Knock Sensors and High Octane Fuels

[m_luke]

Common knowledge is that using high octance fuels should give better mileage and a slight increase in power output.

This is achieved through knock sensors to advance or retard the timing thereby altering engine speed.

With respect to the Gen5 and Gen5.5 Camrys, I have heard that I4 Camry uses only 'passive knock sensors' so it lacks the ability to auto-adjust engine timing whereas the V6 Camry does possess this ability.

Is there any truth to this?

[Tony the Tiger]

I don't think there isn't any sensors that can sense what octane fuel you are using and then advance the timing. Pretty much all knock sensors are there to sense bad gas and still allow proper operation if you do end up using a lower octane fuels that are below the recommended octane rating. The Camry V6 only differs in being quite sensitive so that when it detects irregular knock (which shouldn't happen at all), it knows something is wrong and it pulls back timing for a lengthly period of time; perhaps one driving trip, or maybe after 5 mins since the knock was detected, etc..

You will not gain power by using higher octane fuels with a stock ECU. The stock ECU will never advance timing, but rather allow it to restore and run on optimum ignition maps. If you match the manufacturer's recommended octane level, your car will be knock free and that's the best power the engine will make. You can create a good habit of putting gas with slightly higher octane than the recommended rating to allow a larger margin of safety in case your engine runs slightly different over time (carbon deposits, faulty PCV, poor cooling system, etc...)

Most guys don't get the point of high octane fuels. If your engine has mild compression, mild boost, etc.. regular pump gas will probably allow the engine to make the best power with fuel and ignition tuning. Too much timing and the engine will surpass the optimum combustion period (MBT) and you will make lesser power instead. To use higher octane fuels, you must be running a lot of boost or a lot of compression to the extent that the engine will detonate before it reaches MBT (knock-limited tuning). In this situation, higher octane fuels will allow more timing without knocking thus creating more power

[angeleyesM5]

You will not gain power by using higher octane fuels with a stock ECU.

Are you sure about this? According to Toyota's website, the 3.3L V6 found in the Camry SE/Solara/ES330 makes 225 hp with premium, but only 210 hp with regular, a 7% increase in power. They don't make this claim with the 4 cylinder engine.

According to Toyota's TSB document for 2004-2005 Camry SE V6s (3MZ-FE), after reflashing the ECM, it says "Select premium unleaded gasoline with an octane rating of 91 or higher for optimum engine performance."

* AngeleyesM5

[Tony the Tiger]

Originally Posted by angeleyesM5

You will not gain power by using higher octane fuels with a stock ECU.

Are you sure about this? According to Toyota's website, the 3.3L V6 found in the Camry SE/Solara/ES330 makes 225 hp with premium, but only 210 hp with regular, a 7% increase in power. They don't make this claim with the 4 cylinder engine.

According to Toyota's TSB document for 2004-2005 Camry SE V6s (3MZ-FE), after reflashing the ECM, it says "Select premium unleaded gasoline with an octane rating of 91 or higher for optimum engine performance."

* AngeleyesM5

You've just stated it yourself, the ECU needs to be reflashed so that it can run at its optimum ignition and fuel tables. The ECU will not know how to "sense" higher octane fuel and then advance timing.

If your ECU is flashed for regular unleaded, running anything higher than regular will not yield any more power. Like what I said, run the manufacturer recommended octane level and it will make the best power. Anything higher will be a waste of money on a stock ECU stock engine car.

[91MR2quickNA]

Unless your ECU has JDM fuel maps, then 91-93 octane is all you'll ever need with a stock engine. Using less than 91 octane will cause the MZ's over-sensitive knock sensors to retard the timing. The Solara's 225/240 rating is still under the old SAE rating. The ES330 provides a more accurate picture with premium fuel on the 3MZ: 218hp / 236lb-ft. 8hp and 16lb-ft is a good loss when you use regular fuel, but again, the MZs have over-sensitive knock sensors.

Using too high of an octane will cause the engine to run rich, and the ECU may make some adjustments via timing or fuel control via the FPR. On drive by wire vehicles, like the 3MZ equipped cars, throttle control is also an option to cure a rich running situation. This doesn't improve performance at all, but instead, keeps the engine within it's optimum running capabilities.

Use 91-93 octane on engines with a compression ratio of 10.5:1 or higher. The 3MZ's compression ratio is 10.8:1.