Knock Q's

[Slow91z]

Hey fellas I was wondering if any of you guys have see this before.

I was always under the impression that most occurs either at the shifts or right before them...I noticed that I get about 1.5 degrees of retard on the 3rd gear shift and about 4.5 degrees at 4200 RPM in 3rd, then it goes away. no knock at any other time. Any input??

You don't want to know my combo you'd call me nutz.

 

[Billdogg]

Valve springs getting weak?

 

[Slow91z]

That would cause knock??? Never thougt of that...It still pulls hard all the way to the red line so I don't think that's it.

 

[Billdogg]

I guess not if if it still pulls past 4200. hmmm... Not sure then.

 

[ijames]

It's more typical to get a little knock just after the shift, about when or just after the rpms bottom out and start climbing again. A few reasons conspire to cause this. The wastegate needs a little time to open, and the turbo needs a little time to slow back down so there will be a (usually brief) boost spike when the engine slows down but the turbo is still flowing enough air to keep the boost up at the top of the last gear. Generally the volumetric efficiency is higher at the lower rpm so even if the boost were constant the cylinder pressure would be higher, pushing the engine closer to the detonation threshold. The load on the engine is greater in the higher gear so it can't accelerate as quickly, keeping it at lower rpms longer than in the lower gears, which again pushes the engine closer to detonation than it would be at a higher rpm with everything else the same. Finally, some chips are a little rich at the bottom of each gear, and I've actually seen a car knock on C16 and 20 psi boost just after the 2-3 shift when the tuning was too rich; we thought we were lean and added fuel which made the knock worse cause we were already too rich, leaned it out and all was well.

 

[Slow91z]

AHA!!! I just got through bumping up the FP, Guess it's richer then I thought...Thanks ijames.

 

[BEATAV8]

Hi Collin,
I'm also trying to tune out similar knock retard problems on my wife's 87T. It's been showing 6 degrees of knock retard on the 2-3 shift. Fairly typical. IMO 6 degrees is on the edge of too much. 4 degrees I would not be too worried about.

I tried 2 things recently (with no luck).

I pulled 2 degrees of timing out at WOT (translator plus setup). Still showed 6 degrees on the shift.

Leaned it out from 8% rich to 4% rich. This brought my 100MPH O2 numbers down from 800 to 770 but the knock was still the same.... 6 degrees.