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  • Writer's pictureRoger Maeda

How to Develop a Honda Prelude Racecar - Part 7

Moment of Truth - Dyno Day

When we last left off with this series, we had just installed our new "Spec B" engine upgrades from Bad Guys Worldwide. This upgrade package consisted of a new intake manifold upper, a 74mm throttle body, and a taper bore adapter to connect the two with sub-millimeter precision.

It is worth noting that the switch to the larger throttle body did involve a few extra steps in the installation process:

First, we had to convert from the throttle body-mounted MAP sensor back to the firewall-mounted sensor that came with our 93 Prelude Si. This was by far the easiest change - All we had to do was to reconnect the intake manifold vacuum hoses to the sensor, which we had never fully removed. Easy.

The second step was slightly more involved. Because the taper bore spacer added an extra inch to the length of the upper intake manifold assembly, we needed to move the throttle cable closer to the throttle body so the cable would have enough slack. This involved removing the throttle body cable bracket, drilling new holes about 1/2" from the original location, and adjusting the tension on the throttle cable. Not too bad a modification - Well within the means of an average automotive enthusiast, for sure.

The third step took the most work. Because the Skunk2 throttle body has such a wide mouth, the walls of the throttle body bore come within millimeters of the mounting holes to the intake manifold. The clearances are so tight that standard metric hex bolts wouldn't even fit. Even socket head cap screws and allen bolts are too wide to fit into the limited space provided by the throttle body.

Skunk2 solved this problem by taking some metric socket head cap screws and turning the heads down on the lathe so they were about 1.5mm smaller in diameter. They then recessed the holes for the bolts into the flange portion of the throttle body so they wouldn't interfere with the coupler for the intake tube.

The screws that Skunk2 provided were too short to work with the taper bore adapter in place. But ProjectCRX co-driver turned USTCC competitor Martin Szwarc was able to take some longer cap screws and turn the heads on his mini-lathe down to the same diameter as the ones that came with the throttle body. All we had to do was to remove the OEM studs from the intake manifold and secure the throttle body with these modified screws. Once we bolted everything together, we gave all the moving parts of the throttle body a spritz of Steel Camel spray to keep them lubricated and prevent corrosion.

With our new Spec B engine package firmly in place, we trailered the car to Evans Performance Academy, where Jeff Evans tuned our Hondata S300 ECU on his brand-new Mainline ProHub Dyno.

The results were shocking. In less than half an hour, Jeff extracted 218.8 whp and 162 lbs-ft of torque from our internally stock H22 engine. This was just 1.2hp under the maximum horsepower number that we declared to the USTCC when we registered for the 2023 season. He also raised the redline of the engine to 8200 rpm so we could safely take advantage of our new peak power output.

Jeff commented that these numbers were unusually high for an internally stock H22, while assuaging our fears that the extra-large throttle body would affect drivability. He did recommend that we adjust the throttle stop screw to raise the idle another 200 rpm so the ECU would be less inclined to hunt for a stable idle. We thanked him for his time, brought the car back to the studio, and adjusted the screw accordingly to raise the base idle to 1000 rpm.

Accompanying Aero Tweaks

While the added power was a welcome change, we knew that top speed wasn't our top priority. The other cars in our USTCC Sportsman class had declared much higher maximum power figures, which means that they would always have the edge on us at the end of long straights. We would have to take a slightly different approach from the competition.

Like most 4th gen (and 5th gen) Honda Preludes, the StudioVRM Prelude is strongest through high-speed corners. The sporty coupe's long, low-slung body and large body overhangs all help increase the effectiveness of add-on aero pieces like wings and full-length front splitters. Our best bet would be to increase the total downforce of the car so we could carry more speed through the corners and use our improved corner exit speed to close the gap down the longer straights. And thanks to the flexibility of the USTCC rulebook, this would be possible without blowing our development budget.

Our aero had been built to the limit of SCCA's Super Touring Under rules. And while STU rules are fairly aero friendly by SCCA standards, they feel almost limiting when compared to the US Touring Car Championship's very permissive aero rules. Under USTCC rules, rear wings can extend up to 6" rearward from the center of the rear bumper. Front splitters can also extend 5" forward from the front bumper as viewed from above. Compared to the STU rulebook's requirement of keeping the rear wing within the confines of the stock body shape and a maximum of 3" extension from the front bumper, this was like having a blank cheque to develop the car however we wanted.

There wasn't enough time to cut and mount a new front splitter before Round 3 at Summit Point, but we could change the mounting location of the rear wing. A quick trip to eBay Motors and $65 later, we had a pair of budget friendly wing uprights that placed our existing wing higher and further back - right at the limit of the USTCC rulebook.

We knew that moving the rear wing rearward by such a large amount would result in a significant increase in rear downforce. So, we lowered the leading edge of the front splitter to increase front downforce and counteract the aero gains in the rear.

Drivability and Impressions

Just one short week after we had installed our new engine and aero package, we were back on track at Summit Point. As expected, the Spec B engine upgrade worked brilliantly. Between the increased power, higher rev limiter, and increase in mid-range torque, the car proved much easier to drive than it had been at Round 1. To our surprise, the 74mm throttle body had no impact on the car's response under partial throttle. It seemed that the 2.2L H22 engine had no problem taking in the air from a throttle body that had a 52% larger bore than the stock 60mm unit. 

What we hadn't expected was the speed of some of the Big Bore cars through the corners - or rather, their lack thereof.

Due to a freak overheating issue on Saturday, we failed to set a qualifying time and were forced to start the Feature Race from the back of the grid. This meant that we would have to work our way through the back of the big bore field - No easy feat when most of the cars in the group had over 300hp to the rear wheels and were 5 to 10 mph slower through the faster corners. As a result, we would struggle to put down any fast laps while being constantly balked by bigger, more powerful machinery.

To make matters worse, the new rear wing uprights had completely thrown off the aero balance of the car. The wing was now producing so much rear downforce that it overpowered the 1000 lb-f/in rear springs, causing the back end of our car to squat down so much that the front splitter would point skywards. The result was constant corner entry understeer that would hurt our lap times through low and high speed corners.

The unbalanced handling made it nearly impossible to overtake the more powerful cars in the group. Our car spent the entirety of the Feature Race stuck behind a colorful mix of muscle cars:

Reviewing the Data

The constant traffic and resulting lack of clean laps made it difficult to extract useful data from the weekend. But we were able to salvage a couple of noteworthy items.

The first was that our cornering speeds through the two slowest corners on the track, turn 1 and the turn 6 carousel, were consistently faster with the new setup. A review of the in-car video revealed that this was down to the driver more than the setup. Because of the pronounced corner entry understeer, our driver had gotten into the habit of applying extra steering lock through mid to high speed corners. This habit seemed to have carried over to the slow speed sections as well, resulting in more aggressive turn-ins and higher mid-corner speeds.

Typically, this is something you would want to avoid in a FWD car, as it puts extra stress on the front tyres. Considering that the tyres felt fine after 28 minutes of racing in hot weather, this may not have been the worst thing for outright performance. We will test out this technique again when we return to Summit Point Raceway.

The second was that, while the new setup didn't increase our top speed, it did help us recover speed after a slow corner exit. Even when our turn 10 exit speeds were 2-3 mph slower than usual, the engine's smooth power delivery and higher redline helped cover that deficit and get us to our usual 124mph top speed by the end of the main straight.

This was confirmation of both the effectiveness of the Spec B engine upgrades as well as the fact that moving the rear wing higher into the airflow increased the aerodynamic drag of the car. If we can find a way to reduce the drag from the rear wing, we should be able to unlock even more speed.

Next Steps

The results from Round 3 were clear: The Spec B upgrade package has tremendous potential that would only be unlocked with the right refinements.

The next step will be to restore the front-to-rear aero balance of the car so we can make the most of our Prelude's high-speed dogfighting ability. We are planning to test several different adjustments in pursuit of this new goal:

  • Reduce wing angle Because the wing is producing so much more downforce, we can afford to reduce its angle to reduce the amount of drag that it generates. We plan to reduce the angle by 1 degree, as measured from the flat section on the underside of the wing.

  • Increase front splitter size Regardless of what we do with the rear wing, we need more front downforce. We can make incremental improvements by decreasing the height of the splitter, but the real answer is to extend the length of the splitter to the limit of the USTCC rules. We plan to enlist the help of Braci Racing's fabricators to cut new splitter blades that extend 4 inches and 5 inches from the leading edge of the bumper.

  • Stiffer Springs & Revised Damping Our setup philosophy for this car prioritizes the ability to bang kerbs and drive through bumps and dips without hesitation. This meant running on lower spring rates and using twin tube dampers to help soak up the bumps and ruts that would upset a more stiffly sprung racecar. However, we are now seeing that the aero is completely overpowering the soft suspension springs and causing undesirable changes to the car's aero balance under pitch and roll. We need to switch to stiffer springs to combat this behavior. Thanks to fellow Honda Prelude Racing Group member Josh Grome, we were able to secure a spare set of Tein Super Street dampers. These spares would be the perfect platform to build a new spring-damper setup to complement our improved aero. At the time of writing, these dampers are being revalved to our exact specifications at Tein's Downey, CA facility. This new suspension setup should be ready by the final round at NJMP Thunderbolt.

  • Adjust Alignment Changes to aero, springs, and damping will affect the car's alignment at speed. We plan to adjust our alignment to match the aero upgrades on the car.

We plan to apply these changes as they arrive in preparation for the upcoming USTCC East Series rounds at Watkins Glen and NJ Motorsports Park. Which of these tweaks will be the key to unlocking the potential of our Spec B upgrade? Watch this space.


Disclosure Section:

The Bad Guys (AKA Bad Guys Worldwide) is a Technical Partner of StudioVRM.Racing, and provided the Spec B engine upgrades for free in exchange for testing its effectiveness. Roger Maeda and StudioVRM are not affiliated with Jeff Evans or Evans Performance Academy.


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