I have never seen a single LS engine get hurt by running it fat, and I've been around them for a long time. I keep hearing non sense about fuel being trapped between the top and middle ring and then combusting, blowing pistons up and what not. Running fat is definitely safe, it slows the flame speed and cools the cylinder. There is a reason why OEMs add timing based on EQ. There are even instances where OEMs run an excessive rich AFR to keep cats alive. Try running m1, and you'll be in the verge of misfiring where the engine makes best power in terms of EQ ratio. At the end of the day ignition timing is the main problem, that's what causes your rods to bend, your heads to lift, and ringlands to break. Running rich is a "safe" thing to do for engine longevity. But the better thing to do is to pull ignition timing up to a point where EGT starts wreaking havoc on your exhaust valves and cats.

Next time you have free time, think about the temperatures the cylinder sees during combustion and find the explanation as to why the aluminum piston doesn't melt. You can find the answer in one of those youtube videos of an engine with a see thru head. Pay attention to the flame and what's around it as it propagates.

 

Can you give your opinion on why there is such a thing as rich knock? Multiple people have logged knock on an overly rich/under timed engine.

 

Well, who are the guys logging rich knock ? Why are these people calling it rich knock? Improper fuel mixtures can definitely disrupt a clean pressure wave to develop during combustion but it's not to be confused with knock. It seems like we are describing a rich misfire condition. The fact that the knock sensors are used to log knock (I am assuming at this point the factory knock sensors are being used) then their credibility just straight goes out the door. Once anything different from factory is installed, be it intake, turbo, camshaft, springs, etc etc etc. The knock sensor accuracy goes out the door, no ifs buts or maybe. This is a fact. So logging rich knock already doesn't make sense. Once again, we can have rich misfires I dont question that. That depends on alot of factors, especially chamber design and quench pad area (the flat spot in your cylinder head chamber), injector spray pattern, fuel type. We could go into actual "rich knock" but that applies to GDI, and more specifically when homogeneous injection modes cant cope with injection times so the EOI overlaps with ignition advance, forcing advance to be retarded to complete the injection. We have other injection modes such as stratified injection, which is essentially injecting such a minute amount of fuel (in excess of 20 AFR gas scale) to economize. One then wonders how this is possible? Unlike homogeneous injection, which injects usually during the intake stroke, stratified mode injects ar or near TDC during compression. The fuel is mixed and it is concentrated around the spark plug making combustion possible. I welcome anyone to log a completely factory vehicle (PFI LS) and make sure theres absolutely no knock, then go as rich as possible and seeing knock. I'm confident that at even a low 10 AFR there will be no knock.

For the undertimed question, I never debated undertiming. But I have seen several LS engines go well into the negatives for a small amount of time (Traction control strategies) without damage, misfires, knock.

And finally, what I am saying is not facts, it is a recollection of experiences and information i recall. I hope this can get some discussion going.

 

Interesting points. I for one have logged "rich knock" on the scanner. This was after desensitizing the factory knock sensors as besides being overly sensitive to large cams/mods, they are SLOW to recover making it tough to decipher where the knock actually occurred. One question, this would imply that the "misfire" is occurring at the same frequency as a knock event?

 

True story, plenty of people throw on maggies with the handheld tuner (which runs rich) on otherwise stock vehicles that don't have knock and daily drive the thing for years. I've pegged a typical wideband gauge full rich on first hit when tuning, no knock just down on power. I've also seen some stock turbo vehicles hit 10's AF from the factory, again no knock. Direct injection has changed that a bit which I think is what you were getting at with the stratified charge and runs much leaner then most would typically expect, even from the OEM's.

 

I've seen it on more than 1 PFI LS turbo....not stock OEM. The AFR was in the low 10's and undertimed and there was an aftermarket cam in all instances.

 

I've seen and heard the rich misfire he speaks of, did your O2 sensors pickup any spikes as well? OEM knock sensors go off on all sorts of BS, and the burst knock is even worse but most disable that.

 

So if I put a CAI on my car, the knock sensors are useless then?

Does the frequency(Hz)of knock change when you put a different intake on?

 

Not useless but as he stated the accuracy is diminished. To me they are kinda useless anyways if you know what you are doing, tuned plenty of standalone setups without knock sensors. If you start low and work up you know where the engine wants to be, but instead a lot of tunes I see are aggressive for no reason and ride the knock sensors.

 

I agree with starting low and working your way up. I mean that's just common sense.

Why is the accuracy diminished? Maybe I am completely misguided about how knock sensors work.

 

I meant intake manifold, my apologies. But essentially the OEMs (and I know this from previous conversations with OEM calibrators) calibrate these knock sensors to certain frequencies known to be knock free. How they know its knock free is with pressure transducers and they can trace the pressure curve to determine so. They then "calibrate" the knock sensors throughout various loads, and RPM ranges as the frequency varies based on these and more variables. He (and another worldwide famous calibrator) gave me an example of them doing contract work for a company who specialized in boat mfg. The company needed help developing a knock strategy for their engine/ECU. The calibrator ends up finishing the work. Two weeks later, the calibrator gets a call saying they are seeing a ton of knock and what not. He goes over, and sees that the company had changed from a plastic style intake manifold, to a cast aluminum style intake. The reason if I recall was related to cost. The calibrator knew there was no actual knock, but in fact the "properties" of the engine had been changed, as the harmonics now travel faster/slower/amplify less or more with the different intake manifold/hardware. He pretty much said the knock sensors were useless at this point and had to re-do the whole work again.

Not only does this make sense, but I've seen it happen as well. One time I had a truck on the dyno and it worked great. Later that day, we were driving in the streets, and started logging a TON of knock. Well, we had gotten ourselves into some mud pits, and the oil pan/block was full of dried up mud! The added mass/material, whatever you want to call it changed the emitting frequency of the engine (at least thats what I think,call me crazy ).
At the end all I am saying is, accuracy is seriously diminished everytime you change something on the engine. ****my opinion and that of one or two OEM calibrators I've talked to**** Do not mistake this as a green light to completely ignore your knock sensors.

 

Thank you for the explanation. I was afraid my previous comment would taken as me being a dick that wasn't my intent.

 

You would think it is common sense especially after you hear they pop a few engines. But some tuners start so far above the curve they don't see the progressive gains and the engine not wanting more timing. They keep pressing for minimal gains, then you make a top speed run or boost the car up an incline on the highway and its riding the knock sensors trying to stay alive because a dyno isn't real life.

 

I'd have to go back and check the logs to be sure, but don't recall the O2 sensors picking up any spikes. Burst knock is from the debil!


That's assuming that the tooner didn't just kill them all together too like some do. There is zero reason to kill them altogether unless you're running a rowdy cam.

 

Interesting read guys! Ive only ever picked up knock on a setup over timed for the compression ratio and fuel, or bad knock sensors giving false readings.

Gen3 ls are bad about this with the under intake sensors and water damage etc.

 

7600rpm? Whats the point of the rpm or boost if you run it that many rpm and throw a fat tune in it to keep it "Safe" in your mind. Run it 800rpm less, throw 10# more boost at it and the motor will live longer and make more power. Boost never hurt anything, the same can't be said about rpm.

 

I know a couple people posting here are better tuners and mechanics than I am by a mile. What I am good at is knowing my skill level enough to be careful and patient with tuning. I see no reason to run 10-10.5 afr so if I see that I stop and adjust based upon what I see and trends to get it closer before going further. Also if I see Kr and have any reason at all to doubt that it may be false I read plugs before going further. If no signs of detonation I adjust the sensors a bit and log again.

 

Boost can certainly hurt things as it bends rods and lifts heads, stock engines like 7600rpm more then they like 35psi lol

Just for ***** I went out adjusted my tune to mid 9's AF and ran it up and down the road a few dozen times under boost... zero knock. So 10-10.5 certainly won't hurt anything and I haven't seen it hurt anything in the past.

 

What timing, what CR, what fuel, cam overlap? You can't just say because your motor didn't do it that it's not possible, because we've done it.

 

Just over 11:1, 93/M1, ~17PSI, 13-14*, nothing special about cam, stock knock sensor settings. But I've done the same on stock L76's, LS2/3's, 5.3's of all sorts, some small cams some big cams, some meth some E85, just happens sometimes when making a pull that it goes rich especially on first few hits and doesn't knock. Not like I leave it there except maybe on a SBE running 25+ on E85 then a little richer then normal seems to work well. But I have had some rich misfire, especially in imports but if you know what knock sounds like it has a different sound.