I am curious about the V6 engine in both vehicles. The 7th generation ES runs on regular and produces 302 HP while the IS runs on premium and produces 311 HP. Is the only reason for the HP bump due to the premium gas that the IS requires or does the IS engine have higher compression? Just curious. I have a '24 UL ES350 on order and was wondering if putting in premium in that vehicle would allow the engine to produce more HP.

 

If you check the "Compression ratio"s on the (US) Lexus web site's ES and IS SPEC comparison tables, you'll find all the V6 entries are 11.8:1.

The difference between regular and premium gas is the difference in octane rating - premium's is higher. Octane rating isn't a measure of energy content; rather, it is a measure of the fuel's resistance to premature detonation (engine "knocking"). So, using premium fuel in a modern engine turned for regular gas wouldn't change anything except draining your bank account faster.

 

As I always add when reading about the eternal Regular v. Premium debate, I bought my 2021 350 ES new in 2021 and always enjoyed it. However, there was frequent engine hesitation, particularly when accelerating going uphill. Long story short, I replaced air filter, checked for software downloads, test drove with service manager etc. etc. and nothing helped until, as he suggested, I switched to Premium grade Top Tier Fuels. The problem immediately and completely disappeared and has been gone since—-for over 6 months.
I’ve heard all the responses to my experience from, “It’s your imagination—-placebo effect…” to Top Tier is only cleaning additives and has nothing to do with your issue, to “wasted octane and wasted money in your V-6 engine.” All I can say is that I know what I know. So take my comments as you wish. Just trying to help OP.

 

How do you explain the difference in HP cited by the OP? And why does Lexus recommend premium gas for the IS?

 

The IS is tuned differently and on the IS forum they’ll tell you it gets worse MPG and power with regular fuel. IS might have higher redline than the ES v6 maybe also I haven’t checked but that could account for higher HP.

Wife and I have 2023 IS350 and ES300h. Have driven RX350 and ES350 a few times but don’t own one.

 

Both share the same N/A V6: 2gr-fks.

 

They’re definitely tuned and run differently. Different intake and exhaust also.

Im thankful there’s a Costco near our house so we don’t get hosed too badly filling up the IS350 with premium it’s a bit under $3 a gallon here right now. Our IS gets the same MPG in the city as my old LS430 used to get!

 

I have no reason to think that you are "imagining things", but this story still doesn't demonstrate much unless you repeat it a few times (e.g. stop using premium fuel now and verify that the problem comes back).

 

Well, if I have a symptom and doc gives me a medication for it that stops the symptom, I’m inclined to keep taking the med and declare victory. I have no interest in discontinuing the med to prove to him and me that he got it right.

 

That may be the anti-knock system adjusting the air/fuel ratio to a richer fuel mixture to suppress the premature detonation occurring from using the lower octane rated regular grade gasoline.

Although I don't think it accounts for the all the horsepower rating differences, I've seen some verbiage indicating the rpm at which engine torque is measured and that torque value has a relationship to the engine horsepower rating. The ES 350 torque@rpm spec is 267 lb-ft @ 4,700 and the IS 350 torque@rpm spec is 280 lb-ft @ 4,800 but that alone doesn't seem to account for the differences in horsepower specs. (There's SAE/JIS/DIN testing measurement standards. Supposedly, US sold engine ratings are compliant with SAE standards.)

 

The difference in fuel economy could be from the AWD too. Driving another two wheels will cause the parasitic drivetrain loss to go up thus making the engine work harder. I agree that the tune might be different too because of the IS being a Sportier" car, it needs/deserves a sportier tune. Power numbers stay the same (or close) but maybe more timing would make it peppier thus requiring Premium fuel. The powerband would change though depending on what they did with the timing and where it comes in at.

In the ES, I'll gladly give up some power to get to use regular fuel because in the long run, We save a considerable amount of money over the need to use premium fuel. That's a good trade off for me because if I want a sporty drive, there are better choices of cars.

Just my opinion though!

 

I saw the difference in mileage between the two vehicles. The ES is slightly lighter but not enough to make that big a difference, The IS350 RWD is 3748 and the ES 350 is 3690. The EPA mileage on an IS running premium is 19-22-26 AWD and 20-23-28 RWD. An ES 350 is rated at 22-26-32 on regular. That is a relatively big mileage penalty for all of 10 horsepower on premium fuel.

 

Theyre totally different cars though. I own a 23 IS and a 23 ES. They’re so different they might as well be different brands entirely. The mostly aluminum suspension of the gen 3.2 IS puts the ES suspension to shame yes even the FS Handling. It’s lighter on its feet yet much more sure footed. Which it should be with all that forged aluminum compared to steel in the ES not to mention different hubs that reduced weight a bit in the gen 3.2 refresh.

seeiously, gondrive a new IS359 back to back against an ES350 of any variety. No comparison and very different.

Regardless the mileage difference is probably mostly due to wheel width and weight, the IS350 FS RWD has wider rear wheels even compared to the ES FS. And much wider and heavier than the non FS ES. That leads to more rotational inertia.

The final drive ratio might also be different which would affect MPG. You’re not looking at the whole picture. And the transmission is definitely different.

 

You are fortunate to have both and can choose what ride you want. The mileage deduction might be the wheels and some transmission tuning.

 

My staff and I own 3.5 V6 from 4GS and 6ES.
Mine on premium.
Hers on regular.
They are the same block, but different engines in the end!

Her regular V6 is actually more practical with noticeably more bottom end torque, and her transverse mounting or whatever other factor results in a smoother idle!
My premium gasoline V6 actually lacks her bottom end punch, but my premium gasoline has more top end torque and more top end power!
Unfortunately, the longitudinal mounting or whatever other factor results in more vibrations at idle.

If I could have it my way, I would put the regular gasoline version in the base and luxury trims, while the premium gasoline engine would be used in the F Sport trim.