It is hard to look at the final result and realize that this engine started as a run-of-the-mill straight-six in some work truck built in 1958.

Automotive Specialists and Johnson’s Hot Rod Shop Turn a Run-of-the-Mill Straight-Six into Something No One has Ever Seen Before

By Jeff Huneycutt – Photography by the Author

How many times have you been searching Craigslist or walking the aisles in a junkyard and find what might just be the perfect score only to raise the hood and be disappointed to find a pedestrian straight-six.

Of course, over the decades, Chevrolet put a lot more sixes in vehicles than they did their much sexier V-8s. The straight-six was Chevrolet’s main engine when it was introduced in 1929, going into almost everything all the way up the V-8’s introduction in 1954. And even after the V-8 came around, the straight-six was the base engine for practically every model. Incredibly, the straight-six was still being produced in Brazil as recently as 1988.

This engine build was more than two years in the making because of all the custom, one-off parts that were created for it. But the foundation is a basic 261ci Chevrolet straight-six block that came out of a truck. From the factory it came with a 3.75-inch bore and 3.9375-inch stroke, and it was rated at a measly 148 hp.

So, given all that history, it is pretty darn cool that somebody is finally giving the straight-six its due. Jonson’s Hot Rod Shop in Gadsden, Alabama, builds some of the coolest pieces of fuel-burning art on four wheels. They’ve built some of the most famous hot rods ever made, including a Ridler Award winner and a Goodguys Street Machine of the Year.

One of their latest projects is a ’50s-era Chevrolet Suburban. We can’t tell you any more about the build than that, or show you any photos, because the Suburban has yet to make its big reveal at an equally big car show or event. But we have been given access to the build of the engine that’s going to power this beast. And it is without a doubt one of the coolest, and most unique, straight-six builds you will ever see.

When Johnson’s wants to create an engine that will fit one of their topflight builds—in other words a one-of-a-kind creation—lately, they’ve gone with Automotive Specialists in Concord, North Carolina. Owner Keith Dorton spent years building winners in NASCAR’s Cup Series, but these days Automotive Specialists is more of a custom shop designing high-horsepower builds for a variety of different disciplines.

Like the block, the crank is also an OEM piece. Check out that giant chunk of iron cast in as a counter-balance between cylinders number three and four. There is a ton of material here, but the crank isn’t exactly as strong as it could be because it lacks main journals between cylinders one and two as well as five and six. The rod journals have been cut down to big-block Chevrolet size, and the whole thing has been nitrided to improve its wear characteristics.

Anyhow, Dorton and Johnson’s have spent the last two years developing a plan and designing one-off parts for the straight-six build to be featured in the Suburban. What they’ve come up with is something that, after at least two Google searches, we don’t think has ever been done before.

Despite being in use for 60 years, Chevrolet’s straight-six was never widely considered much of an option for performance. Johnson’s and Dorton wanted to go beyond the typical turbo kit and intake manifold. But what they came up with is so far beyond that, it’s hard to believe. That includes, twin turbos, custom stainless steel exhaust castings, individual throttle bodies, port fuel injection, and more.

The block originally came with rope seals, and one-piece rear main seals aren’t exactly available for the straight-six. But Dorton wasn’t about to depend on a rope seal, so he found a quality unit that’s for a Ford diesel that was at least close and then machined the crank, block, and main cap to accept it.

The only catch is Johnson’s wanted to limit the horsepower to half a grand. In this situation, 500 is nowhere near the power limit of this engine, but knowing the ins and outs of the Suburban build, they felt that was the maximum needed. So Dorton limited the engine to less than 10 pounds of boost, and even then had to pull the handle back at 5,200 rpm. The real power max is almost certainly well north of 750, but even at 500, we know you can appreciate the innovation here.

So let’s have at it.

Twin-Turbo Straight-Six  
RPM Torque Horsepower
3,500 578.6 385.6
3,600 585.2 401.1
3,700 588.0 414.2
3,800 586.1 424.1
3,900 586.3 435.4
4,000 582.8 443.9
4,100 582.6 454.8
4,200 581.9 465.3
4,300 582.3 476.7
4,400 581.6 487.3
4,500 577.7 495.0
4,600 571.8 500.8
4,700 562.0 502.9
4,800 558.7 510.6
4,900 553.1 516.0
5,000 549.8 523.4
5,100 540.7 525.1
5,200 531.2 525.9

Automotive Specialists
(704) 786-0187

Johnsons Hot Rod Shop
(256) 492-5989

Another area where Dorton simply couldn’t find parts to meet his high standard are the bearings. Instead of settling, he used a variation on an old racer’s trick to cut down the main journals and reduce rotating friction. Except this time the goal was to be able to use a modern, coated bearing. Dorton installed a set of stock bearings in the mains, then used a line hone to open up the id on the bearings until they matched the od for a set of LS bearings. The crankshaft was cut down to match. An ancillary benefit is the LS bearings are narrower, which will, in fact, cut down on rotating friction. Also, just below the main bearing you will notice the cam bore now has needle bearings. More on that in a minute.
To make the most of the measly four main caps securing the crankshaft to the block, the center two are new billet units. Dorton kept the stock outer main caps because “they are already huge.”
The pistons are custom Diamond flat tops. They are sized at 3.780 inches in diameter, making the engine’s displacement 265 ci. As we already mentioned, the crankshaft’s rod journals have been cut down to 2.200 of an inch, or big-block Chevrolet sized, which gives him a much greater selection of connecting rod options to choose from. These are 7.100 long forged rods from Molnar Technologies.
There was no way he was going to be able to find a cam to meet his specs. After all, the second-generation straight-six never came with a roller cam, which we wanted to use. So Dorton honed the cam bores out to accept a 50mm cam and then installed high-performance needle bearings. Then he had a custom cam ground from billet steel. This is a solid roller cam with 250 degrees of duration at 0.050 tappet lift for both the intakes and exhausts. Lobe lift is 0.374 of an inch, and with a 1.74 ratio rocker arm, total valve lift will be 0.550 of an inch. Notice the collar clamped to the shaft of the cam just behind the cam gear at the front. That’s for the cam timing sensor, which we’ll explain in more detail here in a second. Also, there is no cam gear here, so an external oil pump will need to be employed.
On the left is the stock solid flat-tappet lifter for this engine. On the right is a pair of modern tie-bar roller lifters that Dorton managed to get to work. This, obviously, is a huge improvement.
The lifters are just barely tall enough for the tie bars not to hit the block when the rollers are on the base circle of the cam, but it works.
Dorton plans to run port fuel injection on the engine and a Holley Terminator X Max engine control unit to control it. That system needs to know both the cam and crankshaft position at all times, so he had to create a way to monitor both. For the cam position sensor, he fabricated this plate that bolts onto the block where the fuel pump would normally go. Then the sensor screws into that.
Here’s an inside look at the cam position sensor system. The collar clamped onto the camshaft contains a magnet that the sensor can read as it rotates past. The fact that the magnet is clamped to the cam is important because Dorton must be able to adjust the magnet’s position once the camshaft is timed in.
The big-block rods are nowhere near as wide as the original connecting rods. You can see the big gaps left once the big ends are secured onto the crank.
To take care of this, Dorton had the Diamond pistons custom made with the wrist pin bosses set to guide the connecting rod’s longitudinally. You can see the shim on the left side used to dial in the minimum clearance.
The cylinder head is a recent recasting of the legendary Wayne cylinder head. This casting converts the straight-six to a cross-flow design that puts the intake and exhaust on opposite sides of the cylinder head. And although the combustion chambers are no longer cutting edge, they are still worlds better than the horrible chambers of the originals. The valves are sized at 1.940 inches for the intakes and 1.550 for the exhausts. The combustion chambers are 72 cc’s.
Up top a set of lightweight beehive valvesprings control the stainless steel valves. The lightweight aluminum roller rockers are shaft-mounted with a 1:47:1 ratio. With an installed height of 1.760, the springs will hold the valves to the seats with 200 pounds of pressure, and that ramps up to 430 at full lift.
On the intake side, Dorton is using six Borla throttle bodies designed to look like old-school Weber carbs. Except these are kitted with fuel injection. The mount onto custom bases that funnel the air into the intake ports.
What you can’t see from the previous photo is the castings that guide the air from the Borla throttle bodies don’t mount directly to the Wayne cylinder head. Instead, they bolt to this intermediate plate, which bolts to the head. The purpose of the plate is to provide ports that connect all six intake runners. This creates a balanced chamber which helps the engine run more smoothly.
To get the pressurized air from the twin turbos into the intakes, this intake plenum provides a way to get the air equally into all six velocity stacks.
Two Garrett Gen II GTX turbos will convert the exhaust into pressurized air. These small turbos have ceramic ball bearings and a super-efficient compressor wheel. They are mounted on custom stainless steel exhaust manifold castings.
Hard lines route oil through this manifold and to the two turbos. In front of that is a Turbosmart adjuster that allows Dorton to fine-tune the maximum amount of boost before the blow-off valves open.
The front drive is an incredible combination of components, practically none of which were originally designed to work with the Chevrolet straight-six. Johnson’s came up with a billet machined front plate to give a mounting point for everything. The water pump is for a BMW, chosen for its efficiency and small size. On the driver side at the bottom is the external oil pump.
And here is the engine on the dyno complete with both turbos and twin air-to-air intercoolers. Although the setup may appear at first glance to be way too spread out, this is actually designed to fit in the Suburban exactly like this.
Another angle of the complete engine on the dyno.
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