I'm having some difficulties stopping knock from occurring at WOT. I am not ready to tune WOT just yet, but I'm trying to get a general spark table down so that I can drive the car and not have to worry about detonation... not that I'd even run the car there without a proper tune, but I just want to be safe.

The table I'm using is from the AUJM bin with more advance at idle (28°). I thought I'd be able to get away with less idle timing, but the ZZ4 cam (208/221@0.050) is smoothest at 28° with 1.8-1.9 ms PW. I do not yet have a WBO2 so that's as good as I can get.

I'm getting knock counts during both gradual tip-in and full-pedal from low RPM. It usually happens between 2600-3400 RPM when the LV8 load is around 220. I'm only running about 27° of timing here, which sounds rather conservative. I also tried disabling the PE spark adder but I still saw knock counts, and that was at 21° advance. Target AFR is about 12.2:1 when it happens.

I'm running 55cc 305 heads on my 350 and the static compression ratio is ~10.3:1. I expect to run lower timing overall due to the higher CR and fast burn nature of the 305 heads, but I can't figure it out. I've attached a picture of my spark advance table. Maybe it's lack of smoothness is having something to do with the knocking?

 

Blue,

Those chambers sound pretty small. Do you know you have enough fuel volume for WOT?

 

I'm not sure... don't have a WB yet. Does an engine with smaller combustion chambers require more fuel at WOT?

 

Might try ramping the timing down faster...
You can have to much (cruise) timing, and it puts so much heat into the chambers, that you can't get enough fuel in there fast enough to get past it detonating, when you get on it.

Never *think* that x amount is too little or too much, the engine is telling you what too little or too much is, you have to tune to keep the engine happy, not to agree with what you think it should be.

There's no reason to run that much timing at idle. Your tuning to make you happy ie smooth, rather then to what the engine wants. It takes a while to get that concept down.

Forget the 36-38d timing nonsense, that's from the days of 103 Octane, and with small chambers, and fast burn designs you just don't want anything close to that.

That's sort of a small cam for that much compression. The higher CR that you run, you need more cam to delay the intake valve's closing point so as to not trap too much air, during the intake stroke. It's the intake's closing point that sets what the effective CR is, since it sets how much air gets trapped into the cylinder to get compressed.

 

Good info, Grumpy... thanks. But could you clarify a few points?

Not to get off-topic, but whenever I tried anything other than relatively high timing at idle I couldn't get the engine to behave smoothly. It just kept stumbling and trying to stall out, which surely isn't what it wanted . A quick blip of the throttle at idle would usually make it die, and that was with anywhere from 1.6 to 2.4 ms PW, open loop. I tried a bunch of combinations of fueling and timing.

With my smallish cam, are you saying that it may be trapping too much air in the cylinders, which would boost my dynamic CR? What would be the downsides of this?

 

Limiting IAC function and also adding/taking away the adder/subtracter timing tables may be of some benefit.

Also when playing with idle its extremely prudent to change things in the chip and also reset the IAC and TPS to make sure everything is in synch with what you want. Also your car may want a slighlty leaner or richer AFR at idle to be happy which u can address using the 02 Constants.

The whole cruise/part throttle to WOT will be experimentation as Bruce has already stated. More than likely you have a lean spot or too advanced timing causing heat buildup. Try pulling a few degrees from cruising or the post you are at before you go to WOT and see what happens.

R u tracking it with a wideband, either stand alone or in a logger?

later
Jeremy

 

Try some higher octane, and maybe some octane booster to see if it reduces the knock.

You do have the ALDL spark adder zeroed out, right?

 

I'm seeing about 10-15 IAC counts at a 650 RPM idle, which I thought was pretty good. I was also under the impression that the spark bias tables were only for cold engine operation. In $6E, the mask I'm using, it seems that the 20° bias is disabled above 113 °F. I don't have the ALDL spark adder zeroed, but after my computer connects to the ECM I remove the resistor from the connector to get out of the diagnostic mode (I rigged up a switch; pretty handy).

I've been using 93 octane, which is just about as high as I can get here in PA. With the high-compression motor I didn't want to take any chances.

Thanks for the advice on idle. I've already reset the IAC and TPS and have used open loop to get the idle to where it seems satisfactory (which probably isn't, since I'm running far too much timing there). No, I don't have a wide-band yet, so I don't truly know what the AFR is. It's on my list, believe me... but I gotta be able to tune something until I get a WB.

There are a few occasions when the timing transitions really quick from ~33° to ~25° when I hit the gas, but from the looks of things most of the knocking occurs as soon as PE mode is invoked (assuming the timing is relatively constant). Maybe PE is too lean? Of course, I can't tell without a WB... .

3.8, I read another post in which you explained the basis of spark tuning. Even if I fix this knocking, another thing that's confusing me is how to get the whole table in line when you're only working with three major regions (idle, part throttle, and WOT). From what I have gathered, idle is one number, WOT remains fairly constant, but part throttle might change a bit depending on load and RPM. And of course you have non-PE acceleration, which falls in between somewhere. After you get those major areas in line, do you just blend the table together and see what works?

 

Join the club, I have 58cc 305 heads, ported, on my 327, which gives around 9.7:1 compression, and a 219/229@0.05 .462/.482 cam. I am also always getting a small amount of knock retard going WOT no matter what (this is a manual trans so the knock counts come on shifting hard, getting back onto the gas WOT after each shift). I was thinking along the same lines, that I'm not ramping timing out fast enough and that the knock is being invoked a lot earlier in my timing curve than I thought. Or I'm completely off base and my PE is lean. I also have no WB O2 and am just guessing until I do.

 

until you guys get a WB...one thing you can do is change the table:

RPM verses %TPS for PE mode

If you change the table to all 100% verse RPM then you won't go into PE mode as long as you stay below 100% TPS. You can observe your BLMs at high percent throttle positions to see if your on the lean side.

This is an academic exercise but could keep you from doing engine damage until you get the WB if you can't stay off the petal.

 

The problem I have with the "turn PE off and get BLMS at 128 at WOT" is that the engine will NEVER see full WOT w/ no PE, so making that mode of operation stoich really doesn't accomplish anything. If you are stoich at WOT w/ no PE then with PE added I'd think you'd have overshot and be way too fat. No?

 

True, but I think what Bethann was getting at was to limit the knocking at WOT. Since neither of us have wide-band oxygen sensors, keeping the engine at 14.7:1 via the NBO2 would be the safer alternative. We don't really know what the AFR is at WOT, and it's been proven that the 12.xx:1 "target" AFR the ECM uses isn't always what it appears to be.

I disabled PE mode and I'm getting far less counts when I press the pedal down. I haven't disabled the PE spark adder, so I'm seeing a maximum of ~20-22° at high TPS voltages. Granted, I'm not comfortable at WOT under any circumstances until I have a WB, but this can foot the bill for now (with severe lack of power). If you zeroed out the PE spark adder table and are using the main spark table for WOT/PE timing, it might pose an issue if the AFR doesn't get richer. Not sure, though.

As for my tune, I went back to my original baseline: a modified ARAP with AUJM timing tables. I then used the AUJM MAF tables, tweaked the closed loop timers a bit, raised the idle to 775 RPM, and killed PE. It idles rather well with 20° advance and NO fueling changes. Hot startups are a pain (it tends to stall out unless I blip the throttle). Surprisingly, fueling was fairly close... 122 being the largest absolute deviation from 128. Idle BLMs are 128-130, too -- do you think the increased idle speed (from 650 RPM) is helping with cam overlap?

Even with PE disabled and 3° pulled from the entire spark table (sans idle), I'm still seeing knock... mostly at points before PE would be triggered, anyway. I'll have to take a closer look at my logs to see where this is happening and what is leading up to the event. Gotta check my plugs for metal, too...