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    • A technical look at Porsche's new '9A2B4' modular turbo flat-4 engines and comparison to the 9A2B6 turbo flat-6 including redline, bore x stroke, compression ratio...

      Porsche refers to their four-cylinder motors used in the 718 Boxster (and soon to be in the 718 Cayman) as the B4 family. These new boxer or flat-four motors are exciting as they will see more much tuner attention due to being turbocharged right from the factory.


      The 2.0 liter motor's output is 300 horsepower and 280 lb-ft of torque and the 2.5 liter motor has 350 horsepower and 309 lb-ft of torque. How do they differ and how do they differ from the larger turbo flat-6 (B6) engines in the 991.2 Carrera and Carrera S?

      The engines are all fairly similar which is by design. Think of the flat-4 as simply missing two cylinders from its big brothers. They both rev to the exact same 7500 rpm redline. The base 2.0 liter motor shares the same 91mm bore and 76.4mm stroke as the B6 flat-sixes from the 991.2 Carrera and Carrera S on which it is based. This means they can share the same pistons and rods.

      The 2.5 liter simply increases the bore to 102mm in order to reach the 2.5 liter displacement. Yes, in theory this means the 991.2 Carrera and Carrera S easily could be bored out to 102mm as well for a displacement of 3.745 liters. Porsche can increase the displacement incrementally over time for the Carrera and Carrera S if they feel like it but they want to keep it safely away from encroaching on the 3.8 liter Turbo model.

      So what else do the B4 flat-4 and B6 flat-6 share? Almost everything. The alternator, fuel injectors, fuel pumps, vacuum pump, camshaft bearings, intake valves (2.5 liter has larger valves), rods, piston rings, camshaft phasing mechanism, iron cylinder liners, same block material, same fuel injector position, same HVAC compressor, and the same main bearings.

      The B4 flat-four is literally the B6 flat-6 but without two cylinders and with one turbo instead of two. The other major difference is the B4 has variable exhaust-valve lift in addition to intake-valve lift and the B4 has a lower 9.5:1 compression ratio instead of 10.0:1 meaning the pistons are not the same even though the B6 pistons of the same bore should drop right in.

      The B4 also uses an air to water intercooler on top of the motor instead of the air to air setup on 991.2 B6 engine. Why? Maybe packaging restraints.

      The 2.0 liter and 2.5 liter also differ beyond just the bore size. The 2.0 liter has a smaller turbo that generates 20.3 psi of boost. The 2.5 liter has lower boost pressure at 14.5 psi from its larger turbo which also features VTG (variable turbine geometry).

      The single turbo setup on the flat-four makes sense due to the firing order. The turbo receives pulses every 180 degrees of revolution. Twins would have to wait on the other meaning 360 degrees of revolution per exhaust pulse.

      Porsche calls the oiling system 'integrated dry sump' which is misleading. Gravity draws the oil in like in a traditional wet sump setup. Those who track the cars may want to be aware of potential oil starvation issues on sticky tires with upgraded suspension components.

      From a tuning standpoint, the 2.5 liter B4 is the way to go in the 718. It's turbo isn't pushed as hard from the factory and the extra half-liter of displacement is hard to ignore. The motor will easily top 400 crank horsepower and Porsche themselves will likely push it over 400 at some point. A GTS model will probably just feature a different tune just as it previously did except with more room to gain power and torque.

      Due to the price point of entry and model volume the B4 and B6 will likely become the most popular motors to tune in the Porsche lineup.




      B4 2.0

      Horsepower: 300
      Torque: 280 lb-ft
      Boost Pressure: 20.3 psi
      Bore x Stroke: 91mm x 76.4mm
      Redline: 7500 rpm
      Compression Ratio: 9.5:1
      Intercooler: Water to Air

      B4 2.5

      Horsepower: 350
      Torque: 309 lb-ft
      Boost Pressure: 14.5 psi
      Bore x Stroke: 102mm x 76.4
      Redline: 7500 rpm
      Compression Ratio: 9.5:1
      Intercooler: Water to Air

      B6 3.0 991.2 Carrera

      Horsepower: 370
      Torque: 332 lb-ft
      Boost Pressure: 13.1 psi
      Bore x Stroke: 91mm x 76.4mm
      Redline: 7500 rpm
      Compression Ratio: 10.0:1
      Intercooler: Air to Air

      B6 3.0 991.2 Carrera S

      Horsepower: 420
      Torque: 369 lb-ft
      Boost Pressure: 16.0 psi
      Bore x Stroke: 91mm x 76.4mm
      Redline: 7500 rpm
      Compression Ratio: 10.0:1
      Intercooler: Air to Air

      This article was originally published in forum thread: A technical look at Porsche's new 'B4' modular turbo flat-4 engines and comparison to the B6 turbo flat-6 including redline, bore x stroke, compression ratio... started by Sticky View original post
      Comments 13 Comments
      1. subaru335i's Avatar
        subaru335i -
        Hmm the 2.5L flat 4 has bigger bore than the subaru EJ25. its real oversquare, it should like to rev. Wonder what the rod ratio is.
      1. Sticky's Avatar
        Sticky -
        I kind of want to pick one of these up to get in on tuning early and promote PorscheBoost.

        The volume here would probably really boost the forum. Heh, get it?
      1. ms335i's Avatar
        ms335i -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        I kind of want to pick one of these up to get in on tuning early and promote PorscheBoost.

        The volume here would probably really boost the forum. Heh, get it?
        I see what you did there.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by ms335i Click here to enlarge
        I see what you did there.
        If you didn't I wouldn't want you operating a car on public roads.
      1. subaru335i's Avatar
        subaru335i -
        Do it. If the bigger Porsche turbo motors are any indication these things will be awesome for tuning potential.
        I am excited, I want a Cayman with the 2.5L. 400-450hp should be pretty easy for it and that platform is awesome.
      1. ms335i's Avatar
        ms335i -
        Click here to enlarge Originally Posted by Sticky Click here to enlarge
        If you didn't I wouldn't want you operating a car on public roads.
        Sadly there are plenty of people out there like that. For example in three days I have seen two drivers run red lights due to texting. One of them took off on red thinking it was green because he saw (from his peripheral) that the left turn lane was going and for some reason he felt the need to take off.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by subaru335i Click here to enlarge
        Do it. If the bigger Porsche turbo motors are any indication these things will be awesome for tuning potential.
        I am excited, I want a Cayman with the 2.5L. 400-450hp should be pretty easy for it and that platform is awesome.
        I'll think about it. The Cayman with the 2.5 would be fun even stock. With a big turbo on there... oh baby.
      1. Zeee_Q's Avatar
        Zeee_Q -
        Tuning should be very fun especially since we know the PDK can take the abuse thrown at it.

        It'll be interesting to see if this segment of Porsche tuning will see piggy back or flash tunes dominating.

        Hoping these motors are E85 tuning friendly as well.
      1. Terry@BMS's Avatar
        Terry@BMS -
        Looks interesting... Click here to enlarge
      1. jyamona@motiv's Avatar
        jyamona@motiv -
        Click here to enlarge Originally Posted by subaru335i Click here to enlarge
        Do it. If the bigger Porsche turbo motors are any indication these things will be awesome for tuning potential.
        I am excited, I want a Cayman with the 2.5L. 400-450hp should be pretty easy for it and that platform is awesome.
        Same!
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by Zeee_Q Click here to enlarge
        It'll be interesting to see if this segment of Porsche tuning will see piggy back or flash tunes dominating.
        That's exactly what I'm wondering. This ECU is probably different from the 981.1 or 991.1 and won't be flashable right off the bat. That means piggybacks have an opening.
      1. John Mclane's Avatar
        John Mclane -
        Huumm... VTG turbines in the 991.2 plus bored out to 3.7L... If tuning alone is bordering 470, imagine that duo...
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by John Mclane Click here to enlarge
        Huumm... VTG turbines in the 991.2 plus bored out to 3.7L... If tuning alone is bordering 470, imagine that duo...
        If going big most guys say better to ditch VTG.

        I'm with you on the 3.7 liter though, would be sick!