Nice numbers on the GT28RS

Discussion in 'Member's Garage' started by ZisLuv, May 19, 2010.

  1. beaver

    beaver southern zeds

    No it

    assumes the intake air is the same temperature, but besides that the pressure and volume of air (hot or otherwise) inside the manifold/plenum has to be the same to reach a target release of 15psi. A bigger compressor wheel, it stands to reason, moves more air than a smaller wheel, but why people think there is somehow a greator volume of air inside the plenum/manifold given by the large compressor @15psi or any other psi is not right imo and i would like that to be proved wrong. Bring on the popcorn.



    that part is not logical to me ...and is the real question over the past debates.
    and would like to get this clarified once and for all....real facts so I can file it in my brain.

    the way I look at it ..
    pushing 15 psi through a straw and 15psi through a garden hose... theres not more volume of air?
  3. buzzniss

    buzzniss Member

    I know tassuperkart gave a good explanation the last time this came up but i cant find that at the moment... Im thinking the larger compressor can move 'x' amount of air without heating it up as much as the smaller compressor (due to lower rpm?) and therefore the 'y' psi which the larger compressor produces contains a greater amount of air because a smaller proportion of 'x' pressure is from heat.

    just thoughts of the top of my head.. feel free to correct if im wrong
  4. buzzniss

    buzzniss Member

    shit i forgot what this thread was about >< yes id like to see a dyno chart too!


    from tassupercart:D

    The turbo is purely the supplier (just like the fuel pump).
    The cylinders are the end users (same as injectors)
    Apart from some minor variations from pumping losses on the turbine side and overall efficiency variations, which will be close to academic at everyday lower boost settings, the engine will still only let *blah* CFM of air thru it at *X* psi at *Y* rpm NO MATTER what POTENTIAL of air supply you throw at it.

    It makes close as fark is to swearing bugger all difference how you SUPPLY that POTENTIAL air pressure/flow because thats all it is....POTENTIAL.
    The engine tune, IE: cams, porting and RPM are the factors that determine how much air will go thru at a given psi.

    there is more volume of air in the manifold with larger turbo
    BUT whether the engine breathes enough to take it in is another story.

    correct me if this is wrong
  6. a2zed

    a2zed Guest

    PSI doesn't matter, CFM matters. Make the system breathe better, and you will flow more CFM at the same PSI.

    A 2860rs will not flow anymore cfm than a T22 on the same setup other than sfa diiference in the turbo restriction itself. They have the potential to flow more cfm at higher restrcition(PSI), but pound for pound on the same setup, they will be showing almost identical numbers. This of course is not true when you punch the smaller T25 out of it's efficiency.


    ok so no matter what turbo on same setup.. the volume of air passing through the manifold is determined by the engine cfm (pump)
    same page?

    just couldn't get my head around the extra flow of air potential with the larger turbo.
  8. a2zed

    a2zed Guest

    Pretty much it, the entire system from the air cleaner to the exhaust tip effects total CFM. Other than frictional losses and heat, both less in the bigger turbo, the numbers will be very close.

    You mention "flow of air potential", remember that is all it is, potential. You need to make other changes to realise that potential, ie make the system more efficient.


    correct this scenario for me

    if the t22 at 15psi at 6000rpm isnt maxing out the engine cfm
    wont gt28rs at 15psi 6000rpm be making more hp due to engine able to flow more?
  10. heavytrevy

    heavytrevy "Hammer time "


  11. a2zed

    a2zed Guest

    If the 22 is maxing at 15psi but the system is not flowing max cfm, the turbo rpm is being limited (boost controller).

    If the 28 is limited to the same 15psi, it will flow more cfm and fill the pump(engine) closer to it's max flow potential. And more cfm means more power.

    The larger turbo will have much less shaft rpm to reach the same CFM. When you limit boost to a specific number, you can throw all the text books out the window. The turbo and engine are no longer at max flow.

    We should be measuring flow rather than boost to get an accurate account of what is actually going on. Like I said earlier, forget psi, cfm is it. You do not build an engine to take 20psi, you build it to flow 700cfm and match that to a turbo map.

    I know what you are trying to work out, but when you limit boost below the turbo's theoretical potential with a boost controller, you are slowing the turbo down and it no longer flows what it should, you are effectively making the turbo smaller and less efficient.


    I'm with you.

    all we need now is the dyno chart for this thread :p
  13. Tektrader

    Tektrader Z32 Hoe, service me baby

    Cubic foot a minute is a bit misleading because it is only valid at 1 air temperature. The reason different turbos can give more power at the same boost level is to do with the boosted air density. That is why most turbo boost maps are actually graphed using lbs/min or KG/min. Its taking into account the actual mass of the air not its volume which varies with temperature.

    A GT28RS A/R 0.64 with all airflow restrictions removed from the system so that the turbo will be the limiting factor to air flow will max out at approx 17 psi. You can push it with more boost but you wont get anymore power just more heat.

    This thread by Greg Dupree explains it pretty well.
  14. a2zed

    a2zed Guest

    It may max out at 17psi with all restriction removed, but that is impossible, it can't happen in a working enviroment.

    You are spot on about mass v cfm. Also Seb is taking a few generic figures and using them in his calculations, no issue with that because thats how most of us do it, but it only gives a ball park figure. Something you use to select a turbo, not calculate VE.


  16. Tektrader

    Tektrader Z32 Hoe, service me baby

    Actually, Sebs car did exist AND had a boost issue ( he thought) the thread I linked to was in response to him not being able to get anymore power out of his new build even though he was only pushing 17 PSI

    He would boost to 21psi and still not get to 650rwHP. More pressure didnt deliver any more power. Greg Dupree stepped in to explain why after he looked at the car.

    Thats why I am pretty confident to say the practical safe maximum you can get out of GT2860RS is under 600 RWHP and in an ideal free flowing full system about 21 PSI of boost.

    Have a further read


    whoa ! :D 650rwhp
  18. Tektrader

    Tektrader Z32 Hoe, service me baby

    LOL, I just fixed it, Put KW instead of HP.

    Lots of power, I would be happy with 350 RWKW at 18 PSI and reliable.

    You have turbos like this you MUST have those high lift cams and if possible the bigger valves. Though IMO the valve size isnt as important as more LIFT.
  19. Pilot.P

    Pilot.P New Z'lander

    Kinda a dragged on thread, hope you guys don't mind me butting in...I'd like to see a dyno graph as well.

    I had my engine break in on the dyno last week; GT28RS, Ash Massives SMIC, TDM 2.5" intercooler piping, Selin dual intake, HPC coated Wiseco pistons & Carillo super A beam rods, 860cc top fed injectors, Z1 TB's, BDE intake cam gears, Tomei exhaust cam gears, JWT R4+ cams, JWT valve springs, SZ expansion Dp's full 3" X pipe exhaust etc etc etc.

    Given that the room temperature was 41.1 degrees C, no timing tweaks at all, base/minimal boost (EBC not installed yet for obvious reasons), running not warrantable 16" stockies in which one was mounted on reverse (slipping on the dyno a little bit) and above all not thrashing it too hard... I only made 309rwkw at 13psi.

    That's quite an achievement you made at 15psi considering I thought I had (nearly?) everything. Despite being on a dyno dynamics "heart breaker" dyno, factoring everything 550rwhp is pretty amazing. I am going for 500+rwkw for the full tune + I have 150 shots of wet Nos on top, might not bother with Nos if I am missing something here!

    In case I get a TTIWWOP, here is a pic of my Z.
    For those who noticed, yup...18x12.5 SSR Gartmaier on 335/30/18s :D


    264/9.5 is what I'm going ...

    I asked JWT to make me 264/9.78 (lift of R5) but couldnt do it due to the way they do the base circle.
    porting and valves are a must to achieve high power with lower boost

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