Intermittent Explosive Disorder (IED) is a behavioral disorder characterized by extreme expressions of anger, often to the point of uncontrollable rage, that are disproportionate to the situation at hand. The violent acts are oftentimes accompanied by a sensation of relief, and, in some cases, pleasure.
If cars could have human characteristics, then Renntech’s C74 Konzept would probably be diagnosed with IED because the amount of violence and rage it delivers is disproportionate to angle of throttle you give it. But unlike some loose canon with a short fuse and anger management issues, the C74’s tendency to go ape is a virtue rather than a liability.
You’d be forgiven for thinking that the “74” in C74 signifies a 7.4-liter engine, perhaps a bored and stroked 6.2-liter M156 V8, but Renntech decided that enlarging the 6.2-liter would be too costly and time would be better spent developing parts to squeeze as much out of the 6.2-liter as possible. The “74” refers to the max amount of rear track width (in inches) allowable after the car is fitted with Renntech’s Full Widebody Aero Package. And that number also pays homage to Renntech’s SL74, an SL that was motivated by a bored and stroked V12 that displaced 7.4 liters and put out 575 hp back in the mid-’90s.
The widebody kit was designed in-house at Renntech’s headquarters in Lake Park, Fla, and consists of front and rear fender flares with integrated vents, a pronounced front splitter, rear diffuser and trunk lid spoiler. The parts blend in well with the lines of the C-Class and the fit and finish look OEM. This particular car, which Renntech calls their “testfahrzeug,” or test vehicle, also wore a Seibon GT-style carbon-fiber hood.
Hunkered down on Renntech/KW coilovers and wide Renntech Signature forged, 9.5x19 (f) and 10.5x20 (r) three-piece wheels and 275/30 and 305/25 Nitto INVO tires respectively, the immediate impression is that of a DTM car modified for street use and, without question, one of the most menacing C63s out there. You can spend an entire afternoon just walking around the car and admiring its details, from the perfect carbon weave with its subtle matte finish to the intricacies of the rear diffuser and weapons-grade exhaust tips.
Although the engine has not been bored or stroked, it’s a lot more than a reprogrammed ECU and high-flow air filters. As we’ve come to expect from Harmut Feyhl and Renntech, just about every area has been optimized to crank out more power. Intake air flows through new air inlet panels, carbon-fiber airboxes and the requisite high-flow filters. It’s then fed through 82mm throttle bodies and eventually makes it way into ported and polished heads that spin more aggressive cams. All of this is controlled by Renntech’s reprogrammed ECU.
After combustion, the exhaust is sent through long-tube stainless steel headers and what the company calls their “sound pipe,” stainless steel pipes that sit in place of the factory resonators for better mid- and top-end power. The exhaust system aft of the catalytic converters was stock when I drove it because, until recently, Renntech never found any gains from aftermarket mufflers. Since then, the car has been fitted with an Akrapovic titanium cat-back system that Renntech said showed power gains.
With the stock mufflers, Renntech claims 605 hp and 555 lb-ft of torque, good enough for 11.4 seconds at 126 mph in the quarter-mile, 185 mph in the standing mile and a top speed of 204 mph. Six hundred horsepower, give or take a few, seems to be the threshold for a normally aspirated M156 6.2-liter as that figure is similar to the engine in Vaeth’s V63RS featured in the July 2011 issue (“Under the Radar”) that also had headwork, cams, exhaust and ECU mods.
But when you hammer the throttle, the exhaust turns sinister and screams a blood-curdling wail that’s dangerously addictive.
You can tell the exhaust breathes more freely with the long-tube headers and sound pipes because it adds a deeper, baritone voice to the exhaust note. It’s more menacing, yet still perfectly acceptable for everyday driving. But when you hammer the throttle, the exhaust turns sinister and screams a blood-curdling wail that’s dangerously addictive.
It’s the engine, though, that’s the real star. There’s plenty of torque throughout the rev range and the seven-speed transmission reacts quickly to throttle inputs by dropping down the necessary cogs to put the engine into its sweet spot. Going through the gears, the engine comes into its own in the mid-range and then, as it approaches redline, there’s an extra surge just before the horsepower peak at 6200 rpm. The power before the surge is impressive enough; the extra surge is just plain indulgent. Its low-rpm drivability and high-rpm power peak means this engine offers the best of both worlds.
On some of the long, straight highways near Renntech’s headquarters, it was all too easy to teleport from relaxed cruising speeds to triple the limit. The engine makes quick work of each gear while the transmission keeps the engine in the thick of the powerband.
Thankfully, we didn’t have to get all of our impressions on public highways because the great people at Palm Beach International Raceway let us have a few hot laps after our photo shoot.
Once off the leash, the C74 simply destroyed the track’s long, front straightaway and the 15.2-inch rotors and eight-piston calipers from their Performance Brake Package slowed the car down consistently and free of fade. Entering the turns, it understeered, but not profusely, you just had to be patient before getting back on the throttle. The Nitto INVO tires gave plenty of feedback and warning as they approached their limits and the Renntech/KW coilovers kept the car stable and controlled through the corners. A little more turn-in bite and grip, though, would help lower lap times.
For C63 owners who are tired of hearing about how the M3 is the better sports sedan, they now have options, enough options to have M3 and RS4 owners seeking restraining orders. Renntech’s fortifications to the 6.2-liter engine puts it into the class of the 600-hp supercharged M3s that have been featured recently in this magazine. Which car is better is as much a matter of taste as it is a matter of lap times, but Renntech’s C74 makes a contender out of an also-ran.
Longitudinal front engine, rear wheel drive
6.2-liter V8, dohc, 32-valve. Renntech cams, airbox, air intakes, sound pipes and ECU reprogramming; ported and polished heads; 82mm throttle bodies; long-tube headers
Renntech Performance Brake Package 3 (8-piston calipers, 15.2-inch rotors)
Wheels and Tires
Renntech Signature forged, 3-piece 9.5x19 (f), 10.5x20 (r)
Nitto INVO 275/30 (f), 305/25 (r)
Peak Power: 605 hp
Peak Torque: 555 lb-ft
Top Speed: 204 mph