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Gasoline Grade For Rx400h


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I know we've discussed about this topic before, but it keeps coming up from other users of whether we should put in premium gasoline (91, 93) or regular/regular plus (87, 89). I certainly don't want to save $.20 a gallon and taking chances of potential engine damage on my $50K hybrid, but then I don't want to throw my money away for no reason by buying premium gas when regular gas is just fine.

According to Comsumer Report: 2008 RX400h gasoline requirement: regular. RX350 gasoline requirement: regular or premium. Motor Trend however, said RX400h requires premium. What you do think? Does only the 2008 and beyond model of 400h can take regular, and all other RX400h models before 2008 have to take premium? What do you currently put in for your RX400h/350?

FYI: My manual said it requires premium, but the service manager at my Lexus dealer told me it can takes regular without any damage to the engine, just reduce in performance.

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Carefully read your manual. It should say that while regular (87 octane) is acceptable, the RX400h will provide higher performance if higher octane grades are used. That being the case, you should determine what is more important to you - having a bit more power when you slam on the gas or spending less on fuel. Incidently, according to many here, using premium will not guarantee higher mileage.

I use mid-grade during the Summer months and regular when the weather cools down. Of course, if I should take a trip to Las Vegas or Arizona, I'd use premium during the time I was there.

Dave

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I think it's silly (for me anyway) to pay a 5% premium for what probably amounts to a sctual 0.1% cost increase in fuel additives.

As our 400h's aren't operating in any exotic compression range (I don't think) I wouldn't expect any performance increase - or certinly not enough to warrent the cost.

If you're having a problem in the summer now with the ICE not cutting off quickly and traling off for a few fractions of a second, then the higher octane should help with this -but again, it hardly seems worth the price. You can probably use the "cheap" :) stuff and use an octance/cleaning additive every so often.

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I think it's silly (for me anyway) to pay a 5% premium for what probably amounts to a sctual 0.1% cost increase in fuel additives.

As our 400h's aren't operating in any exotic compression range (I don't think) I wouldn't expect any performance increase - or certinly not enough to warrent the cost.

If you're having a problem in the summer now with the ICE not cutting off quickly and traling off for a few fractions of a second, then the higher octane should help with this -but again, it hardly seems worth the price. You can probably use the "cheap" :) stuff and use an octance/cleaning additive every so often.

Wow, been down this road before with the people on the Acura MDX forum.

First let me say I am a retired mechanical engineer who used to work for an engine builder. Actually Cooper Bessemer. In my early days I worked in research and development as a technician while I was attending college on a co-op plan.

Anyway to the point, we tested engines running in a steady state enviroment, constant speed and load for periods of time and we adjusted the timing to get optimum performance, ie fuel effeciency for the load. The engines I worked on were very large, the type used in electrical power generation and on compressor sites to move natural gas in pipelines. Typical engine configurations were as large as 12, 16 and even 20 cylinders and up to a maximum of 16,500 HP.

These engines ran very low RPM and piston slap could be easily discerned.

We did a lot of work on cylinder/valve configurations in order to achieve total

combustion. As you might imagine fuel efficiency is very important on a 16,500 HP engine running all day long 24/7 pumping natural gas from Texas to Ohio for example. The companies like Columbia Gas Transmission want the best efficiency they can get.

A little more background here and then to my point. The 2006 RX manual did not suggest you must run 91 Octane premium, also the 2006 manual recommended 5w-30 weight oil; but the 2008 manual says 91 octane premium

is required, and they also changed the oil requirements to 5w-20 and as an alternate 0w-20. ok so what design changes to the engine have occurred, I can only guess. I could look at compression ratios ect and speculate but who knows for sure.

Anyway back to the R & D testing and results:

1. Engines run most efficiently at the proper timing setting, retarding timing for any reason not only reduces available HP, it also reduces fuel efficiency even if you are not needing the extra optimal HP available at the optimal timing.

We could always see the impact of less than optimum timing on fuel effeciency

2.In a large engine like I am talking about, piston slap really sounds terrible, when the engine knock occurs, its a loud and ugly event; but in a small car engine it isn't so loud and to boot it is deadened by other sounds, it is however just as damaging.

3.It's all in the design, how the engine is meant to be run, the compression ratio and the cylinder head design along with valve placement configurations. The gas flame front will advance differently and impact both maximum energy output and fuel efficiency, no matter what the load the design parameters of the engine cylinder, proper timing and yes optimum fuel based on the design/ implemented design goals.

Now having said all that, we need to think about how the hybrid ICE setup works, yes it runs at in a more constant RPM range with a variable load. Also like others have said driving for best fuel economy, we aren't pushing the engine into the extremes in needed HP, but the interface of the ICE and how load is applied by the hybrid system is vastly different from a conventional gas engine only setup. Ping is of course going to be more noticeable and more prevalent when power demand varies more with a regular transmission, but that does not mean that the hybrid setup with a CVT doesn't encounter load variations that would cause piston slap-ping if it weren't for the electronic

countermeasures retarding the timing as needed.

Another point as I understand it, the electronic timing anti knock system is designed to reset the timing curve over a period of time, like when you switch

gas from one tank to the next, instantaneous timing adjustments do occur, but the whole timing curve is only gradually adjusted as you burn thru a change in octane of a new tank of fuel. I am not a Lexus engineer so I am only speculating based on rumors I have heard.

Also I will state now I own a new 2008 RX 400h and I use 91 octane premium, with $4 a gallon gas $0.20 is only a 5% premium and 5% of 27 mpg is only a 1.3 MPG difference and you break even. Plus I have the added comfort of knowing I can tromp on it if I want to,I can go up steep hills without worry, I can encounter really hot summer days with the AC running and indirectly loading the engine for needed electrical power ect. However, the best part is I can go the the Lexus dealer in the case of a warranty related failure and say I followed the recommended fuel requirement and my warranty should be good

if needed.

I know way too long, to summarize though, it's your $49,000/ $50,000 car do what you want to, my 08 RX 400h calls for premium required in the manual and it's only 5% more at todays prices and I am a mechanical engineer with engine testing experience, but I still cant totally evaluate how necessary it is or the total impact of not using premium. So I will error on the side of caution following the factory recommendations and enjoy the ride when I might otherwise worry. If I want to feel the power I can and I can climb a steep hills without having to reduce speed going up it without concern.

I am older and not a heavy footed driver, I drive for economy most of the

time. I still however find times when I want to use my car close to it's design potential.

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What we need are some dynamometer tests on our RX400h's to determine the effect of various oil viscosities and fuel octane ratings. I suppose that this is unlikely, considering the type of vehicle.

Tom

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i agree with mind machine's post that the premium fuel does provide enough extra mpg, making the extra cost covered by the higher mpg with the premium. also there is a small amout of extra power with premium. i have experimented with both octane levels and still use the premium. Ironically the higher octane makes the 400h cut in and out MORE abruptly than with regular gas. I noticed this right away and others confirmed it when i posted it online at that time.

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Nice write-up by mindmachine. Wonder how he came up with that name? :lol: ;)

We use 93 octane in our 2008 400h. I also use it in my 2001 BMW 330Cic, it's probably even more critical in that car which gets a lot of performance from a 3.3 liter six.

Even at today's gas prices, the price difference isn't THAT great. I just figure it's better for the engine. One relatively minor repair could negate the savings garnered in 100k miles.

We've averaged better than 27 mpg with my light-footed wife doing most of the driving. That's pretty darn good for a 4000+ pound SUV that has plenty of power! I'm always impressed when I stomp on the 400h, it delivers the goods.

We'll continue to use 93 octane until someone provides tangible evidence that it's a waste of money.

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I'd like to again mention that ambient temperature plays a large part in how an engine performs with a given grade of gasolene. For example, driving almost any vehicle when the ambient temperature is say, 35F and you will notice a significant difference in power output compared to driving the same vehicle on a 95F day. To some extent, the same analogy can be presented when it comes to the viscosity of oil that is used.

Consumer Reports often advises consumers that they can save money by using regular fuel IF the vehicle's owner's manual states that this is acceptable. This is especially true if the driver does not often "push" the vehicle and if normal ambient temps are not very high.

I have to laugh every time I read that the tiny Smartfortwo requires premium gas. For the same amount of money, anyone can buy a Yaris that has a backseat, proven reliability, and runs on regular gas - negating any small advantage in fuel economy the the Smartfortwo offers.

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"i agree with mind machine's post that the premium fuel does provide enough extra mpg, making the extra cost covered by the higher mpg with the premium. "

I have never seen any written evidence besides someones opinion on an internet forum to prove better mileage with higher octane gas. I would be interested in reading the studies that prove this. I remember a while back reading an article on the Toyota Hypermilers forum that showed the exact opposite.

I certainly see no evidence of this myself. My mileage over the last 30 days is 31.3 mpg on 87 octane. I have certainly not done better on the tanks of 91 octane I have tried.

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There was an old saying I remember from what my father told me when I was a kid. A guy goes to a Caddy showroom and asks the salesman how many MPG the Caddy gets. The salesman says something like people that can afford a Caddy don't worry about MPG. And now 40~45 years later I hear a similar thing from Lexus owners

I don't understand why people that drive a Lexus are worried about a ten to twenty cent differential between grades of gas. I consider myself to be cheap. When I moved to SC I searched for the stations with the cheapest gas price [i did the same thing when I lived in TX and NY] and buy my gas there.. I bought gas yesterday and the price was Reg 3.79, med 3.89 and super for 3.99. So every time you buy 10 gallons it only comes out to $2 more between super and regular. To me $2 isn't a big deal and I bet most people leave more than that as a tip when they go out to eat.

When I lived in NY I saved every soda can and returned it to get my nickel back. I try to be thrifty and use no-name brand gas but it is always 93 octane

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Many years ago, premium fuel was indeed special in the sense that it did have ingredients that ensured the engine was better "cleaned". However, today, you'd be hard-pressed to be able to prove that an engine that uses premium lasts longer or requires fewer fuel filter changes.

Many of you may have read articles about habits of the very rich - especially those who worked very hard to make it "big". Almost all question every sales pitch with suspicion and subsequently, do not buy something just because it has a "name" and is supposed to be better. The common misconception about the rich is that they frivolously buy anything without much thought. I've found that the exact opposite is true. To this day, I tell people who drive their Corvettes very leisurely that almost any tires will do. But if the owner is someone who pushes the car to its limit, paying more for tires can be very advantageous.

I think the same can be said about fuel. If your owner's manual states that using regular fuel is acceptable, and you aren't the type of person who occasionally races X5s, then spending an extra 2 or 3 dollars may be equivelent to throwing it away - something few of the hard-working rich would dare do.

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I've tried to calculate my MPG for both 91 and 87 octane, and found no differences. Any prove out there that premium gas gets better mileage? Please post.

Also, my manual said premium gas required, it didn't say anywhere about regular as an option. I've looked at it really careful. But I agree with some of you, saving $2-3 per fill is not much, but why throw away that money when I can I spend it where else, unless...of course...it proves to benefit me.

Anyways, anybody knows anything about Top Tier Gasoline? I have read a lot about it and it looks like they added extra detergine/addictive to the gasoline, maybe it's worth trying? Maybe that is more important in a gasoline than the grade of the gasoline?

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I've tried to calculate my MPG for both 91 and 87 octane, and found no differences. Any prove out there that premium gas gets better mileage? Please post.

Also, my manual said premium gas required, it didn't say anywhere about regular as an option. I've looked at it really careful. But I agree with some of you, saving $2-3 per fill is not much, but why throw away that money when I can I spend it where else, unless...of course...it proves to benefit me.

Anyways, anybody knows anything about Top Tier Gasoline? I have read a lot about it and it looks like they added extra detergine/addictive to the gasoline, maybe it's worth trying? Maybe that is more important in a gasoline than the grade of the gasoline?

My personal feeling is it is more important to me to buy 93 grade than it is to buy brand name gas. I can get 93 at stopaminit gas for less than it would cost me to buy 87 grade down the road at Shell or another name brand. I honestly don't feel as if I am paying more if I buy no name 93 for less than brand name 87. But I guess a person could make the argument save even more and to buy no name 87 grade

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"i agree with mind machine's post that the premium fuel does provide enough extra mpg, making the extra cost covered by the higher mpg with the premium. "

I have never seen any written evidence besides someones opinion on an internet forum to prove better mileage with higher octane gas. I would be interested in reading the studies that prove this. I remember a while back reading an article on the Toyota Hypermilers forum that showed the exact opposite.

I certainly see no evidence of this myself. My mileage over the last 30 days is 31.3 mpg on 87 octane. I have certainly not done better on the tanks of 91 octane I have tried.

it takes about three tanks of premium to notice the mpg improvement, the power improvement you notice right away.

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Guess I'll weigh in here. I use Premium simply because my manual states it's preferred over regular. I believe the manual puts it that way so that if you're in an area where there is no premium, you can feel free to fill up with regular. Ultimately, it really boils down to why Lexus wrote it that way and whether or not you choose to believe them. In my working life, we follow the operating manual "to the letter". Our engines can burn a variety of different fuels but some are preferred and that's what we use if at all possible.

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I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

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Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

I agree 100% on the Ethanol, back in NY we didn't get much choice and we wound up with a summer an winter blend, if you bothered to read the sign on NY pumps it clearly stated they increased the Ethanol for cleaner air but by doing so it would lower your MPGs. It didn't matter because we had no choice, all the pumps were mandated to add Ethanol with the seasonal change. I didn't notice any signs on the pumps when I lived in TX or where I now live in SC. so I assume there is no Ethanol [or a much lower percent] in the gas here except for the E-85 they sell. E-85 is a lot cheaper, I bet some people use it without knowing the car has to be designed for it

I guess the fact that some states require Ethanol and some don't could account for some people getting better MPG readings than others [assuming they both have the same driving habits]

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I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

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I have used regular 87 Octane in my 2006 RX400h for two years. for the last 12 months my MPG average is 27.6 (I do not reset so that I can see winter/summer averages over time, but the dealer reset it about a year ago). Using 87 octane I usually do about 8% better in the summer than in the winter for mileage. I tried higher octane for several tanks across the same weekly driving habits and did not get any improvement worth the extra $.20 per gallon. The manual says minimum 87 octane or higher so I am comfortable that there is no extra engine wear due to knock. I have good driving habits. My '99 RX300 averaged 21.0 MPG. I am enjoying the gas savings.

I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

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I have used regular 87 Octane in my 2006 RX400h for two years. for the last 12 months my MPG average is 27.6 (I do not reset so that I can see winter/summer averages over time, but the dealer reset it about a year ago). Using 87 octane I usually do about 8% better in the summer than in the winter for mileage. I tried higher octane for several tanks across the same weekly driving habits and did not get any improvement worth the extra $.20 per gallon. The manual says minimum 87 octane or higher so I am comfortable that there is no extra engine wear due to knock. I have good driving habits. My '99 RX300 averaged 21.0 MPG. I am enjoying the gas savings.
I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

If you don't have x-ray vision you won't see the dammage till it is too late.

This is what can happen to a piston

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A couple of additional thoughts....

From what I've read, the RX400h engine compression ratio has been lowered, since the electric "boost" is substantial enough so that the ICE doesn't need to be as powerful as the pre-hybrid engine. That being the case, the requirement for premium fuel is not nearly as urgent - hence our (2006) manual's statement that regular fuel is acceptable.

Also, the engine's computer has the ability to make changes (due to differences in fuel octane) very, very quickly. I have a 1989 Mazda MX6 GT Turbo that can use any grade of gasolene from 87-93 (according to the owner's manual). I have used premium and midgrade for more than 11 years now (more than half of those years using mid-grade) and the engine is still passing all smog tests with flying colors (low HCs). As far as I can tell, the engine will suffer no damage that is even remotely discernable during the life of the vehicle. That tends to be proof enough to me that if a 1989 engine can use varying grades of fuel without issue, in all likelyhood, so can my 2006 RX400h.

Dave

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The manual says use 87 octane or higher

Which is why I stated:

If the owner's manual says to use a higher octane, then listen to the Good Book.

I am not familiar with the requirements on a 2006 RX400, but if you say it recommends 87 octane, then your ok, as using a higher octane then what is required is useless also. Only engines that need higher octane, or have knock issues, need to use higher octane.

As for the fuel mileage, I would attribute the difference in MPG's to the way you drive during the summer vs. the way you drive in the winter as the major contributing factor.

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A couple of additional thoughts....

From what I've read, the RX400h engine compression ratio has been lowered, since the electric "boost" is substantial enough so that the ICE doesn't need to be as powerful as the pre-hybrid engine. That being the case, the requirement for premium fuel is not nearly as urgent - hence our (2006) manual's statement that regular fuel is acceptable.

Also, the engine's computer has the ability to make changes (due to differences in fuel octane) very, very quickly. I have a 1989 Mazda MX6 GT Turbo that can use any grade of gasolene from 87-93 (according to the owner's manual). I have used premium and midgrade for more than 11 years now (more than half of those years using mid-grade) and the engine is still passing all smog tests with flying colors (low HCs). As far as I can tell, the engine will suffer no damage that is even remotely discernable during the life of the vehicle. That tends to be proof enough to me that if a 1989 engine can use varying grades of fuel without issue, in all likelyhood, so can my 2006 RX400h.

Dave

This is a completely different scenario though. Mazda has been "barrowing" technology from Ford for many years now. And truth be said, the adaptability of the Ford ECU programs are much greater than Toyota, Honda and Nissan. I should include BMW and a few others, but to keep this short, the biggest "tuner" obstacle has allways been the ECU fuel mapping parameters in Toyotas and other Japanese mfgr's. I have a hand held "tuner" for Ford and Mazda made by SCT. ( Superchips, Diablosport, Hypertech and a few others also exsist) via my computer at home, I can datalog, and make timing alterations, A/F adjustments and even control my idle speed directly. In that one unit I can have a high MPG setting, a normal setting, and a performance setting. By plugging directly to the ODB II and pumping that info right into the stock ECU. Non of that is available for any Toyota, Nissan or Honda that I am aware of unless the ECU has been replaced by an aftermarket system such as AEM, Haltech or a few others that exsist. I wouldn't say that Mazda, Ford , or anyone else is more advanced or higher tech, I think it has more to do with the philosophy behind it.

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I just noticed that I didn't address the other portion opf your post though. You are correct in that the stock ECU does change or adapt, but only by so much. The issue is that the lower octane could still ignite before the spark plug ignites it, which would still defeat any timing alterations, therefore preignition would still be occuring. The adjustments have more to do with temputures and maintaining Stoic A/F mixtures to maintain the cats. Remember, no amount of timing adjustment will ever create spark before TDC occurs.

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