Tag Archives: EJ257

BOV: Blow off Valve Subaru FAQ

BOV: Blow off Valve Subaru FAQ

Term usage: “Blow off valves” go by several names, among them are compressor bypass valve (CBV), air by-pass valve, bypass valve (BPV), blow off valve (BOV), Diverter valve, and possibly a few others. BOV is the common and incorrect term that lumps true blow off valves and bypass valves under the same term. For the sake of correctness, this post will refer to either aftermarket BOV, aftermarket BPV or OEM BPV as these are the most correct terms.

What is the function of a blow off valve (BOV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to releases the air to the atmosphere.

What is the function of a bypass valve (BPV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to recirculate the air back into the intake before the turbo inlet, but after the airflow sensor.

A stock Subaru BPV (not a BOV).

What is the purpose of a BOV/BPV? When the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and in the absence of a BOV/BPV, a pressure wave will travel back to the turbocharger. The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly. This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.

Are aftermarket BOVs necessary with Subaru turbos? No. The OEM BPV is perfectly fine up to 20psi of boost. For applications using higher boost levels, an aftermarket BOV/BPV should be considered.

Can I mod my stock BPV to hold higher boost? Yes. I’ll be creating a post giving detailed instructions on how to do so.

Is the STi BPV better than the WRX BPV? No. They are the same. However, the JDM STi BPV will hold more boost as it is physically different than both the USDM STi BPV and the WRX BPV. The specific PSI rating of the JDM STi BPV is unknown, but users have reported it is good up to 25 PSI.

Is an aftermarket BPV better than the stock BPV? No. Unless you are considering an aftermarket BPV solely for the purposes of holding higher boost levels. An aftermarket unit should sound just like the OEM unit.

2nd Generation Subaru Impreza WRX STi United Kingdom varients

Impreza WR1

A limited edition of 1000 Japanese MY04 WRX STI‘s with DCCD were exported to celebrate victory in the 2003 WRC drivers’ championship. 500 were sold in Europe, Australia and South Africa as the Petter Solberg edition; the other 500 were further modified by Prodrive and sold in the UK as the WR1. They had 316 bhp (236 kW) and 309.8 ft·lbf (420.0 N·m), an incredible 0-60 mph time of only 4.25 seconds and a top speed of 155 mph (249 km/h) (electronically limited). They were equipped with Prodrive WRX STI springs, Pirelli PZero Nero tires, mesh grilles, special Ice Blue metallic paint and Prodrive PFF7 Pewter wheels. The new ECU and exhaust were not EU-homologated, so they were fitted after registering the car. The retail price was £29,995.

EJ257 Block info

So you bought an EJ257 block to build on the side. The seller says it’s “good” and “was running.” I mean, they can say anything, right? That is until you pull the engine block apart and the crank suffered major rod bearing damage from either oil starvation or debris from other failing components.

Here we go to look at the bore housings of the main bearings on the engine block. They look perfectly round to the eye but only measuring tools will reveal how oval they really are. Heat isn’t your friend and is the major cause in warping the bore housings. Rod bearing failure will cause excessive heat if driven on. Even brand new blocks from Subaru can be out of round.

The pictures show the honing marks after a pass on the honing machine. You can see where the color is more dull and has minimal light reflection. There was no contact there with the honing stones. You can see the dark defining line, which was where the bearing sat. Half of the bore housing number 3 (middle) didn’t make contact while honing as with bore housing number 4, which did not make any contact except where the block halves met.

What do we do now? The issue here is you’ll have bearing clearances that are too small or too large. This will cause oiling issues to areas where they most need it. Also the warped mains will not be straight. This can cause bearing to crank journal contact. So when looking at a block to buy, ask about the block’s history. If the seller has the block apart, look for yourself before purchasing. An experienced machine shop that has an align hone machine will be able to straighten the bearing housings as long as they aren’t too out of round.

EJ257 honing Marks
EJ257 honing marks