It’s no secret that here at EngineLabs, we love naturally aspirated engines. Nitrous is cool, and boost is awesome, but at the end of the day, being able to make big horsepower numbers with nothing more than the atmosphere the Earth gives you is the sign of a great engine build. So, when we saw this 429 cubic-inch, 351 Windsor-based engine built by ZSR Engines, making 2.1 horsepower per cubic inch on pump gas, pop up on our Facebook feed, we had to know more about it.

Zach Smith, the founder of ZSR Engines, decided to specialize in Blue Oval powerplants when he hung his shingle out in 2015. “I worked in shops that did all makes, but I was always more knowledgeable about the Fords,” Smith explains. The decision to focus on Fords was more than just based on passion and knowledge. By specializing in one make, he felt it would actually expand his reach, instead of just being the local engine shop. Specialized customers allow him to focus on what he enjoys, as well as streamline the shop’s operations.

This particular engine came to him as a pump gas street/strip engine project for a 1968 Mustang Fastback out of Montana. “The goal was a bit of a moving target,” says Smith. “The engine came in with an issue and evolved over the build process. It ended up with different cylinder heads, intake, carburetor, pistons, and so on.” What the engine turned into is a monster of a small-block, making over 900 horsepower on simple 93-octane pump gas.

Building A Brutal Pump-Gas Windsor Bottom End

While not requiring the same brute strength in the block that a boosted combination requires, a combination like this still requires the benefits of an aftermarket block. In this case, the ability to open the bores up all the way to 4.185 inches in diameter. The Dart SHP iron block allows for that as the maximum bore size. It also provides steel four-bolt main caps, with the three inner mains having splayed outer bolts.

Into those four-bolt mains went a set of coated Clevite main bearings, which allow the Callies Magnum crankshaft to spin freely. The 3.900-inch-stroke Magnum crank is a forged, heat-treated, 4340 steel crankshaft with gun-drilled mains and windage-reducing profiled counterweights. Besides being strong, the Magnum cranks also utilize Callies’ “Magnum Mass Correct” counterweights, which spread the balance load throughout the crankshaft, not just at the ends. This is a huge advantage in a high-RPM naturally aspirated combination.

The short-block looks deceptively simple on the outside. It’s just a 351 Windsor with a belt drive, right? Wrong. It displaces 429 cubic inches with some of the best parts available inside of it.

Hanging off the Magnum crank are an octet of Callies Ultra I-Beam connecting rods. They are made from Timken 4330V steel and have a shape that has many hours of engineering built into it, optimizing every possible stress path and load area for a truly optimized design. Attached to the Ultra I-Beams are a set of Diamond Racing pistons, that have been custom-designed for this application with a 12cc dish.

With the custom piston design comes the ability to design whatever ring pack your heart desires, and in this case, Smith specified a .043-inch top ring groove, .043-inch second ring groove, and a 3mm oil ring groove. Those grooves are filled with a set of Total Seal’s Advanced Profiling piston rings.

Sealing the bottom of the short-block is a Stef’s fabricated five-quart oil pan with an integrated crank scraper and windage tray in order to keep all of the oil in the proper locations (off the crank and unaerated). Moving all that oil efficiently is a Melling Select oil pump. Yes, this is a standard-location wet-sump engine. Pretty impressive for this level of performance.

A Killer Cleveland Top End

In order to make killer power naturally aspirated, you need a set of killer cylinder heads. Without the ability to force air into the cylinders, having a perfect port design that will move air at atmospheric pressure is absolutely crucial. In order to accomplish his goals, Smith looked down under, turning to Higgins Race Heads in Australia.

Smith opted for a pair of Higgins’ 3.3 Cleveland heads. The “3.3” in the name denotes the 3.3-inch minimum cross-sectional area of the intake port. On these heads, the intake port flows a whopping 411 cfm at .900 inch of lift, while the exhaust port flows the 255 cfm at the same lift. This is thanks in large part to the Victory 1 2.230-inch titanium intake valves and the 1.600-inch titanium exhaust valves.

Once the heads and valvetrain are on, you start to notice that this is a serious small-block. Those high-dollar parts can only mean one thing… lots of power.

In order to control those valves’ opening and closing events, Smith had COMP Cams grind a cam to ZSR’s specs. A set of Jesel tie-bar solid-roller lifters rides on the lobes, with a set of Manton Series-5 7/16-inch diameter connecting the lifters to the T&D shaft rocker setup. Keeping everything in time with each other is a Jesel belt drive system.

Topping off the killer top end is one of Higgins’ 4500-series intake manifolds that has been fitted with a Davinci 1,175-cfm 4500-pattern carburetor. Exhaust gasses are scavenged from the engine via 2.0-inch primaries, which are stepped to 2.125 inches, flowing into a 4.0-inch merge collector. This induction and exhaust combination isn’t for the faint of heart, as is all comprised of seriously hardcore components.

On the engine dyno, with simple 93-octane pump gas from the local pump flowing through the system, the engine spun to 8,500 rpm on the dyno, making an amazing sound (seriously, speakers up with the video above) and producing a wild 904 horsepower and 611 lb-ft of torque. Doing the math, that makes the output of the engine 2.107 horsepower per cubic inch. That’s an incredible figure for an engine that will be run on the street.

Look at the tachometer on the dyno. The engine spins to the 8500-rpm rev limit effortlessly.

“In the final stages of the build, the customer asked if 900 horsepower was possible,” relates Smith. So we did meet the goal, and I left the dyno in a good mood, which is rare.” Asked if he would do anything different next time, Smith admitted that there were some compromises made from the early desires of the customer to run a little nitrous.

“I would go with a thinner ring package next time, since it evolved into a more N/A build. I’d also like to add cubic inches to it. It was short on cubes because we reused the original crankshaft. Switching it to a longer stroke would be ideal, but we used what we had. Looking back, that would have been a marginal financial increase and would net a nice performance gain. We were just trying to reuse what we could,” he says.

While the engine is back in his customer’s care, ready to tear up the asphalt, Smith is confident that he’ll get his chance to add some displacement and power sooner or later. “I am sure it will be back eventually,” he says. “Every racer wants more power and that is why we are here.”

ZSR knows how to make the 429-cube small-block Ford combo sing. This engine isn’t restricted by the need for pump gas and makes even more power.