, you writin' all this down?
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  #4  
Old 08-10-2001, 10:26 AM
BradT
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great stuff, though not Subaru's first one

Actually, Subaru had a dual-stage induction system two years earlier, in its Japanese 2.0L turbo engine.

(For those who are keeping track. ;~)

--Brad
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