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K-Series Engine Family Breakdown - K-Series Refresher

Three flavors of i-VTEC: Which one's right for you?

Aaron Bonk
Feb 13, 2014

It's been more than a decade since Honda introduced its K-series engine family and a lot's changed since then. What was first met with resentment from Honda fans all-too complacent with B18Cs and LS-VTECs has developed into it becoming the engine of choice, especially for naturally aspirated hard-liners who pine for cubic inches and a modernized valvetrain. Thirteen years later, though, and there's a whole lot of K-series engines to sort through, which can make choosing the right one downright baffling.

You don't need to be told that not all K-series engines are the same. Pick the wrong one and you might have been better off putting up with whatever B-series you've shunned as its predecessor. And none of this has ever been more true than now, especially when considering Honda's new fleet of K-series engines that care more about reduced emissions and fuel consumption than they do horsepower and upgradability. Follow along as we break down Honda's most popular K-series powerplants, from past to present.

K-Series Development

But first, a little history. We Americans were first introduced to the K-series with the help of the 2002 RSX, Civic Si, and CR-V. Following a 12-year reign, the B-series was finally nixed and, in late-2001, replaced with the then-unfamiliar K20A2, K20A3, and K24A1. The new platform's most notable differences when compared to previous four-cylinder Honda mills is its direct-fire ignition system, reverse layout, and clockwise rotation. To improve catalytic converter light-off by positioning the engine's exhaust side toward the rear of the vehicle, Honda developed a clockwise-rotating architecture, which is a whole lot different than the decades worth of counter-clockwise-rotating engines the company's become famous for.

The number of K-series engine designations aren't few, but fortunately almost all of them can be traced back to Honda's initial K20A2 and K20A3. Although similar in many respects and both featuring Intelligent Variable Valve Timing and Lift Electronic Control (i-VTEC), the engines' valvetrains and VTEC implementation are where they differ most.

Performance I-Vtec

Unofficially known as performance i-VTEC, such engines feature Honda's most impressive valvetrain to date. At the heart is something you've heard of: VTEC. Like its predecessors, inside the cylinder head lies Honda's hydraulically operated variable valve timing system. Simple and elegant, VTEC allows the engine to alternate between two different camshaft profiles depending on various conditions. In terms of i-VTEC engines, though, there's more, and it's called Variable Timing Control (VTC). Here, a specialized camshaft gear allows for continuously variable intake camshaft phasing throughout the RPM range. Based upon all sorts of things that the driver needn't worry about, like camshaft position, ignition timing, exhaust oxygen content, and throttle position, a 50-degree range of camshaft phasing can occur (25-degree range on K24A2). Much like VTEC, the camshaft gear is electronically controlled and hydraulically driven, resulting in reduced timing at idle and advanced timing at higher engine speeds, which increases valve overlap and power. Fittingly, the "i" in i-VTEC stands for intelligent and, together, VTEC and VTC offer an unprecedented balance between performance and emissions.

The advantages up top span beyond the valvetrain. In terms of airflow, performance i-VTEC cylinder heads shame even the best of B-series top ends. This is, in part, due to oversized valves—2 mm larger than B-series VTEC engines. Performance i-VTEC engines also feature more optimally shaped intake and exhaust ports when compared to the B-series and larger ports when compared to other K-series engines. On the intake side, they form a nearly straight path. On the exhaust side, they're free of the humps typically found within B-series ports. K-series engines also benefit from sophisticated roller rocker arms, which reduce valvetrain friction and allow for all manner of billet camshafts. This is in stark contrast to older Honda engines that feature standard, pad-style rocker arms.

Down below, every 2.0L engine features a 86mm bore and 86mm stroke. Also known as a square design, such engine architectures are a good compromise between decent low-end torque and adequate top end power. The 2.4L engine's over-square design, which features a longer stroke and slightly larger bore, results in more torque, but at the expense of any additional top end power. In terms of crankshafts, each is forged from high-quality steel, but only performance i-VTEC versions feature a fully counter-weighted design. Performance i-VTEC engines also feature under-piston oil squirters for increased cooling, stronger connecting rods and, generally, higher-compression pistons.

In terms of North American-spec and Japanese-spec K-series engines, the K20A2, K20Z1, K20Z3, K24A2, K20A, and K24A all share a similar i-VTEC architecture. Although engine geometry and ancillaries such as intake manifolds, throttle bodies, and exhaust manifolds may differ between them, they all share more in common than not and make up some of the most desirable K-series engines.

K20A2

  • 2002-2004 RSX Type-S: 200 hp, 142 lb-ft torque

The most established of K-series engines, the K20A2 is based off of an all-aluminum block and DOHC, 16-valve cylinder head. Compared to economy-minded K-series engines, the K20A2 features larger intake and exhaust ports, a higher, 11.0:1 compression ratio, and a single-stage aluminum intake manifold that's designed for top-end performance. As is the case with all performance i-VTEC engines, a stiffer, cast-aluminum oil pan is also incorporated that increases overall rigidity.

K20Z1

  • 2005-2006 RSX Type-S: 210 hp, 143 lb-ft torque

As Acura performed its mid-model refresh on its RSX, a revised K20Z1 was placed underneath its hood for the model's final two years of production. Generally speaking, there isn't much difference between the K20A2 and the K20Z1 other than slightly more aggressive camshafts, a higher-flowing catalytic converter, as well as a larger intake duct and exhaust, which altogether net another 10 horsepower.

K20Z3

  • 2006-2011 Civic Si: 197 hp, 139 lb-ft torque

The Si's powertrain is unsurprisingly similar to the K20A2 with the exception of its electronics and ancillaries, the most notable of which are its free-flowing intake manifold and drive-by-wire throttle body. The K20Z3's intake manifold, also known as the RBC manifold, which is indicative of Honda's parts naming system, has gone on to serve as one of the most popular bolt-ons for non-Z3 K-series engines because of its exceptionally short runners, straight-through design, and high-flowing capabilities. K20Z3 engines also feature revised camshaft profiles when compared to older engines as well as internal balance shafts, which, in an effort to minimize engine vibrations, sacrifice a bit of power. The K20Z3 also features a deeper, larger-sumped aluminum oil pan when compared to other K-series engines.

K24A2

  • 2004-2005 TSX: 200 hp, 166 lb-ft torque
  • 2006-2008 TSX: 205 hp, 164 lb-ft torque

Honda began producing 2.4L K-series engines for the 2002 model year but didn't release a proper performance i-VTEC version to the United States until the TSX was unveiled. Unlike the K20A's square-bore configuration, 2.4L K-series engines feature a significantly longer 99mm stroke, a 1mm larger bore and, in the case of the K24A2, a lower, 10.5:1 compression ratio. What's more, 2.4L K-series engines are the largest four-cylinders the company's ever produced. Compared to other North American-spec 2.4L K-series engines, the K24A2's internals were made stronger in order to cope with its higher redline, and like most other 2.4L K-series engines, its crankshaft girdle is slightly thicker than those found in 2.0L engines. Like all 2.4L K-series engines, the K24A2's oil pump is counter-balanced, making it less effective at higher engine speeds. Fortunately, the K20A2 oil pump transfers over relatively easily following slight modifications made to it in order to clear the K24A2's thicker crankshaft girdle. Up top, all K24A2s feature cylinder heads similar to other performance i-VTEC engines but with slightly narrower ports that yield increased air velocity. All K24A2 engines also feature single-stage aluminum intake manifolds with long, narrow runners, and drive-by-wire throttle bodies. Several changes for the 2006 model year, like 1 mm larger intake valves and a higher-lift, longer-duration intake camshaft that's arguably better than some Type R camshafts, result in an additional 5 hp. Other mid-model changes include even stronger connecting rods, a more thoroughly counterweighted crankshaft, and additional oil passages within the block, which help reduce windage. A larger throttle body and freer flowing exhaust components were also added.


The accompanying three images break down the three different types of K-series engines into three easy pieces: performance i-VTEC engines that feature VTEC on both camshafts, economy i-VTEC engines that feature VTEC only on the intake side, and the other economy i-VTEC engines that feature integrated exhaust manifolds.


K20A

  • 2001-2005 Civic Type R: 212 hp, 150 lb-ft torque
  • 2001-2006 Integra Type R: 217 hp, 152 lb-ft torque
  • 2007-2011 Civic Type R: 222 hp, 159 lb-ft torque
  • 2009-2012 Civic Type R Euro: 198 hp, 142 lb-ft torque
  • 2002-2008 Accord Euro R: 217 hp, 152 lb-ft torque

Honda didn't disappoint when it came to developing its top-of-the line K-series—the K20A Type R. The company produced several versions of its Japanese-spec K20A, though, of which only some were designated for Type R-badged chassis. It's important to mention that since Japanese-spec Honda engines use slightly different nomenclature when compared to the rest of the world (note the absence of any number following the K20A), that there are several variations of what, at first glance, might appear to be the same engine. For example, Honda fitted its top-of-the-line 2007-2011 Civic Type R with the K20A yet placed a similarly named K20A inside of its Integra Type-S, which isn't much different power-wise than the North American RSX Type-S. The K20A Type R engine family features the same square-bore architecture of other K20A engines and compression ratios ranging from 11.5:1-11.7:1. Power varies slightly between the mix but remain among the most sought after K-series engines to date. Also making up the Type R engines are even more aggressive camshafts and a performance-tuned, short-tract, aluminum intake manifold. As with previous Type R engines, in stock form they're difficult to beat, but if significant modifications are planned, starting with a less-expensive North American-spec engine isn't a bad idea.

K24A

  • 2004-2006 Accord 24S, 24T, 24TL: 197 hp, 171 lb-ft torque
  • 2006-2008 Accord 24TL, Type S: 197 hp, 171 lb-ft torque
  • 2008-2013 Accord Type-S: 206 hp, 171 lb-ft torque

Similar to the Japanese-spec K20A, Honda also made a variety of 2.4L performance i-VTEC engines available in its homeland. Like the North American-spec K24A2, the K24A shares a similar design yet produces slightly less power and slightly more torque, and like the K24A2, the K24A features a drive-by-wire throttle body and a modest, 10.5:1 compression ratio.

Economy I-VTEC

Although similar in many ways, the economy i-VTEC version of Honda's variable valve timing is different enough to command its own nickname. Much like Honda's emissions-conscious VTEC-E of the mid-1990s, impressive power figures were not the primary concern when developing these engines. Most notably, their exhaust camshafts are stripped of VTEC and their intake camshafts feature only two lobes and two rocker arms per cylinder instead of three. Prior to VTEC engagement, economy i-VTEC engines make use of only a single intake valve per cylinder, even though they do indeed feature 16-valve cylinder heads. The remaining intake valve cracks open slightly—just enough to prevent unburned fuel from pooling up behind itself. Also known as valve idling, the process allows the engine to operate using a conservative amount of fuel at low engine speeds with improved power at higher engine speeds when both valves open and close normally. Even VTEC is tuned differently, with low emissions paramount. As a result, a unique swirl develops inside of the combustion chambers as well as an extremely lean air/fuel mixture, resulting in impressive combustion and fuel efficiency, but not a whole lot of power. At a predetermined engine speed, the secondary intake valve opens, at which time the valvetrain operates, as you'd expect. Unfortunately, overall lift and duration don't increase like with traditional VTEC engines. Unlike performance i-VTEC cylinder heads, economy i-VTEC top-ends also exhibit an awkward groove cut into the walls between each intake valve, which plays a role in their single-valve VTEC operation, but costs in terms of airflow and performance. To the disappointment of Honda fans everywhere, as of the 2012 model year, only economy i-VTEC engines are offered.

Underneath, economy i-VTEC engines share some of the same characteristics as the performance-oriented K-series. The blocks are essentially the same, the crankshafts are similar but not as thoroughly counter-weighted, and their connecting rods share similar geometry albeit a generally thinner and weaker design. Their pistons are where most of the differences lie, though. To help with emissions, many economy i-VTEC pistons feature a rounded dish on top that, together with their curious intake valve operation, help further promote the swirling effect. Economy i-VTEC engines are also missing under-piston oil squirters (although provisions for them are still there), which help promote lower temperatures but aren't necessarily needed when upgrading to forged pistons. Underneath, a more conventional stamped-steel oil pan can be found on most economy i-VTEC engines, which is in stark contrast to performance i-VTEC engines' more rigid aluminum versions.

K20A3

  • 2002-2005 Civic Si: 160 hp, 132 lb-ft torque
  • 2002-2006 RSX base model: 160 hp, 141 lb-ft torque

The K20A3 doesn't immediately look all that different from the K20A2. As it turns out, the important differences between the two are mostly internal. Because of the K20A3's conservative implementation of VTEC and lower 9.8:1 compression ratio, only 160 hp is registered. Worse yet, the Civic Si version yields even less torque. Despite all of this, considering their entry-level cost, there are far worse engines. Although internally the same, the RSX's and Civic Si's K20A3 engines share different intake manifolds, which results in the Si's torque loss. The RSX features a composite, two-stage intake manifold, which increases low-end torque by essentially adjusting its plenum volume with a series of throttle plates depending upon engine speed. The dual-stage design incorporates a short runner and a longer runner for each cylinder. Below 4,600 rpm, the long runners deliver air exclusively. Above 4,600 rpm a rotary valve placed inside of each short runner opens, allowing additional air into the cylinders. The Si's K20A3 features a more conventional, single-stage, aluminum intake manifold.

K24A1 / K24Z1

  • 2002-2006 CR-V: 160 hp, 162 lb-ft torque
  • 2007-2009 CR-V: 166 hp, 161 lb-ft torque

The CR-V's K24A1, at least in sentiment, is very much like its predecessor's 2.0L B-series engine. For example, both engines were designed with low-end torque production in mind, and neither seem to feel all that comfortable at higher engine speeds like the more nimble B16A, B18C, or K20A engines do. The 2.4L engine's additional torque is provided by means of a slightly larger bore and significantly longer stroke, similar to the K24A2, geometrically speaking, but with entirely different components. Surprisingly, its compression ratio is a modest 9.6:1 yet the 2,394 cc K24A1 still achieves impressive performance. Like all 2.4L K-series engines, the K24A1's connecting rods are stronger than 2.0L economy i-VTEC versions and their crankshafts are better counter-weighted. A composite, two-stage intake manifold is present on the K24A1, which, along with its larger displacement, ensures respectable power and impressive torque. Although the K24A1 doesn't feature VTEC on both camshafts, its cylinder head port configuration and flow characteristics are very much like the K20A2 and K20Z1, which make it an ideal candidate for cost-effective 2.4L engine swaps. The K24A1 also shares much of the K20A2's casting underneath, allowing for the popular RSX Type-S oil pump to bolt on without modification, unlike with other 2.4L K-series engines. Finally, much like the B-series' popular LS-VTEC configuration, the K24A1 makes for the perfect candidate for such conversions, allowing for easy adaptation with performance i-VTEC cylinder heads like the K20A2 thanks to its nearly flat pistons. In 2007, Honda updated its economy i-VTEC K-series for the new CR-V. The K24Z1 marginally beats out the K24A1 horsepower-wise although its bottom-end internals remain the same. The K24Z1 also features a cast-aluminum oil pan that's proven to increase the block's overall rigidity, and a cast-aluminum, single-stage intake manifold. All K24Z1 engines are also equipped with drive-by-wire throttle bodies.

K24A1 bottom end 05 Photo 5/5   |  
Swapping out the original K20Z3 block for a built K24A1 bottom end resulted in a large increase in torque for the HT Project Si sedan. This combination of engine parts has become common practice, especially for 8thgen Civic and RSX owners.

K24A4 / K24A8

  • 2003-2005 Accord: 160 hp, 161 lb-ft torque
  • 2003-2006 Element: 160 hp, 161 lb-ft torque
  • 2006-2007 Accord: 166 hp, 160 lb-ft torque
  • 2007-2008 Element: 166 hp, 161 lb-ft torque

The K24A4 and K24A8, structurally, aren't much different than the K24A1 except for the fact that K24A8 Accord engines feature a drive-by-wire throttle body, single-stage aluminum intake manifold, and slightly different cylinder head port configurations that don't flow as well. The presence of an exhaust gas recirculation valve as well as a single-stage intake manifold, unlike the CR-V's dual-stage system, also separate the Accord and Element engines from the K24A1. Marginally different from the K24A1, the K24A4 yields similar power while the K24A8 betters both slightly. The K24A4, in particular, was designed to meet stringent low-emissions standards, which is an impressive feat considering power wasn't sacrificed. The K24A4's VTC mechanism also functions more for the sake of emissions-consciousness. Here, VTC allows for a lower idle and reduced pumping losses and significant valve overlap in the upper RPM range through an exhaust gas recirculation effect, much like how older Honda engines' exhaust gas recirculation valves worked, only now internally. As redline is approached with the throttle fully opened, timing is continually retarded to allow for better cylinder scavenging and pumping efficiency. The K24A4 also features internal balance shafts located within the oil pan, which help minimize second-order harmonic vibrations for a smoother ride.

The Other Economy I-VTEC

With their integrated exhaust manifolds and non-existent performance-oriented VTEC, the case for choosing any of the following engines instead of Honda's performance i-VTEC or more conventional economy i-VTEC engines isn't a good one. In an effort to further improve catalytic converter efficiency, Honda eliminated its standard exhaust manifold and instead integrated it into the cylinder head. The results are reduced emissions but a severely limited platform in terms of exhaust modifications. Honda also went on to eliminate all 2.0L configurations from its lineup, now exclusively relying on the larger, 2.4L platform, which isn't necessarily a bad thing.

K24Z2 / K24Z6

  • 2008-2012 Accord: 177 hp, 161 lb-ft torque
  • 2010-2011 CR-V: 180 hp, 161 lb-ft torque

The K24Z engine family (excluding the 2007-2009 CR-V's K24Z1) builds upon the already successful tenants of its predecessor's K24A yet now places even more emphasis on low emissions. These 2.4L i-VTEC engines produce more power and torque than earlier engines, but the K24Z engine family makes for a much more challenging engine to build upon thanks to their integrated exhaust manifolds. No longer is adapting a true header or exhaust manifold possible with these types of cylinders heads. Still, the K24Z engine family can make for a solid foundation. Aside from an all-new, downward-facing oil filter placement, bottom-end-wise, the new 2.4L family of K-series engines isn't much different than its predecessors.

K24Z3 / K24Z7

  • 2008-2012 Accord: 190 hp, 162 lb-ft torque
  • 2009-2013 TSX: 201 hp, 172 lb-ft torque
  • 2012-2013 Civic Si: 201 hp, 170 lb-ft torque
  • 2012-2013 CR-V: 185 hp, 163 lb-ft torque

Honda once again updated its K-series engine platform for the completely redesigned TSX and Civic Si. To better suit the second-generation TSX's and ninth-generation Civic's slightly larger size and heavier chassis, the all-new, larger-displacement K24Z3 and K24Z7 were selected. Similar to the older TSX's K24A2, both feature 2,354 cc of displacement by means of Honda's standard bore and stroke yet benefit from a higher 11.0:1 compression ratio. The results are slightly more power and a lot more torque. In a seeming effort to confuse everyone, Honda went on to develop another K24Z3, this time for the 2008-2012 Accord, and another K24Z3, now for the fourth-generation, 2012-2013 CR-V. Although a K24Z3 and K24Z7 by name, their compression ratios, horsepower, and torque figures all pale to the TSX's and Civic Si's. What's even more curious is that none of these engines benefit from performance i-VTEC or performance-orientated, aluminum intake manifolds, which TSX and Si enthusiasts have thus far come to expect. It isn't just the TSX and Civic Si that are being singled out, though; by the time the eighth-generation Si's final production run was completed in 2011, so was performance i-VTEC.

K24Y2

  • 2012-2013 Crosstour: 192 hp, 162 lb-ft torque

For the 2012 model year Honda released its K24Y2 engine for its 2012-2013 Accord Crosstour, now offering consumers a four-cylinder option. In most respects, the K24Y2 isn't all that different than what you'll find in the K24Z lineup. Although it produces an impressive 192 hp, the K24Y2 also features an exhaust manifold that's integrated directly into its cylinder head and Honda's limited version of i-VTEC.

By Aaron Bonk
408 Articles

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