Taking a race engine, and turning it into something that will live on the street seems like a relatively simple task, but in fact, it’s more of a challenge than you might think. Race engines are usually designed to perform in a very narrow operational envelope and driving on the street is the opposite of that, with a wide range of conditions and variables that need to be taken into consideration.
As Jeff Huneycutt of The Horsepower Monster explains, Pro Motor Engines (a.k.a. PME Engines) has spent the better part of the past four decades focused on building engines for the top NASCAR racing series, ARCA, and SCCA, to name a few. They excel in those arenas and simply haven’t had the bandwidth to dedicate to anything outside of those areas, until recently.
With an internal expansion, the company has created room to extend its focus to the high-performance street market. Since they have focused so heavily on the NASCAR powerplants, it seems only natural that inspiration for street performance comes from those powerplants. Repurposing NASCAR components that have outlived their competition lifespan is nothing new. However, this is the first time anyone has offered an R07-based street engine program.
Getting into the Guts
Obviously, the base of this engine build is the Chevrolet Performance R07 engine block, which has been designed to have only the exact amount of metal needed to be reliable and nothing more. As such, it definitely has a lean exterior appearance. Further increasing strength is the fact that its cast from Compacted Graphite Iron, which is significantly stronger than traditional gray iron.
The block features an extra 0.100-inch of bore spacing, coming in at 4.500 inches instead of the traditional small-block Chevrolet 4.400 inches. The bores have been opened up .005 inch from the NASCAR maximum size of 4.185, which between the wider bore spacing and the CGI material, poses no problems at all.
A Bryant billet crankshaft, identical to a Cup crankshaft, but with a longer 3.600-inch stroke is used in this combination since there is no rulebook on the street. The block and crank use 2.00-inch main journal diameters to reduce bearing speed, and due to the additional stroke, the R07’s piston oil squirters need to be removed.
Carillo forged-steel, 6.200-inch, tapered H-beam rods are utilized in this combination with a small 1.850-inch rod journal diameter, again to reduce bearing speeds when engine speeds approach 9,000 rpm. Speaking of bearings, this engine uses standard Clevite coated H-series bearings in the rods and mains to keep everything moving freely.
Of course, since this is still a high-end build, high-end custom pistons are being used. 4.190-inch forged 2618 pistons from MAHLE are fully coated to reduce friction and feature a reverse dome milled into the crown to bring compression down to a manageable 12:1 while also acting as a valve relief. While piston ring specs aren’t disclosed you can see in the video that they are super low tension, ultra-thin pieces, as evidenced by the complete lack of effort required to get the piston into the bore.
Additionally, the pistons feature a reduced-width wrist pin and the pin boss is designed to locate and guide the connecting rod, allowing for much-increased rod side clearance on the crank journal. Combined with the 3.600-inch stroke, the total displacement of this short-block comes in at 397 cubic inches in a very oversquare design.
Top End and Valvetrain
The heart of the valvetrain is the engine’s NASCAR-based solid-roller camshaft. With narrowed lobes and journals, the billet steel bumpstick has all the friction reduction tricks still in place. It also has the R07’s unique front timing gear, designed to reduce timing changes based on camshaft twist. The specs weren’t shared, exactly, only confirming duration in the 280-degree range and lift just shy of an inch, with a 110-degree lobe separation angle. The camshaft rides in a raise cam tunnel outfitted with roller bearings to further reduce friction and is driven by a fully adjustable cog-belt drive system and outfitted with a carbon fiber front cover.
The engine uses .935-inch solid-roller keyway lifters, which are guided by keyed bronze lifter bushings, in order to handle the aggressive cam lobe designs used in the engine. Double-tapered 7/16-inch pushrods with a .165-inch wall-thickness are used, with a nominal length of 8.000 inches. Between the lifters and the rocker arms.
The roller rocker arms are a set of steel 2.0:1 ratio shaft-mount units without adjustment hardware in order to save precious weight. The stands are shimmed to fine-tune the stand height geometry, and then the valve lash is fine-tuned with minute adjustments in pushrod length.
The dual valvesprings used to control the valves are in the neighborhood of 780 pounds of open pressure and are held in place by titanium retainers and tool steel lash caps. The lash caps are both for increased roller contact area, improved wear resistance, and used for final lash adjustment. Primary lash adjustment is achieved through pushrod length, with the goal being to achieve the desired lash with a lash cap right in the middle of the range of available thicknesses (.070 inch) so that there is plenty of room in either direction available through lash cap adjustment.
Topping off the combination are a set of aptly named R07 cylinder heads. They feature an LS7-style valve arrangement and valve angles of 11.5 degrees on the intake and 7.5-degree exhaust. Both valves are made of titanium with a lightweight 7mm stem diameter, with head diameters of 2.210 inches on the intake and 1.625 inches on the exhaust. The combustion chambers are quite small by street-engine standards at 40cc.
Sixteen head studs sit in the block, along with another four which thread into each head, and are secured to external flanges on the block. The 20 fasteners per bank provide the clamp load and sealing comes from a set of copper-sprayed multi-layer steel head gaskets. Topping the heads off is a set of cast aluminum valve covers with built-in spring oilers.
Stepping completely away from the NASCAR mold, the intake manifold is a sheetmetal dual-quad tunnel ram fitted with a pair of out-of-the-box Holley HP 750 carbs and a one-inch open design carb spacer under each carburetor. Underneath the intake manifold sits a cast-aluminum valley cover slash water manifold specific to the R07 cylinder head design.
On The Dyno
Since the R07 has no provisions for an internal oil pump, a six-stage dry-sump system was fitted for the dyno session. A set of race-style 1-7/8-inch stepped-primary headers were fitted for the dyno testing, as well. Once broken in, the engine was spun all the way up to 8,800 rpm on the dyno, with impressive results. The dual-quad setup yielded a peak of 955.1 horsepower at 8,300 rpm and 655.5 lb-ft of peak torque at 6,700 rpm.
One thing to note, is this version of the engine is still a high-winding, pissed-off powerplant that is probably better suited to a track day than a cruise night, unless you are super hardcore (which this editor has no doubt plenty of you are). Pro Motor is working towards releasing a version that has much better street manners thanks to a reduced compression ratio, which will also allow it to perform on standard pump gas and still make in the neighborhood of 750 horsepower. With the expected revamp of NASCARs engine programs in the next couple of years, there is sure to be an influx of R07 components, and PME is planning on being ahead of the curve.