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Mysterious power flatline from 5k rpm - tuner is stumped


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Hi all,

My R34 GTT has been really weak on power across three (different people) tunes, across three turbo setups, both before and after a moderate engine rebuild. I started with ~180kw on a high flow turbo with a nistune after the rebuild.

I made 245kw using a hypergear ATR45 for RB25, Stock manifold, china FMIC, 3" turboback, Link G4X. Naturally my spool was slow with this hefty turbo (GT3582 .63 IWG equivalent afaik). That tuner's explanation for the low power was my terrible intercooler. Dyno chart for reference, see red line:

image.thumb.png.efbc54bac15a299f3812b17f3313fff6.png

 

My endeavour to have a low boost threshold with a minimum 280kw (ideally 300+) lead me to install:

  • 6boost with EFR7064 T4 TS 0.92 IWG, 4" dump to 5" high flow cat, 3" straight through catback
  • Plazmaman Pro Cooler, 2.5" piping, 60mm (stock size) bosch e-throttle

Here is some pics - I am pleased with how it looks at least :)

No description available.No description available.

I am now getting my car tuned. I received a call that my torque curve craps the bed from 5000 rpm - I consented to further diagnosis. He sent me this, showing my dyno result so far, versus a nearly identical R34GTT they have in the shop to emphasise the issue. Note how I flatline at 5k, whereas the blue lined car with a 3076 turbo flies to the moon and 400+ hp. Tuner says my poor performance is NOT normal - I totally agree and am really getting sick of slow car.

image.thumb.png.94f4d16ddd5ce1515936073e1b2063b1.png

 

Tuner has already tested the following:

  • Good, even compression and leak down test.
  • Very low dump pipe back pressure.
  • No significant pressure difference pre vs post throttle body
  • No 'basic' problems e.g. spark, fuel pressure, base timing etc
  • Tested disabling VCT - only lost low end and midrange with no change up top.

Further notes

  • Needs ~30% wastegate duty at moderate RPM to maintain 18psi, and it rockets up to 70% at high rpm to maintain.
  • Turbo speed continues increasing with RPM and I am far from the turbo speed limit.
  • Tuner says the nissan manual recommendations of 0.3mm and 0.5mm for intake and exhaust valve clearance is crap, and I should be looking at 0.3 max on both sides on stock cams. I've read similar things on the forums - is there ANY chance that having 0.5-0.55mm exhaust valve clearance may cause this?
  • His next best guess is confirm that the VCT cam gear isn't damaged and moving 'out of time' without moving the cam.

He was honest and admitted he is stumped for now, but has further ideas and would like to sleep on it - I do appreciate the transparency. Alas, further diagnoses will cost me fat stacks as it progresses. Does anyone have any suggestions on what might be the issue here? I suspect an EMAP test may be enlightening, but I have no bungs on my manifold.

Edited by CowsWithGuns
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What do you mean by 'in the wrong spot'? Just that it's out of time with the timing belt?

I've reinstalled the cams twice myself, during rebuild few years ago and in october 2023. Didn't notice a difference before and after taking them out recently. I'm ready to learn I'm a weaponised dickhead and the source of my own misery, give me the good news GTSBoy

Edited by CowsWithGuns
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Yeah, like a tooth out, or something similar.

This is why I said that "It's a lot to suggest that it survived the engine being taken apart and reassembled" because for such a problem to have existed before, and persisted after reassembly, it means a real sneaky and unexpected problem or substantial incompetence.

But when you get right down to it.....a really flat top end reeks of insufficient valve overlap - when all your other diagnoses suggest nothing wrong with the rest of the engine. Well, either that or a boost rag!

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Video of first assembly (on tune 1 and 2)

Second time I had a second person look at it too... man, it's an interesting one to be sure. Tuner already has his suspicions on the camshafts so I shall let him cook - done a few hundred RB's so I'm in good hands :)

 

 

Edited by CowsWithGuns
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Yea... 

You've changed everything else and gone above and beyond here, so it's """""safe""""" to say this is in somewhere you haven't touched. The manifold _is_ something you have touched and changed. It's unlikely you have some massive boost leak somewhere that you somehow missed while changing every single component that could potentially leak, but if that was the case your turbo speed would be higher than expected for 18psi.

Cams can do weird shit, and if you're confident you've covered everything external to the motor it's time to look inside. To be so down on power is so odd that there'd be no other problems it's causing.

Unless the 7064 is a dog on 98/mismatched, it's not a commonly run turbo.... I remember a 7163 running out of puff/nosing over at high ranges on a SR20, though that was due to Poncams running around. I don't wanna be 'that guy' but a 3076 is also a bigger turbo, compressor size, though the 7064 has a larger turbine wheel. Point being the 3076 may not be the most likely thing to match it against. Someone point @Lithium here.

....maybe it just needs much more boost to get the wheel speeds up there and the curve won't so much flatline as just nose over as one might**** expect. This would be after verifying "omg the cams are actually okay, wtf is goin on here"

It reminds me of highflow days and IWG days. Something could be running out of puff here at high RPM on a 2.5.

That thing could be the exhaust cam as previously mentioned though :D

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1 minute ago, TurboTapin said:

Check your timing and do a boost leak test. 

Yeah I think a deep dive into timing is what tuner is taking care of as we speak. Regarding boost leaks, I pressurised to 30 psi and was losing ~1psi per second into the crank case which I believe is normal, especially on a cold engine (all coming out the oil cap). No other external leaks per the soap water test. Tuner also has done a boost leak test during pre-dyno inpsection and was satisfied.

13 minutes ago, Duncan said:

It is also possible the harmonic balancer has spun if it is a factory balancer, which means the crank timing mark can be in the wrong place

Makes sense, but it's only 2-3 years old and this implies it would have been spun or mismatched from factory. Not impossible. I believe the tuner is now 'mechanically' inspecting the timing by looking at where the lobes are pointing when the piston is at the top - or something like that.

18 minutes ago, Kinkstaah said:

Yea... 

You've changed everything else and gone above and beyond here, so it's """""safe""""" to say this is in somewhere you haven't touched. The manifold _is_ something you have touched and changed. It's unlikely you have some massive boost leak somewhere that you somehow missed while changing every single component that could potentially leak, but if that was the case your turbo speed would be higher than expected for 18psi.

Cams can do weird shit, and if you're confident you've covered everything external to the motor it's time to look inside. To be so down on power is so odd that there'd be no other problems it's causing.

(unless the 7064 is a dog on 98/mismatched, it's not a commonly run turbo). I remember a 7163 running out of puff/nosing over at high ranges on a SR20, though that was due to Poncams running around. I don't wanna be 'that guy' but a 3076 is also a bigger turbo, compressor size, though the 7064 has a larger turbine wheel. Point being the 3076 may not be the most likely thing to match it against. Someone point @Lithium here.

....maybe it just needs much more boost to get the wheel speeds up there and the curve won't so much flatline as just nose over as one might**** expect. This would be after verifying "omg the cams are actually okay, wtf is goin on here"

If it really is just a mismatched turbo, I might cry a bit (a lot). Lots of effort went into this lol. But even then, surely I'd be looking at maybe 270 nose offs rather than 240 flat lines? Matchbot gave me the good news regarding the compressor map. But yes no doubt I was chasing a 7163 - but they were 5.5k AT BEST, whereas I snagged the 7064 for 2.5k within Australia.

I pray for a faulty head at this point, that would feel 10x better than wrong turbo selection.  😂

Thank you everyone for your feedback, I will report back when we get somewhere.

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A final observation. When I did boost leak tests, I would have to rotate the crank to *exactly* a sweet spot, otherwise air would be blasting into the crankcase as fast as I could fill it. Then it would hold quite reasonably.

Does this mean anything - I assumed it was normal.

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I don't think the 7064 would be _that_ bad. The 2871 and GTX3071 were actually pretty decent performers (at least on E85) and some smart heads would pick the GTX3071 over the GTX3076. So having a ... "GTX3270" is not the end of the world. Probably. Given it's a EFR it'll likely be great. 

Just maybe (and you've implied you wouldn't be) don't be upset if it makes 299.9kw when a GTX3076 may make 320 on 98.

It just reminds me of highflow days, but those were .63's not .92's and as mentioned, the exhaust side of the turbo is relatively large compared to the comp size. Just also remember a lot of results are on E85 which makes a huge difference. I'm making the assumption this is all on 98.

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2 minutes ago, Kinkstaah said:

I don't think the 7064 would be _that_ bad. The 2871 and GTX3071 were actually pretty decent performers (at least on E85) and some smart heads would pick the GTX3071 over the GTX3076. So having a ... "GTX3270" is not the end of the world. Probably. Given it's a EFR it'll likely be great. 

Just maybe (and you've implied you wouldn't be) don't be upset if it makes 299.9kw when a GTX3076 may make 320 on 98.

It just reminds me of highflow days, but those were .63's not .92's and as mentioned, the exhaust side of the turbo is relatively large compared to the comp size. Just also remember a lot of results are on E85 which makes a huge difference. I'm making the assumption this is all on 98.

Yeah, I was chatting with you a while back on turbo selection and you definitely made it clear to me that 300kw on the small EFRs may not be happening. I am very much at peace with that! On the other hand, it's also what motivated me to go a bit 'extra' on things like a fatty dump pipe because I thought it would be a cool post to share here if I did crack 300kw on an IWG 7163 or 7064 on 98.

If an underlying issue gets rectified and I get it back at say 290, I will be VERY happy, and also motivated to minmax things (e.g. supreme intercooler ducting, heat shielding out the ass etc) for science and to flex my 'big' power small turbo ;) Hit the winter time dyno for internet points ;) :) 

 

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54 minutes ago, CowsWithGuns said:

A final observation. When I did boost leak tests, I would have to rotate the crank to *exactly* a sweet spot, otherwise air would be blasting into the crankcase as fast as I could fill it. Then it would hold quite reasonably.

Does this mean anything - I assumed it was normal.

that is not normal at all. The intake air is sealed from the crankcase by the piston rings (a little should pass but not much) and valve stem seals. However, if it was heading out the exhaust (which might sound similar) that is perfectly normal on the part of the cycle when exhaust valves are open.

I normally do an intake pressure test up to the last point in the intake I can cap off so I can bring it up to pressure and leave it there to check for slow leaks.

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13 minutes ago, Duncan said:

that is not normal at all. The intake air is sealed from the crankcase by the piston rings (a little should pass but not much) and valve stem seals. However, if it was heading out the exhaust (which might sound similar) that is perfectly normal on the part of the cycle when exhaust valves are open.

I normally do an intake pressure test up to the last point in the intake I can cap off so I can bring it up to pressure and leave it there to check for slow leaks.

Yep sorry, I believe it is going out the exhaust in generous volumes, except in a very narrow crank angle range. Then it slowly exits out the oil cap and nowhere else. Is that what you would expect then?

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Ye, when people pressure test the system they test the system (itself, only) by capping the pipe at where your TB is and pressurizing it from Turbo outlet pipe to TB. Or at least I would, because what you're realistically trying to test is your intercooler piping.

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Have you tried a new CAS?

torque going flat, then requiring massive amounts of duty cycle to close the gate leads me to believe your timing is all over the shop.

Perhaps try to do a quick load hold and see if shit glows OR if your tuner is game hold a timing light on your balancer with the car loaded up (easier on a hub dyno, a bit scary on a roller) to see what the actual timing is.

Another thought, post up your dyno pull ECU logs, sometimes you might have some weird sensor (like IAT doing weird shit) and there's an ignition compensation being applied and pulling out shit tons of timing.

Another thought, valve float? 

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Close of business, got report from the tuner. No diagnosis - fat bill for the week. But they have tested everything advised so far before I mentioned it.

  • CAS wheel has been changed a year ago to a 24 tooth (nissan opto in da bin)
  • Timing light is steady under load on dyno
  • Visual inspection of mechanical timing is good (including faulty balancer)
  • VTC gear and electronics work correctly
  • Boost leaks retested, BOV inspected
  • Valve float assumed a non issue due to aftermarket heavy valve springs, boosted only to 18 psi
  • No boost rags ;) Cross checked by adding a MAF pre throttle body
  • Statement that my loose valve clearances would not cause this issue (though they don't love it).

I will request ECU logs but I won't get these til Tuesday.

Times are tough mates. The team said they will give it a good think over the weekend and maybe there will be some eureka moment - but at this point I'm getting myself prepared to accept that I'll be running a fully sick 240kw for another year.

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Maybe pull the inlet cam pulley off and check the bolts are tight, the outer pulley might have slipped on the inner, retarding the inlet cam.

VCT still works, but timing up the shit

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