http://www.sae.org/servlets/productD...D=J2723_200504 for a mere $59.00 you can get all the info as to test requirements.

 

You are thinking maybe the 2005 engine on premium tested at 108 HP also because it was bare, and the 2006 engine on regular tested at 103 HP also because it was loaded with peripheral equipment?

I thought testing of bare engines to get "Gross HP" was ended years ago. The high power ratings made good advertising, but were misleading for consumers.

 

To put a more practical and less theoretical perspective on this, after switching to premium from regular I immediately notcied two things:

1) rough idle disappeared
2) engine runs quieter

What I thought before was "normal" was actually detonation ocurring in the engine. Perhaps the gasoline in California is worse than average and the regular here is especially bad which is all the more reason to use premium fuel.

 

Idle is not affected by octane.
And detonation from low octane only occurs under heavy throttle or high rpm, and the xB's ECU would immediately suppress it, so I don't see how you could have heard it long enough to think it was a normal unquiet running noise.

 

A lot of times when people WANT to notice a difference, they do! simple as that

 

"Placebo effect." :D

That's why pure seat-of-the-pants, subjective "results" don't usually impress me.

Something that is reproducible because of limited direct control by the testor - does the auto transmission upshift/downshift when maintaining a fixed speed under specific load, for example - is more likely to convince me.

I've seen "expected" results show up so many times where no change has actually occured to cause them, and in so many different technical areas, that the placebo effect seen in medicine seems the obvious parallel.

Many "results" seem to be purely psychological.

Or, in other words, "What he said! ^ ^ ^ "

Tomas

 

Hi,

Idle is affected by timing. Timing is affected by knock. Knock is affected by the compression ratio, fuel/air mixture, fuel quality/octane, and engine temperature. In "theory" knock would only occur at high loads and rpm where temperature is higher, but in "practice" this is not the case. I have heard very loud and undeniable knock at low rpm in several other cars. This is first hand experience. We should only post things we know from "practice" and not what we think or want to believe in "theory".

 

Guys, just don't forget that this is a 4 cylinder camry engine. My mom has the 4 cylinder camry and my step dad the 6....THEY ALL TAKE 87 OCTANE!! remember, the tC is not TECHNICALLY a sports car! - don't get me wrong, i love it, but it doesn't need more than 87 octane and you all know it. It seems that we all WANT it to be a sports car SOO badly that we WANT it to take higher octane gas. Supercharger & Turbo aside, 87 octane is all we need.

(quality of the gasoline is another story)

 

Yes, idle speed is affected by timing.
No, timing is not affected by knocking. Knocking is a result of timing too advanced for the conditions.
Your other statements about knock are OK.

It is possible to cause knocking at idle in an older engine by turning the distributor to greatly advance the timing. It is likely that the engine will die if it knocks at idle, as the spark is so premature that is stops the rising pistons before they can pass top dead center.

It is not uncommon for older engines to knock at idle and run-on (diesel) when shut off, if their timing is too advanced or they are running an octane that is too low for their compression. That is probably what you heard in the other cars.

There is not going to be knocking in a modern engine at idle unless something is wrong with the ECU. You could not have heard your xB knocking on regular gas except momentarily under load.

I do have practical experience with timing to avoid knocking. I have replaced and adjusted the ignition parts on two of my BMW 2002s and many of my old BMW motorcycles, and kept them timed right for years. I set timing statically with a buzzer and check the advance dynamically with a strobe light, and have adjusted the advance curve of old centrifugal advance units by using springs and stop bolts.

 

Riddle me this. I've tried all 3 grades of gas and only with the highest grade does my idle not bob up and down like a yo-yo.

 

Fluctuating idle speed is not caused by timing, as timing at idle does not fluctuate.
Fluctuating idle speed is also not caused by octane differences, as the octane is not fluctuating.
It is likely the ECU sensing a variable input from something like the mass air flow sensor, and making the idle vary.
I would put a couple of tanks of regular gas in the car to get the gas in the tank back down to 87, and show the dealer what is happening.

 

My gas mileage also increases by 2 (mpg) when using premium. That about offsets the additional price of premium.

 

Unfortunatly, there must be another factor that gave you the increase. Fuel grade will not give better gas milage on a non FI Scion.

 

How carefully have you determined that?

I can think of only one condition under which premium gas could give better mileage. That condition is whenever the ECU would otherwise intervene on regular gas to de-tune the timing to prevent knocking. That would only occur at heavy throttle and/or high rpms.

For premium gas to improve a tank's mileage, you would have to spend a large part of that tank driving very hard and fast. Then 2 mpg difference would be down in the 20's, like the difference between 25 and 27.

 

This de-tuning is done by a "knock sensor". Reference Diagnostic Trouble Code P0325:

"Knock sensor is fitted on the cylinder block to detect the engine knocking. This sensor contains a piezoelectric element which generates a voltage when it becomes deformed, which occurs when the cylinder block vibrates due to knocking.... When spark knock occurs, the sensor picks-up vibrates in a specific frequency range. When the ECM detects voltage in this frequency range, it retards the ignition timing to suppress the spark knock."