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Race Spec Pro Shortblock

The Race Spec series short blocks are designed and built for the demands of motorsport. The block is CNC machined and fitted with Darton sleeves. The Race Spec Pro is our top level motorsport short block option and features components of the absolute highest quality. Built using an Arrow lightweight billet crankshaft, King XPG Tri-Metal bearings, Omega 2618 forged pistons, RalliSpec-developed H-beam rods machined from 300M (ultra high strength aerospace alloy) and fitted with ARP Custom Age 625+ bolts, and ARP high tensile block hardware. Assembled from all new parts and fully balanced/blueprinted. Optionally, we have a used block exchange program whereby we build from a good used block and take yours in trade. Available in two compression ratio options.

Click here for shortblock comparison chart.

Usually ships in 3 to 4 weeks.

Manufacturer: RalliSpec


  • Subaru

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).

Why do you use Omega pistons in the Race Spec and Cosworth in the Street Spec series? Omega is a highly regarded piston manufacturer in the UK with experience developing pistons for racing teams running at the highest levels ranging from F1 to WRC to MotoGP. The Omega forged pistons we are using are a lightweight slipper design precision machined from a 2618 heat treated aluminum. This alloy offers maximum strength and superior thermal conductivity (potentially reducing piston crown temperatures). This makes it the optimum choice for racing applications. Cosworth pistons, also a slipper piston but a litter heavier design, are machined from a Cosworth proprietary alloy similar to 4032. This is a low expansion high silicon aluminum with excellent wear resistance and the piston is designed with a pin offset for reduced noise making it ideal for street driven applications. Both Cosworth and Omega offer pistons with a level of quality and engineering far superior to other options of the market. Yes, there is a premium to be paid for this level of quality. However, we feel strongly that achieving maximum performance and maximum durability requires a no compromise approach when it comes to piston selection.

What is the purpose of sleeving the block? The factory uses a thin steel "liner" that is cast in to the aluminum cylinder wall. The top of the cylinder is supported in only a few locations where aluminum bridges the cylinder wall to the main block material (semi-closed deck design). However, due to the EJ25's large bore size the total cylinder wall thickness is quite thin and to make matters worse the cylinder is notched in some locations to allow access to the block bolts which further reduces that thickness in those areas. Increasing the engine's power output will lead to increased peak cylinder pressures. Peak pressures occur only 10 or 20 degrees after top dead center so they will be trying to balloon outward the upper portion of the cylinder. If the pressures exceed the strength of the cylinder the liner can crack leading to a loss in compression and performance. Additionally, abnormal combustion events such as knock, detonation, or pre-igntion can cause a pressure spike to really push things over the edge and can blow apart a portion of the cylinder wall. Generally it becomes very difficult to achieve good reliability on the EJ25 at extreme power outputs due to the weakness of the cylinder wall. Sleeving the block involves machining out the existing cylinder liner and a good portion of the aluminum wall behind it and fitting an iron sleeve to provide the necessary strength. The sleeves we use are stepped so the upper portion of the sleeve where the pressures will be greatest is quick thick and in direct contact with the cooling jacket.

Do you ever have problems with dropped sleeves? Dropped sleeves occur when the machining or installation is done poorly or incorrectly. It can occur when the sleeve is not sitting directly on the machined ledge that is meant to locate it at the time the block is decked, and is often due to material or debris trapped underneath. Our blocks are currently being sleeved by Darton's own machinist and we have had no issues of any kind.

What about cooling issues with a sleeved block? It is true that the increased liner thickness and the machined interface between the iron and the aluminum block will reduce the heat transfer slightly when compared to the factory cast-in liners. Additionally, the sleeve is susceptable to corrosion and cavitation issues. We strongly suggest using Evan's waterless coolant in all of our engines to maximize cooling and eliminate corrosion and cavitation issues but it is particularly critical in our sleeved engines.

What is the advantage of the billet crankshaft over the factory one? The factory crankshaft is quite well engineered and has good strength at fairly low weight. It is forged with a generous journal fillet and will suffice for moderately high power levels. In more extreme applications the billet crank improves on the standard forged crank in many ways to maximize its strength and durability while reducing its rotational inertia. Most importantly, however, the oil passages are redesigned to improve the flow of oil between the main and rod journals particularly at high rpms where rotational forces are working against the flow of oil in the crankshaft.

What is the balance process used when building this short block? The Race Spec short blocks have pistons matched to within 1/4 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 boring and honing processes 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 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. Piston clearances on the Race Spec value are set to 0.0035".

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.