Vet

Quick question... filled up the tank yesterday (93 octane of course), then not more than a quarter mile later, I heard what seemed like a touch of detonation at the very beginning of 2nd and 3rd gears... just for a half a second each time... this behavior continued. I know the car will automatically retard the timing when it senses detonation, but is the system slow to the point where you actually HEAR a bit of detonation BEFORE the timing retards (which would explain my symptom)...? Or do I have some other issue? I always thought that the car's knock sensor etc would react faster than that (to the point where the timing would retard BEFORE the driver would hear any detonation). But, I guess it makes sense that it would take at least a half a second or so for the system to "hear" the knock and then retard. Man, I had only pumped in about 6 gallons of expensive Exxon 93, filled the tank... they must have given me some real p*ss quality fuel. I still maintain that regardless of where you get fuel and how much you pay etc, you still never really know what's going into your tank.

BigGeek

Lower octane numbers can cause preignition. The lower octane numbers fire at a lower pressure/temperature. This is one of the reasons why GM recommends only 93+ with our higher compression ratio engines.

Some smaller gas stations are guilty of mixing low octane with high octane in order to make a bigger profit margin. Could your station be guilty of this? (I personally never ever had a problem with Exxon gas.)

It's also possible there's water in the gas, which causes it's own sort of problems in a cold engine.

Michrider

It can retard timing extremely quick.

However, not all spark knock can be stopped by anything you do in the programming. Example, preignition. Preignition had nothing to do with the timing, the fuel mixture ignited on it's own.

So if it's spark knock caused by advanced timing and poor fuel, sure timing retard will correct that extremely quick. But preignition which sounds like a hundred ball peen hammers, or a real quick metallic crunch, nothing is going to stop that except removing foot from the throttle and getting better gas in it.

lvjetboy

Let's talk about this.

Without side-tracks debating detonation vs pre-ignition.

I know what pinging sounds like, pretty sure Vet and others do too. It's not ideal combustion. Mechanically, mild and temporary ping maybe ok....over time who knows. But extreme detonation certainly can damage parts.

Smooth combustion is better.

That said, why does my brand new 2007 Vette detonate mildly under certain throttle conditions even though I always use the recommended 91 octane? We're not talking carbon build-up on an engine with only 3500 miles. In fact, I noticed this the first time I drove it (3 miles on ODO) Why doesn't the spark knock sensor and electronics compensate faster? My old 92 Vette would ping maybe once then adjust. My new 2007 pings at times (lightly) for a significant time, maybe 3 or 4 pings maybe more or until either rpm or throttle is changed. I don't get it. Why doesn't the knock sensor or timing adjust for this?

I understand detonation (pinging) and many reasons why it can happen. I also understand what damage can be done to an engine when detonation becomes severe...mine's not severe...but it's there and annoyingly so.

I'm sure the dealer will tell me it's normal.

What I'm wondering is have others with new 2007's experienced this. And gear heads out there...Shop dog? Is this now considered "normal" for GM engines or does my new vette have issues?

jer

OregonC6

Why? IMO because your brand new '07's engine has a very high compression ratio. So high, in fact, that the knock sensors and the range of available adjustments are NOT sufficient to prevent knock.

The engine was built with very high comp to make more power. Consider the Z06: The minimum recommended fuel for it is 93 octane I believe and that grade of gas isn't even available on the entire West Coast. 92 is tops out here.

So, IMO, GM designed the engine to make lots of hp given the displacement and design of the normally aspirated LS2. it designed it, IMO, to make too much hp in fact for available fuels.

Consider this: when the engine does adapt to gasoline with inadequate octane it is not making the advertised hp. So basically every time the knock sensing and adjustment system steps in it is because of the engine was designed to run on unavailable fuel.

i think a better discussion would be what , if any, really safe and effective octane boosters are out there that would keep engine hp at the rated level and make the automatic adjustments unnecessary.

Last point. GM says that the system will compensate for a range of fuel grades. They also just increased the factory warranty. My $.02 worth is for you to run the engine as hard as you like. If it knocks so what? Let it break. Let it throw a piston through the head. So what?

The manufacturer says that the engine can run on 91 or 92 octane and will not be damaged. Let em replace the engine if damage occurs.

IMO the LS2 should have more cubic inches and a lower compression ratio so that it would never knock even without the sensing and adjustment system operating.

lvjetboy

Oregon,

Not interested in octane booster threads. Been there done that long time ago and many times since.

I think you're probably right about GM design stretch and compression, yet my base C6 manual says 91 recommended not the Z06 93. Ya there's the the warranty thing...but.

Is my experience with the C6 normal? And if so, why can't the engine management system (fuel and ignition) compensate for 91 octane which afterall is the recommended octane not even the minimum? If true that seems wrong to me.

jer

Michrider

Your just going to have to accept the fact that not all spark knock can be stopped by anything you do in the programming. When I was a tech I had several customer angrilly confront our service mgr and myself and other techs swearing up and down that they must have bad knock sensors because they can hear spark knock. I could show them on a Tech 2 scanner that knock sensors were working perfectly and detecting knock and pulling back tons of timing, yet you could still hear knock. It's because not all knock is initiated by the spark plug firing, preignition - that knock self ignited way before the piston was at TDC and before the other flame front was started by the spark plug. The two flame fronts collide and its hammer time.

Every one of those situations were ultimately resolved by using better gas. 93 from Shell is a beautiful thing. Or any other brand on the top tier gas listing. Do a search for toptiergas.com and use one of those brands of premium. If your area doesn't allow 93, but a bottle of octane booster in a tank of 91.

StrayDog

My A6 2007 runs great on 93 no nocking ..took 1,300 mile spirited road trip through Nova Scotia , had nocking at part throttle especially on Cabot trail hills was useing supreme Esso , the esso guy said " 'just as good as american fuel ".resolved when returned to US.& filled up with 93 >> My 1966 B/B nocks on some 93 brands >> shell whoops maybe sunocco getting old need to take a good look at sign works best 93 no nocking just 2 cents from the Dog. woof.possibly there is some octaine varience on diff brands. Woof

Irv

I notice pinging at times with my '06-MN6. Not as bad as the '03-LS6 (which often sounded like a can of marbles), but it happens, especially at <2000rpm, 3rd or 4th gear, on a hill, with 1/4 or more throttle. A/C on appears to make it happen more consistently. I suspected high IAT readings, but my laptop says no way - intake air temp was within 10F of ambient on the two occassions I measured. I always run 92-octane gas.

I'm guessing GM programs a slow reaction to pinging on purpose, and that's why we're hearing it. I haven't had my car tuned yet, but I'd be surprised if it wasn't running lean under the conditions I mentioned above. I've wondered whether or not CAFE targets were indirectly at fault here... whether GM was tuning for leanness at small throttle openings when a bit of richness might cure the pinging, to get marginally better EPA mileage numbers.

mlongo99

Maybe you did get a bad batch of gas. Burn that stuff off and fill up from a different place. I'm stuck with 91 octane and I've NEVER heard my engine knock. It shouldn't be happening at all. Even the Z06s here have to run on 91 and they're fine.

Gearhead Jim

2006 C6, A6 tranny, 13k miles. Always use best gas available, around here that's 93 Shell or BP, Phillips 66 is also Top Tier. Never hear any knock or ping, but perhaps the automatic tranny helps- it downshifts before the engine can lug very much, and when using the paddles, I keep the RPM up.

Some C5 guys used to have a lot of knock problems but ours never experienced it.

boosted_z06

Knock depends on a lot of factors and the ECM has several tables as what should happen when knock is reported.
ECM can pull as much as 15 degrees of timing worst case.
Knock signal does not mean it is really knock but anything that can cause the sensors to report a A/C frequency above the compare frequency GM set to.
As to time elapsed depends on the decay and attack values in their tables and can be adjusted through a ECM tune.
GM has no way know what each brand of gas, what is made of or even difference of summer and winter gas makeups so it looks at many conditions and simply attacks the timing to limit piston slap.

If you want to rid knock or prevent it then prevent carbon buildup, keep engine temps cooler, have ECM tuned and use water injection
My C5 Z06 and C6 Z06 has 11.0:1 compression, 91 octane gas and zero knock in doing the above.

In tuning you can have the ECM attack knock sooner and decay it faster
and control the tip-in knock
ECM has two timing tables, one is used when there is only slight knock and another timing table with lower timing when it hears more knock so either table values can be used plus timing values between them

Mark06

Could you add a shot of acetone to the fuel for more octane?

aaaaa

You would need more than a 'shot' to make a difference.

Mark06

I know that, just not sure how much but I did not want to make it sound like " Go and get a can and dump it in scenario " I fear someone would misunderstand and do just that and I would feel like S!%t. I think I read somewhere you could go like 5% to total fuel volume.

lvjetboy

My mild knock at part throttle happens somewhat regular (but for relatively short periods of time) no matter where I fuel. Don't think it's a "bad batch" thing. Like you, I'm stuck with 91 octane...the "optimum" recommended by GM for my car. I've been careful to fill it with so-called top tier detergent gasoline as recommended in the manual since it was new. I fuel in Nevada and California...91 octane is max on the road unless you limit your cruising to about 200 miles from your nearest "specialty fuel outlet"

Although we do have a few specialty fuel vendors in Las Vegas...this is NOT an option for me!

I've had this problem with 3 miles on the ODO so I'm thinking maybe not pre-ignition from carbon deposit, what do you think?

Octane booster's are another can of worms. We could debate claims of performance versus results on and on. If an octane booster is needed for 91, then I'm dissapointed the GM manual doesn't recommend a brand of octane booster, or how much should be added, or if adding straight acetone will void my extended warranty. Sorry for the sarcasm but comm'on guys.

jer

lvjetboy

Why accept that when GM recommends 91 octane for my car?

Ok Toptiergas.com lists Shell and Chevron. That's where I fuel with premium 91 as recommended by GM. See my comments on octane booster.

jer

lvjetboy

I'm thinking your post may be why I'm getting detonation. And not the just live with it and be happy posts.

jer

knkali

i have th knock too. 0^ a6 no mods and I know the dealer isnt going to fix it. So............I am thinking about a tune

lvjetboy

Anyone else have this? I'd rather not start a poll, but so far seems few have experienced any detonation or knock so maybe there IS something wrong with my 2007.

jer