Subaru Myth Busting

Commonly held incorrect ideas about Subaru Cars And Engines

Myth #1WRX crankshafts and connecting rods are “cast” not forged.

Fact: All Subaru EJ motor crankshafts and connecting rods are made using forged steel material. The crankshafts being made from forged high-carbon steel and rods from forged steel.

From 2008 the USDM STI EJ257 engine featured a heat-treated and nitrided crankshaft. JDM EJ207 engines also had a nitrided crankshaft in the 2007 model year. All other Subaru crankshafts are untreated forged carbon steel.

The difference in the WRX connecting rods vs the STI connecting rods is the big end cap and small end design. Both are made using the same material and forging process however the STI rods feature pin registered big end caps with high strength rod bolts. The WRX rods have pressed in studs which position the caps and rod nuts. The WRX design is not as robust as the STI engines so we advise no more than 300WHP with the stock EJ205 engine block.

The small end design of the STI rods is tapered to accommodate the beefed-up pin boss of the stock STI cast pistons. The WRX EJ20 piston has a thicker crown thus no need to add material for the pin boss strength. No particular advantage to either design and in our opinion OEM STI rods should only be used with OEM STI pistons. Aftermarket forged pistons do not require a tapered small end design to fit.

Overall, if you have an EJ205 or 2007 or newer EJ255 engine and are targeting power above 300WHP it is a good idea to upgrade the pistons and connecting rods before going further.

Myth #2VF Turbos Can not be rebuilt

This one is a laugh riot for us as you could imagine. RHF55 series turbos which comprise the VF30, VF37, VF39, VF43, VF48, and VF52 can be readily rebuilt and we do it every day. There is however no global factory-supported repair parts infrastructure for any IHI turbo so a business such as ours has to make parts and get parts from specialist companies. That’s what we do and we can repair almost any RHF55 or RHF5H turbo in almost any condition.

IHI only supplies new complete turbocharger assemblies. There are no cartridges or individual parts available from the factory or main distribution channels. For new turbos we are supplied by IHI Japan and for rebuilds we manufacture our own cartridge and components which meet or exceed factory quality.  Our current rebuild service includes upgraded thrust bearing and journal bearings available exclusively from Six Star Speed.  

Myth #3: Using “thick” gaskets on a Hybrid EJ25X/EJ205 is perfectly fine.

We know that lots of folks have done so-called hybrid builds and a lot of them have been done quite wrong in our opinion. Th main issue is the difference in chamber size between the S20 and the 25X heads of the EJ255 and EJ257 engines. 48.4 cc vs 51 cc. The smaller EJ205 (S20) head requires a different shape of piston dish as well.

If using an STI piston with a -22 cc dish with the S20 heads and OEM STI gasket thickness for example, you end up with a compression ratio of 8.9:1. A little high for an EJ motor on pump gas. So what some builders advise is going to a thicker gasket, say 1.3 mm which will bring the compression down to 8.5:1. That’s all fine and good but what you have done is increase the quench distance by 60%. Quench, the distance between the piston and head deck at Top Dead Center is a vital specification for a performance engine. Increasing the quench distance creates a prime environment for detonation which will kill even the strongest motor very quickly. Increased quench also slows down the burn rate of the charge and promotes slower agitation resulting in an uneven mixture and more erratic combustion. This is critical for a boosted engine.

Increased quench also costs power. You will lose about 5-7 hp for each .25 mm increase. Going from the recommended 1 mm to 1.6 mm costs 12-17 HP and increases the chances of detonation destroying the motor.

Decreasing the quench distance burn rate and combustion stability improve which adds power and resistance to detonation. Recommended quench for an EJ turbo motor is 1 mm for 2618 pistons and .8-1 mm for 4032 pistons. With other materials such as MMC it may be possible to go as .5 – .6 mm quench.

Now for our properly built hybrid motor it is best to use pistons designed for the application. These are available from manufactures such a JE, which is the piston we use in our hybrid short blocks. The dish volume and shape is made specifically for the hybrid application and proper quench of 1 mm can be set with the OEM gasket. A hybrid build with proper pistons and quench will be powerful and detonation free given proper tuning and supporting mods. We advise against using thicker gaskets that take your quench over the recommended distance.

Myth #4: There is a difference among RHF55 turbochargers.

Over the years we have specialized in the repair and performance improvement of RHF55 turbos.  Not to toot our own horn but we are the greatest repository of knowledge about the venerable RHF55 turbo and its wide range of derivatives across not only the Subaru platform but it’s origins as a diesel truck turbo outside of IHI itself.  And you heard right the RHF55 and most other stock location Subaru turbos have their origins in the diesel truck industry.  More about that down the page.

The RHF55 began life as a journal bearing diesel/industrial turbo in the late 80’s and has been developed into many variations and applications for roadgoing vehicles.  The RHF55 turbo comprises many name variants used on Subaru.  As far as the STI goes there have been the VF30 VF37 VF39 VF43 VF48 and VF52 on the WRX.  There are a few others which we will leave out for now.  All the turbos just mentioned use the same CHRA and aero (turbine and compressor wheel)  No differences.  That’s correct, and it means for 18 years the JDM and USDM STI turbos have not changed at all.  Now there have been variations such as the twin-scroll adaptation which mainly enhances the low-end response of the turbo but no real significant changes in the turbos in 18 years!  Among the USDM applications, the only change to the family of STI turbos has been the slight change in the compressor housings with the advent of drive by wire throttle.  The position of the compressor outlet had to be pushed back for clearance.  Turbos built after the US ’05 model year all share this feature and are otherwise identical internally.  The VF43 has a slightly different casting to accommodate the ’07 STI oil line.  So any RHF55 STI turbo made from 2005 can be used on any STI from that year to the present without any difference in performance.

The VF52 has an entirely different compressor housing casting and slightly different bolt pattern on the mounting end to work on the 05-09 Legacy 08-13 WRX and 09-12 FXT.  That’s the only difference there.  Internally the same RHF55 CHRA and aero.

RHF55 Turbine housings are also all the same and they all crack.  The reason they crack is the concentration of stress around the wastegate area.  After a number of temperature cycles above 1,400 degrees cracks start from the inside and propagate outward.  Sometimes many cracks appear.  Cars used at higher elevations are more prone to cracking housings as exhaust temps are generally higher.  The good news is that the cracks are basically benign and do not pose any problems for the operation.  They are unfortunate cosmetic problems but don’t cause issues with performance.  If you do however have a turbine housing that is cracked on the outside of the housing allowing an opening for gases to escape outside the exhaust system then that housing should be discarded.  This is an extremely rare case and is unlikely to  be encountered in street use.

In any event, being a diesel truck derivative turbo the wastegate on the P18 (single scroll) Subaru housings is quite small.  The flapper is 31 mm and the port is 25 mm.  In point of fact, the 08 WRX TD04L turbos have larger 33 mm flappers and 27 mm ports!  Needless to say that on the 2.5-liter USDM applications the wastegate is way too small to dump enough gas at high RPM.  There is where the dreaded “boost creep” comes in.  It’s not really an issue for 2-liter cars however because of the smaller flows.  On the 2.5 liter cars, there really needs to be enough wastegate capacity to dump 40% or more of the gases.  External wastegates are an imperfect solution as they are totally emissions illegal, are expensive and require fabrication in many cases but they do work well.  They also wear out rather quickly because the entire wastegate mechanism is exposed to the full heat of the exhaust.  The stock wastegate needs to be modified slightly to improve the ability to relieve the exhaust gases but you can only do so much.  Simply put larger internal wastegates are needed.

In conclusion, all USDM RHF55 turbos are the same, with the slight differences pointed out above.  Don’t get hung up on the VFdesignations as those are just superficial.  When/if there is a significant change made to the series of turbos we will be sure to point it out and let you know.  In the meantime keep watching this space for more Subaru enlightenment!

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