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The K20A3 intake ports are smaller and have specially shaped aluminum
cutouts in the port floor to enhance turbulence. This probably helps low
end torque at the expense of top end power.
The VTEC mechanism differ significantly between the K20A2 and A3 head.
The K20A3 uses only two cam lobe profiles on the inlet.
Under 2200 rpm, this engine is a effectively a 12 valve engine, and
over 2200 rpm it is a 16 valve. There is no VTEC mechanism on the K20A3
exhaust valves.
Basically, this head does not breath well compared to the K20A2. It
seems designed for low end torque, emissions and economy.
The K20A2 head uses three cams per pair of inlet valves on both the
exhaust and inlet. The low lift cams on this head are probably very
similar to the high lift cams on the K20A3. The high lift cams engage at
5800 rpm and disengage at 5600 rpm.
The K20A3 uses plastic dual length intake runners similar to the GSR
The short runners provide an additional 5-10 ft. lb of torque between
3000-5000 rpm. The short runners contribute to top end power from
5000-7000 rpm.
The switch to short runners is made at 4700 on increasing revs and back
to long runners at 4500 on descending revs. The throttle body is 60 mm in
diameter. The long runner is 38mm in diameter by 540 mm. This is quite
narrow and provides a high intake velocity, peaking torque at 4000 rpm.
The short runners measure 40 by 270 mm.
In this dyno chart, the long runners were locked on and tested
against their normal operation. 10-15 Ft/lb of torque was lost. If only
the short runners were used a loss in high end power would be seen.
The K20A2 Type S (Integra) and Civic Si share the same intake design
with 42mm by 210mm runners and a 62 mm throttle body. Completely
aluminum this manifold heats the incoming air by contact with a metal
gasket to the head and by transmitting heat from the hot radiator air.
Gains are to be made by cooling this manifold. Two options are to us the
Hondata intake gasket and to ceramic coat the intake. This manifold
seems to breath very well.
Unlike VTEC operation VTC operates continuously throughout all engine
speeds and throttle positions. The intake cam can be advanced by as much
as 50 degrees. Advancing the cam at part throttle generates an EGR effect
where some exhaust gas enters the intake. This helps the RSX attain LEV
(low emissions vehicle) status.
Oil pressure fed to either side of the lobes in cam advance mechanism
allows the ECU to continuously advance or retard the cam to meet either
emissions or power requirements. As far as power is concerned this allows
Honda to optimize torque across the entire rev range. For the technically
minded, this is a complex rapidly acting closed loop feedback system.
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