Compression of our motor and octane

I have reached 10K miles. For the last 8K miles I've been running Mobil Supreme 93 octane. 1st couple thousand miles I ran regular. To me the engine runs considerably smoother on 93 octane compared to 87/89 Research on web leans toward I should not notice a difference. So this week I switched to mid grade 89 octane. I can definitely notice a difference after 3 trips to work and back home. Exhaust note is slightly louder and engine not nearly as smooth. I'm thinking the higher compression of this engine compared to typical is the reason. I'm thinking the ECU will slowly advance timing if not sensing engine knock. This advance in timing can be achieved do to the higher octane resulting in smoother performance. What do you guys\gals think or experience?

 

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87 for me. It does run a bit smoother on 92, but either has the same MPG (on OVT) or loses mileage (stock).

Below is a lot of good info about the engine. The octane noted (91) is non-US Research, not AKI.

 

You're wasting your money, the exhaust note is not changing from running 93 octane and any smoothness is just the lighter wallet making you sit flusher to the seat.

 

The compression ratio for the 3.5l V6 is 11.8:1 compression

 

Do what you wish.

But at the delta of reg vs premium you'll never have a ROI based on MPG or service life.

Although the oil companies will thank you for funding their business.

 

My theory is low MON sucks.

87 AKI=91 RON=83 MON
91 AKI=95 RON=87 MON

Read up on RON vs MON testing...makes sense to me.

 

You can always run this !!!!!!

https://vpracingfuels.com/vp-street-legal-fuel/

 

You not the first guy on here to claim to know more than the engineers.

 

I don't think there's one good answer here. We do know the engine compression ratio is 11.8 to 1, but we also know that varies due to variable valve timing. That valve timing is adjusted based on engine load AND other factors, including fuel quality. The ECM determines fuel quality constantly, so in theory if it detects higher octane fuel, it can use different valve timing for more power/efficiency.

Just my .02

 

“Premium” labels got your panties all moist ay? Don’t worry lots of others on here that believe that shit too.

Here we go again boys. Let’s ride the shit out of this bitch! Woo Hoo!

I mean Friday close isn’t it?

 

I notice a difference in power delivery and smoothness in my truck running premium vs regular. It cures most of the shifting and drivability problems for me honestly, and I get roughly 0.5mpg better. I think the stock ECU pulls a ton of timing, valve and spark, to compensate for regular gas which leads to the drivability problems so many 3rd gen owners complain about.

 

If you monitor OBD2 data, the "adaptation" to premium is pretty darn instant. Its not like the old days when you either experienced no difference in performance or drivability OR it took multiple tanks before you'd notice a difference. These finely tuned knock sensors are ALWAYS advancing timing until knock, then holds, then will continue to attempt to advance periodically to maintain a -3 knock feedback value. You can run down a tank of 87 octane and see the knock feedback timing go from 18 degrees to the high 20s within minutes of filling up w/ premium and yes - this debate never ends on here, but the truck runs smoother on premium. It will survive on 87 and was designed to do so, but if you can justify the cost / afford it, this engine loves to drink the top shelf stuff. period.

 

Just a small clarification, VVTi does not change the compression ratio. Infiniti has the only motor in production with a variable CR.

 

I remember years ago reading about Saab working on some sort of weird variable compression engine, the Head actually slid up and down, seemed like that would have been a reliability nightmare.

 

For older vehicles a 11.8:1 compression ratio would require race fuel only

 

Yep. It was a nice science experiment. I don't really trust Nissan / Infiniti either.

Mazda seems to have solid footing on very high CRs. Maybe they'll bring back the Wankel someday?

Now, the Saab 2.0l HOT was a monster of a motor. I kinda miss it at times. If only it was hooked up to the proper wheels, we might still have it.

 

I generally run 87 because it's cheaper where I am. But I swear the few times I've run 91 in the tank, the engine tick has been dramatically lower. I've even had other people notice it. And the gas is from the same station

 

Yep, I made the switch from 87 to 93 a couple months back and the difference is night and day. The engine runs better and I get a little better mileage.

 

Yeah! I was gonna say my dirtbike has an 11.5:1 piston AND a stage 2 cam and only runs on 110 octane.