CefiroA31

Convert single scroll to bigger twin scroll, yet spools 500rpm quicker and more hp everywhere. Imo, twin scroll works great.

Your pistons will be 1mm below block deck which means less ideal squish/quench so may be easier to knocking. Stock 25det : 121.5mm rod length + 31.5mm piston height + 35.85mm half stroke = 188.85mm. Your combo : 119.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 187.85mm. Sell your pistons and buy rb26 pistons, mill down its dome to get proper CR for rb25det neo head, that way you have proper piston to deck clearance/squish and uses common rb25/26 121.5mm rods which also have better rod/stroke ratio than less available shorter 119.5mm rods. If have to use rb25 piston with 121.5mm rod, pistons will be 1mm above block deck : 121.5mm rod length + 31.5mm piston height + 36.85mm half stroke = 189.85mm Can use thicker head gaskets to compensate pistons above block deck, cometic has rb25det 87mm head gasket thickness of 1.8mm, 1.9mm, 2mm, 2.5mm, that will work fine with pistons 1mm above block deck : https://www.cometic.com/applications/automotive/nissan/rb-series/rb25det--152ci25l-i6

I assume you have non vct rb25de with greddy 3 bolt turbo flange manifold, if it must be trust turbo, get trust td06sl2-20rx. As tested by trust on td06sh, 20rx spools like 20g but peak hp close to 25g, so higher average hp than 20g or 25g. By trust pictures, 20rx compressor looks like similar size as 20g compressor with smaller center hub, taller + extended tip blade, so its good as it has 20g spool but close to peak hp of much larger 25g (20g 52.6mm/68mm compressor, 25g 60.5mm/78mm compressor). What doesn't show on dyno plot is spool up/response time and lag, 25g being larger will has more inertia thus lazier than 20g and 20rx, and 20rx nearly match 25g peak hp so I see no pros of 25g vs 20rx. Turbine side I suggest to go with smaller td06sl2 54.1mm/61mm it should be enough to support 20rx compressor if using 10cm turbine housing (my experience with holset twin scroll 10cm spools quick and not choking on rb20, so single scroll 10 cm which flow more than twin scroll 10cm should match well with rb25, while single scroll 8cm better match rb20 but may choke on rb25). Td06sh is just regular mhi td06h turbine, its bigger at 58.8mm/67mm, also has more inertia thus lazier than td06sl2. Due to bigger wheel has more inertia thus lazier spool, for similar max flow I prefer smaller wheel + bigger housing (td06sl2 10cm) vs bigger wheel + smaller housing (td06sh 8cm).

If wanting quickest spool/response then its twin scroll efr set up. For >750hp, efr8474 or larger. If not efrs, g35-900 or g40-1150, if wanting cheaper, gtw3884r-67mm or gbc37-900, all shoud be in twin scroll set up for better spool/response. Skip g35-1050 at it has mismatched wheels, compressor too large for turbine.

For straight bolt on no change to cold/hotside piping, check these options : https://hypergearturbos.com/product/rb25dethighflow/ https://www.rapidperformance.co.nz/products/nissan-rb20det-rb25det-rb25det-neo-recore-turbo https://www.rapidperformance.co.nz/products/nissan-rb25det-td06-bolt-on-td06-turbo https://turbocare.co.nz/shop/tc-performance-turbochargers/tcr-performance-range/rb25-bolt-in-400-450hp-upgrade-turbo/ All are larger than stock rb25det turbo, so will be laggier on rb20det, thus only worthwhile if you have bigger injectors >440cc and nistune or stand alone ecu to tune for at least 300 hp on engine, for worthwhile hp increase vs laggier spoolup/response and loss of lower rpm hp/torque.

Looking at HKS GTIII-SS specs and as they are MHI based, then 56mm compressor exducer and 52mm turbine inducer should be from TD004H or HL turbine family : TD04H turbine : inducer 52mm exducer 44.2mm. TD04HL turbine : inducer 52mm exducer 45.6mm. TD04 turbines are usually 12 blades fom MHI, but also available aftermarket upgrade with 11 blades and 9 blades. Compressor exducer 56mm can be 15T (inducer 41.9mm), 16T or 15G (same inducer 43.4 mm but different blade profile), which compressor maps show max flow of around 29-31 lb/min at 1-1.5 Bar of boost. However its possible that HKS uses newer TD04 compressor from MHI which may flow a bit more. That BMW S55 MHI upgrade should use similar compressor as above with TD04L turbine inducer 47mm exducer 41.3mm. I have read TD04L maxes out about 220-250whp in other engines on e85 or pump gas + methanol mix. OEMs such as Volvo, Saab, uses these sizes TD04s for engines around 220-280 max hp usually at lower boost than 1 bar. Biggest OEM TD04HL is a TD04HL-19T which has 46mm/58mm compressor with max flow around 37 lb/min at 1.3 Bar of boost Garrett turbo closest in wheel sizes to TD04HL is GBC22-350 which has 44mm/56mm compressor max flow 37 lb/min at 1.7 Bar of boost and 50mm/46mm 9 blade turbine and also journal bearing as TD04s or next step larger GT28 series in journal or ball bearing. Other close sized option is Holset HE221W which has 43.4mm/61mm compressor max flow 38 lb/min at 2 bar of boost and 52mm/45.7mm 11 blade turbine, with T2 internal gated turbine housing sized at 5cm/6cm/7cm, so will bolt on to RB26 stock exhaust manifolds, but piping and downpipe will need to be custom made. Compared to GT28 ball bearing turbos, TD04s has smaller turbine inducer, exducer, shaft and compressor bore diameter so should have lower inertia which may allow it to spool comparable to GT28 ball bearing turbos, dependent of housing sizes. I have read infos from various platform which said TD04HL-19 T is close to GT2860R in response and max power. So my take on HKS MHI based GTIII-SS is it should work well if hp target around max 500-600 engine hp. Although if insisting on twin turbos then GBC22-350, TD04-HL-19T and HE221W will work just as well if not better and definitely cheaper. However, if no legal/emission compliance reasons, instead of upgrading twins, going to a similar rated power twin scroll single turbo such as GTX3071R/GTX3576R Gen 2, GTW3476, S257SXE, Holset HX35/HE351W, will be simpler and lighter although likely similar cost wise as need to buy proper t3/t4 divided manifold and external wastegate, and with similar HP should be faster car. This is what I would do.

RB26 crank, rods and pistons with machined down domes modeled after RB25DET NEO lower smaller piston domes to lower compression about 9.0:1 = RB26DET NEO.

On my rb20 sohc (so worse flowing than dohc heads), I had HX35 with smaller 50mm 7 blade compressor inducer and 0.82 a/r t3 twin scroll Garrett turbine housing on a twin scroll single wastegate exhaust manifold with proper divider wall up to the wastegate. On 95 octane gas at 4th gear it started spooling around 3500 rpm and reach 15 psi around 4200 rpm. Change only the hot side to smaller hx30 52 mm turbine with t3 twin scroll 12cm housing, also on 95 octane gas at 4th gear it starts spooling around 2800 rpm and reach 15 psi around 3500 rpm with good transient response, still on my car after 5 years. HX35 with the usual 54mm 7 blade compressor inducer which compressor map flows around 55 lb/min and should spool about 300 rpm slower. Yes, i agree that rb20det/rb25det factory manifold modified to twin scroll with 25-30mm spacer plate to pair it with a t3 twin scroll turbo is a simple, cheap and effective set up if target hp is within flow limit of factory manifold. You can weld the wastegate pipe to the manifold or mill waste gate ports to the spacer plate then weld the wastegate pipe to the ports on the spacer plate. Spacer wastegated.jfif

You can buy t3 divided housings up to A/R 1.25 for turbos up to EFR 9280, S372 SXE, GTX3584RS, GTW3884, and 6766, from AGP Turbo for usd 245-305 + shipping. Those turbos and housings should be enough for 700whp. If you decide to use t4 divided turbos, you can buy t3 divided to t4 divided adapter-spacer from customfabshop for usd 90 + shipping. This way you can reuse your t3 manifold, but will need mods to the piping as the turbo sits higher due to the adapter-spacer.

Straight from the holset hx repair manual .pdf: 8. Normal oil temperature is 95+/-5°C (203+/-9° F). It should not exceed 120°C (248°F) under any operating condition. 9. Any pre-lube oil must be clean and meet the minimum CD classification. 10. The orientation of turbine housing, bearing housing and compressor cover is fixed according to application. During installation, do not attempt to rotate these components. Inclined turbocharger installation is not recommended. If an installed angle is necessary, oil inlet centreline must be +/- 10 degrees from vertical and rotor centreline +/- 5 degrees from horizontal. 11. Holset permits oil return pipes to decline at an overall angle of not less than 30 degrees below horizontal.All turbocharger applications require a pipe of internal diameter greater than 19 mm which has integrated connectors. To ensure oil returns into the engine under all operating conditions, the return connection into the engine sump must not be submerged and the outlet flange of the turbocharger must be 50 mm above the maximum oil level of the engine sump pan. Crankcase pressure should be limited ideally to 0.8 kPa (0.12 lbf/in2) but 1.4 kPa (0.20 lbf/in2) can be accepted by reference to Holset. 12. Oil pressure of 150 kPa (20 lbf/in2) must show at the oil inlet within 3 - 4 seconds of engine firing to prevent damage to turbocharger bearing system. A flexible supply pipe is recommended. 13. The minimum oil pressure when the engine is on load must be 210 kPa (30 lbf/in2). Maximum permissible operating pressure is 500 kPa (72 lbf/in2) although 600 kPa (88 lbf/in2) is permitted during cold start up. Under idling conditions pressure should not fall below 70 kPa (10 lbf/in2). 14. Recommended oil flows for the turbochargers are 2 litre/min at idle and 3 litre/min above maximum torque speed. Those are the oiling requirements for Holset HX30-32-35-40 turbos, if you can meet those and you use quality oil with reasonable oil change frequency, your HX32 should last years even at 2+ bar boost where they often operate in original diesel applications, assuming its not already damaged. Turbosmart Oil Pressure Regulator on the oil feed is a great way to control oil feed as requirement above. Minimum 19 mm inside diameter of oil drain hose. I have read that for Holsets on Mitsubishi 4G63, when oil pressure/volume is too high they do fit restrictors on the oil feed, although can't remember what size they used. I have used HX32, HX35, and HX40 on both RB20 and 2JZ, only once had turbo oiling problem. First time was I try HX35 on the 2JZ. I use same oil feed set up that was used for Garrett T04R journal bearing turbo that was on it previously, just put bigger 19-20 mm oil drain hose. No issue, has great spool/response. After a while I had the HX35 leaking oil from compressor, found the engine (stock 2JZ from junkyard) has broken ring land thus high crank case pressure preventing oil to drain from the turbo. Rebuilt the 2jz and upgrade HX35 to HX40, its been healthy since, and made more power but more lag as expected vs HX35. So do check your engine health. I used T3 twin scroll HX32 on RB20, it has 54 mm compressor inducer as HX35 ( but compressor housing was smaller than regular HX35 compressor housing) and 52 mm turbine exducer + turbine housing of HX30, made good power with good spool/response, though IMO the turbine side will choke before the compressor if maxed out. If your HX32 is actually broken you can buy HX30 CHRA and fit it to your HX32 housing, move the HX32 compressor + housing to HX30 CHRA, effectively making a HX32. HX35 CHRA don't use v-band clamp to joint CHRA to turbine housing like HX30-HX32 use, so HX35 CHRA won't fit HX30-HX32 turbine housing and HX35 use larger turbine with 60 mm turbine exducer, so will be laggier but support bigger hp due to lower back pressure and able to max HX35 compressor which can flow around 57-58 lb/min. HX32 on RB25DET Neo will be a responsive 400-450 whp turbo, due to smaller turbine not able to max the compressor.

Other options : Supertec/Platinum Racing and Spool have spline driven oil pump gear kits for Tomei oil pump.

Ok, thank you for clearing that.

Ok noted thank you.

So either stay stock on lower boost/rpm if go bigger get those valve springs, thank you for this.

Thank you for the input, to clarify on first paragraph you mean replacing with new OEM damper or ATI/Fluidampr/Ross damper?