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Thread: changing compression from 8:5:1 to 9:1

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    changing compression from 8:5:1 to 9:1

    I put some 9:1 pistons in as I blew the ringlands on my old stock pistons.
    the plan was to bash on a big turbo / 740's and chase some numbers but reality has put a stop to me chucking more money at the car for now so I put my old injectors and turbo back on and stuck it all back in the car.

    am I going to be ok to use it with the higher compression pistons fitted? its a track only car so I only really want to go and do a few hundred laps of my local banger racing oval to try to bed the engine in. plan is to accelerate the straights and coast the corners.

    it has z32 AFM/ s15 injectors/ s14 T28 and nistune. (plus all the other associated modz)

    is the increase in roughly half a compression point going to be a real deal breaker??

    it had decent compression test results when it was mapped on the 8:5:1 compression ratio a few years ago, and I bloomin hope it has outstanding compression test results now.

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    Guest big_mac's Avatar
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    I doubt it will really have much of an impact if you are in t28 land, the problems will be when you try and chase numbers imo. You might find you constantly det, struggle with inlet temps etc etc.

    It sounds like you just want to enjoy the car again, in which case I'd go with the high compression pistons and get some use out it. Once you have fallen in love again you can decide if it's worth the stress and expense to push it's limits

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    Guest ANDY black s13's Avatar
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    its not that big a number difference imo it will make too much of an issue,it may slightly drive better off boost,like a tiny bit more grunty
    and maybe afr's/timing will be affected at say over 18 psi (t28 won't last long if wound up higher imo),it should be fine @ 1 bar and 99 ron fuel
    you can tweak it on the nistune if needed, a wide band if not fitted would be a good investment for when run in and you tune it
    over the years a lot of tuner's have tried higher comp + boost,like 10:1 or more and some worked well,
    some have to pull so much timing to stop det it is a disadvantage to ultimate power compared to lower comp/more boost

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    Guest zeppelin101's Avatar
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    Quote Originally Posted by big_mac View Post
    I doubt it will really have much of an impact if you are in t28 land, the problems will be when you try and chase numbers imo. You might find you constantly det, struggle with inlet temps etc etc.

    It sounds like you just want to enjoy the car again, in which case I'd go with the high compression pistons and get some use out it. Once you have fallen in love again you can decide if it's worth the stress and expense to push it's limits
    For half a ratio on an SR, I disagree.

    The difference isn't that severe in terms of in-cylinder temperature. You're talking a peak increase of maybe 11 °C during compression. At sensible performance levels it won't make or break the engine.

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    Guest big_mac's Avatar
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    It really depends what chasing numbers means. 3-400hp might be fine but you may run into issues if your plans were above that.

    Out of interest where did you get an 11deg increase from?

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    my end goal is to get something like a gtx2867R or a gtx2863R so around 400 as I like driveability and not having to wait till 4k rpm for something to happen. that's why I chose to up the compression abit to give it a touch more power off boost.

    im still a bit undecided on what turbo to actually get so putting some miles on it while I have a long hard think wont be a bad thing.

    ive had so much fun drifting on the last setup at only 15psi of boost, but after 6 years or so on the same setup it would be nice to have a bit more grunt.

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    Guest zeppelin101's Avatar
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    Quote Originally Posted by big_mac View Post
    It really depends what chasing numbers means. 3-400hp might be fine but you may run into issues if your plans were above that.

    Out of interest where did you get an 11deg increase from?
    Polytropic compression calculation and an approximation of temperature at BDC (50 °C which is ~ball park with a good intercooler) and pressure (boost pressure is not a bad start which assumes 100% vol eff - but this figure will be the same for both cases).

    Constant = PV^n.

    P is pressure
    V is volume
    n is the polytropic coefficient. For an air / petrol mix this is circa 1.3 (but its the same for both CRs so makes no difference to the calculation).

    With that its possible to calculate what the pressure is at compression TDC (no combustion) and approximate temperature for any CR you like.

    There are other noise factors, but they won't be an issue at 9:1, especially going from 8.5:1 to 9:1. For instance, at higher CRs (above 10.5 give or take) cam profile and flow capability becomes more important because of the rapid increase in volume after TDC. The amount of work required to get the air and fuel into the cylinder with a sub-optimal cam profile and port flow increases which means that more combustion work is also required, which in turn means more air/fuel and a greater chance of knock. That's when the performance benefit of higher CR disappears without counter measures (like water injection).

    If I were in your position I'd go for the most responsive 350-360hp setup I could manage and enjoy the driveability. It doesn't sound like you need the power as such, more the broad torque shape. More easily achievable with slightly less top end.
    Last edited by zeppelin101; 02-02-2017 at 15:01.

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    driveability is champion. Im just not interested in massive turbos 1:1 CR and 80psi of boost with small powerbands.

    the stock t28 picks up well but falls off around 5500rpm.so in a drift situation where your on the limiter in 3rd then shifting to 4th would see you bogging down so I would drop the revs back down to 5500 before upshifting to ensure it wouldn't bog and spit you back straight.

    I think a smaller turbo like the gtx2863r (.64) would not suffer boost creep type problems on the standard wastgate, so I could stay bottom mount. Americans suggest they come into action at about 3300rpm so coupled with VVT and maybe some tomei pon cams 256 in/ex should see a real decent performer with no obvious compromise.

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    Guest zeppelin101's Avatar
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    The problem with that is that the GTX compressors want a lot of boost to get working properly. I think with the 60mm GTX wheel they can flow around 340hp worth of air but with the larger hot side shouldn't choke up as early but should be pretty responsive.

    BW have added a lot of turbos to their line up recently, I'm yet to look through and see whats there at the sensible and

    Sent from my Nexus 5X using Tapatalk

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    I know ive got to run 20psi+ to get decent results. deep down I now I will have to go external wastegate but ive had major faff with those in the past and would prefer to Keep it Simple S.

    I did look into the borg warners and came back with efr 5758 and the 7163.

    turbo selection is a chore as most results are from years ago or peoples build threads etc.. they fizz out and you get no dyno graphs or accurate feed back.. we want to know what's best now not way back in 2012..

    I did mess around with holsets a few years back but found the hx35 took forever to boost and the hx30 came on song real quick but made small numbers, the 32's that seem to be floating around might be a nice compromise of the two but im loathed to try it to find out as its major faff chopping up the rear housing and fabbing downpipes etc....

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    Guest zeppelin101's Avatar
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    I was thinking more about some of the update Airwerks units but those have quite large turbine wheels so would suit something with more capacity better. They are the same deal as the GTX units too in that they don't come with a turbine housing.

    As an alternative, you could try and rescue a GT2560 (roller bearing T28 in other words) with a larger/better compressor. Just having a look at the maps and to my eye it looks like the GTX60mm wheel is about 7% more efficient for the same mass flow and pressure ratio, which will give you a little more top end feel with similar response to what you have at the moment with a T28. Will probably only make 320-330hp but ought to rev out a little more happily.

    Someone like AET or Turbo Dynamics will bolt almost anything you ask for together.
    Last edited by zeppelin101; 03-02-2017 at 08:14.

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    i had a 2560 originally but the plastic bearing cage let go, i still have it and somehow there is no blade to housing contact even though i found the debris inside my sump pan.I put the original t28 back on as the bearing cartridges are the same price as the turbo.

    I was looking at the airwerx last night. I guess I will have to make some enquiries to see how they all match up price wise. some of the twin scroll EFR series have some flappy valve inside them to divert all of the initial flow into 1 of the scrolls to aid spool up or something like that.

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    Guest zeppelin101's Avatar
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    Are you talking about the compressor bypass valve? You want the exhaust flow separated at low speed to get the turbo to spool faster (exhaust pulses are more defined so impart more energy to the turbine wheel) the compressor bypass valve is like an integrated BOV.

    The bearing cages can be upgraded so if the turbine wheel and housing is intact that could be a good option.

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    I seen It was on the inlet side. is that what it does? it was late when I was looking so I obviously got the wrong end of the stick.

    TBH its been ages since I messed around with turbos so will have to take some time and relearn what all the graphs etc.. mean.

    thinking about it my mate got a 'souped' up t28 made by bernies blowers for his mx5 that he said was a touch bigger/better than a disco potato so I might speak to him about that. im not going to be turbo shopping till at least the summer so its no rush.

    if I can getaway without having to loads of fabwork and plumbing I will be real happy that's why I was looking at the garret fit and forget jobbies. the £1500 price tag also puts me off.

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    My SR (straight cam) has about 145psi compression across all 4, i have a gtx2867r which is doing 1.3bar at about 3750rpm with pon cams and a large intercooler. With what you're saying, my friends SR (straight cam) has a solid 160psi on all 4, would his engine spool the turbo faster than mine would? And would he make more/less power for the same boost/mods?

    I'm just thinking of when i rebuild my engine as it is feeling a bit tired, i have no idea on the mileage and its had its head kicked in by myself for the last 3 years and god knows who else before that lol.

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    tbh adam if you have good oil pressure and your cam lobes etc.. look good then I wouldn't take the engine to bits.

    I kicked my engine to death for 7 years. till the head gasket went..

    what rear housing do you have on your 2867? 0.64 or 0.86? or tial 0.72, internal or external wastegate and do you have a dynograph/who mapped it? how come you stopped at 1.3 bar?

    your mate would make more power if his engine is more healthy.

  17. #17
    Guest zeppelin101's Avatar
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    The cams will change the result of a compression test as well, unless you both have the same valvetrain.

    Sent from my Nexus 5X using Tapatalk

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    Quote Originally Posted by green_rs13 View Post
    tbh adam if you have good oil pressure and your cam lobes etc.. look good then I wouldn't take the engine to bits.

    I kicked my engine to death for 7 years. till the head gasket went..

    what rear housing do you have on your 2867? 0.64 or 0.86? or tial 0.72, internal or external wastegate and do you have a dynograph/who mapped it? how come you stopped at 1.3 bar?

    your mate would make more power if his engine is more healthy.
    Oil pressure seems good, the car runs pretty efficiently i've never had a problem with the car getting too hot. Might test it again soon with a better quality tester as it was about this time last year i got the 145 results.

    I'm running the stock 0.64 housing with the wastegate welded up and running a turbosmart compgate 40mm external. Stopped at 1.3 as the car started missfiring due to the coilpacks not being able to take it (tuners words). I still haven't got round to upgrading the coilpacks, and the car was road mapped so no dyno graph unfortunately. Was going to upgrade the coilpacks and go back for a re tune and then go on the dyno.

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    Quote Originally Posted by zeppelin101 View Post
    The cams will change the result of a compression test as well, unless you both have the same valvetrain.

    Sent from my Nexus 5X using Tapatalk
    His readings were with stock cams, although the engine has had a mhg fitted. Mines got BC valve springs, tomei poncams and cosworth 1.2 HG.

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    Guest immy21's Avatar
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    My S15 unopened SR went from stock cams 150ish-153psi to poncams 160ish-164psi
    That confused me to ****

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