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Shift Knock?


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Recently I've been dealing with a lot of false knock in between shifts after harder acceleration, typically from 2nd to 3rd but honestly it can happen between any gear. I've sent multiple sets of logs to my tuner and he insists that it is false. I do believe him but I cannot find what could be causing this.

 

I also get low feedback knock readings from -2 to -3 when I pull into my steep driveway staying in 1st gear and dancing with the pedal to get to my spot in the garage. Seems to trip the knock sensor when engaging the clutch at times.

 

I figure that it can't be exhaust shield rattling since I don't have any being while being catless. I've checked bushings (everything that whiteline offers, I basically have). My motor mount and trans mounts are STI Group N and had them on for a while now.

 

My guess is that it might be drivetrain/transmission related but not sure where or what to check. Can a wearing clutch/flywheel cause this? I have no signs of issue with my clutch though. I can upload some logs if that is helpful.

 

Also, nothing registers on Fine Knock Learn and DAM never changes below 1.

 

Has anybody dealt with this before? Thanks.

Edited by Notorious
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Bump. I guess this is something either nobody has similar experience with or something everyone just deals with if it’s not too big a deal? Feedback knock spikes to a max of -12 sometimes after shifting into 4th and not even pressing gas yet. Might have to do with lift off or during gear engagement but idk.
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I had subscribed because I was hoping for responses :-D

 

I have something similar I think. I've always had -2.11 come and go for shifting, but after I swapped in the lower mileage engine and a SB Stage 3 Daily, I can get -5.98 and higher from time to time.

Most of them come at lower speeds/throttle, which is why I wasn't too concerned about it.

After tuning what I could and driving for a while, I'm pretty convinced it's noise, but would still like to figure out how to get rid of it.

I've gone over it several times trying to find something lose/knocking without luck.

I also put blast plates on the tranny, so I'm wondering if that contributed or not.

Edited by Infosecdad
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Bump. I guess this is something either nobody has similar experience with or something everyone just deals with if it’s not too big a deal? Feedback knock spikes to a max of -12 sometimes after shifting into 4th and not even pressing gas yet. Might have to do with lift off or during gear engagement but idk.

 

Similar to you I get the same at low load operation and shifting from 1st to 2nd, 2nd to 3rd and in parking lots putting around. From my readings on RomRaider and other tuning forums/sites (Subaru specific), esp if you are like me and "high mileage" feedback knock in those scenarios is not too concerning. I will have less than 20 events in some cases and 34 events in other cases the worst is when I'm on and off the clutch a lot in tight spaces or crammed in city traffic.

 

I expect this as I have 194K on the original engine and most/all the related sensors. Do you have a LWFW? Are you on the stock clutch or stock profile clutch disc? Any stiffer bushings? Short throw? Any metal on metal touching? I ask this b/c the feedback microphone is known to pick up false knocks as a result of the above. On my 09 LGT (still miss that LGT) I had a failing harmonic balancer giving me knock fits when the shop went to replace it they found the alternator had a lot of forward/aft play which could also result in what the senor hears as knock.

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Not sure if this helps but I was experiencing a similar situation with my 5eat. I needed retune for some new hardware and brought it to my tuners attention. Since the retune I’ve had only -1.3 fbkc on very limited occasion at low boost. What I noticed but haven’t confirmed with my tuner yet is when I come off the gas my wide band goes to about 12:1. So it would appear that it’s adding fuel so not to go lean on lift. IIrc it used to go to 25:1 on lift. I would imagine that when you lift to shift a manual it goes lean. Not sure if it can be tuned similarly with manual trans but you may want to ask a tuner.
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I'm in the same boat. I get feedback knock every now and then when cruising around town and it's usually in the -1 to -3 range but sometimes hits -5. No FKL and DAM has never dropped below 1 so I don't pay too much attention to it. I still have the original exhuast manifold heat shields at 174k miles and always figured it was those or some other drivetrain noise.
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Similar to you I get the same at low load operation and shifting from 1st to 2nd, 2nd to 3rd and in parking lots putting around. From my readings on RomRaider and other tuning forums/sites (Subaru specific), esp if you are like me and "high mileage" feedback knock in those scenarios is not too concerning. I will have less than 20 events in some cases and 34 events in other cases the worst is when I'm on and off the clutch a lot in tight spaces or crammed in city traffic.

 

I expect this as I have 194K on the original engine and most/all the related sensors. Do you have a LWFW? Are you on the stock clutch or stock profile clutch disc? Any stiffer bushings? Short throw? Any metal on metal touching? I ask this b/c the feedback microphone is known to pick up false knocks as a result of the above. On my 09 LGT (still miss that LGT) I had a failing harmonic balancer giving me knock fits when the shop went to replace it they found the alternator had a lot of forward/aft play which could also result in what the senor hears as knock.

 

Stock flywheel, Stage 1 Exedy Clutch. I do have stiffer bushings and Kartboy short shifter. I guess I can't get rid of those but I'll check the alternator and such parts around that area. Could even an alternator cover cause this? Mine doesn't fit perfectly (modified Perrin) and sometimes loosens over time but I've tightened it and no change.

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I also wonder if the RacerX FMIC piping is causing this. It does hit the metal at some of the bends. I wrapped the all the piping with the titanium exhaust wrap just to help combat metal on metal contact but maybe the contact still is causing knock readings. I would think though that it would register knock when I give it gas from idle in neutral, but it doesn't. I can't get any knock changes in neutral which is why I thought it was drivetrain related.
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I was in the same situation as you when I put together my car, except my FBKC pulls were usually between -8 and -12 degrees. Freaked the everloving shit out of me the first time it happened. Talked to Cryo, and he indicated it was probably false based on where we were at with the tune. I took two steps to make myself feel more confident that it was false, since I'm open source - I grabbed a pair of shift logs with both IAM cut to 0.5, and MAF curve scaled 10% to force the AFR rich. Saw the exact same FBKC pulls, which to me was enough to confirm that it was false.

 

I spent many a night knocking around above and below the car with a deadblow, tightening down everything that I could possibly think of, and couldn't get rid of it. Even went as far as swapping my Group N motor mounts for some low mile JDM EJ20 mounts, which are softer. No change.

 

Then, I installed a SPT turbo heat shield. Started hearing a nasty vibration on decel, and some on accel. This indicated that there was a resonance in the up-pipe. Mounted an accelerometer on top of the compressor housing and watched it with my little pocket scope, and saw it ring when I heard the sound. When I put the motor in the car, I had left off the big bracket that bolts the up-pipe to the trans and block, to clear the IAG oil lines (since they don't install right on B25 heads without grinding down part of the head). Ground the head, made the oil lines fit right, and reinstalled the up-pipe support bracket. No more noise, shift knock will now and again be in the -2.11 to -5.98 degree range, but is just as often 0. But..... like said before, I've come to expect some from these cars since everyone I've owned (including a bone stock 13 WRX) have done it.

 

Moral of the story being, don't sweat the small stuff, you won't always find an obvious cause for false knock, and if you can take a measure like pulling timing and adding fuel for a couple of logs just to see if the shift knock goes away. Cryo also mentioned that on E85 cars you can desensitize the knock sensor to reduce the maximum FBKC timing pull and increased bleed-off rate, but only if you're positive the knock isn't real. I was almost ready to go that route when I found the up-pipe bracket.

Edited by awfulwaffle
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I can't get any knock changes in neutral which is why I thought it was drivetrain related.

 

FYI - there is a minimum engine load threshold for both feedback and learned knock correction, which would prevent you seeing anything in neutral. IIRC stock is 0.6 g/rev for FBKC and 1 g/rev FLKC.

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