Over the weekend I spoke with an acquaintance who seems to have come up with a very clever economy build with K series engines, one that makes me a bit jealous. I'll do my best to explain what he has assembled in his garage, but it will require some background.
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K series engines are very modular. The blocks are essentially all the same, and it's what's bolted to them that determines their character. Broadly speaking, they tend to fall into the categories of 1) economy vs performance head, and 2) 2.0L vs 2.4L.
Back in the 90's, Honda made a 3-stage VTEC engine, with a total of 3 different cam profiles.
In "Stage 1", the engine operates in 12v(ish) mode, with one of the intake valves opening based on the "medium" cam lobe, and the other riding on the "low" profile, barely opening (to prevent fuel pooling behind it). This causes a swirl effect which improves very low RPM combustion.
In "Stage 2", a pin locks the rockers for both intake valves together and both ride on the "medium" cam lobe.
In "Stage 3", both valves switch over to the "high" cam lobe. The exhaust valves also switch to a "high" cam lobe.
To my knowledge, only one engine was ever made that had all 3. All other Honda engines either operate in stages 1 & 2 (economy head), or 2 & 3 (performance head).
As to why you want different cam profiles in the first place, here's a chart of the fueling values for my engine, for the "low" cam and "high" cam, at various camshaft angles:
It may be hard to interpret at first glance, but essentially what happens is:
The "high" cam lets in more air at high RPM but at low RPM it produces less power, for various reasons. If the car were to run on the "high" cam at all times, it would idle poorly, have high emissions and very little power until around 4000RPM.
The "low" cam is timed to open and close more optimally for low engine speeds. Combustion quality is much better between 0-4000RPM, producing a cleaner, more economical engine with a smooth idle. However, above around 4,000RPM, the valves just don't open wide or long enough to fill the cylinder, and power starts dropping off.
Most engines without VTEC choose a single cam profile that is intermediate between these two.
My chart also includes cam phasing. Take the high cam: advance (open it sooner) by 40 degrees and it produces more power at 4000RPM. From 4250-5000RPM, it produces the most power with 30 degrees of advance. Beyond that, it should be dialed back to 20 degrees of advance. The same applies at lower speeds, but it's harder to see. 0 degrees advance creates the cleanest and smoothest idle, while different resonances produce more power at different points between 0-4000RPM with the throttle wide open.
As for part throttle, opening the intake cam sooner can allow the intake valves to be open before the exhaust valves have closed, causing some of the exhaust to be pushed into the intake manifold. This is a kind of "EGR", allowing exhaust gases a 2nd chance to let their heat do some useful work, and reducing the amount of vacuum created for a given amount of power. Too much exhaust gas mixing, however, and combustion quality drops. So, VTC can improve economy as well.
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Between 2002 and 2011, Honda had essentially four major engine variants:
Economy K20 - found in the base model RSX and 02-05 Civic Si, 160HP peak, 12v operation, "low" compression (9.8:1)
Performance K20 - found in the RSX Type S and 06-11 Civic Si, 200HP peak, has "high" cam, high compression (11:1)
Economy K24 - found in the Accord, Element, CR-V, 160HP peak, 12v operation, has EGR valve, "low" compression (9.8:1)
Performance K24 - found in the Acura TSX, 200HP peak, has "high" cam, high compression (10.5:1)
The performance K24, from the TSX ("K24A2") is generally the most desirable engine of the lot for those looking to make big power numbers. They're so overbuilt it's possible to extract 700HP with a large turbocharger without ever opening the engine. I scored a good deal on one of these and couldn't pass it up. As it comes from the factory, it's probably the worst K series for fuel economy, with its only redeeming feature being that it's a high compression engine which makes most of its torque not too high in the rev range. To remedy this, I build an extremely tall transmission, removed the balance shafts, underdriven oil pump, lightened the flywheel, used an intake manifold meant for low-end torque rather than high end horsepower, swapped in a 50 degree VTC gear (from factory 25°), added an oil "warmer" and feed it mostly warm air. I'm also considering retrofitting the EGR system from the Accord.
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What my acquaintance has done is very different, and in retrospect I feel it's closer to the engine I "should" have built, given my goals.
His build:
* Performance K20 block (K20A2)
* Economy K20 head (K20A3)
* Very small turbocharger (Garrett GT2554R)
Why this head?
The economy K20 head has the smallest combustion chamber, and raises compression on whatever block it's attached to. It has 12 valve operation below 2300RPM, where I operate my engine most of the time. It's only good for ~6800-7000RPM and doesn't breath all that well above 5k (hence 160HP in stock form), but the turbocharger makes up for this.
Why not just use the economy K20 engine block?
*The performance K20 block has a heavier, fully counterweighted crankshaft, and stronger rods designed to rev up to 9000RPM, It can typically take 500HP+, compared with <300HP for the economy block. Neither of these help with economy, but shouldn't harm it either, and it makes this block VERY forgiving with high compression and a turbocharger.
* Highest compression pistons of any engine block. Paired with the economy head, it has an outrageous 11.7:1 compression ratio, which should be good for ~3% economy over my engine.
* Shorter stroke than a K24. It has a more favorable rod:stroke ratio, meaning there's less side loading on the piston sleeves (lower friction), and the engine vibrates a lot less without balance shafts.
* Smaller displacement than a K24 = higher load = better economy.
* From the factory it already has an oil warmer, 50 degree VTC, no balance shafts, and underdrives the accessories (even the oil pump) compared with 2.4L engines.
Best of all, people want to get rid of these blocks and swap them for K24 blocks for the larger displacement, so they're not very expensive.
With an off-the-shelf 4.389 Honda final drive it would have only marginally higher load under normal driving conditions than my K24 with my custom (expensive) 3.4 final drive. Paired with my extra tall transmission, it would not surprise me to see 70+ mpg, so long as you stay out of boost.