Source of false knock

[WS6]

other than obvious downpipe contact. Where would be some other souces of metal to metal contact. Reason I ask is that I still have this false knock problem under hard acceleration (19.9 down to .7 then 0.0) usually at about 39 or so MPH. I've got zero knock 3rd through 4th all the way up to 110 and I'm fat at about 812 ish.

I inadvertanly caught the knock gauge light up 2 greens when I was off the throttle and rolled over a bridge expansion joint at about 40 MPH. Now it didn't register on the scan master and I found a bumpy blacktop road and was able to get it to light up one green light when I hit multiple washboard cracks in the road. That time I was under light throttle,in traffic, at about 55MPH.

Do you guys think it's far fetched to blame the front suspension? like the struts? I have one that is leaking and is more than likely given up the ghost.

And, are the F bodies more prone to false knock like this because they are uni-bodies and without body bushings like our G body brotherin' ? And generally a more forgiving suspension?

 

[RatFink]

You ask great questions. I've always thought that the better the question the brighter the author anyway. Also, I have no answers for you, sorry. DontKnow:. Your car sounds a lot like mine. It will do some funky things with the scan master and the casper's knock gauge. On occasion they read in parallel , sometimes the don't say the same thing, sometimes they do. Never blown a head gasket, the plugs always look pretty, no indication that it has ever knocked (2nd set of plugs in the car from new), though I know it has. When it knocks it is very audible, which back in the day, there was no knock gauges, I always put race gas in it, shortened the rod and listened close and with nothing other than a cutout on the stock DP and stock chip, it would run low 8.20's on freakin rock hard Goodyear Gatorbacks (the stock tires)!!!. So when I tried a few different chips with regular gas, I'd hear knock early on VERY AUDIBLE. I really believe I've only heard audible knock maybe 5 times in the last 19 years and always let out instantly. That's with having a scanmaster only about 5 of those years. I think the best tool is the powerlogger. I don't have one and DO NOT speak from experience, but the guys I talk to at the strip say they are the best tool to have. That comes from Buick GN people going low 10's high 9's (street-able, well lets be honest..he trailers it to the strip), but almost any ricer will tell you more about gadgetry than their exhaust system, so I'm pro technology stuff. I think the scanmaster is good, If I had to do over, I wouldn't even have bothered with the casper's knock gauge in my 3 gauge pillar. I would have instead put in a A/F ratio gauge in and got the powerlogger and delete the scanmaster as well. About the same money, except for the laptop. I've stated what I've stated many times before. I need to take my own advice, but damn I got like a $1.37 to my name!:cry::drag:

 

[WS6]

I arrived at this question because I generally over complicate problems,real or imagined. And I was just thinking that F bodies have no real layered defense when it come to absorbing resonate noise throughout the chassis. Tight engine bay,tight exhaust routing,no body biscuits, less hangers for the exhaust,only one control arm up front,per side,Etc.
I have never heard this car audibly knock and I'm pretty adept at hearing detonation. But that doesn't mean it isn't happening.

I know why the caspers guage picks it up first and subsequently why the scan master doesn't jive with what the knock gauge reads.
And that is where my question comes in, is the knock gauge picking up "road noise" and then then it's gone before the ECM can react to it? Thereby explaining why the scanmaster displays knock retard late or not at all.

So, I take it $1.37 doesn't get your transmission repaired? or is that what's left after the rebuild.

 

[RatFink]

So, I take it $1.37 doesn't get your transmission repaired? or is that what's left after the rebuild.

Ha Ha! yeah, broke ass me. I just closed on a house about 3 weeks ago, so it goes without saying that I've spent more than I had planned. The trans may get fixed this winter. Do you have sub-frame connectors? Strut tower brace? Those things would probably help.

 

[WS6]

Do you have sub-frame connectors? Strut tower brace? Those things would probably help.

Yes, I have SFC's,strut tower brace,tubular panhard bar and tubular lower control arms all with poly bushings.

I sometimes wonder if that actually contributes to false knock. You know, stiffening up an already stiff chassis.

Funny thing I noticed, the car, when I first got it would lift the front end some under launch. Now it just sits and launches very level.

 

[Quiky One]

Flextplate?
Tranny mount?
Motor mounts (doubtfull unless it failed)
Do you have a DS loop? Check it.
Pull and tug on the intercooler. I've seen bolts back out a few turns and cause false knock.
Check ALL the stupid things you think it wouldn't be. You would be surprised.

 

[WS6]

Flextplate?

New and recently torqued down

Tranny mount?

Just bought one today for the helluva it.

Motor mounts (doubtfull unless it failed)

Brand new and just installed

Do you have a DS loop? Check it.

Possibly! I'll check that..

Pull and tug on the intercooler. I've seen bolts back out a few turns and cause false knock.

Yes, a constant problem especially with the lower RH one. I made a bushing to shim it out so I could really sung it down.

Check ALL the stupid things you think it wouldn't be. You would be surprised.

I'm taking the the whole crossmember down currently. I see where the intermediate pipe hanger has been making contact with the crossmember and the upper transmision mount maybe making contact because the bushings look a little tired.
Also with the replacement of the flex coupler off the downpipe, that has moved it about 3/8" up ward and now it clearly is whacking the sub frame connectors.

Hey aren't you in college this year? how's that going?

 

[Quiky One]

Not yet. One more year of HS.

I have all my application stuff in so now it's a waiting game.

 

[WS6]

So after all of it went back together with new transmission mount and relocation of exhaust and all I still have a whopper of a false knock problem. Like an immediate 19.9 KR then down 10 then down to 0.0. This happens either from a dead stop launch or a 45 MPH roll on blast from 3rd gear.
But! I had some time today to put my old alternator back on 'cause the New Delco crapped out last night in less than 100 miles since I bought it. That went fast so I went under the back to recheck the rear exhasut hangers and I found the passenger side one to emmulate a knocking noise when I moved it up and down. So I took it all apart and reassembled it differently than it was (hooker cat back) and low and behold I took it out and the knock is all but gone! I still get some knock under hard launch(tires spinning) but it's minor like 4.0 then goes away by next refresh.
So I made a ton of progress for 15 minutes work, and the car seems faster becasue of the abscense of the false knock.

 

[Quiky One]

I bet the 4.0 is false too. Every once in awhile I'll get 32 or so doing a burnout in the box but I never get it on the track.

That is great you found it!

 

[WS6]

I bet the 4.0 is false too.

I think you're right. It shows up as 4.0 and disappears next refresh.

I don't want to declare total victory just yet. I need to get some more test launches and roll-on launches.

I was really starting to get discouraged as I was making very little progress.

Thanks for the help!