What happens when you take an old Chevy straight-six workhorse engine and treat it like a modern-day powerhouse? Well, that’s what Keith Dorton and the team at Automotive Specialists wanted to find out. So, they took the vintage inline mill, added a pair of turbochargers, gave it individual throttle bodies, and EFI, along with a host of modern upgrades to answer that question.

Now, as Jeff Huneycutt points out for the more pedantic viewers, technically this engine isn’t a proper “Stovebolt” engine, as those were the engines built specifically between 1929-1936. The core engine used here was built in 1958, making it a second-generation inline-six. If you’re interested in the history of the engine, you can check out the article we published on it, here.

Old School Building Blocks

As the base, there is no getting around using the stock block on a project like this. Well, I suppose anything is possible out of billet these days, but for the purposes of this project, the original block will be plenty. It’s a 261 straight-six casting from 1958, making it 63 years old. Dorton opened up the bores .030 for a 3.780 final bore (does that look familiar at all?).

Since there are no high-performance bearings available, for a Chevy straight-six, Dorton simply installed a set of standard replacement bearings, and then align bored the mains — with the bearings installed — to the proper diameter to allow for the use of high-performance coated LS bearings. The original bearings now simply act as a spacer between the block and the LS bearing.

Since there are no performance bearing options for a Stovebolt 6 engine, Dorton simply opened up the stock bearings to hold a set of LS bearings. The original bearings simply act as a spacer for the LS bearings.

Sitting in those bearings is an original crankshaft that Dorton sent out to be tweaked. The main journals were recut to 2.559 inches to match the LS main bearings. The rod journals were also turned down to 2.200 inches (again, look familiar?), but were not offset during the process, maintaining the original stroke. Once recut, the crankshaft was nitrided to harden the journals.

Since he was modernizing it anyway, Dorton also did away with the traditional rope rear main seal and machined the block and crank to convert to a modern one-piece rear seal (from a diesel Ford, of all things). One thing there was no getting around was the heavy counterweights and the only four main journals. So to try and reinforce things, Dorton used doweled billet-steel center caps on the two center mains.

Some New School Concepts

For the pistons, Dorton went with a set of custom forged pieces from Diamond Racing. Using .990 wrist pins, he was able to use off-the-shelf Molnar 7.100-inch big-block Chevy connecting rods. The modern forged-steel H-beam rods and much narrower than the original connecting rods, which presents an issue, as the sides of the crankshaft journal located the rod.

To rectify this, Dorton had Diamond move the piston pin bosses inboard, for a piston-located rod configuration. While all that additional side clearance might concern you at first, check out this article we published on the true relationship of oil flow and side clearance to put your mind at ease. While it was believed that excess side clearance would lead to an internal oil leak in the past, we know better now.

In decades past, this amount of side clearance would cause some to freak out. Fortunately, we now how to control oil flow with a piston-guided connecting rod setup.

Dorton opened up the cam bore to be able to fit 50mm roller camshaft bearings. Since the camshaft is gear-driven directly off of the crankshaft in a straight-six, Dorton fabbed up an oil squirter to keep the gears lubricated under hard use.

Obviously, the original camshaft wasn’t going to be an option for this build, but it’s not like the 261 straight-six has pages of camshafts in the local speed shop’s catalogs. So, Dorton had one cut to his exact specs out of billet tool-steel. Designed for a solid roller lifter, Dorton spec’d out the same lobe for the intake and exhaust, which measures .550-inch of valve lift with 250-degrees of duration at .050-inch.

An original solid flat-tappet Chevrolet inline-six lifter on the left with the modern tie-bar solid roller lifter lifters used in this build.

Another modern touch to the camshaft (as if a billet camshaft and roller bearings weren’t enough) comes by way of a trick clamp-on camshaft position sensor reluctor to provide data to the Holley Terminator X ECU Dorton will be using to control the beast. The sensor itself resides in a custom-fabbed mount that fits into the OEM fuel pump boss.

The Terminator X requires both a camshaft and a crankshaft position signal. So, to generate the latter, Dorton added trigger magnets to the flywheel and mounted the sensor into the bellhousing adapter. Now, the engine designed in the 1920s and built in the 1950s is able to accurately report the precise location of the crankshaft and camshaft, electronically.

Besides the badass custom billet tool-steel camshaft for the engine, note the clamp-on camshaft position sensor collar on the left. That allows the vintage engine to operate a modern EFI system.

Futuristic Top End

Surprisingly, there is a non-original, non-billet cylinder head option for this engine. The casting a reproduction of the Wayne cylinder head that ruled the inline-six game back in the day. It features a radically different combustion chamber design from the original Chevrolet cylinder head.

Besides utilizing an almost-hemispherical 72cc combustion chamber, the Wayne cylinder head converts the engine to a cross-flow design with radically better flowing port designs. The new chambers house 1.94-inch stainless intake valves with 1.55-inch exhaust valves.

Actuating those valves is a set of 1.47:1 shaft-mounted aluminum roller rocker arms. The valves will be controlled by a set of beehive valvesprings with 200 pounds of pressure on the seat and 430 pounds of open pressure.

The intake manifold for this engine is a completely custom-built piece, developed by Dorton and Johnson’s Hot Rod Shop. A custom base smoothly converts the rectangular intake ports to the round shape of the Borla throttle bodies as well as providing a common plenum between all of the cylinders. From there, three custom-cast adapters mate three pairs of Borla throttle bodies to the port spacing of the Wayne cylinder head’s intake ports. Each Borla throttle body has an individual injector in its runner and will provide incredible throttle response.

A pair of custom cast-stainless-steel turbo manifolds sit on the other side of the cylinder head. With a simple three-runner design, the two manifolds are different sizes, but both feature V-band attachments for both the turbocharger and a wastegate. A part small of Garrett Gen-2 GTX turbochargers will feed the engine, while a pair of Turbosmart wastegates control the amount of boost produced.

A pair of small Garrett GTX turbochargers provide more than enough boost to reach the customer’s 500 horsepower goal. In fact, on the first start, they made way too much boost.

On The Dyno

Once together, this engine package, which looks absolutely nothing like a Stovebolt 6 anymore, was mounted to the dyno and plumbed with a pair of air-to-air intercoolers. Right out of the gate, the combo was making way more boost than anyone expected, way sooner than expected. So with a quick wastegate spring and boost controller adjustment, the combo was fired back up.

The team at Johnson’s saddled Dorton with a 10psi boost limit and a 500-horsepower maximum, in order to preserve parts, since this isn’t an all-out performance engine. That means the test only ran up to 5,200 rpm. Peak torque of 588 lb-ft occurred at 3,700 rpm, and Dorton blew past the 500 horsepower mark at 4,600 rpm, with a peak of 526 horsepower at 5,200 by the time the handle was pulled back.

Without the artificial limits imposed by the customer, Dorton believes this setup could hit the 750-horsepower mark on pump gas. Not bad for an 80-plus-year-old engine design that was never designed to be a performance engine.

While 526 horsepower at 5,200 rpm and 588 lb-ft of torque at 3,700 rpm seems conservative, Dorton feels that if he were turned loose on the combination, he could easily get that horsepower number to 750 on pump gas.