RK951

Could someone explain what is happening during engine knock? With the CV disabled running a MBC/EBC don't you lose knock protection? Is this ever a concern?

Oddjob

The Cycling valve has nothing to do with Knock Detection, although both are controlled by the KLR.

You do not loose knock detection or protection when bypassing the CV or using other means of boost control.

So you are safe.

RK951

Thanks Jim. What affect or control does the KLR have upon boost when or if it senses knock?

Rip It

i think it cuts your fuel flow ?

toddk911

I beleive a small ignition retard. But not really enough to save if you you get an overboost situation.

TurboTommy

"You do not loose knock detection or protection when bypassing the CV or using other means of boost control."

Well, I don't know about that.
One of the ways the KLR controls knock is via a safe boost mode. How will it be able to do that if the CV is out of the loop?

Oddjob

Yep, thats true, it cant cut the boost - my mistake (typing faster than thinking). But the the KLR will still detect knock and it will still have the DME retard timing.

I cant remember exactly, but the factory manual describes the sequence for knock detection/protection. Something like: if the KLR detects knock it will retard timing by 3 deg. If it still detects knock it will then retard timing by 6 degrees and cut the boost (which will then be limited to approx 1.2-1.4 bar absolute). If you have manual boost control, the KLR wont be able to drop the boost, but the DME will still retard the timing. And Im guessing that you would have to have some pretty big issues (really bad gas, very poorly mapped chips, etc) to continue knocking after retarding the timing by 6 degrees, even without cutting the boost.

The fuel cutoff is the overboost protection due to higher than stock boost levels, not the knock detection. Overboost protection is mapped in the factory DME chips. If you have aftermarket chips, you dont have any more overboost protection.

Excessive boost (overboost) and knocking are not the same thing. Excessive boost does not necessarily cause knocking, and the damage from running lean at high boost is not the same as damage caused by knocking.

Perry 951

I believe I am currently having a knock issue at full throttle with my car and the KLR is noticing. The DME will pull a total of 9 degrees timing (as said above, 3 then 6). To me, when it happens, it sounds like a horrible missfire, only on more than one cylinder. I see a 200 degree temp drop in the exhaust and I also shoot flames when it does it. (also confirming it's retarding timing so much it's now burning most of the fuel in the exhaust).

Based on this, I'd think it'd be pretty easy to notice if you are getting full retard from knock.

As for boost and fuel cut, DME won't cut fuel since it's not as safe as shutting down or retarding the ignition. CV would open and limit boost, if you have one, in overboost or limp mode.

Oddjob

I couldnt find the explanation I was thinking of in the factory service manual.

But, for what its worth, I did find this explanation from an article published in Panorama, excerpts from a presentation given to SAE by Paul Hensler, director of porsche power train design, DEC 85.

"In the 944 Turbo, with the onset of knock, the ignition timing of only the knocking cylinder is retarded and boost pressure is reduced in direct proportion to knock frequency. At the onset of knock, timing is retarded by 3 degrees on the knocking cylinder and then stepped back to normal. If further knocking in detected, ignition is retarded further to a maximum of 6 degrees. Boost pressure reduction occurs at throttle openings above 50 degrees. This is done by a timing difference which is compared to an rpm dependent value. If the value is exceeded, boost is reduced as necessary by 40mbar steps to a minimum."

Perry 951

Interesting.. so 6 degrees max retard. I've always heard it was 3 then 6 for a max of 9. Guess the words from the horse's mouth are better than the "pass the word" info I've been told.

Sure would be nice if someone could decode the KLR chip to see what the lookup values are and for how long they hold the retard.

hally

Perry how much boost & what a/f are u running on your k27/8 to trigger these suspected knock scenarios?

toddk911

Perry:

That sounds like a straight missfire issue, not necessarily a knock issue.

Are you plugs/plug wires ok???

Perry 951

15psi, 12.5 AF. My fuel is spot on thru the range.

Cap and rotor are brand new, wires, plugs, and coil are in good shape with about 12k on them. It's possible I filled up with some junk and could have a small amount of 2 year old gas that remained in the tank from storage. I plan to run out the current tank and then see what happens. If it still does it's thing, in goes the tolulene. If it keeps up after that, I'm gonna have to really dig into the issue.

I first thought it was a misfire, but it's too irratic and too specific to throttle position. I can run 15psi at 3/4 throttle and she goes like stink. The exhaust note changes too much for a missfire, so I'm pretty sure it's a timing issue. I can't hear knock, but my exhuast and intake are pretty friggin loud at full tilt.

I'll check back after I get a chance to run this tank out over the weekend.

toddk911

Have you tried in several gears?

The higher the gear the more load and therefore a missfire would be more noticable.

If it does it more or worse in say 3rd, or 4th when WOT, but not in 2nd, I would say just some sort of missfire. But if the gear you run doesn't make a differene, then I guess that leaves a knock issue.

I just have never heard of knock causing a missfire. I guess if it is a fuel cut issue from the knock, then that could cause a missfire????

Also, I have had expierence where the age of the cap/rotor/plug wires, etc. does not discount having some sort of arching somewhere.

TurboTommy

"The higher the gear the more load and therefore a missfire would be more noticable.

If it does it more or worse in say 3rd, or 4th when WOT, but not in 2nd, I would say just some sort of missfire. But if the gear you run doesn't make a differene, then I guess that leaves a knock issue."

Todd,
it's the other way around.
Higher gears wouldn't do anything with the misfire issue, but the higher would make it more susceptable to knock.