adamsocb

Recommended, not required

See Specs here:
http://www.c6registry.com

Does this mean those of us in 91 Octane States won't get 400 HP?

Scissors

It means you won't get 400 for torque. Whether or not you hit 400 HP depends on how much it retards timing.

Plus, the LS2 is underrated.

The LS6 has the same issue.


[Modified by Scissors, 9:46 PM 1/1/2004]

Low Flight

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.
The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together. It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

In general, automotive octane requirements are lower at high altitudes.
Therefore, in some areas of the United States, such as the Rocky Mountain States, the octane ratings for the gasoline grades may be 2 or more
octane points lower.

That was probably far more information than anyone wanted or needed, but some may find it interesting.

Miaugi

Hey Low Flight, thanks for the excellent write up! I learned something today! :thumbs:

rcolling

Compression Ratio 10.9:1

scorp508

Possible, yes, fact, not necessarily.

Rocketblock

Thanks Low Flight. That was very succinct and to the point. :thumbs:

BHP

Too high a CR, IMO. With the crappy gas out West, 91 Octane with additives, the LS2 will be in constant knock retard. On hot days an LS1 barely makes it.

2ZS

:thumbs:

larrysb

I think that's only part of the importance of octane, what you describe is pre-ignition.

There's another important factor, perhaps more important and that's knock. Detonation (knock, pinging) occurs when the cylinder charge ignites when it is supposed to with the spark, but the flame front turns into a rapid explosion rather than controlled burn. Detonation is very damaging to engine components and is what the computer is really trying to avoid when it retards timing. If you have pre-ignition, you can retard timing all day long and it won't help. Timing retard helps to avoid knock by reducing peak cylinder pressure and temperature.

Since the LS2 has greater compression, I suspect it will indeed be more sensitive to fuel quality than the LS1.

jinki

Low Fright and larrysb, thanks a million !! I learned something today :cheers:

WhiteDiamond

Some very good information on fuels has been posted and that is a large part of the equation, but not all of it. Depending on how GM has designed the CC and the piston can also help tremedously in running lower octane. If the General has left the CC more open and without possible hot spots, then one could run lower octane fuel at the same compression ratio.

I think GM needs to re-examine the fuel availability in the US, as I don't know that 93 octane is commonly available in most. I see 91 Octane just about everywhere I travel, but 93 only pops up here and there. If the octane truely turns out to have major impacts on the engine performance, I think a lot of owners will be burning up the service departments.....

Todd

Corvette_fetish

Not true. You explained it in a way I can totally understand. Appreciated!

yellow01

What concerns me the most about the C6 and I'm interested to see how it develops is the combination between lighter piston rings and higher compression.

On my C5 (547th off the line of 2001s) I had the lighter rings and oil consumption issues. I also developed a significant ping at high RPM. My gut feel is they are related (due to carbon left in the engine due to significan oil consumption). I ended up getting rid of it before fully investigating.

It's all conjecture, and not intended to be bashing, but I'm interested to see how the LS2 behaves as production starts and we see a few thousand in daily use.

C6-XTSY

Since GM says "recommended" 93 octane, I think that it really is designed for 87 or 91 octane. However, the engine software might have enough programming to make use of higher octane for even more power if 93 is used. If that's the case that would be really cool.

coolcat

One nice thing about living in the buckeye state. I get to put Sunoco 94 Ultra gasoline in my tank. Simply the best. :cheers:

Another Yellow

They changed the Sunoco Ultra to 93 octane here in NY about a month ago. :mad :cry

need-for-speed

While I agree that octane is the name for C8H18, I don't think it is correct that 87 octane means it's 87% C8H18. Gasoline is a very complex blend of hydrocarbons, oxygenates, etc. In fact, they tweak the mix for various climates. For cold weather they increase the lighter ends (C2's, C3's, and C4's) to improve volitility and vaporization. In hot weather, too many light ends in the mix can cause vapor lock. In addition, octane requirements decrease with altitude (you did say that). That's why in Denver, Super Unleaded will most likely be 91 octane (but no price break ;) ) As a sidenote, I had an Organic Chem prof who spent years working for Exxon and he told us that the additive that affecxts octane the most is 2,2,4 tri-methyl pentane. As you know the octane number posted on the gas pump is an average ((R+M) / 2) of the theoretical number based on composition, and the "M" , or motor method which is determined by a "knock" motor. A knock motor is a motor in which spark advance is increased (** edit** compression ratio is increased, not timing, thanks Duke / end edit **) until the point of detonation. That spark advance is correlated with octane content. The two numbers are averaged.

That peak HP is based on a motor running the recomended fuel. From there, it only get lower. Sorry.

Also, as stated above, pre-ignition is ignition before the timed event (spark). Retarding timing will not help but higher octane fuel will. Detonation is an uncontolled, or too fast burn. Reatrding the timing and increased octane does help.

p.s. I'm in the "methane" (natural gas) business myself.


[Modified by need-for-speed, 8:21 AM 2/5/2004]

Alex D

For all of those who fight knock retard:
Try 1 oz. of "Fuel Power" additive per 5 gallon of fuel and you won't have any more retard or knocking. PERIOD. I have been using it with great success in my tricky Harley (113 inch S&S engine, 10:1 compression, big hot engine)and now can run 89 or 90 octane fuel in my 2002 Suburban Z71 with the 92 octane HPP program loaded. Before Fuel Power I had to run 91 or better to avoid knock retard and severe pinging on WOT upshifts. Now I can run cheap gas and still have no pinging or knock retard.

I challenge everyone with LS Edit or other sofware to check this stuff out.
This stuff absolutely rocks and is not expensive!! it's been discussed at length at BITOG too.

Check it out: http://www.lubecontrol.com

SWCDuke

The octane rating of a gasoline is its detonation resistance compared to the percent iso-octane in a blend of iso-octane and normal heptane. Thus, 100 octane gasoline would have the same detonation resistance as pure iso-octane, and 90 octane gasoline would have the same detonation resistance as a 90/10 blend of iso-octane and normal heptane. The blend of iso-octane and normal heptane is a reference fuel. Actual commercial gasolines are mixtures of hundreds of different hydrocarbon species.

Detonation is measured on CFR (Combustion and Fuels Research) engines manufactured by Waukesha. They have a variable compression ratio, and the basic design hasn't changed much since the 1929s. All refineries have these engines and continuously sample finished gasoline product to ensure that it meets the octane specification.

There are two methods used to rate gasolines (except avgas, which uses slightly different procedures)- the Motor Method and Reseach Method. The Motor Method is more severe and incorporates higher load, coolant temperature, and inlet air temperature. Back when the government decided to require octane numbers to be posted, the bureaucrats couldn't figure out which method to use, so they decided to require the arithmetic average (R+M/2) to be posted. The difference between typical commercial blends, called the fuel's "sensitivity", is usually in the range of 8-10 points, so today's 93 is about equal to 97 RON, which is a little less than typical sixties vintage premiums which were about 99-100 RON.

Above 100 octane, the control blend is 100 percent iso-octane plus a measured concentration of tetraethyl lead (TEL), so a 101 octane gasoline is equivalent in detonation resistance to 100 percent iso-octane with one cc/gal TEL, 102 octane, 2 cc/gal, etc.

Knock sensors on modern engines allow the use of low octane fuels by retarding the ignition timing, but full power will not be available at all engine speeds, and it's more of an issue at low revs than high revs. Most people who use regular fuel in late model high compression engines where premium is the preferred fuel can't tell the difference in performance.

Duke