Hybrid How-To No. 15
K24 short block with K20A2 VTEC cylinder head
By Randy Hasson
Photography: Chris Felser
What and why
There's no substitute for displacement, except maybe technology, so in a perfect world you'd have both. If you're driving a Honda powered by the new K-series engine, there's a relatively easy way to get both. Take the big 2.4-liter block from an Element, CR-V, or Accord and mate it with the high-flow head from the K20A2 found in the RSX Type-S. It's almost that easy. Almost. This swap can be called a head swap or a block swap, depending on what you start out with and what you want to achieve. It can also be dubbed a "poor man's TSX engine." Regardless, you'll wind up with the intake and exhaust side VTEC system on top of a 2.4-liter block, and as we'll see, the marriage works.
There's VTEC, and then there's VTEC
Honda has used two very different VTEC systems on K-series engines. In spite of the engine's differences, both happen to be called i-VTEC. The first system, found on the K20A2 (RSX Type-S) and K24A2 (TSX) has three cam lobes for each pair of valves. At low rpm, each valve follows its own mild cam lobe. At high rpm, they both start following the third lobe, which has higher lift and more duration for better high-speed cylinder filling. This is the VTEC you're used to. The second system, found on the K24 (Element, CR-V, Accord) and the K20A3 (base RSX and Civic Si), is designed to minimize emissions and maximize driveability and gas mileage. In this system, there are two cam lobes, a normal one and a puny little atrophied one, for each pair of intake valves (nothing happens on the exhaust side). At light load and low rpm, each lobe opens one valve, so most of the intake air goes through the valve that's opened more. This creates a swirl in the combustion chamber that happens to be great for combustion efficiency. Floor it, though, and one valve won't be enough, so both valves follow the bigger lobe. Notice there's no screaming high-rpm race lobe here.
The "i" part of i-VTEC stands for VTC (don't ask us how they came up with that) the Variable Timing Control system that graces all of the K-series engines, whether they have the go-fast VTEC or the sissy model. VTC simply advances or retards the entire intake cam.
K24 blocks are ready to make big horsepower numbers. The crank and rods are similar in construction to the K20A2 parts, but with beefier rods, bigger rod bearings and additional counterweights on the crank. The K20A3 engine, sourced from the base RSX and Civic Si, is built for less power and lower rpm. Honda got the extra 400cc for the K24 from both an increase in bore from 86mm to 87mm and an increase in stroke from 86mm to 99mm. The additional stroke forced a 19.7mm increase in deck height, which might only become apparent when you let the hood drop, so check those hood clearances carefully. Remember, this also means your exhaust manifold will move up 19.7mm, so make sure you have that kind of room, or prepare to make adjustments. Sales figures for the 2002-and-up CR-V, Accord and Element are already nearing a half-million, so you shouldn't need to stay glued to eBay for a chance for one. Expect to pay between $650 and $1,000 for a complete K24 engine in good condition. The K24A1 that sits in the CR-V has a 9.6:1 compression ratio, while the K24A4 found in the Element and Accord have 9.7:1. The other differences between the K24 applications, most notably the presence of EGR, used on the Accord and the Element, and a variable length intake runner system, found on the CR-V, are irrelevant since we're not using the K24 head.
Although we haven't seen any yet, it's possible Honda will come out with ultra-efficient K24s for tighter emissions standards. Keep a watchful eye out for these as they may be trimmed down (bad) for lower friction and weight. Also beware of pre-2002 CR-Vs, which had a B-series variant. You can also find the K24A2 in the TSX, and it's different enough to need its own paragraph. The TSX mill is a mixed blessing. With a 10.5:1 compression ratio and a 7100-rpm redline, it makes 200 hp (179 hp at the wheels on our dyno). To cope with the higher revs, it has stronger rods and a unique crank. It also happens to have basically the same head we're swapping on here. Sounds great, so why not just transplant the complete TSX engine? One, it's rare and expensive. Two, it comes with throttle-by-wire and an ECU that's not compatible with other K-series-powered vehicles. To make the TSX engine more "swappable," install an RSX Type-S or Civic Si intake manifold and throttle body on it. You'll have to engineer a block-off plate or plug for the EGR port on the TSX head, since it will be exposed with the new intake manifold. So the big question is: Should you buy a TSX engine if you have the chance? If you intend to stick with natural aspiration or light boost, it would be a good choice with its high compression. Additionally, the only other part you'd need to find is a short runner intake manifold. The choice boils down to what you want to do with the engine and what parts you already have.
The RSX Type-S is the only North American source for the K20A2 cylinder head, so you might have to widen your search to find one in the United States. JDM engines are a good alternative. In this case, the engines we're interested in are called K20A and are found in Civic Type-Rs and Integras. They're typically shipped with a transmission (which has a limited-slip diff) and an ECU, all costing between $3,500 and $5,500. The K20A engines are trick, stuffed with pistons that will squeeze an 11.5:1 compression ratio. The cylinder head you'll wind up with is arguably better as well, utilizing longer duration intake and exhaust cams and dual valve springs on both the intake and exhaust sides, no doubt to cope with 8600-rpm blasts. Don't get the K20A2 or K20A confused with the K20A3, which is the base-model RSX engine that you'll be disappointed with if it shows up at your doorstep.
If at all possible, start with two complete engines. You'll miss out on a few important parts if you buy a K24 short block like the timing chain and timing chain cover, unique to the tall-deck K24. The same goes for the K20A2. They're nothing that a costly trip to the parts counter won't fix, but just be aware before you jump at the first K24 block that you see holding the door open at your favorite junkyard.
Mix 'n' match
Let's start at the bottom. Either of the oil pans will work, but the RSX Type-S part is cast aluminum vs. the K24's stamped-steel part. The cast pan is stiffer and braces the pan to the transmission bell housing, making it the better choice. You should also use the full-length windage tray from the K20A2, which will do a better job keeping the oil where it needs to be. The K24 uses a pair of balance shafts cleverly incorporated into the oil pump assembly ("pump holder set" in Honda-speak), but you don't need their extra weight or the friction required to turn them. Hasport recommends ditching the K24 oil pump assembly in favor of the simpler K20A2 part. The oil pump drive chain and tensioners are identical, so take your pick, or get a new one if in doubt about its condition.
You can avoid the machine shop if you don't want the oil-to-water oil cooler that's used on the K20A2. An oil cooler of some kind, however, is a good idea; at least, Honda thought it was on the RSX Type-S. The stock unit has the advantage of being able to warm the oil in cold weather. To plumb the stocker, have the K24 block machined for the coolant return line, which is right above the oil filter boss on the K20A2. Any decent machine shop can do this. You'll also need to use the K20A2 water pump since it has the coolant supply fitting for the oil cooler. Although the factory system serves its purpose, you could do just as well with a sandwich-type oil filter adaptor and a decent air/oil cooler.
On the front of the engine, the K24 timing chain, tensioner, guide and timing chain cover will be used due to the taller deck height. The K20A2 crank pulley is smaller than the K24's to keep the accessory rpm down on the higher-revving engine. Think of it as a free underdrive pulley. On top, use the K24 head gasket, which is sized for the 1mm-larger bore diameter. The head can be bolted on with either set of head bolts; they're identical.
Start by stripping both engines to short blocks. There's no need to touch any of the rod or main bolts unless new pistons or a rebuild requires it. With a few exceptions, we're tucking a K24 block between all the good parts of the K20A2. As always, arm yourself with a factory service manual for assembly procedures and torque specs. Every thread in the motor is tapped in aluminum, an unforgiving material to a tyrant with a breaker bar. Begin assembly with the bottom end. Install the K20A2 pump holder set on the K24 block. Top it off with the K20A2 oil pump and oil pump drive chain. Next, install the water pump housing, using the K20A2 part if you want to retain the factory oil cooler. Hondabond is used on this joint to make the seal.
Install the K24 cylinder head gasket and lay the new cylinder head in place. It's your choice to reuse the head bolts or the gasket, but again if you're in doubt, or if you have plans for forced induction, buy new parts. Note the head bolt torquing procedure is different between used and new head bolts. Consult the manual for details. Once the head is torqued down, you can install the casting that holds the rockers, followed by the cams and the cam bearing caps. Install the K24 timing chain, tensioner and guide and replace the crank angle sensor wheel, making sure to put it on in the right direction. Cover it all up with the K24 timing chain cover (and more Hondabond to keep the oil in).
Both Hasport and its friends at Hondata have found that K-series engines heat their intake air significantly, causing issues with engine management and power output. Hondata now sells a heatshield intake manifold/cylinder head gasket to replace the factory part. It has measured intake manifold surface temperatures from 15 to 45 degrees Fahrenheit lower than stock with this gasket. If you're going to try one, it will never be easier than right now. Because of the extra deck height, the K20A2 intake manifold support bracket that stretches to the block won't quite reach. We fabricated a 19.7mm tall aluminum spacer to take up the slack. On the cheap without a lathe, you can do something similar with a stack of washers. Don't worry, we won't tell.
All of the bits that still need to go on are straight off of the K20A2, another good reason to avoid the cheap K20A2 cylinder head being used as a jackstand for your friends' del Sol. The one exception is the K24 dipstick, which is longer to reach down the taller block into the pan. Surprisingly, the stock RSX Type-S ECU will make the new K24 run well enough for grocery runs, but hard driving would be unwise. The ECU bases fuel delivery on rpm, manifold pressure, and the amount of air that it knows a K20A2 ingests under those conditions. Since it doesn't know about that extra stroke, it won't add the fuel to go with it. The timing is also optimized for 11.5:1 compression, and can now be advanced significantly for the low 9.5:1 or 9.7:1. Conveniently, Hondata offers a modified ECU the way we like 'em, plug-and-play. Just like on the Sci-Fi channel, your brain will get modified and shipped back with a USB connector on the back, along with some software that will let you tune the Honda ECU yourself. It'll also supply an assortment of base fuel and spark maps to get you started. No extra wiring, no extra sensors, no stress headaches. Well, until you start tuning it.
Sometimes we run across a mod that's so easy and effective, we wonder what kind of cost-management weenie would nix the idea. This is one of them. If they'd shipped the RSX Type-S with our hybrid, it would've been a fierce competitor, even against the turbocharged clan (beat by a Dodge...ouch). Our 2.4-liter mustered 30 lb-ft more torque across the rev range, leading to a power increase between 15 and 30 hp all the way from 2000 rpm to redline. Is this free power? Basically. The added displacement is good for more than a 25-percent increase in torque throughout the rev range, with no loss of driveability. The hybrid can even drink the cheap gas, thanks to the lower compression ratio. True, you can't impress yourself with an 8000-rpm redline, but that's a small price to pay for going faster. Using the K24 block not only adds torque, but the hybrid's lazy 9.6:1 compression ratio cries out for supernatural aspiration. And remember, it doesn't take much boost to make insane power with 2.4 liters. We like this mod because you get great results from stock parts. Especially if you already have an RSX Type-S or anything else with a K20A2 stuffed into it, you'll be hard pressed to find another bolt-on that nets as much power for the price. So the adage is correct; there is no replacement for displacement. We've just combined it with a dose of modern engineering to prove it once and for all.
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