Flame Hoops? The Weird Science Behind the World's Fastest Car's Wild Engine

We've come to expect speed records to be set by deep-pocketed global car companies like the VW Group's Bugatti brand, which has the budget to produce crazy complicated quad-turbo W-16 engines. So when the Washington state-based and independentSSC Tuatara managed to set an official two-way average speed record of 315.7 mph (!) running a small-block engine boasting half the cylinder and turbo count and a single (billet) camshaft, we were desperate to learn more about this record-setting engine. Who better to tell us all about it than its architect, Tom Nelson, of Nelson Racing Engines?

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Small-Block Geometry, No GM Parts

Tom was quick to note that this is not a GM engine block. It merely shares some key geometric dimensions with the iconic and quintessentially American V-8. But every single detail is optimized for this extreme duty cycle. The block is cast, not billet-machined, because this is the only way to properly heat-treat a block. Billet blocks must be built, run, and then completely re-machined after that hard running completes their heat treatment. The three main secrets to generating 1,750 horsepower from this 5.9-liter twin-turbo V-8 include a flat-plane crankshaft, an 8,800-rpm redline, and E85 ethanol fuel (power drops from 1,750 to 1,350 horsepower on 91-octane gasoline).

Why a Flat-Plane Crankshaft?

As we've previously noted, the world's first V-8s had flat-plane cranks because they were essentially two four-bangers sharing a crankshaft; Cadillac's invention of the cruciform crank (cross-plane, with throws offset by 90 degrees) in 1923 enabled more even firing for considerably smoother operation. Flat-planes have been reintroduced on high-performance V-8s because the exhaust pulses within the header on each bank of cylinders encourages more efficient scavenging at high engine speeds. They alsosoundawesome, which is largely what founder Jarod Shelby was after. As General Motors demonstrated in 1970, that sound can be replicated with elaborate, long, spidery "180-degree" exhaust headers that combine exhaust from two cylinders on opposite banks of a V-8 running a cruciform crank. Nelson's team looked into such a header for the Tuatara, but the runner lengths are so different from cylinder to cylinder and the packaging is so tortured that they scrapped the idea in favor of a flat-plane crank.

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How Can This 5.9-Liter Flat-Crank Engine Survive 8,800 RPM?

First, this is a very large-bore (4.125-inch), short-stroke (3.375-inch) engine, which greatly reduces piston speeds at high rpm. Next, the rotating mass is reduced to the max, with short-skirt aluminum pistons and titanium connecting rods that end up with a total rotating mass per cylinder of about 1,600 grams, including oil allowance. Then the team spends two full days precision balancing the entire rotating assembly ("We can spin it at 10,000 rpm and set a wineglass on the balancer," Nelson said). Finally, to isolate what vibration there is from the rest of the vehicle, the engine is mounted using proprietary oil-filled mounts instead of urethane. Oh, and Bryant Crankshafts supplies the crank, which is machined from billet TimkenSteel and is said to cost $10,000.

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Patented Dual-Volute Turbochargers

To take full advantage of those evenly spaced exhaust pulses in each header, they are plumbed into separate passages so they hit the turbine wheel 180 degrees apart. This allows the larger turbos to spool up as quickly as a smaller one would with all unevenly timed exhaust pulses all hitting the turbine wheel in the same spot. The patents cover novel aerodynamic shaping of the exhaust turbine and compressor blade designs. Another novelty—the left and right turbochargers are symmetrically opposite and rotate in opposite directions. This ensures that the exhaust-flow is identical on each bank. Nelson claims these turbos have demonstrated 82 percent turbo compressor map efficiency (up from 70 percent for most turbos). Boost pressure can go as high as 30 psi, but the record run indicated boost peaking at 19 psi. Also note that this isnota hot-vee design; the turbos sit outboard, with the intake manifold in the vee.

Extreme Intercooling

Generating big boost is great, but only if you can cool the charge back down. Here, twin air-to-water intercoolers are integrated into the billet-machined intake manifold, which is fed by twin electronic throttle bodies. On the record run, engine sensors measured 260-degree air exiting the turbocharger, with that temperature reduced to 130 degrees after the intercooler. Nelson notes that every 10-degree (F) drop in temperature is good for about 1 percent power increase. These temperature sensors are but a few of the many employed to measure every aspect of this highly strung engine. There are 11 exhaust gas sensors, four air-fuel-ratio sensors, and myriad pressure sensors. The safety parameters of the engine are very closely monitored to ensure it never blows itself up. A 10-percent variation in any spec triggers an alarm/limp-in mode. All this sensing assists in communication to manage torque during shifts and in conjunction with the Light Racing-developed traction control, which is absolutely essential to put this much power down through the rear tires.

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Other Engine Odds and Ends

The oiling system is dry sump design with a heavily baffled oil pan to enable the extreme track driving the Tuatara has been developed for. The engine cooling system is pretty unremarkable, except that there is a booster water pump in the nose of the car to assist in moving all the coolant to the forward radiators. A "flame hoop" helps secure the block to the cylinder head and to contain the immense in-cylinder pressures associated with generating 1,750 horses in a 5.9-liter engine. This "hoop" involves boring a registration groove around the top of the cylinder, into which a tempered steel ring is inserted. This ring indexes into a receiver groove in the cylinder head and locks the head gasket in place to prevent it from blowing. And finally, during test runs on the Friday before the record run, the team was unhappy with exhaust temperatures and traced the problem to coils that were not returning to ground quickly enough above 6,000 rpm. This effectively over-advanced the ignition timing. Inserting a resistor between the coil and ground rectified the problem, reducing temps from 1,900 to 1,700 degrees. And that's how this development engine has survived so long, and why SSC has enough confidence to sell it on to a consumer. For its part, Nelson Racing looks forward to building the next several iterations of this wild V-8.

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