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  • Writer's pictureRoger Maeda

How to Build a Honda Prelude Racecar Part 9

When we last left off in the series, the StudioVRM Prelude was back with powertrain wizard Robert Oliver for a top-to-bottom rebuild of its battle-worn H23A1. In addition to giving the block a long-overdue refresh, we also installed high compression pistons to take advantage of the cams we installed during the 2018 season.


Why stick with the non-VTEC H23A1?

At this point, you might ask why we didn't just do a motor swap and start over with a H22 instead. After all, the higher performance 2.2L engine comes with more power out of the box, a better flowing head, and the awesome power of Honda's coveted VTEC technology. Plus the 2157cc engine would allow us to run with a lower minimum weight by virtue of the SCCA Super Touring Under class' displacement-to-weight rules.

As always though, the devil is in the details. Details, such as:

  • The fact that our lightly modified H23A1 is already making more power than a stock US Market H22

  • We lose the ability to hit SCCA STU's 12:1 compression limit using OEM pistons*

  • The oiling system in a high revving H22 is much more highly stressed than in the milder H23

  • VTEC isn't useful in a racecar where the gearing can be adjusted to keep the engine in a certain RPM range

*Installing Type S pistons in a H22 will get you to 11:1 compression, which is close but not quite the same as you get from installing USDM H22 pistons into an H23A1.

That's not to say that the H23A1 is a better engine than the H22. We just happen to be at the point where starting over would cost far too much for what gains we could achieve. Besides, what would be the fun in that? StudioVRM is all about taking the road less traveled.

So with our H23 upgrade plan firmly justified in our minds, we set off on the road less traveled and went about improving the Prelude's lesser-loved non-VTEC engine.


Building a Better Head

The StudioVRM Prelude has come a long way since its days as a near-stock ITS car. As of this build, the following aftermarket parts now occupy the top end of the engine:

Most of these modifications were actually installed early in the 2018 season as part of our stress test of PTH racing oil. The purpose of this round of modifications was to unlock the power that these modifications gave us.


Rebuilding the Bottom End

It may come as a surprise that the StudioVRM Prelude's block has never been disassembled, much less rebuilt. In fact, even after a decade of racing, our racecar still had the original bearings as it had when the car rolled off the assembly line in Sayama, Japan.

This finally changed this winter, after 25 years of faithful service. Over the off-season, the bottom end was rebuilt with:

By combining the H22A1 pistons with the H23A1 rods, we were able to bump the compression of our rebuilt motor from 9.8:1 to just shy of 12:1. The theory is that the increased compression would help maximize the power output of our high-overlap custom cams. The brand new King bearings would help keep the motor running safely through sustained high-rpm sessions on the racetrack.


Sidebar - Wrist Pin Weirdness

During the course of assembly, powertrain wizard Robert Oliver stumbled across a strange problem. When he tried to install the H22 pistons on our H23 rods, he found that they were too big for the pistons. This seemed like an impossibility considering that wrist pin sizes for Honda H series engines are very well published. According to all known literature, both engines should use exactly the same pins. So what was going on?

As it turns out, our H23A1 is not the USDM Prelude Si engine that we thought it was. It is actually the Japanese variant of the engine, which came with the larger wrist pins and rods from the VTEC-enabled H23.

Wrist Pin Diameters for reference:

  • USDM H22A1 Wrist Pin Diameter Spec = .8649" -.8654"

  • USDM H23A1 Wrist Pin Diameter Spec .8649" -.8654"

  • JDM H23A1 Wrist Pin Diameter = .8657"

This little known fact caused a major headache for the powertrain wizard.

The H22 wrist pins were the perfect size for a floating pin application in the H23A1 rods, but that would require precise machine work to bush the rods and modify the pistons to properly retain the pin. We could go out and use aftermarket forged pistons, but that came at a substantial cost. Plus most forged pistons aren't compatible with the H series' FRM cylinder liners. Finally, we could try and get a different set of H23 rods. But Honda hasn't made new H23 rods in years. And there was no guarantee that we wouldn't run into the same issue with someone else's used rods.

After pondering the options, Robert came up with a very simple solution. He first drilled a set of small oil passages into the small end of the H23 connecting rods. He then pressed the larger H23 pins into the H22A1 pistons using his pin press. This allowed us to keep the pressed-pin setup of the stock H23 without having to do any machine work at all.

Because we effectively forced the wrist pins into a hole that was 0.004" too small, this does mean that we may not be able to disassemble the pistons and rods in the future. But practically speaking, this isn't much of an issue. H22 pistons and H23 rods are cheap and readily available. If we ever needed to replace the pistons or rods, we could just assemble and replace them as a set.

In addition to teaching us a fun new fact about the H series engine, the experience taught us a valuable lesson - Never underestimate the power of Occam's razor.


Induction, Exhaust, and Electronics

Of course, none of this would do any good if we can't get air in and out. As the powertrain wizard keeps reminding me, an internal combustion engine is just a giant air pump. The key is to get as much air flowing through it as efficiently as possible. Here's what we have to make that happen:

Check out the previous installments of the series for details on these items.


Dyno Tune

After a few weeks of careful assembly, I picked the car up from Robert and brought it back to our go-to Honda tuner, Jeff Evans. Since our last visit to his shop, Jeff had transitioned his highly regarded tuning shop into Evans Performance Academy, an innovative tuning school. Fortunately, he still makes time to tune the odd racecar, as you can see here:

Since the motor was totally unproven at this point, we played it safe and tuned the car on Sunoco 260GT, a 100 octane unleaded race fuel.

So how much power did it make? Well, see for yourself:

Jeff's initial assessment was that we should feel a substantial difference compared to before. While the peak numbers were modest, the car gained 30+hp at 6000 RPM. This will make a huge difference in a racing situation.

Better yet, Jeff thinks there's more power to be unlocked from this motor. He thinks our stock intake manifold and mildly ported head are choking out the engine and preventing it from reaching its full potential. While we won't be doing that this time around, it's encouraging to know that there's more to be had out of our lowly non-VTEC powerplant.


Conclusions and Next Steps

180hp and 169 lbs-ft of torque might not seem earth-shattering, you have to remember that different dynos read very differently. The only way to get an accurate comparison is to compare with another car on the same dyno.

By comparison, a stock H22A1-powered Prelude reportedly makes around 165hp and 135 lbs-ft of torque on the Dynapack at Evans Performance Academy. While a modified H22 would produce more peak power, they would be hard pressed to reproduce the same low-end grunt as our fully worked-over H23A1.

Of course, all of this is all theory at this point. The only way to find out how fast the car is to run it on a full size racetrack. As I write this, the StudioVRM Prelude is getting some final suspension adjustments before it goes off for corner weighting and an aggressive race-friendly alignment. As soon as that's done, we will be ready for our first track test of 2019.

How much faster will this car be in the 2019 season? Stay tuned.

See you at the track.


Disclosure Section:

Roger has no affiliation with the manufacturers or companies mentioned above and therefore paid full price, out of pocket, for everything mentioned.

StudioVRM is now an Amazon Associate, which means that we get a tiny bit of income to support our site if you buy a product using the Amazon links above. We would be tremendously grateful if you did just that. The site gets a lot of traffic nowadays and hosting isn't that cheap.


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