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N75J/N75C/MBC Comparison - with graphs

Discussion in 'A3/S3 Forum (8L Chassis)' started by Welly, Aug 2, 2010.

  1. Welly
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    Welly VX220 SC Driver :)

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    I posted a few weeks ago about how I had changed from the N75J I was using for the standard item, the N75C, and was very happy with the results.

    Now, I like to follow these things up if at all possible.... so that's what I've tried to do.

    Thanks to Bill @ Badger5 for the use of the dyno for this testing.... :thumbsup:

    I have dyno print outs of both runs below.

    The comparison in the boost curve between the two valves is incredibly different.

    Anyway....

    J Valve

    The J valve is very spiky as we all know. Gives good initial boost - around 20-21 psi on my setup @ around 4000 rpm, which then fades away very sharply to around 14 psi by 6000 rpm.

    Based on this, the torque delivery suffers badly after the initial spike, dropping like a stone through the rev band. This will be seen on the dyno print I will post later.

    So really, the J valve is a trade off. Peak PSI is increased earlier, but to the detriment of the later stages. The turbo gives it's all early, and runs out of puff much sooner. Not good IMO as really affects driveability of the car when on the road. This is due to the torque curve following the boost curve – the sooner the boost dies, the sooner the torque dies.

    On another note, I have also found that long term fuel trims were more out when on this valve.... But may just be my motor.

    Graph here:

    [​IMG]

    Sadly, there isn't a breakdown of this one as there is below, however it does show Flywheel power, Wheel power, torque and boost.

    Note that boost is down to ~16 psi by 6000 rpm.

    C Valve

    Mine is original and old.... lol.

    Noticed an immediate difference in the power delivery when using this valve.

    Boost is less spiky, and comes on in a much more progressive form. Peak boost is less, however holds that peak boost for a much longer period. As can be seen from the graph below, the C valve doesn’t actually achieve peak boost until much later in the band. Sustainable boost is held from ~3600 all the way to 6000 and beyond.

    Torque when using the C valve is reasonably stable through the same band. IMO much better.

    Graphs here:

    [​IMG]

    [​IMG]

    Boost hold much better, and peak torque when compared to the J valve (225 lb/ft) is much higher.

    MBC

    Will add this when I have fitted it and had it re-run.
    Last edited: Aug 2, 2010
    #1
  2. Matt82
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    Matt82 Active Member

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    peak bhp after 6kprm is actually quite late for a 20vt from what ive seen.. well done!

    i desperately want to get the boost to hold for longer up the rev range on mine. not even checked which n75 i have
    #2
  3. DJ_Troopa
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    DJ_Troopa K1 Driver

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    Don't we have the N75 F valve as standard? What's a C?
    #3
  4. Welly
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    Welly VX220 SC Driver :)

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    The F is the later version of the C valve, so the same thing really.
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  5. Matt82
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    Matt82 Active Member

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    jesus christ. big difference in those plots. its not hard to see which is going to be the quicker car

    i need MORE up top. i want more between 5k and 6k. i lost out twice on sunday to a stripped 182 but a few hundreths. at the top of third mine was going limp and hers carried on like a proper hot hatch should.

    get those mbc plots up because i may have to "cheat"
    #5
  6. Welly
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    Welly VX220 SC Driver :)

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    Will do - ordering one tomorrow..... I am interested to see also.
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  7. Madeley
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    Madeley Member

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    shropshire carrot cruncher here!! whats a MBC?
    #7
  8. m8arx
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    m8arx Member

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    manual boost controller
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  9. s3_kev
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    s3_kev Stage2'd!!!!!!

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    im considering going MBC will it effect fuel trims though?? having boost issues atm have no leaks but going to replave the one way valve in the crankcase brether pipes as this may be causing my issue i replaced all the pipes recently but did'nt change the valve so wounder if its faulty as my long term fuel trims are out also??
    #9
  10. s3dave
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    s3dave TFSI Hybrid

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    just make sure you set the mbc low on standard internals and work up bit by bit as it could potentially wiz you upto 28+ psi
    #10
  11. superkarl
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    superkarl MAN OF STEEL

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    where can you get hold of an MBC??
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  12. s3dave
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    s3dave TFSI Hybrid

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    gts chris on ebay does good ones or of course there is the super expensive forge
    #12
  13. Welly
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    Welly VX220 SC Driver :)

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    Agreed, and thats bad! Thats getting close to bendy bendy moments....
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  14. Welly
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    Welly VX220 SC Driver :)

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    The UNOS isn't cheap, but it isn't cheap for a reason. Because it is VERY good.
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  15. s3_kev
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    s3_kev Stage2'd!!!!!!

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    yeah i plan to keep it to 20 psi 21 max thats where it was and where i want it, my crappy AMD mapp used to boost 25 psi when i first had it mapped then eventually settled at 20 psi now its gone down to 18 and even as low as 16psi recently so need to try and sort that out 1st.. but one time after disconecting the battery and then driving pretty hard i saw boost go to around 27psi before i crapped my self and came off boost quick
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  16. s3dave
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    s3dave TFSI Hybrid

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    £20 ones are just as good, its a ball and spring at the end of the day,once they are set the cheaper ones dont fluctuate i had one on my supra for years and it never lost or gained psi, we all no why forge are expensive... because it is "forge"
    #16
  17. superkarl
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    superkarl MAN OF STEEL

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    Would anyone care to explain how the MBC works? I've found the forge item on boostcontrollerstore.com in $ tho, not as expensive as i imagined. How would you know what boost you are turning it up to? its just a knob, would you have to log the car?
    thankyou
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  18. superkarl
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    superkarl MAN OF STEEL

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    BTW welly, thanks for doing this for everyone, theres me thinking getting the N75J is a good way to go!
    #18
  19. s3_kev
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    s3_kev Stage2'd!!!!!!

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    would you completly replace the n75 with the mbc?? i saw speedy steves the other week and looked like the n75 was still in line with the mbc??
    #19
  20. Welly
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    Welly VX220 SC Driver :)

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    Maybe as an item they are as good, but adjustability isn't fantastic on the cheap ones. I use the £29.99 R Spec one personally, and rate them very highly, had one on my A3 for about 6 months and I agree, was good as gold. Have used a UNOS though, and they are great, well made, look nice....

    I don't not like the cheap ones, they are what they are.
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  21. Welly
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    Welly VX220 SC Driver :)

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    If you are planning on getting one you NEED a boost gauge to set it up.

    Have a look here: http://www.boostcontroller.com/display_page.php?i=19. Some good reading there.
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  22. Welly
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    Welly VX220 SC Driver :)

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    I will be yes, Steve's MBC is there to stop the initial bost spike, and then the N75 valve controls it from there.

    I will be removing my N75 and blocking the TIP hole for it up. Should eliminate wastegate creep as well as giving me the boost level I decide.
    #22
  23. Welly
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    Welly VX220 SC Driver :)

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    Thats the opposite to my conclusions - I think the standard valve produces better power all through the rev band, and that is in fact what the dyno has shown too....

    BUT, it depends on how you want the car to drive at the end of the day. I don't want a huge boost spike and then less boost later, I want more stable boost through the rev range.

    It's purely personal opinion IMO.
    #23
  24. S3 Nattie
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    S3 Nattie Quattro ist für sissies

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    I think you would get different readings if you was running a standard map.

    My car with the n75j valve spikes quite high, roughly 18psi (have seen 20psI on a cold evening) it dies off to about 15psI at 6k which is higher than the n75c valve would ever go, would spike to 14 psI and die to 11 @ 6k.

    So for a my standard mapped car the n75j valve holds more boost than the standard c does throughout the whole of the rev range.

    Nathan
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  25. s3dave
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    s3dave TFSI Hybrid

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    exactly a ball and spring like the dear ones in fancy wrapping.....
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  26. Matt82
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    Matt82 Active Member

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    thats forum myths for you ;)
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  27. Welly
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    Welly VX220 SC Driver :)

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    Thanks for adding that Nath, that's interesting to hear. How long have you been using the J?
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  28. S3 Nattie
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    S3 Nattie Quattro ist für sissies

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    Been running a J valve for about 2/3 month. I think it is harder for the ECU to control and sustain high, spikey boost on a remapped (generic map) cars due to them running quite a bit more psi than a standard map and thats maybe why the J version isnt as good as a standard item .

    I would defiantly recommend a J valve for a standard car, will have to see how the car performs when I get my car at stage 2 level. From looking at your results, it does appear that the standard C/F valve does have a better area under the curve so more boost over the rev range, be good to see what readings you would get with a J valve and a MBC.

    Nathan
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  29. Welly
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    Welly VX220 SC Driver :)

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    J Valve dyno plot is up there now, first one.
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  30. S3 Nattie
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    S3 Nattie Quattro ist für sissies

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    Sorry, wasnt meant to just say "be good to see what readings you get with a MBC".
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  31. Welly
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    Welly VX220 SC Driver :)

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    D'oh! OK... no worries....
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  32. Welly
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    Welly VX220 SC Driver :)

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    OK - MBC arrived this morning and has been fitted.

    I went with a cheap-ish ebay one just for testing purposes - it was only £21 delivered and made by Mumby Engineering.

    Made of brass, it is a stainless steel ball and spring boost controller. Fitting took about 30 seconds.

    A point to be made here is that I have been using my liquid gauge to look at a live graph of requested vs actual and found that the actual never meets the requested using the N75 valves (either of them).

    Anyway...

    Initial results appear good - I have set the boost to 22 psi peak. It achieves and holds 22 psi reasonably well, tailing off to around 18/19 by redline.

    I have used the liquid gauge to gauge results in the interim period.

    Boost as I said is 22 PSI and holds up much better using the MBC. Spool up time is also improved.

    Figures are interestingly different. I used the liquid to test power before and after.

    Before fitting: 258 bhp and a peak of 242 lb/ft.
    After fitting: 267 bhp and a peak of 286 lb/ft.

    I have gained around 40 ft/lbs of torque at peak, however that is a spike. It then holds torque better through the rev band due to the increased boost later on.

    BHP increase is minimal as it isn't really flowing all that much more air.

    As said, these are only interim figures, the dyno will be a much better way of showing.

    Hoping to get it on the dyno at some point next week all being well....

    Watch this space.
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  33. s3dave
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    s3dave TFSI Hybrid

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    very interesting, did you try get it higher?
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  34. s3_kev
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    s3_kev Stage2'd!!!!!!

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    how have you fitted yours then just mbc and electrionicly conncted the n75?? i fitted mine yesterday went for inline with n75 still in series for now set the boost to 21 psi peak and it holds 18 ish but as the revs gather it tails off which is strange as it would peak at around 19-20 and hold about 17 all the way on the n75 and mapping, might try just mbc see if it holds boost higher to redline.

    however im yet to have a proper drive from 2nd gear through to 5th or 6th im planning to tonight if i feel neccasary ill go just mbc,
    #34
  35. s3_kev
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    s3_kev Stage2'd!!!!!!

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  36. Welly
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    Welly VX220 SC Driver :)

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    I don't need to - I have run such tests in the past and had up to 32 psi boost. I may have a fiddle over the next few weeks, but am having other drivetrain related problems which mean I don't want to hammer it around too much at the moment.

    Yep, I have blocked off the TIP and just fired in the MCB in it's place.

    I need to turn my 007P back around to the normal way as at full boost I get a chatter which I think is coming from there...
    #36
  37. s3_kev
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    s3_kev Stage2'd!!!!!!

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    what spring are you using?? i have recently gone to yellow with a shim seems ok
    #37
  38. Welly
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    Welly VX220 SC Driver :)

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    I'm on yellow too - but it's fitted in reverse at moment. I think that under full boost it's pushing the valve open and allowing boost loss.

    Thats the chatter sound I think I can here, the valve being battered by inlet manifold pressure in the top, and charge pressure in the bottom.

    Either that or it's my TIP collapsing which I also suspect it could be.
    #38
  39. s3_kev
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    s3_kev Stage2'd!!!!!!

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    i also run mine in reverse but i have a silicone tip so thats the only difference we seem to have get a new tip maybe?
    #39
  40. Welly
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    Welly VX220 SC Driver :)

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    It's on my ever lasting list..... mine is poorly at best, it remembers when Jesus was born it's that old.
    #40

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