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High horsepower Hondas are nothing new this day and age as vast improvements in vehicle engineering have empowered the H-badge enthusiasts to build everything from the mildly modified to the more wildly modified 700 hp street driven machines. A new age of computer-literate teenagers and young adults use the internet to find ways to squeeze every fraction of power from their cars. Unfortunately, with every wheel-spinning and tire-roasting component added onto the vehicle comes the necessity of obtaining large amounts of cash. Most owners end up spending their hard-earned dough on what we call "the rolling money pit".
Gary Castillo, owner of Design Craft Fabrications in Lake Forest, California, is a true import enthusiast. He has dabbled for years in everything from participating in engine buildups to being the head mechanic for the infamous RS*R S2000 drift car piloted by Tyler McQuarrie. Castillo is never scared to try something out of the ordinary. During routine engine maintenance for the S2K, Castillo came across an interesting find when disassembling the F20C powerplant a few months back.
"I took some measurements and found some fascinating information between the Prelude H22a and the S2000's F20c engine. Combining these two engines components and using a TSX crankshaft enabled me to create a true 2.4-liter engine using the original F20C's 2.0- liter block. The main reason why this 2.4-stroker engine is such a perfect combination is the stroke of the K24 crankshaft in conjunction with the H22a's connecting rod center to center length and dead-on piston height matches perfectly with the F20C's deck height," Castillo said.
With Design Craft Fabrications taking credit as the pioneers of structuring this engine combination, Castillo made a valid point that was sure to stir up a frenzy within the S2000 community when he stated: "You know the best thing about this 2.4-liter stroker buildup is we're using all factory Honda parts which anyone can purchase over the counter. All you need is some minor machining at a reputable shop, and assembly skills. This is a stroker setup that won't kill your bank account."
Check out the following photos for a detailed illustration on how Castillo was able to convert the 2.0-liter F20C engine into a 2.4-liter stroker engine.
- An untouched H22 rod overlaid onto the K24 crankshaft offers a visual perspective on the difference in size between the two.
- The factory H22 rod is measured at 23.85mm in width when gauged with the dial caliper.
- Arbitrary measurements show the machined rod with 4mm (2 mm off each side) machined off the factory H22 rod.
- Castillo states, "If you decide on going the naturally aspirated route, then modifications to the rod (2mm on each side) similar to what was done with this project motor is ideal. If it's a high horsepower motor, then it's recommended that you open up the rod journals by cutting out 4mm on the crankshaft. Our initial goal with this setup was to experiment on the rods themselves by lightening up the rotating mass on the engine and further expand our testing for this engine ona street turbo motor application. If you grind the crank, then it's possible to use all Prelude components." Here's a close-up image of the machined H22a end cap fitted with a K24 rod bearing.
- On the left is the new rod end that was re-cut with a new "tang" to fit the K24 bearing. Notice the difference in size with both the K-bearing and the factory H22 unit.
- Pictured to the left is a K24 crankshaft plucked from an Acura TSX. Sitting adjacent to the K-crankshaft is the factory F20C unit. Prior to installing the 99mm K24 crankshaft into the F20 block, DesignCraft Fabrication sent the crankshaft to their specialized machinist for servicing that removes the vbeveled lip section on the crank, which is used as a source for centering the K24 flywheel. Grinding down the lip is crucial to accommodate the new S2K flywheel.
- In comparing the compression height (the distance from the center of the pin hole to the deck of the piston) between the H22a and F20c piston reveals the difference in dome height when lining up both pistons on the same center pinhole. The F20c compression height is measured at approximately 1.81in or 30mm while the H22a is set at 1.219in or 31mm.
- While the never ending battle of long-rod versus short-rod debates between mechanical engineers and Internet nerds continues to brew, Castillo took to his grinder and notched the lower skirts of the piston sleeve in order to make clearance for the newer, yet shorter H22a rod. The H-rod's length from cetner to center is 5.63in or 143mm while the F20c rod is measured at a longer center to center length of 6.02in or 153mm. Creating a shorter rod ratio causes an increase in rod angle which is highly debatable in terms of efficiency and "dwell" time. A few taps on the calculator show the F20c's factory rod ratio at 1.82:1 and the H22a calculated at 1.58:1.
- Although the factory oil squirters can be used if modifications to the nozzle are made, CASTIllo recommends using AN plugs to seal the factory squirters. "The less shit dangling around the piston area, the better. Besides... you can make up for more oil pressure and horsepower without those things."
- Using the K24 crankshaft the F20c block is as simple as dropping in the correct sized bearings, assuming you've taken the correct measurements and topped them off with some assembly lube.
- Like a set of Legos, the mix and match process works to perfection for our stroker engine as S2000 thrust washers were positioned on journal number two. The K24's washers are located on the same journals as the S2000's F20c engine.
- One of the key reasons this is such a successful cross breeding of Prelude and S2000 components is that both engines use the same FRM cylinder wall material, which is different from the usual B-series steel lines on the sleeve. This is what makes it possible to interchange pistons from H22 to and F20 block.
- Fiber-reinforced metal (FRM) cylinder sleeves contain fiber-based material lining on the cylinder walls of the block. This material can create some serious problems for those who attempt to bore the cylinder walls or plan to use a set of aftermarket pistons and rings. Boring out cylinder walls to fit oversized pistons can punch through the FRM lining and expose the soft metal; while aftermarket chromoly and titanium piston rings cause gouges to the cylinder walls and tear them to shreds.
- With both the H22a and F20c sharing the same 87mm bore, you can use either service manual to find the proper ring gap.
- Using the proper tools, like this ring filer, will ensure that your pistons properly seal and will eliminate unwanted blow-by.
- Combining the K24 crank with H22a rods and pistons gave the newly revised S2K engine a new rod ratio of 1.44:1 versus the factory S2000 ratio of 1.82:1. Numbers show the piston speed on the stroker motor is significantly quicker, while the factory S2000 is a lot slower.
- All-motor fanatics have the option of using Prelude Euro-R spec pistons to further bump up the compression ratio on the new 2.4-liter engine to 11.6 to 1.
- Sharing the same piston bore size of 87mm and a larger 99mm K24 crankshaft within the F20 engine block rewarded us with a new displacement figure of 2.355 = 2.4-liters. For shits and gigles we did some calculations with the same setup and found that using 30 thousandths overbore pistons on a re-sleeved block would net a total displacement of 2.515 = 2.5-liters.
- "The main reason why this 2.4-liter stroker engine is such a perfect combination is the stroke of the K24 crankshaft in conjunction with the H22a's connecting rod center to center length and perfect piston height matches perfectly with the F20c's deck height," said Castillo of DesignCraft Fabrication.
- Purchasing the factory Prelude rods, bearings, and pistons from your local Honda service center is cheap. Besides the expense of purchasing a new or locally found K24 crankshaft, one of your only other expenses will be the time you spend on rebuilding the factory F20c into a stroked F24 power plant.
Text and Photos by Scott Tsuneishi, Turbo Magazine