The HyHi manual states that it requires at least 87 Octane gas, but higher octane is preferred.

I've been running 87 Octane only, since I'm a cheapskate.

I've always understood that if you're not hearing engine-knock, then there's no reason to buy premium gas... But I think this gets more complicated with computer controlled variable timing engines. Does this old rule still apply?

And more specifically, is there really any reason to run high octane gas in the Hybrid Highlander? Will it increase my mileage? Will it increase my mileage enough to pay for the more expensive gas?

Thanks,

_Am

 

Exaclty how are you being a cheapskate? Higher octane rating DOES NOT mean better gas.

If your car requires 87 (such as mine... HCH I), putting anything higher than 87 is a waste of money and MORE importantly robs the car of performance. Not to mention could damage your engine.

 

I can't find it online in the forums, but I recall that somebody did their own study to see if the higher octane made a difference in their MPG results. Their conclusion, if I recall correctly, was that there was such a negligible difference that it didn't matter if you stayed with 87 as the owner's manual says or put in "the good stuff".

As other discussions here have said, the main things to be sure to do are (a) keep your tires properly inflated; and (b) use the Toyota-recommended oil -- 5W20 or 0W20... not the 5W30 that's printed on the oil cap.

 

Can you give a pointer to where toyota recommends 5w20 or 0w20 over instead of whats printed in the manual
\
Thanks

 

Here is a forum post with a scan of the TSB from March 2006

5W20: This oil has been set as the Toyota genuine engine oil, starting in February 2006 for the US. Series: AZ, GR, JZ, MZ, NZ, RZ, UZ, VZ, 1ZZ-FE, 2TR-FE

0W20: This oil's initial application is in the 2006 model year 2AZ-FE engine and will be used in new engines to be developed. 2AZ-FE Camry, 2AZ-FE Solara, 2AZ-FE Highlander, 2AZ-FE Rav4 (ACA3#)

 

Lower compression engines that are designed for 87 octane only will not benefit from premium and will actually get lower MPG and less engine power with premium. Also, using premium in such low compression engines may contribute to carbon deposit buildup.

However, Toyota has designed the Highlander Hybrid engine with a very high compression engine to take advantage of the properties of premium fuel if it is used. Toyota recommends premium fuel in the owners manual for best performance in the HiHY. Note that using 89-93 octane in the Highlander is optional. Using 87 octane will not cause any drivability problems.

If you use premium in the Highlander Hybrid, the engine will be more much more responsive as expected. However, the engine will also run more efficiently when driven conservatively such as when you are driving with cruise control on at highway speeds. The engine actively advances the ignition timing as much as allowed by the fuel grade used under light engine loading and part throttle to maximize fuel efficiency.

I have a Scan Gauge II in my Highlander and it digitally displays the engine timing advance every few seconds. I have noted that the engine is at it’s most advanced timing settings when driving down a level highway at 65mph with cruise control on and the engine at a low and stable RPM. 89-93 octane will lead to improved fuel economy if you have the self-control not to use the added reserve horsepower.

The other part of the equation is the differential in price between regular and premium. When gas was $2, 91-93 octane was typically $0.20 more or a 10% surcharge over 87 octane. With gas closing in on $4 per gallon, premium is still only typically $0.20 more resulting in a reduced 5% surcharge over regular. Thus the percent cost difference between premium and regular is narrowing considerably as fuel prices rise. This may tip the balance for the premium’s extra fuel efficiency paying off. With a typical 28 MPG for the Highlander, you need to improve fuel mileage with premium over regular by 1.4 MPG to break even. When gas was $2/gallon, A 2.8 MPG improvement was needed to break even.


The Highlander Hybrid has a very high compression ratio of 10.8. This predisposes it for higher octane fuels. High compression ratio engines are more efficient than low compression ratio engines all else being equal.

However, the higher pressures in the cylinders cause the gasoline to be more difficult to control from a combustion standpoint. Gas will ignite more rapidly at higher cylinder pressures which can result in engine knock if the combustion peak pressure occurs too early in the downward stroke of the piston. Engine knock can damage an engine.

Modern engines all have knock sensors tied into the vehicle’s engine controls which operate with lightning speed to retard (delay) the ignition spark timing to head off this knock condition before the driver could ever notice it. This automatic dial back of engine timing and engine efficiency is what allows 87 octane fuel to be used in the Highlander’s high compression engine without any adverse effects.

Premium fuel requires higher temperatures and pressures to combust properly and it burns more slowly than regular fuel. Thus, premium fuel behaves more controllably under the higher compression conditions in the Highlander engine and will allow the engine to advance the spark timing more than with regular fuel resulting in higher engine efficiency.

The standard internal combustion engine is terribly inefficient. Only about 30% of the gasoline you buy is actually converted into mechanical energy to make the vehicle move. A whopping 70% of the fuel you pay for is just converted into wasted hot air that escapes from the radiator, the tailpipe, and radiates off the engine itself.

The engine ignition timing is extremely important in extracting the maximum amount of mechanical energy from the burning fuel. It’s a complex choreography but in simple terms the fuel burn should be timed to reach maximum piston force at approximately 15 degrees of crankshaft rotation past top dead center of piston travel.

When lower octane fuel is used in a high compression engine, the fuel burns more quickly, reaching peak forces much sooner than high octane fuel. This potentially causes knock. The spark timing is retarded to reduce the knock but this results in less of the fuel energy being captured by the piston as mechanical energy and more being ejected from the engine as waste heat.

 

very good post don.

the only variable i don't quite comprehend is the ecu - how 'adaptive' is it?
does it follow a standard timing map as written into the s/w and simple advance as much as the fuel in the tank will allow? if we run 5 tanks of 87 and then throw in a tank of 93 will we see timing advanced instantly, or is there a 'learning' period for the ecu to adapt?

 

I was just checking out the manual, and I do not see where it says a higher octane is preferred. Specifically, it says:

At my altitude, 85 octane would be fine. I will consider using a higher octane if I can convince myself it will make a measurable difference in my fuel economy.

 

The ECM adjusts ignition timing advance via stored memory maps with inputs of engine speed, intake air volume or manifold pressure, throttle position, engine coolant temperature etc.

A corrective ignition timing control function in the ECM makes a final adjustment to the pre-mapped timing settings based on such conditions as engine speed transitions, cruise control operation, traction control activation, knock detection, etc.

In simple terms, the corrective ignition timing algorithm for knock retards the timing in relatively quick discrete steps until the knock just disappears. If knock is no longer detected, the ECM then begins to advance the timing again but now in slow discrete steps. If knocking occurs during these slow advance steps, it again retards the timing in quick steps.

The engine is most efficient using a particular grade of fuel when the ignition timing is fine tuned to the ragged edge of advance just before knock occurrence. At this setting, the engine achieves the best fuel economy, engine power output, and lowest exhaust emissions. The advance timing correction is not glacial in response. The vehicle should correct for a higher octane shortly after filling the tank with 93 octane but not instantly.

 

Thanks Don,

That was a great reply and largely what I was looking for.

After reading your post I tried running with 91 instead of 87.

I saw an almost immediate, meaningful improvement in mileage.

While before I rarely got over 27 MPG, now I can easily get 31.

To me, I saw clear and convincing evidence that the Hybrid Highlander will run much more efficiently on premium gas.

Some numbers:

Cost of regular gas $3.40
premium premium $0.20

With 87 Octane.

Best mixed mileage 27

Cost of 100 miles = 12.59
Distance per tank = 464.4

With 93 Octane
Best mixed mileage 31

Cost of 100 miles = 11.61
Distance per tank = 533.2

Differences:

Running with 93 Octane at a cost of $3.60/gallon vs 87 at 3.40/gallon
Saved me $.98 every hundred miles and adds an calculate extra 68.8 mile range.

As the saying goes, Your Mileage May Vary.


_Am