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Rb25det Slight Missfire And Power Fluctuation

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Bit of back round.

RB25DET S1 perfect compression stock internals

progressive water/Meth injection


Walbro pump (will get changed to an external 044 bosch with surge tank in the next 3 months)

Nismo regulator

PFC w/datalogit

Wide band

greddy intake mani

Q45 maf

Q45 TB

splitfire coils

range 7 NGK iridiums


Plenty of supporting mods

and so on,

All installed in a pretty light S14

At low boost .9 bar it runs loverly, very linear power up to 7500rpm, just a slight stumble at low cruizing speeds even with a perfect AF and timing (changes dont realy make a difference)

at high boost its goes up to 1.5bar nice and fast (before 4500rpm) and pulls well and clean, but when I get to 6800 to 6900rpm it takes off to 7500rpm almost like its got a 150shot of nitrous, Boost stays stable and my AF is remains at 11-1 stable, timing also doesnt change much at all at that RPM to make such a prominent power change, I am seeing FC datalogit knock figures of 11 to 14 (ahh i love Meth injection) and nothing changes there, no other noticible log changes happen, everything looks 100% normal on the ECU, just a massive leap in actual power.

Im thinking that maybe my plugs are gapped a tiny bit to tight, but would like to know if you guys have ever had somthing like this? happen?

Car is also has a bit of a crapy idle, if i recall right my plugs are gapped at around .68 to .7mm which i know is tight, but my previous coils were going belly up so i kept them tight and didnt bother re-gapping when i put the splitfires in.

What you guys recon?

Edited by Rezlo
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get it on a dyno and find out where and when it is happening, Also how old are the plugs?

The power increase comes in at between 6800-6900rpm and holds right threw to 7500rpm red line in any gear at WOT in high boost, it repeats its power increase every time i go up towards red line.

Plugs are about 4000km old.

Finding a good dyno in South Africa is a challange!!!

Edited by Rezlo
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oh, yeah i can imagine it would be!

Get a set of cheap coppers and gap them down to .5 or so mm, chuck them in and run high boost and see if it goes away, if it does, then you've got a prob somewhere in the ignition system. might be an ignitor perhaps?

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kinda seems like the opposite, like its to tight.

Im going to go over all the grounds and yank the plugs and check them, and go up in gap and see, im not blowing out spark up top so im going to go up in gap till i just start to blow spark out at high boost & RPM and see if that sorts out mid range,

Im thinking that maybe the top end is right and everything else is a bit off key due to a to tight plug gap.

Anyone else willing to share their plug gap running split fires at 1.4 to 1.5bar on heat range 7's?

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Just got it off the dyno, 352wkw and 482nm @6987rpm (both still climbing)

power and tq were just rocketing up, he wouldnt take it into that power surge into the 7000rpm range,

Will regap them tonight and see if i can tell a diff.

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problem solved, my wideband LSU4 sensor is going bad, started taking fuel out of it and it got better and better, taken 15% away from the injector map and its cleared up PERFECTLY, WB still reads 11-1 under boost...

Edited by Rezlo
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