As per the comment above regarding energy content (btu) of higher octane fuel, paste from Wiki

Octane ratings are not indicators of the energy content of fuels. (See section 4 of this page and heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.

....end...

So an engine is designed to operate at a certain compression ratio... it is tuned based on a given Octane of fuel, which higher octanes combust at higher pressures. The fuel type determines the compression pressure for the engine. So higher octane fuel in engines with higher compression ratios can extract a higher % of the fuels nrg content due to less pre combustion of the fuel...however, its my guess, as these computers and sensors have gotten better, the lower octane fuels can now be combusted at a very high efficiency.

Of course, with todays computers, using lower octane fuel vs. engine design requirements.... the computer can tweak some of the engines parameters accordingly ...but it makes perfect sense, that using lower octane fuel in an an engine designed for higher octane fuel can generate unpredictable losses in fuel economy, as fuel economy now prob. now depends on so many other variables, such as the computer program, reliability of sensors, sensor reaction times, temperatures, pressures, how low the octane is, etc.

Reading more about this.... a lower than suggested octane fuel causes auto igniting, or, pre ignition (before it' designed to combust)....this is due to the higher compression the engine is operating under, vs. the pressure combustion point of the fuel... this pre combustion is a loss of nrg, and can react the stroke cycle of the engine, ..... a potential double whammy as it relates to nrg output. (of course, during pre combustion events only, not the entire tank of gas)

I think most people assume lower octane fuel in a Prem. fuel car is still a better deal, as the higher octane fuel cost more...but it only takes a 15-20% loss of fuel economy to easily pay for the added fuel cost. For those with poor fuel economy, who are using lower octane fuel vs. manufacturers recommendation, try a few tanks of premium, see if the added octane raises fuel economy enough to change your mind. It is also easier and safer on the engine... mis firing produces counter forces on the stroke cycle, slowly stressing all the stroke components....

I read on another thread, the compression ratio of the 2010 RX's is such, where its not soooo dependent on Premium fuel....hence why so many owners don't notice a big difference...

What year did the RX no longer require Prem. fuel? 2012 ?

 

^ you got it backwards.
WE have been using premium for decades. But when gasoline prices hit the $5 mark here in California, WE did our research, talked to some mechanics and decided to try Regular unleaded (87) instead of the premium (93).
Objectively speaking, we didn't notice any drop in mpg.

I bet my zero to sixty is about half a second slower than most people who still uses premium.
But who drives like that everyday?

 

Mark, it was my understanding (I am NO RX expert by any means, my first RX).... in the past, RX350's required Prem. fuel. Then recently, I think read somewhere, 2012, Prem. fuel was no longer required as per Lexus. Do I have this wrong? OH, BTW, should have clarified, I was referring to Non-Hybrids only. ??

 

Your right about the premium requirement for 2011's and older models of the RX 350/330.
There was no change in the engine.
However, there's a decrease in the "claimed" horsepower.
If i remember the sticker correctly, my 2011 promises 275 hp on premium fuel while your 2012/2013 have 270hp on regular unleaded. Again, it's still the same engine.

I'm pretty sure some tweaking were made to accomodate the change in fuel.

Anyway, i don't really pay attention to the horsepower, im more interested with the torque which is a better gauge of power. And how's the torque at 2,000, 3,000 or even 4,000 rpm. But this deserves a different thread and another day of discussion.

 

just got back from a little trip, the best i got was 26.3 mpg on dash readout, that was running 70 - 80 mph, fil it up here at home and it read 376 crusing range, held right at 18 galon. i was happy

 

Thx Mark, interesting information. It makes sense, same engine de-tuned, a few less hp, but lower cost fuel, a wise formula for better sales. The RX is not a performance car.... cant imagine one someone not buying it due to 5hp loss from prior year. :-)

In this current era of 4cyl options for 4k lb cars, huge premiums for Diesels, Hybrids, etc, I think Lexus did well with this small 6cyl for the past 5 or so years..... good power, good (not great) fuel economy. My bet is, the next generation in 2015? will at least "offer", if not come standard, with a super charged 4 cyl to raise fuel economy for the next decade. As Lexus is slow to change anything.
BTW, is the RX350 a copy of a Toyota engine? Venza?

 

Pretty much, the RX350/450 use the Toyota 2GR 3.5L engine. While this line of engines has several variants and tweaks done per model, it is a very common line of engines used in many vehicles, Venza, Camry, Sienna, Highlander, RAV4, etc.

 

Toyota has been very reluctant to use F/I. They have valid reasons - more stress on components equals reduced reliability. For a company that has a reputation for building reliable people-movers, it is not something they take lightly.
I don't know of any current gas Toyota vehicle that is not N/A, particularly in NA region (pun intended).
Even when Subaru built Scion FR-S using their own engine, Toyota engineers insisted to share D-4S direct and port injection technology instead of turbo-charging, which Subaru is so familiar with.
I always say, Toyota is a very conservative company. They get something to work and they stick with it for a long time.
My guess is going to be them introducing direct injection to the 2GR-FE engine family across the board.

Here is the list of vehicles using this engine (from Wikipedia):

2005 Toyota Avalon (GSX30)
2006 Toyota Aurion (GSV40)
2006 Toyota RAV4 (GSA33/38)
2006 Toyota Estima (GSR50/55)
2006 Toyota Previa (GSR50)
2006 Toyota Tarago (GSR50)
2006 Toyota Sienna (GSL20/23/25)
2007 Toyota Camry (GSV40)
2007 Lexus ES 350 (GSV40)
2007 Lexus RX 350 (GSU30/35)
2007 Toyota Harrier (GSU30/31/35/36)
2007 Toyota Vanguard (GSA33)
2009 Lexus RX 350 (GGL10/15/16)
2007 Toyota Kluger (GSU40/45)
2007 Toyota Blade (GRE156)
2007 Toyota Mark X Zio (GGA10)
2008 Toyota Alphard (GGH20/25)
2008 Toyota Vellfire (GGH20/25)
2008 Toyota Highlander (GSU40/45)
2009 Toyota Venza (GGV10/15)
2009 Lotus Evora (280ps & 350Nm using Lotus engine management, Sport Pack package redline increased to 7000rpm)
Toyota Corolla (E140/E150) (for Super GT use)

 

Interesting Vlad.... makes sense, NA engines have less stress, which increases reliability over their lifespan....but would you agree, prob. not much difference in reliability in the early life cycle of the car, say first 5 yrs or 60k mileage range?

BTW, this goes back to one of the laws of nature...there is no free lunch out there.. you want better fuel economy, (for a given weight of course), you pay more for repairs over the vehicles life, prob. throwing back any savings in fuel economy into repairs. Of course, maybe its the 2nd or 3rd owner taking the hit, whereas the first owner got the benefits of the improved fuel economy with no repair costs. thoughts?

 

I read somewhere that Toyota estimates their engines to last 200K miles. That puts certain limitations on designs, such as F/I and redline limiter and more aggressive tuning.

That is correct. Case and point - direct injection (certain mfgs). First owner reaps the benefits and the 2nd is stuck with expensive valve cleanings from carbon deposits and potential engine rebuilt.
I'll take worse fuel economy and reliability as a package deal.

 

Well, your mechanics were wrong. Even though the ecu can retard the timing itself, the mechanical parts of the engine can not adjust such as,Valves, Crank, Cam...Therefore the only thing the ECU does when you put lower Octane into your engine would be the Sparks timing. This prevent Detonition where the cylinders will be melted by extreme heat + down Force = a Hole Burnt into you engine

The ECU will Advance the timing, make it happens earlier than the time the Air+Fuel Mixture could get to Top DeaD Centre, where the Engine mechanical parts were designed, and Fixed for a complete Seal, and operate at it Best Efficiency.

By advance the Ignition (burning) Before the Valves completely shut, the Pistol got an Expanded Force, which Force the Cylinder to go down, on it way going up top as designed (knock), and the Valves have not been completely sealed, the engine will lose it efficiency, and more likely to over heat the cylinders over an Extreme long Hours of operation, and under Heavy Load, or Higher RPM. It will burns a hole in the cylinders, and your car will be Wasted. This shorten your engine life dramatically

Let make it simple. You can burn Premium in an engine which require Regular, and you still lose efficiency, but it is not as Dangerous as, Burning Regular in an Premium fuel required engine.

Best gas grade, and for the longest life, and the most efficient, Please use the Required Gas Grade stated by the manual book. Your Engine was Specifically Engineered for that.

 

X2. A lot of folk spend in the 50k range for a car requiring premium and try to save maybe a couple of hundred dollars a year on gas by burning regular. Now the regular 350 has been de-tuned through the ECU as stated but the compression was also reduced to 10.8-1 see chart below. The 450h still requires premium and maintains the 12.5-1 compression ratio. That 'may' be as it uses the Atkinson cycle. No way I'd burn regular in that unless it was just an emergency (on an occasion shouldn't hurt). At some point there may be someone who posts what an engine rebuild cost them due to burning an octane too low on a regular basis.

 

Thank you Cruiter for the awesome screenshot :-)

 

If you drive 15K miles per year in your vehicle, and if the difference between regular and premium fuel is 30¢ per gallon, assuming no difference in MPG between the use of the two fuels -- say we use 20 MPG as a round figure, then you are talking $225 a year in additional fuel costs or about $18.75 a month.

Now, that is what... 3 or 4 coffees in a month at Starbucks? One dinner or two lunches out? Not really all that much when you look at the big picture.

Until just recently, the 3rd Generation RX350 required the use of premium fuel according to the Owner's Manual and all other documentation one could find. The RXh has always stated that premium fuel is required. As Cruiter mentions previous, when the requirement for premium fuel was dropped recently it was accompanied by a drop in compression ratio for the engine. So, when talking about 3rd Generation RX you have to make sure everyone is on the same page for the discussion as what is right for one RX is problematic for another. If your RX says it needs premium I would be running that as best possible regardless if you do not notice any performance difference, drop off in MPG, and any savings you might see in using regular.

Of course, if you are just holding on to your RX for three or four years and then trading in, what should you care. This is a problem for those who are purchasing used or those who want to hold their RX long term.

As for the topic of this thread, the OP was using premium 93 octane fuel so that is not a factor in their low MPG calculations. I did recommend they try using a different brand of fuel, as I stated previous, since I have noticed some vehicles I have owned do not like certain brands of fuel regardless of octane. More than likely, their problem is a combination of their driving habits and driving location and the nature of driving a two ton SUV. Little they can do about that. They may see some relief as the winter blend fuel is phased out for the warmer months.

BTW, does anyone know that when the government MPG figures are calculated the fuel being used does not contain any ethanol at all? So, right out of the gate you are at a disadvantage in trying to reach the sticker MPG numbers as the fuels you are using more than likely contain up to 10% ethanol.

Also, here in Florida unless you have a special needs vehicle/equipment requiring "Recreational Fuel" (boat, motorcycles, antique car, etc) it is illegal to dispense ethanol-free gas into your standard car. Makes you wonder some times.

http://tbo.com/news/business/isnt-it...ol-fuel-202968

 

Compression ratio is a statically calculated mechanical ratio of cylinder volume to combustion chamber volume, but the Atkinson cycle is designed NOT to take a full intake charge into the cylinder, so in everyday use the realized compression is not quite so high as might seem to be indicated. That is why these engines (even though specification wise are a higher compression engine normally associated with higher performance engines) don't develop as much torque or HP as their conventional twins. They burn cleaner and more efficiently, and rely on the electric motors to add back the missing torque and HP.

If you take a regular RX350 motor, up it's compression to 12.5 and throw in some electric motors, now that would be awesome, they'd have to call in an RX600h.