I read some of your posts on the cost difference and it made a lot of sense. In my opinion, I would not cheap out on a lower grade engine oil in my luxury cars, why would I used a lower grade gas.

 

So because something useless is cheap, it makes sense to buy it?

 

If there is a quantifiable test difference, then using the recommended octane is not useless. Lexus recommends 91 octane for the NX350h, no where in owner’s manual does
Lexus recommends 87 octane, it just say you could use it with reduced performance. I tend to use what the manufacturer recommends, especially if the cost is not a big factor.

 

Yep..... for the x time use what the fuel door calls for

 

One possibility is that in Japan the regular is about 85.5 AKI (using US units for consistency).
And the higher grade is about 93.5 AKI.

So it's possible the Japanese manual started saying this because of the lower octane of their regular fuel.
Then the translated manual would have kept the message even though the US regular has a bit higher octane rating.

In AKI we have:
US: 87 or 91
JP: 85.5 or 93.5


But other cars using the A25A-FXS are fine with 85.5 AKI in Japanese manuals...

 

An engine management system could be tuned to run on different octane. Putting in high octane on a car that is not tuned for high octane will do nothing. In the NXh’s case, the engine is tuned for 91 octane and will definitely be able to take advantage of that.

 

My questions is, is been able to take advantage of the higher octane 91 the same as using 87 is not good for the engine? I believe the specs on the 2.5 on the RAV4 & NX350h are the same, HP & torque, & the RAV4 recommends 87. It looks like they are "tuned" the same? Not sure what extra do you get by using 91.

 

This has gotten into studying the "left hind leg of a flea" territory

 

HAH -- hahaha

That's why I checked out back in Post #44...

 

If you are comparing the 2026 rav4 2.5L vs the 2025 nx250, the nx250 is not tuned for 91 octane, Lexus recommends 87 octane for the nx250 and 91 octane for nx350h, nx450h and nx350.

 

I wonder why if the car is tuned for 91, still has the same HP & torque specs. I was under the impression that the 2.5 L in the 2025 NX250, is the same as the 2.5 L in the 2026 NX350h, Ie. same HP & torque; differences in HP is because of the electric motor. I would expect that with the higher octane you should get @ least more HP.

 

Here is what I found online, 2025 nx250 203hp on 87 octane, 2026 nx350h gas engine alone 189 hp on 91 octane. Guessing the engines are tuned for different purposes. The traction motors specs are below:

• Front Motor (MG2): 134 kW (180 hp) and \(270 \text{ Nm}\) (\(199 \text{ lb-ft}\)) of torque.
• Rear Motor (MG3): 40 kW (54 hp) and \(121 \text{ Nm}\) (\(89 \text{ lb-ft}\)) of torque.

 

Not only tuned differently, but they are also mechanically different engines. The short block (ie block, crank and pistons) are substantially the same (the A25A part of the engine designation), but the cylinder heads are different (the following 3 letters in the engine designation, FKS, FXS, etc designations). Hybrid engines (well, conventional ones designed for economy) use a cylinder head with a higher static compression ratio but at the same time the valve timing is much wider (the range during engine rotation where the valves will be open or closed). The purpose being the engine can be controlled to take in a lot less air, so needs less fuel, but the higher static compression still supports good combustion in this situation while having the benefit of the larger space within the cylinder for fuel to cleanly burn and expand more thoroughly to extract a wee bit more power from the tail end of the combustion process (gas only engines tend not to do this because they can't afford to wait around all day for this to happen, LOL).

I guess a simpler analogy, during casual/calm usage the 2.5L engine sort of acts more like a 1.6L engine when drawing in air and injecting fuel, but acts more like a 2.5L engine when the air/fuel mixture is being burned. I suppose you could think of it partially similar in purpose to a cylinder deactivation technology some manufacturers use, but a lot less stupid. This creates substantial increases in economy and lowering emissions, but the side effect tends to be lower HP and torque (which is handled by electric motors). Gas engines have traditionally not used this because the drop in HP and torque was excessive. Over the years the design development coupled with computer control have allowed the differences to narrow, but they are still present (otherwise all cars would use hybrid engines, whether actually hybrid or not)

One of the reasons that hybrid trucks and performance hybrids are now often using turbo engines, despite the benefit of instant torque electric motors for assistance, its hard to beat the torque of a forced induction engine once it gets spinning while meeting economy and emissions targets (EVs with giant batteries aside).

 

Very good explanation of the Atkinson cycle engine. A normal compression stroke and a larger expansion power stroke, Use less gas on compression strokes and gets every bit out of the power strokes. Getting that last drop of power tends to slow things down down, hence the nx350h gas engine alone has lower hp than the nx250 but I am loving the gas milage.

 

These FKS, FXS engines seem to be quite robust and of high quality, so far. Class leading thermal efficiency too. I know there's next generation 1.8 and 2.0 litre engines forthcoming based on FKS, FXS technology from Toyota.