V6 Guys Only

[esperman]

Originally Posted by cnofear

Thanks ! !
And talking about the compression ratio well then this is where I'm a little puzzled about when it comes to the compression ratio being high in the V6 and I do believe it is even higher then the SS's (which calls for premium) so it only makes sense to use the higher octane due to the high CR in the V6 but I guess with the technology used in the SIDI Engine and the way it is designed then I have to go with what the GM Engineers tested and developed by using 87 octane to be the RIGHT OCTANE FUEL for this Engine ..

I am TEMPTED to use and try the Higher Octane and see if in fact their is some difference in the overall performance for the V6 but then again it could only be the " Placebo effect" setting in after I fill it up with the Higher Octane...

do it. It's not going to hurt anything, and you will notice a difference in the upper rpm band. trust me.

[67 GTO]

Pump in what you like. The manual says it all, 87 or higher.

[Mankind]

The main reasom you can use 87 in these engines is because when the computer pulls timing it also affects the timing of the injectors opening with 500+ psi behind them.

The SS will without a doubt knock audibly if using 87 octane when you have it under load. The injection system is different and is not as percise with the moment the injector opens. This is due to the lower fuel pressures. 87 will predetonate and cause the "knock" to be rather aweful.

Maybe they need to look into the V8 not being a push rod engine and doing away with the fuel being injected into the intake port.

[spyderbuddy]

This is were the v6 is better than the v8 I think. The v8 is port injected wereas the v6 is direct injected. If i am right port injected the gas is injected before the valve wereas the direct injected is gas is injected straight into the cylinder. So with that said that is party why we can use 87 octane gas.

[Doc]

Octane is a retardant. It's not a measure of how powerful the gas is. The purpose is to raise the temperature required to ignite the gas so it doesn't pre-ignite in high compression engines which can destroy the engine. Any "hotpoint" inside the combustion chamber, whether a tiny bit of carbon or even the end of the spark plug itself can ignite a compressed fuel mixture if the hotpoint is hot enough. If this happens before the spark plug actually fires, which causes the compressing mixture to explode before it's supposed to, then you hear "knocking" which is the jolting of the engine trying to push a piston up when a pre-ignited explosion is trying to push it down. Needless to say this is VERY BAD for the engine.

In order to avoid this problem retardant is added to the gas to raise the temperature required to ignite so only the spark plug is hot enough to ignite the gas. This is known as octane rating. The higher the compression ratio in an engine, the more octane needed to prevent pre-ignition. This is why racing gas is 103 octane; because their engines are compressing the mixture to such a high degree it's hyper critical that ONLY the spark plug can ignite the mixture.

Street engines don't need 103 octane because their compression ratios aren't high enough to need it. A high performance street engine is going to be anywhere from 10 to 12-to-one compression and will definitely benefit from 91 or 93 octane whichever is available to you. Both the V6 and V8 in the Camaro will benefit from premium gas. Especially if you've improved the intake and/or exhaust.

The reason why our cars CAN run 87 octane is because the computer retards the timing to compensate for the possibility the gas can ignite too early. In essence because the retardant isn't in the fuel, the computer retards the ignition timing instead to protect the engine. As you can guess this will reduce the performance and efficiency of the engine somewhat.

To get the best performance run premium in both the V6 and V8; you can run 87 in a pinch if you have to and have no choice but try and use at least 89 if you can. Keep in mind it will take at least 100 miles of driving for the computer to accumulate enough readings to start readjusting the timing back towards performance mode. The programming is written to be EXTREMELY conservative in favor of protecting the engine; which is a good thing.

[rhd]

Originally Posted by esperman

do it. It's not going to hurt anything, and you will notice a difference in the upper rpm band. trust me.

+1 i noticed that when i run 93 and down shift to pass it seems to feel stronger....nothing crazy but i do feel it, but only on the highway for some reason

[ALLTRBO]

Doc is almost right on as are some of you, but I'll try to help clear up some misconceptions, as I see many.

Pre-ignition is not the same thing as detonation (by the automotive definition... a.k.a. knock or ping), though they're often confused with one another.

Pre-ignition is when the mixture ignites for various reasons well before the spark plug fires, as mentioned, but in most cases this is very quickly detrimental to your engine because the air/fuel mixture often ignites while the piston is well on it's way up still, therefore causing the explosion to pound down onto the piston that is headed up the cylinder, which is obviously a very bad thing.
This scenario ignites the mixture well before the normal BTDC (Before Top Dead Center) few degrees of timing advance that is required to make the engine run efficiently at the power stroke (it needs to start igniting the mixture slightly before the piston reaches the top because the fuel takes a certain amount of time to fully ignite so the process has to be started sooner than TDC - Top Dead Center -, therefore you have degrees [in crankshaft rotation] of "ignition timing advance".)

Detonation (or "knock", or "ping") is when a hot spot in the cylinder ignites, as Doc mentioned, without the spark plug igniting it, but that spot (or spots) ignites the mixture so close to the timing of the spark plug ignition that the two (or more) "flame fronts" BANG into each other. With severe detonation, particularly on older carb'd engines or EFI engines without "knock sensors" to tell the ECU to retard ignition timing to back power off and cool things down, you can hear this as a knocking or pinging sound constantly coming from the engine. It's easiest to hear at low-rpm because the engine noise and the exhaust noise don't drown it out, and it usually happens at high-load, IE: driving up a hill with the accelerator pressed down decently to maintain speed, or much more regularly in our particular cases, at WOT (Wide Open Throttle).

Detonation is much more common and usually much less harmful than pre-ignition, though will still kill your engine eventually (or near-instantly) depending on the severity and conditions, and on the strength of your engine parts (usually the pistons). It's still a very bad thing to have.

More coming up in the next post... (Is there a technical write-up section to sticky this type of info in for easy reference? I guess I haven't looked around this huge site enough, yet)

[spyderbuddy]

Y'all, I think you are missing a critical point. the reason the V6 can use a Higher comperssion ratio. The V6 is direct injection, that means there is no fuel mix in the intake or port, the mix is injected directly into the cylinder at time of ignition or a split nanosecond before. This is therefore called a dry intake system, the trucking market has been using this design for years.

[ALLTRBO]

So, here's where octane, air/fuel ratio (AFR), compression ratio, forced induction, elevation and temperature, knock sensors, engine construction material, piston/combustion chamber shape and volume, quench clearance, rod/stroke ratio, and a million other variables come in to control, reduce, or cause detonation. I can't cover them all, but here are a few things worth explaining for the purposes of this thread.

As mentioned (which, in one post, appeared to be a copy+paste from another source without attributing it), higher octane reduces the occurrence of detonation (hereafter referred to as the more commonly used term, knock). I think that's the obvious now, but blanket statements about when and why it occurs are mis-informative at best, dangerous at worst.

GM has stated that our LLT's are designed and tuned specifically for 87 octane, and that there will be no performance increase by going up in octane, because their factory tune is optimized and max'd out based on 87 (I need to find the source, but I know several links are around this site somewhere).
Most of their, and other manufacturers, performance-oriented engines are tuned to run well on 87 octane, but will increase power from running higher octane by way of raising ignition timing to a more efficient point because the higher octane will allow this without causing detonation.
So, again, they're saying that this engine specifically will not gain anything from raising the octane from 87.

However... Apparently at least one other and I have both seen measured indications that this is a faulty assertion, and that they didn't tune the LLT for all conditions as well as they had believed. Don't get me wrong, I'm sure these engineers are smart, very smart, and they build in lots of margin for error to control knock (by way of a normally-rich AFR, to name only one method) but they're far from infallable, and they can't possibly cover EVERY condition that might happen around the world, right from the get-go. Even they know this, which is why they often slightly change calibrations after a model year or even sometime within a model year (usually unnanounced... at least this is the way it was in the not-too-distant past).

In addition to the aforementioned findings, I dyno'd my car a few weeks ago, and 2 of the 4 pulls (2 in 3rd gear, 2 in 4th gear) showed a nasty square-wave type of power curve, indicative of serious knock retard and ramping back up when the knock goes away, over and over several times throughout the runs. Both the dyno operator (and shop owner, who has tuned many many very fast cars, N/A and boosted) and I instantly said that it looks like knock. We could be wrong (though he knows more than I) because there is so much going on inside this ECU that has yet to be unlocked and figured out by anyone save the GM engineers, but it follows the traditional and very common pattern.
That being said, my LLT Camaro only gets 93 octane now, and it does 'seem' to have a more consistent powerband in this summer heat than before, though I don't have hard evidence since.

We have a compression ratio of 11.3:1, which by all means has been generally considered way above the level that 87 octane can handle.
But... you can't compare one engine to the next. The LS3 is apples-and-oranges away from the LLT.
The largest reason these engines can handle all that compression on 87 octane (most of the time) is because of the direct-injection, as has been mentioned. I don't know all of the details yet, but injector timing is one reason as mentioned, as is excellent fuel vaporization via the super-high fuel pressure.
The static compression ratio (SCR) is actually what is 11.3:1, but there's also the dynamic compression ratio (DCR). It is a more important indicator of knock control than the SCR. DCR depends on valve timing, which is dictated by the cam(s), and in our case, varies considerably due to the continually-variable-valve-phasing. Just that variable throws the DCR from a 2D map into a 3D map. Lots to get your head around!
The LS3 has none of this, but is more akin to the traditional engine as we know it from the last 20 years, and that, combined with many other factors, dictates that it must use 91 (or 93?) octane and it will lose power from any lower value, even though its SCR is lower than the LLT's.

Next post coming up...

[spyderbuddy]

But, agian the fuel dose not get injected into the cylinder untill right before ignition. The cylinder, not the port or manifold

[ALLTRBO]

For a much more extreme apples-to-oranges example, my old '08 ZX-10R (RIP :( ) had a 12.9:1 SCR and 'only' required 91 octane. 12.9:1, and it made 200hp with the ram-air! It didn't have direct injection, either. There are many reasons this particular ICE (Internal Combustion Engine) in this particular application was able to have such a high SCR. It just shows that you really can't compare one ICE to a totally different ICE in this way.

Ultimately, the ability of our engines to run such a high static compression ratio (as compared to many engines in the past) on 87 octane (most of the time) has to do with the whole package. It all adds up. DI, VVT, aluminum block and heads, efficient combustion chamber design, tight quench, rod/stroke ratio, etc. It's all great stuff.

Another question was about summer vs. winter...
Ambient air temperature is another variable that can affect knock. Hotter air into the engine = even hotter air inside of the chamber. Heat causes knock. Higher octane reduces knock. Therefore, if the car knocks in the hotter temps but not in the lower temps, a higher octane value must be used in the hotter temps, but not in the lower temps. It's as simple as that, but how much knock you get and how much octane you need to control it isn't as easy to pin down. That depends on many things, and can be mostly pinned down by way of datalogging everything the computer sees and deciphering that information into useful knowledge. Learn all you can. Knowledge is power.

Now... my headache is ever-worsening, I'm about to pass out. That's it for now. I hope I help someone by having spent all this time typing!

[ALLTRBO]

Originally Posted by spyderbuddy

But, agian the fuel dose not get injected into the cylinder untill right before ignition. The cylinder, not the port or manifold

Originally Posted by ALLTRBO

The largest reason these engines can handle all that compression on 87 octane (most of the time) is because of the direct-injection, as has been mentioned... injector timing is one reason as mentioned

I was typing that before your last two posts, referring to your much earlier post. There were more 'points' in this thread to be missed than just that one, and it isn't the end-all to knock prevention, otherwise they would have upped the CR even more, they wouldn't have incorporated individual knock sensing for every cylinder, and we wouldn't be showing large differences in engine output and timing advance depending on whether we use 87 or 93 octane.

Yes, the 'trucking market' as you speak of has been using DI for years, but those are diesels, and they RUN off of detonation. Different animals entirely. Also, that's why GM officially calls this SIDI (spark-ignition direct injection), so as to differentiate it from diesels.

[spyderbuddy]

Originally Posted by ALLTRBO

I was typing that before your last two posts, referring to your much earlier post. There were more 'points' in this thread to be missed than just that one.

So, i see you were getting to it, and something i did not take into consideration is VVT. Sounds like you know what i was trying to express. That there is more going on with the V6 than just a octane boost

[spyderbuddy]

So, just I understand, 93 octane is not going to hurt my engine or systems. And 87 octane is not going to hurt anything either since the car was designed to run on this fuel. We know the gas mileage will not really change unless you change your driving habits. I haven't really seen a difference. But we could see a performace difference though it may just be because the timing is not backing off.