K Series VTEC Breakdown
K Series Motors
by Kent Douglas
K20a-JDM ITR and CTR both come in with 11.5:1 compression and 2 liters of displacement. The Type R motor is rated at 220hp at 8,000rpm and 152lb-ft at 7,000 rpm. Both of these motors have the same VTEC mechanism as the older B series motors but with the addition of VTC. It also has larger cams, higher compression and larger intake manifold than the Type S motor. This motor comes with a 6 speed transmission and LSD. The 6-speed manual transmission uses multiple synchronizing cones for all gears, including a triple-cone synchronizer in 1st and 2nd gears and a double-cone synchronizer in 3rd through 6th gears.
K20a2-Acura Rsx Type S has 11.0:1 compression and 2 liters of displacement. The Type S is rated at 200hp at 7,400 and 142 lb-ft at 6,000 rpm. This motor also comes with a 6 speed transmission with no LSD. It has the same VTEC mechanism as the older B series motors but with the addition of VTC.
K20a3-Acura Rsx Base and Civic SI models come in with 9.8:1 compression and 2 liters of displacement. The Base motor is rated at 160hp at 6500rpm and 141lb-ft at 4000 rpm. This motor does not use the conventional VTEC mechanism from the earlier B series motors but does have VTC. Instead it only has 2 cam lobes on the intake side and 1 on the exhaust lob. This motor has no VTEC mechanism on the exhaust cam and runs on 12 valves before 2200rpm with the other 4 slightly opening. It also has smaller ports than the Type S and Type R. The motor comes with a 5 speed transmission instead of the 6 speed and has no LSD.
K24a1-Honda CRV comes in with 9.6:1 compression and 2.4 liters of displacement. The motor is rated at 160hp at 6000rpm and 162 at 3600 rpm. This motor does not use the conventional VTEC mechanism from the earlier B series motors but does have VTC. Instead it only has 2 cam lobes on the intake side and 1 on the exhaust lob. This motor has no VTEC mechanism on the exhaust cam and runs on 12 valves before 2200rpm with the other 4 slightly opening. It also has smaller ports than the Type S and Type R. The motor also comes with a 5 speed transmission instead of the 6 speed and has no LSD.
K24a2- The Acura TSX comes in with 10.5:1 compression and 2.4 liters of displacement. The motor is rated at 200hp at 6800rpm and 166lbs-ft at 4500rpm. This motor uses the conventional VTEC mechanism as the older B series motors but with the addition of VTC. This motor uses a drive by wire throttle and has EGR castings on the manifold. This motor comes with a 6 speed transmission and no LSD.
K24a4/k24a3- the Honda Element and Accord come in with 9.7:1 compression and 2.4 liters of displacement. The motor is rated at 160hp at 5500rpm and 161lbs-ft at 4500rpm. This motor does not use the conventional VTEC mechanism from the earlier B series motors but does have VTC. Instead it only has 2 cam lobes on the intake side and 1 on the exhaust lob. This motor has no Vtec mechanism on the exhaust cam and runs on 12 valves before 2200rpm with the other 4 slightly opening. It also has smaller ports than the Type S and Type R. The motor also comes with a 5 speed transmission instead of the 6 speed and has no LSD.
The K20A3 does not have a standard DOHC VTEC valvetrain as we know it from the B-series engines - the K20A3 should actually be called a “DOHC i-VTEC-E” engine, because it uses a VTEC-E cam setup. The K20A2 is the “real” DOHC i-VTEC engine, utilizing the standard DOHC VTEC cam setup we’re all familiar with. To help you understand the differences between the K20A2 and K20A3 engines, I’ve included the following information from a post I made elsewhere:
Allow me to evaluate. Let’s start out by defining some terms:
VTEC - Variable valve Timing and lift Electronic Control. At low RPM, a VTEC engine uses a normal cam profile to retain a smooth idle, good fuel economy, and good low-end power delivery. The VTEC mechanism engages a high-lift, long-duration “race” cam profile at a set RPM value (i.e., ~5500RPM on the B16A) to increase high-end power delivery.
VTEC-E - Variable valve Timing and lift Electronic Control for Efficiency. This system isn’t really VTEC as we know it. At low RPM, the VTEC-E mechanism effectively forces the engine to operate as a 12-valve engine - one of the intake valves does not open fully, thus decreasing fuel consumption. At a set RPM value (i.e., ~2500RPM in the D16Y5), the VTEC-E mechanism engages the 2nd intake valve, effectively resuming operation as a normal 16-valve engine. Note: in a VTEC-E engine, there are no high-RPM performance cam profiles; this engine is supposed to be tuned for fuel economy, right?
VTC - Variable Timing Control. This is a mechanism attached to the end of the intake camshaft only which acts as a continuously variable cam gear - it automatically adjusts the overlap between the intake and exhaust cams, effectively allowing the engine to have the most ideal amount of valve overlap in all RPM ranges. VTC is active at all RPMs.
i-VTEC - intelligent Variable valve Timing and lift Electronic Control. This is a combination of both the VTEC and the VTC technologies - in other words, i-
VTEC = VTEC + VTC. Currently, the only engines that use the i-VTEC system are the DOHC K-series engines.
Now this is where things get tricky - Honda uses the term “DOHC i-VTEC” for two different systems: The first system is used in the K20A2 engine of the RSX Type-S. The second system is used in the K20A3 engine of the Civic Si.
The First System (K20A2):
This system is pretty close to the older DOHC VTEC engines. At low RPM, the K20A2 uses a normal cam profile to retain a smooth idle, good fuel economy, and good low-end power delivery. At 5800RPM, its VTEC mechanism engages a high-lift, long-duration “race” cam profile to increase high-end power delivery. The only difference between this i-VTEC engine and the older VTEC engines is the addition of the VTC system. The intake camshaft has the automatic self-adjusting cam gear which continuously optimizes valve overlap for all RPM ranges.
Here we see an image of the intake cam lobes of the K20A2. Notice there are 3 lobes; the two side lobes are the low-RPM profiles, and the center lobe is the high-lift, long-duration profile which engages at 5800RPM. Basically the same setup as the old VTEC engines we are familiar with.
Now here we see the VTC mechanism - the gear on the end of the intake cam that adjusts valve timing (overlap) automatically on the fly.
This system is used in engines powering the JDM Honda Integra Type-R, Civic Type-R, Accord Euro-R, and the USDM Acura RSX Type-S and TSX.
The Second System (K20A3):
This system does not really conform to the “DOHC i-VTEC” nomenclature, as Honda would like us to believe. As I mentioned in my previous post, it actually should be called “i-VTEC-E,” because it uses a VTEC-E mechanism rather than a standard VTEC mechanism. At low RPM, the VTEC-E system effectively forces the engine to operate as a 12-valve engine - one of the intake valves does not open fully, thus decreasing fuel consumption. At 2200RPM, the VTEC-E system engages the 2nd intake valve, effectively resuming operation as a normal 16-valve engine. There are no high-RPM performance cam profiles; this engine is tuned to balance fuel economy and power, rather than provide pure performance. On the intake cam, there is the VTC mechanism which basically is an automatic self-adjusting cam gear used to continuously optimize the valve overlap for all RPM ranges. This being a VTEC-E system - and not a true DOHC VTEC system - is the reason the K20A3 redlines at a measly 6800RPM, while the K20A2 is able to rev all the way to 7900RPM.
Here we see an image of the intake cam lobes of the K20A3. Notice there are only 2 lobes - there is a nearly round one used only for the low-RPM disabled intake valve, and then there is the regular lobe used by the other valve at low-RPM and by both valves at high-RPM:
This system is used in engines powering the USDM Acura RSX base, Honda Civic Si, Accord 4-cylinder, CRV, and Element.
Special note: The K20A3 engine used in the Acura RSX base has a slightly different intake manifold design from the K20A3 engine used in the Civic Si. The RSX engine uses a dual-stage manifold, similar in concept to the manifold of the B18C1 in the old Integra GSR. It uses long intake runners at low-RPM to retain low end power, and switches at 4700RPM to a set of shorter intake runners to enhance high-end torque. This accounts for the extra 9 ft-lb of torque in the RSX (141 ft-lb, vs. 132 ft-lb in the Civic Si).
Myths:
1. The i-VTEC engine engages VTEC gradually, and not suddenly like in the old VTEC engines.
Wrong. The i-VTEC engine “engages VTEC” at a single set RPM, like always. Read the definitions above.
2. VTC engages at a set RPM.
Wrong. VTC is always activated. Read under “VTC” above.
3. The K20A3 engages VTEC at 5000+ RPM.
Wrong. Technically, there is no “VTEC” (as we think of it) in the K20A3 engine - it uses a VTEC-E technology, which engages at 2200RPM. Read under “The Second Sytem” above.
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Civic 92 DX EG6
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