The Street Spec Pro is designed for more serious weekend track users who demand maximum performance and durability, or for applications generating more extreme power outputs than our Standard short block can handle. The Street Spec Pro is now a fully closed deck block design. "Closed Deck" refers to closing of the open spaces of the head gasket sealing face of the block. The process involves milling pockets in the block to accept press fit inserts precision machined from a cast aluminum material to closely match the expansion rate of the base block material. This deck design serves to reinforce the most highly stressed portion of the cylinder bore reducing distortion and the potential for cylinder cracking associated with extreme power outputs. It also helps to reduce head gasket failures by stabilizing the sealing surface under extreme conditions.
The Street Spec Pro rotating assembly starts with a salt bath nitride hardened crankshaft that is precision balanced for smooth running and reduced bearing loads. We fit our Pro Series tri-beam 4340 connecting rod with ARP L19 bolts. The tri-beam design is a hybrid between an H and I beam design . The rod design is FEA optimized for excellent strength and rigidity with minimal weight. The crankshaft spins in coated ACL tri-metal race series main bearings and low-friction Calico coated Clevite rod bearings for maxiumum durability. Pistons are RCM/Omega forged from 2618 alloy. Pistons and rods are weight matched to within 1/3 gram.
Every short block is professionally assembled in our dedicted engine building room by our in-house builder who has over 20 years of motorsport level Subaru engine experience and has built hundreds of Subaru high performance engines. Every short block is blueprinted to carefully designed specifications that balance extended service life with extreme power capabilities. Every dimension is logged and a build spec sheet is provided for reference. The block is assembled with all typical short block hardware, plugs, and oil separator plate.
The block is optionally available machined for RCM 14/12mm or 14/14mm Superstud oversized head studs. Note that the upgrade price includes a complete set of head studs. 14/12mm head studs use a 12mm head side thread which fits the OEM gasket hole size and cylinder head holes but is an extremely tight fit (modification may make assembly easier). 14mm Superstuds use a 9/16in head side thread which requires drilling the cylinder head holes and enlarging the center two head gasket holes (or running a gasket intended for 14mm or 9/16" stud size such as the RCM ones we sell).
Click here for shortblock comparison and specification chart.
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What is the recommended break-in process for the engine? This information along with warranty details can be downloaded here: Engine Warranty & Break-in Instructions (PDF).
At what power level is a closed deck reinforced block necessary? We receommend any application above 500whp be utilizing a reinforced block due to the extreme cylinder pressures and increased possibility of detonation. Applications in the 400 to 500whp range may not absolutely require a reinforced block from a failure prevention standpoint but may see advantages in ring sealing and overall longevity.
Will a closed deck block run hotter? The addition of material around the top of the cylinders can cause that area of the cylinder as well as the piston to run somewhat hotter due to the extended path between the cylinder/piston and the cooling jacket. Its unlikely that you will see any actual increase in cooling temperatures unless the cooling system was very marginal to begin with. However, there may be a small loss in performance due to the higher piston crown temperatures. This may be offset by the improved piston ring seal in cases where cylinder distortion was significant with the standard block.
What is the purpose of the piston oil sprayers and how do they work? The piston oil sprayers (or oil squirters) are an optional upgrade designed to provide additional cooling of the piston. We machine the block to accept an oil jet that sprays oil in the direction of the underside of the piston once the oil pressure is sufficiently high. The oil for these jets is supplied from the #2 and #4 crankshaft journals. The oil spray provides a cooling effect to reduce the piston temperatures. This technology is borrowed from the original Subaru factory closed deck blocks which were fitted with these sprayers from the factory.
When is it necessary to run a larger head stud? Our general suggested guidelines for head studs is as follows: Up to 400whp - ARP2000 11mm Head Studs, 400-550whp - ARP CA 625+ 11mm Head Studs, 450-600whp - 1/2in Head Studs, 600+whp 14mm Head Studs.
How does cylinder o-ringing help? The purpose of o-ringing the cylinders is to put additional pressure on the head gasket fire rings (the reinforcement in the head gasket designed to contain the cylinder pressure). When using a multi-layer-steel (MLS) head gasket a copper o-ring is used. With a copper head gasket a steel o-ring is preferred. The ring is pressed into a machined groove and sticks up slightly above the block surface. With a copper o-ring the copper material crushes down when the head is torqued but retains some residual pressure on the gasket. The o-ring should be replaced any time the head is removed. A steel o-ring, on the other hand, does not crush down instead crushing the copper head gasket. The most ideal situation with the steel ring/copper gasket is to run a matching receiver groove in the head allowing the gasket to be forced into that groove forming a robust interlocking seal. We are no longer utilizing this feature with out short blocks since head gasket technology now offers superior sealing without the need for this. Athena Cooper Fire Ring and Fire Lock head gaskets are preferred.
Do you use more than one closed deck design? Yes, we offer 2 variations of the closed deck block depending on the short block. For our Race Spec 2.5L series we offer a pro version of the closed deck design which features increased thickness of the closed deck inserts as well as special threaded plugs to close down the block access holes and provided additional rigidity.
Do you use the nitrided factory crankshaft? Yes, the current crankshaft we are using is nitrided which increases the surface hardness or wear resistance of the crankshaft.
What is the balance process used when building this short block? Street Spec Pro short blocks have pistons matched to within 1/3 gram and connecting rods matched to within 1/2 gram end to end. The crankshaft is dynamically balanced to 0.25 oz in.
Is the block honed with a torque plate? Yes, we always fit a torque plate during the honing process to simulate the distortion created by the cylinder head bolt torque. We suggest you indicate to us what cylinder head hardware, torque values, and head gaskets will be used so we can most accurately replicate the loads. When this information is not supplied to us or we are not upgrading to larger studs then our standard process is to use ARP standard head studs torqued to 90ft-lbs with an OEM head gasket.
What clearances are used for piston-to-cylinder and for crankshaft bearings? The clearances we use are based on extensive testing and experience. Oil clearances are increased over the factory settings to allow for significantly increased loads and component distortions. The engines require a 5W40 / 10W40 (road) or 10W50 / 15W50 (race) oil viscosity and a minimum of an 11mm oil pump. Target piston clearances with this short block range from .0027" to .0037" depending on the expected power output (or head stud diameter used if power information is unknown).
What is the maximum boost pressure for this short block? We are often asked this question. Boost pressure on its own will not overstress an engine. Peak cylinder pressure after the spark plug fires will be significantly higher than any useable boost pressure. Increased boost pressures generally lead to increased cylinder pressures and heat loads which can overstress an engine. But the exact correlation depends greatly on the tuning, the type of fuel, and other factors. Its impossible to tell you what the maximum boost pressure the engine will handle will be.
What is the maximum rpm of the short block? There are many factors to consider when establishing a maximum rpm for a particular engine. Our shortblock comparison chart offers a suggested maximum rpm provided all other contributing aspects of the engine assembly have been designed to support that rpm. It is also important to understand that it makes no sense to attempt to increase revs if the engine's power is already falling off significantly or is running outside the efficiency range of the turbo. Additionally, there are often limitations in the factory oil and cooling systems that make significant increases in rpm unwise without a proper re-evaluation of those systems.
Can I substitute different pistons or rods? Our engines have been developed using what we believe are the finest components available within the target price range. In particular, when selecting pistons and rings for our Street Spec engines we took into account the desired characteristics of low noise, maximum longevity, and reduced blowby/oil consumption under typical street use drive cycles. Although we are happy to build a short block or long block to your specific requirements if you feel strongly about a certain setup, we treat any deviation from our standard engines as a custom job. We feel that our off-the-shelf engines represent the best possible combination of components and processes.
