Effect of oversize valves on powerband

Discussion in 'Z32 Technical' started by J3bba, Oct 12, 2017.

  1. J3bba

    J3bba He Who Struggles

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    Hey all,

    Was wondering if anyone had any experience with oversize valves and what effect they have on the powerband. Are they a small boost along the entire thing or effect high end or low end more?

    What other benefits do they have? Do they improve spool time at all?

    Cheers
     
  2. Fists

    Fists Active Member

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    Roughly, all other things being equal, bigger valves shift power to the right. As with cam lift and duration, and intake runners/manifold/head ports valve sizing is about tuning intake velocity and pressure to the rpm and swept volume X volumetric efficiency of the cylinder.
     
  3. J3bba

    J3bba He Who Struggles

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    Is it a significant gain? Does it affect spool time for the turbos at all?
     
  4. Fists

    Fists Active Member

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    It depends what you're applying it to, your sig suggests you still have stock turbos so for your engine, no. Gain will be marginal and would probably slow spool a little although may make it spool 'harder' which feels fast.

    For builds that already have big turbos and stuff moving from stock cams and valve size can improve spool by having the peak efficiency of the valve train overlap more with the efficient areas of the turbo, for those cars it spools earlier but compared to stock turbos it's still pretty late.

    If you're asking for your car and you're not trying to set a stock turbo power record then either highflows, GT2560s or E85 is a far better way to spend money. If you want gains across the board you can do 2.5" intake and intercooler pipes but gains are again marginal and those kits cost as much as a set of turbos.
     
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  5. J3bba

    J3bba He Who Struggles

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    Yeah, I've got a set of cams to put in and was thinking what else I can do at the same time. Planning on throwing some turbos on as well while the engine is out early next year.
     
  6. Fists

    Fists Active Member

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    What cams and what turbos are you thinking? Any piston or other lower block changes? This stuff is all about having a good matched set up with the power band in the right place for the way you use the car. There isn't really enough proper sciencey info on 300zx mods to sit down and calculate exactly which parts to put together, much better off finding someone else's build that's about right and learning from their wins and mistakes, few builds to study on this forum and quite a few comparative dyno graphs on twinturbo.net
     
  7. J3bba

    J3bba He Who Struggles

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    Cams are 220/9.5mm and was thinking of gt2860r/s or gt2871r for the turbos. Wasn't planning on doing anything lower block at the same time but in the future definitely.

    Plan was to make real good power under the curve instead of a ridiculous peak number hence the shorter cam duration.
     
  8. rob260

    rob260 Moderator Staff Member

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    Wow that’s less duration than a stock cam, if nothing else I think O/S valves will help your assembly breathe a little better up the top.
     
  9. BLACK BEAST

    BLACK BEAST SLICKTOP TT R-SPEC

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    220 must be a typo?
    stay away from 2871 if you want a good power band
     
  10. ProckyZ89

    ProckyZ89 Senior Member

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    Good power band...or usable power band :p

    Maybe he means 270- odd thing is, valves aiding spool time is often countered by the fact you've moved spool time xxxrpm with the turbos anyway.
    Reason I wanted them was to increase the engines capability to move air at higher rpm or velocity
     
  11. J3bba

    J3bba He Who Struggles

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    Sounds like you put some in? Did you go +1mm or +0.5mm? What were your impressions of them?
     
  12. Fists

    Fists Active Member

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    I'm guessing the 220 duration is at/over 0.050" lift not the full lift duration which would put them about half way between the JWT mild and R4 cams. These cams and 2860 turbos especially with larger valves wouldn't really fit my definition power band over peak power, not that they have a bad power band but it's just up the rev range a bit, sort of a 400kw racing set up and 30 year old piston rings and bearings aren't likely to play for long, not to mention fuel requirements and the cost of tyres that allow you to do anything with the power.

    Pretty sure Rob, Procky and Beast have all run the 2860s with some headwork and cams at some point in their builds making pretty impressive power on 98 octane if you want to trawl their build threads. That kind of set up is definitely sacrificing some low end off the lights spool compared to stock-highflows or GT25 turbos which will do 300kw though.
     
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  13. AndyMac

    AndyMac Better than you

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    Another option, if adventurous, is to look at A/M cam phasing and ditching the OEM VTC setup, you generally have 3-4 phases so you can basically tweak your advance / retard to suit the power band you want. I'd imagine having control over cam phasing would allow you to balance your mods a bit better. Can't recall anyone doing this on the VG, so can't help you there, but you could be the first :)
     

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