The nailhead has more in common with a pair of four-cylinder engines than a V8. Like a four, the nailhead is a torquey motor, making all its torque at idle and then, as the rpms go up, the torque goes down and the horsepower rises.
Engine builder Guy Henson recently did a custom, high-tech build on a 401-cid Buick nailhead for a ’39 Ford Standard street rod. There aren’t many Ford Standard Tudor Sedans left today, so the owner of this unusual car wanted an equally unusual engine to power it. Henson built him an engine that he could cruise with after pulling up to a gas pump, filling the tank and driving away.
He rebuilt the bottom end, then started to think about his engine management choices. Since he already had a Whipple supercharger he planned around it. Henson hand-carved a billet intake manifold out of five different chunks of aluminum, put the Whipple on top, added a custom plenum and capped all this with eight Hillborn throttle bodies with spun aluminum velocity stacks and filters.
Ribs were carved into the intake that matched those in the supercharger. It looked nice, so Henson had it powder coated. He played Pinocchio with the Whipple, extending the nose piece four inches to get the belts to align. Then, he carved a water pump pulley out of billet and put a crank sensor on it. Since the customer had air horns, Henson glued some 2 x 6-in. wood pieces together to mock up a plenum. He then laid it out and carved out half of it on his milling machine, then sent it off to be digitized and replicated via CNC machine.
Henson added an elbow on the back of the plenum to hide an IAC (Idle Air Control) motor in, so he can control the throttle blades. He can completely close them shut if he wants to, bringing the engine down to a nice purr at idle. The throttle bodies can be positioned at any point from straight out to a 30-degree angle, which allows different “looks” with the same equipment. Guy went for driveability over brute horsepower. The car owner can hit the button and get instant starting. The engine makes 20 in. of vacuum at idle and when it comes up it has eight pounds of boost with the Whipple supercharger operating.
For engine management, Henson used a FAST (Fuel Air Spark Technology) system that will allow him to “drive” the completed car with a laptop once he has developed a base program on an engine dyno. Using a laptop, he’ll be able to tune the motor to behave itself the way that the owner actually drives the car.
The Hilborn velocity stacks suck air, but the injectors that fuel the engine hide between the custom intake manifold and the finned polished aluminum lifter cover. There’s a rubber vacuum line running around the motor where the Hillborn throttle bodies would normally be. That’s where Henson will tap off vacuum for things like vacuum gauges and windshield wipers. The finned polished aluminum Offenhauser rocker covers and steel headers came from Speedway Motors.
He designed the cam in the engine and had CNC grinders make it for him. Other parts are standard items. The Hillborn throttle bodies are a current edition of a classic design made to modify Chevy 283-cid small-block V8s for drag racing. Henson made his own throttle blades, changed the degree of angle and designed a more rounded shape that allows closing them off. They do not have to be slightly open to achieve idle, since the IAC technology takes care of that.
Henson talked to Andy Starr at Hillborn before selecting the throttle bodies. Starr told him Hillborn had sold 11 of that model in the last five years and that Henson had bought eight. Starr is hoping he sells a bunch more! Since this was a pilot build for hardware that Henson hopes to package for other engines, he figured out a number of different ways to run linkages to the Hillborns.
The Nailhead uses a cogged belt drive off the front of the plenum with tensioners. The throttle linkage is in the back and runs through the center of the plenum, so it actuates the throttle plates on both sides. The same setup can be routed out the back, so that both sides are separate and operated by a linkage. Return springs are incorporated in turret barrels mounted below the throttle bodies on both sides of the engine and the spring pressures are adjustable.
The Whipple 2.3L supercharger came off a kit for a 6.0L Chevy truck engine. Whipple has since changed the design for Mustangs and other late-model muscle cars. However, the bolt pattern on the back is the same, so Guy’s parts will bolt to any newer Whipple. The market niche he really wants to serve is hot rod versions of the Ford 4.6 and 5.4L Modular V8s.
With the Ford Modular V8, the stock cams will work if you degree them a little differently. The cams will accommodate the proper amount of boost and give you the most affordable hot rod engine you can imagine. And it will look great hanging out both sides of a car with the right “goodies” mounted on top of it.
In the ’50s and ’60s, factory pistons in hopped-up Nailheads tended to break. Buick used a “skeleton” piston with a hollow area above the wrist pin for expansion. They were designed for a big, heavy car with a two-speed transmission that never exceeded 3,000 rpm. A modified nailhead that’s revved up when cold will break pistons. Henson used custom Ross Racing pistons because he likes to run 8.5:1 compression compared to the factory 10.25:1.
Guy prefers the lower compression ratio and flat top or dished pistons in the nailhead’s huge 125cc combustion chambers. He feels that with 413 cubes, his engine fills such a large volume of space that compression can go too high.
“If I build some monster that no one can drive, I’ll hear about it,” says Henson. “I learned long ago to design something that I can tune to behave for the customer. The trick with any engine is to move more air through it, because it’s an air pump. The more efficiently you can mix the fuel into it, the better it will perform. I design for efficiency and horsepower is just a by-product.”
In initial dyno testing, the Nailhead hit its boost on the bottom and measured eight pounds of boost in the plenum. Henson plans to open up restrictions in the plenum to increase airflow. The dyno test produced 460 lbs.ft. of torque, so Guy expects to wind up with about 350 hp and 580 lbs.ft. of torque when the engine is dialed in. The low-end torque is so high that when combined with the six-speed Tremec transmission it’s going to purr down the road. The car owner will basically be going on engine alone when he’s running down the road.
Henson’s long-range plan is to manufacture and market the setup used on the Nailhead for Ford Modular V8s and any other engines that a Whipple supercharger will bolt to. The Buick engine has really become his prototype. And it’s the innovative approach to business survival that typifies today’s successful engine builder. 
