CTS 3.6L Fuel Octane
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Cadillac CTS First Generation Forum - 2003 - 2007 Discussion, CTS 3.6L Fuel Octane in Cadillac CTS Coupe, Sport Sedan and Sport Wagon Forums; Picked up my new 2004 CTS with the 3.6L engine and very much enjoy the car. Owned both BMW's and ...
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    tzoid is offline Cadillac Owners Member
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    CTS 3.6L Fuel Octane

    Picked up my new 2004 CTS with the 3.6L engine and very much enjoy the car. Owned both BMW's and M/B's and this car drives and handles very similar on most highways and roads. Cadillac really hit the mark with this design. The issue on fuel octane requirement has me puzzled, though. All of the advertising and marketing on this engine says 255hp, regular fuel engine. The owners manual has some suttle comment about how the engine will operate better with higher octane (89 to 93) fuel. Asked the dealer and even contacted Cadillac on the hot line....nothing but b/s answers like..."whatever the manual says". What's the REAL story? Either the car makes the 255hp with regular, 87 octane fuel, or it doesn't. Is Cadillac now saying you need to run hi-test fuel to get the 255hp? Don't talk about ECM's and knock sensors that measure fuel detonation and self adjust to make more hp if fuel octane level is higher, because that is not what has been marketed. This is SUPPOSED to be a 255hp, regular grade fuel, engine....what is it really? What grade of fuel do you REALLY have to run to get the 255hp? What grade of fuel is required to get the advertised mpg ratings? Come on Cadillac, tell us the FACTS, don't play with marketing hyperbole.

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    c5 rv's Avatar
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    Re: CTS 3.6L Fuel Octane

    87 for the 3.6, per GM. 91 for the 3.2.

    There was a good thread on this in the past you can search for. Here's a link to GM from the previous thread:

    http://media.gm.com/division/2004_pr...dillac_CTS.pdf

    Oh, and welcome!

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    Mbai2's Avatar
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    Re: CTS 3.6L Fuel Octane

    It says the gear ratio for the reverse and fifth gear are the same for the 5MT. That can't be right...

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    tzoid is offline Cadillac Owners Member
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    Re: CTS 3.6L Fuel Octane

    Appreciate your response c5, but the owners manual and the GM tech on the hotline BOTH reiterate the manual as being the design criteria..."for the 3.6L V6 engine use unleaded gasoline with a posted octane of 87 or higher. However, for best performance and trailor towing, you may wish to use middle grade or premium unleaded gasoline." That is a direct quote from the owner's manual. So, what is the octane fuel you really should use to get the 255hp that the engine is advertised to have ??? Come on GM, it's your turn at the podium....

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    Re: CTS 3.6L Fuel Octane

    I think you might be reading too much into this.

    The CTS is rated at 255hp with 87 octane fuel. As with just about ANY car, you will get better performance (though to me it's not THAT noticeable) from higher octane fuel.

    If you read the complete "Gasoline Octane" section on 5-5, it reads that the 3.6L is rated at the 87 octane requirement, and the 3.2 and 5.7 is rated at the 91 octane requirement. It even says on the latter two engines that performance will degrage if you use less than 91 octane.

    In case you were not aware, most states alow up to a 3% variance in octane reporting. So an 87 octane rating can be 85 or 86 octane and still be listed as 87 (but most good places are actually higher than 87).
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    tzoid is offline Cadillac Owners Member
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    Re: CTS 3.6L Fuel Octane

    Well, Miscreant, I guess you put me in my place with your mini "flame". So it's "obvious", huh? Let's see now....here's what I see is obvious. Onboard engine management systems, I believe, are sometimes capable of advancing and retarding timing depending on the octane level of the fuel being fed into the engine. If indeed the engine is designed to run on 87 octane, it would be fairly unlikely that the use of a 89, or 91-94 octane fuel, would cause the engine to generate much more than 5 or so more horsepower. Very difficult to feel in the normal "seat of the pants" tests that most owners use. Most manufacturers quite often tell you that the use of lower than specified octane (e.g. a mid grade designed engine being run on regular grade) will still work OK, but with less power. Conversely, cars designed to run on regular fuel will very often run poorer if you use mid or premium grade fuel. Yes, I know I'm anal on this subject, and I'll bet you've already figured that out. But, then again, I've owned quite a few cars over the years, many of which were the high performance variety from all 3 major U.S. manufacturers. I think reading the owner's manual is a very important step in operating a new car. It just seemed a bit strange to see the owner's manual worded the way it was regarding the fuel requirements...I'll take your word for it on the regular since your signature says your with GM and burn off into the sunset.

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    Re: CTS 3.6L Fuel Octane

    Quote Originally Posted by tzoid
    Well, Miscreant, I guess you put me in my place with your mini "flame". So it's "obvious", huh? Let's see now....here's what I see is obvious. Onboard engine management systems, I believe, are sometimes capable of advancing and retarding timing depending on the octane level of the fuel being fed into the engine. If indeed the engine is designed to run on 87 octane, it would be fairly unlikely that the use of a 89, or 91-94 octane fuel, would cause the engine to generate much more than 5 or so more horsepower. Very difficult to feel in the normal "seat of the pants" tests that most owners use. Most manufacturers quite often tell you that the use of lower than specified octane (e.g. a mid grade designed engine being run on regular grade) will still work OK, but with less power. Conversely, cars designed to run on regular fuel will very often run poorer if you use mid or premium grade fuel. Yes, I know I'm anal on this subject, and I'll bet you've already figured that out. But, then again, I've owned quite a few cars over the years, many of which were the high performance variety from all 3 major U.S. manufacturers. I think reading the owner's manual is a very important step in operating a new car. It just seemed a bit strange to see the owner's manual worded the way it was regarding the fuel requirements...I'll take your word for it on the regular since your signature says your with GM and burn off into the sunset.
    Oops. That was NOT intended to be a flame, and I have changed the wording so. I appologize. I completely understand your question.

    If this helps, heres a page for you:
    http://media.gm.com/division/2004_pr...cts/specs.html

    You'll notice that the 3.2L produces 220hp with 91 octane fuel, and the 3.6L produces 255hp with 87 octane fuel.

    I've tried 91+octane fuel, and only notice slightly better gas mileage, but unlike other cars I have done this with, the CTS does not show enough of a gain to make the higer price of 91+ octane fuel a benefit...
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    billsCTS is offline Cadillac Owners Member
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    Re: CTS 3.6L Fuel Octane

    Remember that octane ratings mean "at least" 87 is at least 87 but could be higher. Same with 89 and 91. If you average 15 gallons per fill up, the cost difference between regular and premium is about $3.00.

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    Re: CTS 3.6L Fuel Octane

    87 Octane fuel is recommended for the 3.6L from GM because it gives optimum flash point on your gas under most driving conditions. This means that the fuel will burn most efficiently and, ideally, all the fuel would be burnt... causing maximum power stroke. The Manufacturer's ratings are generally conservative and they will recommend a slightly higher octane than what is actually the most efficient ignition point for the engine... never lower.

    GM says the car produces 255Hp with 87 octane.... think about it ... would they put that into writing if it produced 260Hp with 91 octane? Not so much. In the real world, HP rating = money in their pockets.

    The whole octane system came about to deal with auto-ignition of unspent fuel (knock) due to using too low an octane. Older engines were not very flexible on octane rating... so different engine needed different octanes to prevent knock. Back in the day, higher performance engines with higher compression ratios needed the higher octane to prevent knock and create optimum power. Somehow, this wrongfully came to be interpreted as "more octane = more power"... it all depends on the engine.

    By changing the octane... you are not changing the quality of fuel going into the car... you are changing the when the fuel explodes (you are actually changing the timing of your engine). Any deviation from optimum only decreases engine performance and gas mileage over the long run.

    Any small deviations from rated octane due to manufacturing is really not an issue... modern engines compensate for this with engine management systems that can adjust and operate efficiently over a wider range of octanes... however, there is still only one optimum octane.

    For long term reliability of the engine... you should always use the recommended octane rating. Using higher octane doesn't hurt your engine... but you are waisting money on octane that your car isn't using effectively. Using lower octane is bad however... not only will this reduce performance and gas mileage... but can lead to knocking which can damage your pistons, valves, etc...

    So...

    Since the 3.6 is rated for 87... you can't put the "wrong" gas in it. The higher octane will not increase your performance... it will increase your gas card balance.

    The 3.2 is rated for 91... so you have to pretty much only put that in the engine. The effects of using low octane can be very bad.

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    silver bullet is offline Cadillac Owners Fanatic
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    Re: CTS 3.6L Fuel Octane

    My own unsientific test on the octane issue is that the engine deffinately puts out more power with 91 . But runs perfectly fine with 87. My test was same section of road t/c off in sport mode floor it lots of tire spin, with 91. same test with 87, results in much less wheel spin, ie less power.I think the 3.6 makes more like 275HP, with 91 octane. The engine computer takes care of the details. .

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    Jon's Avatar
    Jon
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    Re: CTS 3.6L Fuel Octane

    Temperature of your tires would have been a factor... Were you driving for teh same amount of time before your runs each time?

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    Re: CTS 3.6L Fuel Octane

    Somebody put it on the dyno

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    CTShead is offline Cadillac Owners Member
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    Re: CTS 3.6L Fuel Octane

    87 octane = 87% octane + 13% (heptane or simliar or ethanol)

    Octane is the longest hydrocarbon chain and the most expensive to refine
    Octane detonates at a higher pressure/temp than all the other shorter chains
    (and even better when you add tetraethyl lead to it)

    87 or 91 as long as they are composed of 100% hydrocarbon chains, they have the same amount of energy in each tank, but have different detonation pressures/temperatures.

    I too am curious if the engine management software is able to maintain the exact same HP even though it must alter the timing to account for different detonation points when using different octane ratings?




    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.

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    Re: CTS 3.6L Fuel Octane

    The limiting factor here is your engine's computer... it controls the engine management system and is engineered to run the engine at optimal conditions. It can adjust for things like altitude change, air intake temp change, octane, etc. Using a less than optimal octane causes it to advance or retard the timing to compensate for the change in flash point of the gas... small deviations will not cause noticeable changes in gas mileage or performance because the computer can handle the change and stay near optimum. Larger changes will lose efficiency. All this being said... realize:

    The can be only one! (optimum efficiency point)

    It is possible to tune to a different octane. You'd have to buy a good chip that is designed for it... then use a dynamometer to check Hp and exhaust gas analyzer to see if all the gas is being detonated.

    ------------------------

    Another thing to think about is the VVT. Everything in your engine was designed with a certain octane in mind... including the variable valve timing on the 3.6L. This engine is very sophisticated and it changes the intake and exhaust timing as the engine speed changes. They intake opens sooner and exhaust closes later as the speed increases... optimizing power and torque over a wide rpm range. If you mess with the octane you are messing with this also.... it's all in the timing.

    ------------------------

    In the past, I can understand having some doubts about GM engineering. I myself have previously had some bad experiences with a certain GM product... to remain nameless here.

    However, I really believe that GM did their homework on this one... What sold me on this car was the chassis and the 3.6L engine. It's really an incredible engine. I personally believe the smart way to go is to trust the engineers and stick the 87 octane.

    There will be someone who inevitably puts enough time, energy, money... into squeezing every possible Hp out of their engine... Meanwhile, I'll be enjoying my nice ride!

    My 2 cents...

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    CTShead is offline Cadillac Owners Member
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    Re: CTS 3.6L Fuel Octane

    Someone needs to get a Cadillac engineer/designer to start reading this board, then we can get some answers

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