Low RPM, high map knock

[LZeppelin513]

I keep getting one or two knocks below 2400 rpms and from 75 - 100 kpa. I have pulled a lot of timing and fueling is good.

Should I keep pulling timing until no knocks? Or is it ok to have a couple knock counts...

It feels like i have lost a tad bit of tq down there from all the timing pulled but that could just be in my head.

Also, I think I am having to run less timing than other 3500's. At 100 kpa I go from 19* at 2400 rpm to 23.65* at 4800 RPM. Is that low or normal? Any more advance and knock counts go up. I still have a knock or two in that range so I may even need to pull some more timing.

[robertisaar]

well, you need to determine if it's false or real knock. the two big ways of doing that are to pull a LOT of timing and see if it goes away entirely or run a higher octane fuel and see if that gets rid of it.

you can definitely feel a difference in spark in the way the motor reacts.... i can feel nearly all of the times the knock sensor decides to call some random noise(probably due to an exploded lower mount) knock in the MC, it almost feels like a miss for a couple of revolutions... and that's ~3* getting pulled and added back in most of the time.

due to the way the knock sensor works, a misfire can get sensed as knock, so make sure your plugs, wires, coils and ICM are in good shape.

[3400-95-Modified]

I had a big issue with false knock... It was at lower map values but similar low RPM... I would get major timing pulled when I was crusing on the highway. To fix it I ended up insulating the outside of the knock sensor. I think the housing of it was picking up noise from the headers possibly and registering as KR.

[LZeppelin513]

Ok thanks for the responses. I will look into that.

What about my WOT spark being only 22*-23*? My fuel is getting closer to good, ~12-12.7. But more advance at WOT and i get knock. From what I have read, most with 3500 run 26-27* at WOT.

What could be a reason I have to run so much less? In this case its likely not false knock since it goes away when I pull timing.

[robertisaar]

running a cam/headwork? that can dictate the amount of advance you can run.

[LZeppelin513]

Nope. Bone stock motor.

Nothing fancy, CAI, free flowing true dual exhaust. 5 speed getrag, 2700 lb car.

[LZeppelin513]

does cam/headwork mean engine wants more timing?

[caffeine]

When I was tuning my 3500 I was getting some knock at low-mid rpm no matter how much timing I pulled, so much so I felt like it was losing power, so I advanced the timing back up a couple degrees to get the power back and left it.

When I ran nitrous through the engine a few months later on 94 octane I didn't get any knock.

[caffeine]

Keep in mind I'm using a knock sensor for a 3.1L, the way I understand it, a 3.4L knock sensor is a better match for the 3500 because the cylinder bore is what the knock sensor is matched to.

[Superdave]

KS just has to be matched to the knock filter on the memcal. I've had the same 3.1 sensor on 3400's and 3500's and never had an issue.


for both stock and wild 3500's i keep the advance above 4K between 26 and 28 degrees, below it's usually in the low 20's. I'd add a little more fuel to get it into the low 12 AFR range and bump the advance up. If it's still knocking then post up a datalog, something isn't right.


some 3500's had a false knock problem caused by the valve springs, there may have been a recall but in your case a quick spring swap might be the answer.

[LZeppelin513]

for both stock and wild 3500's i keep the advance above 4K between 26 and 28 degrees, below it's usually in the low 20's. I'd add a little more fuel to get it into the low 12 AFR range and bump the advance up.

The other thing that seems weird is that my fueling seems way different then yours.

I set my BPC down to 150 like you were saying but I ran out of table. I have been upping it to bring my AFRs down and now I am at 176 BPC. My VE is 100 at 4000 RPM 100 kpa, and my AFR vs RPM for 4000 up is in the 9's. It doesn't make a lot of sense to me because it sounds way different then you are running.

Its running good though and feels pretty strong. And I really think my wide band is working correctly but I could be wrong.

[Superdave]

you're using the stock 3500 injectors right?

I know some of them are flex fuel engines, i wonder if the injectors are different than the 24lb/hr rating that was posted.

I had tune issues with a guy that had those injectors in his 3500 Fiero, it was lean popping at idle and surged really bad.

[LZeppelin513]

Yes I am using stock 3500 injectors.

[Superdave]

what year is the engine?

any chance you can get the part number off of the injectors? Maybe we can put this to bed, the issue i had with that Fiero owner drove me nuts.. first problem out of 100+ swap chips i've sold.

[caffeine]

I don't know the difference but the injectors in my Fiero and the injectors that came out off my Firebird's engine physically appear different. Both engines are from 06 G6s. The second motor I bought also is missing a vacuum port next to the brake booster port and had different UIM gaskets.