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Use 93 Octane Or Not?

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93 Octane? The debate continues...

21621 Views 47 Replies 21 Participants Last post by  pL2014
I have been reading all of the internet, mostly on this forum, over the debate of filling up the Cruze with the highest octane fuel. I have had my 2012 Cruze LS 1.8L since December of 2011. Until the last two times I have filled up(past 3 years), I have used regular gasoline. But recently, I was told I need a fuel injection cleaning and told that if I simply put in a bottle of fuel cleaner as well as use Shell V-Power, that would do the trick.

I did do that and the two times I have done it, I have noticed much better performance out of my car. Not only better performance, but better millage too. I did read in the Cruze manual and it states about using at least regular unleaded fuel (87 Octane level). But the manual states you can use higher octane if you wish.

Everyone I have spoke to about this thinks, "it's a bad idea", "it's not good for your engine". Both people who know cars and others that have just had more experience with cars.

I don't see what can be the downside to using higher octane gas in the Cruze.

What are your thoughts? I am stuck with what to believe.
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I use 91 in Denver and 93 where available. I have tested 91 in my son's LS MT and there is a definite improvement in drivability, low end power, and fuel economy.
Unless specifically called out for your car, don't waste your money on higher octane fuel. Octane is merely a measure of a fuel's resistance to autoignite (i.e. knock). It has nothing whatsoever to do with the energy content and will not provide more MPG in and by itself.
And it begins again...

Yes, you are correct that octane is a measure of resistance to premature detonation. However, the Cruze's engines (definitely the 1.4T and very likely the 1.8) were designed for 91 octane. GM's marketing department realized that as an economy car the Cruze wouldn't sell in the US if it required premium gas. So GM's engineers designed and implemented a system to monitor the engine for predetonation and pull the ignition timing to protect the engine from knock when driving on regular (87 octane) gasoline. There are two ignition maps in the ECU, one designed for premium and one for regular. This works, but anytime the engine is under load the ECU will pull timing to protect the engine from knock. It does so very well but at the expense of optimal ignition timing for fuel burn. You feel this as a pulsing in the throttle.

The side effect of pulling the ignition timing to protect the engine from knock is a noticeable reduction in power and fuel economy. Using a higher octane fuel eliminates the need for the ignition timing adjustments with the attendant reduction in power and fuel economy. The real question that each driver needs to answer for themselves is "Is the additional price at the pump more than made up for by increased fuel economy and drivability?" For a long term/distance driving review of the Chevy Cruze running regular vs. premium take a look at The Ultimate Hot Weather MPG Test - Regular vs. Premium - 2011 Chevrolet Cruze LTZ Long-Term Road Test.
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The funny part is the post is just below my post.
You mean the one showing the RPO code for 91 octane. :)
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Thanks for the replies, guys. But, did you read my post? After getting this rep statement, I think not:

"You have no idea what you are talking about. In this application, it makes a difference. Do your research before you post."

My post said absolutely nothing about ignition timing. If 91 RON indeed does produce better MPG than 87 RON, then certainly ignition timing might explain it. But, please read my post again before whoever you are send me such nonsense rep lines. And, as background, I started as a mechanical engineer working for Ethyl Corp (experts in spark knock) at their research facility in Ferndale, Michigan, and there is no power to be had from octane in and by itself. Ethyl's R&D facility is long gone now, but octane still by itself does not produce more power.
Yes, I read your post. However, unless you understand how the Cruze's ECU manages the fuel ignition there is no way to understand why the Cruze can safely run on 87 octane despite the engine being designed for and carries the RPO code for 91 octane. There are two ways to reduce/prevent knock (premature fuel detonation) - one is to run higher octane - the other is to change the timing of the spark in the cylinders to prevent excess fuel from accumulating. GM's engineers appear to have detuned our engines in addition to adding serious knock sensors to give the ECU immediate feedback so it can protect the engine from too low of octane.

As with all newer cars there is a lot more engineering in the Cruze than just mechanical. The last estimate I saw was that the average new car has nearly 10 million lines of computer code.

Just as an FYI, the Cruze will run on 85 octane here in Denver but it really stutters at the throttle and has zero passing power. This is exactly what the ECU is programmed to do to protect the car from knock.
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So you are saying 89 doesn't even make the grade? I found 91 overkill and at 40 cents a gallon extra. 89 is 20 cents extra and works just fine? Perhaps the key is to have a new engine like mine with 3K before the system gets gunked up, like every computer I have owned? I don't trust Shell after what they did to Florida and Canada about 10 years ago. Interesting GM twist too?

Toronto Star: Shell shells out for tainted gas: Canadians denied help for problem

Jun. 4, 2004.
Sulphur harms gas gauges in Florida
Oil giant Shell is reimbursing motorists in Florida for fuel gauge repairs made necessary by contaminated gasoline, something its Canadian subsidiary wouldn’t do here last year.
An official for Shell Oil’s refiner in the United States has confirmed it will start sending cheques today to customers who complained about faulty gauges after buying gas containing corrosive elemental sulphur from the company and Texaco.
Johan Zaayman, a spokesperson for refiner Motiva Enterprises, said yesterday Shell has received more than 9,000 claims for the cost of repairs in less than a week from customers in Florida and nearby states after the company acknowledged a problem.
“We’re accepting claims from customers to get reimbursed for the cost of the repairs of the gas gauges,” Zaayman said in a telephone interview from Motiva’s offices in Houston.
“The cheques will begin to go out tomorrow (Friday),” Zaayman said.
In the last week, hundreds of Shell, Chevron and Texaco service stations have sold gas with traces of elemental sulphur, which quickly corrodes the contacts on sending units in tanks and, in turn, triggers faulty gauge readings.
Motorists have run out of gas and stalled when fuel gauges read full but tanks were actually empty.
Like Shell, Chevron says it has set up a special customer hotline and will cover the cost of repairs. Costs could range from $300 to $600 (U.S.), dealers say.
The reaction by Shell in the United States differs from how the company handled customers in the Toronto region last spring and summer.
A Shell official in Canada said yesterday the company handled customers appropriately here by sending them to their dealers for reimbursement of expenses.
“When we learned of the problem, we co-operated with our mutual partners (the auto industry) to understand it,” said Simone Marler, manager of public affairs for Shell Canada Products.
“We then added an ingredient to our gasoline. We directed our customers to their dealers for assistance. Our goal was to make sure our mutual customers were looked after. And they were. The issue has been resolved.”
Marler said she was unaware of the circumstances in the U.S. incident.
Unsuspecting motorists bought gas tainted with elemental sulphur at Toronto-area Shell and Petro-Canada service stations in the spring and summer of 2003. They got inaccurate gauge readings and stalling, a safety hazard on busy roads.
After a lengthy investigation, Shell and Petro-Canada concluded months later their gas contained elemental sulphur.
The two oil companies denied responsibility for the faulty fuel gauges. They stressed their products exceeded industry and government standards.
At one point, a Petro-Canada official said General Motors was responsible because it used an inferior part, but later the oil company said it wasn’t sure what the problem was.
Shell sent customers back to their dealers, primarily GM, for reimbursement of repairs. But many customers expressed dissatisfaction with their treatment by dealers and GM’s refusal to pay for repairs.
A Star investigation showed that GM had encountered the problem in 2000 in parts of Canada and the United States.
GM sent a notice to customers warning them about the problem and corrective action but it did not issue a recall.
It redesigned the fuel sensors so they could tolerate elemental sulphur. However, the company only started using the new sensors in models last year. GM confirmed it accelerated the installation of the redesigned part in more models late last year.
The investigation found GM notified dealers on how to handle customers in last year’s incident, but the company never warned customers.
Earlier this year, GM offered a free oil change and a Petro-Canada cash card worth a total of about $55 to customers who had gauge problems.
In Tampa yesterday, Skip Greaney, fixed operations manager at Gordon Chevrolet, said the number of repairs of fuel gauges in his shop shot up to 42 in May and 10 to 12 in the first two days of this month. There were only nine similar repairs in the first four months of the year, he added. “There’s been a huge spike,” he said.
Meanwhile, two women from Fort Lauderdale, Fla., who say the sulphur in the gasoline damaged their cars, have filed a lawsuit against Shell Oil Co. and Motiva, alleging the companies violated state law by engaging in unfair and deceptive trade practices. Shell owns half of Motiva.
The lawsuit says the companies “knew or reasonably should have known” that the gasoline with the sulphur would have damaged the vehicles but they continued selling the fuel to motorists.
First, that article is 10 years old. Second, it really highlights the differences in customer service in the US vs. Canada, one that we still see today with Chevy Dealerships.
So you are saying 89 doesn't even make the grade? I found 91 overkill and at 40 cents a gallon extra. 89 is 20 cents extra and works just fine?
My recommendation to everyone is as follows:

If you notice throttle pulsing and/or crappy acceleration in the heat, move up to 89 octane. Run this for two tanks to get a good feel. Then repeat at 91 or higher octane. Now that you have a butt/foot dyno feel for how your Cruze behaves on 87, 89, and 91(+) pick the octane that gives you the best performance vs. cost.

If you are running any performance tune from any tuner, run 91 or higher as that is what the tune will require. GM "detuned" our engines via ECU programming as part of the changes to allow an engine designed for 91 octane to safely run on 87. Aftermarket performance tunes undo this change.
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At the opposite end of the spectrum from pL2014 is my experience. The first time I drove up I-70 west of Denver I was running 87 octane. Even on Cruze control the car was bucking and stuttering to the point that I was concerned I had a clutch and/or transmission problem. The same drive on 91 octane is smooth as glass when it comes to the transmission. No bucking, stuttering, etc. This stretch of road is hard on most vehicles and you always see at least one on the shoulder dead from exhaustion. Here's the profile (US then Metric):

Text Line Slope Rectangle Pattern
Text Line Slope Rectangle Parallel

This is from the C-470/I-70 interchange (the initial drop is coming down off C-470) just west of Denver and south of Golden to Floyd Hill west of Genesee Park, Colorado. Most cars cannot maintain the posted 65 MPH up this hill. pL2014's driving environment has predominately low engine loads so the higher octane, while improving the car's performance, doesn't appear to improve it sufficiently to justify the extra pump cost. A high load environment (the graph above is an extremely high load) can easily justify the higher pump cost in terms of drivability and fuel economy. Thus my recommendation on how to determine the price/performance sweet spot for each driver.
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