I have been using premium gas in the past 7 years. As the gas prices have shoot up substantially, I am curious if it would be OK to use 89 instead of 93.
I know it won't burn as clean and may affect the sparkplugs and other internal parts, but like to know whether others have used lower Octane gas and have experienced any adverse effect.

 

I've been curious about the same thing. Gas prices are going up where I live too and I've been considering switching to regular, but I also am not 100% sure if it's good to do.

 

Compression 10.5: 1 allows you to use 89 octane without harming the engine. But with a low octane consumption will increase, you will not save money and will have to refuel more often at the gas station.

 

Have a 2013 RDX with 119k miles, have used regular grade gas since it was new and still runs great. Premium is recommended, not required. This is one reason I prefer a naturally aspirated V6 over a turbo charged V4. This thread comes up regularly in all car forums and always gets advocates on both sides. I would think the biggest variable in any mpg/cost comparison would be the price difference between regular and premium. Around here it fluctuates between 40 and 80 cents a gallon.

 

I have been switching between 91 and 87 with each tank. My daily drive is only about a mile and a quarter each way, so it's hard to see much difference, although I do think that the 91-octane premium gets me slightly better mileage. If so, it is not enough to make up for the difference in price, currently $.50 per gallon where I buy mine. Nonetheless, I do buy premium every other tank.


RFT!!!
Dave Kelsen
--
"There is nothing wrong with America that can't be fixed by what is right with America." -- Bill Clinton

 

Cost aside, I think lower octane gas also burns less cleaner than premium. So I am more concerned about carbonized plugs or interior cylinder chambers.

 

if you are concerned about that, then disable the VCM and put your foot in it.

Here in California we have the grade in the middle, 89. This is what my wife often uses in her. RDX. I think it is more important to make sure the fuel you are using is Top Tier.

https://www.toptiergas.com/licensed-brands/

 

This is the strangest suggestion I've ever heard, what does the VCM have to do with cylinder chambers?

 

I'm also very confused myself...

 

What does Variable Cylinder Management have to do with cylinder chambers?

I'm not sure how to answer that, but I'll give it a shot...

Disabling the VCM means that when you "put your foot in it", i.e. push the accelerator pedal to the floor, all six cylinders will be actively working; the goal is to ensure that both the spark plugs and the cylinders are being used at high RPM, in order to (hopefully) burn off any accreted carbonization.


RFT!!!
Dave Kelsen
--
"We live in a tragic world, but there is grace all around you. So attend to the grace." -- Bruce Springsteen

 

In any case, in order to burn all deposits, you need RPM and load on the engine. VCM operates at low RPM and at a load lower in the middle, that is, VCM does not affect anything at all.

 

On the cylinders that shut down when VCM is active, some oil gets by the rings (more on that in a moment) As said above, this causes carbon
build up and spark plug fouling. As VCM goes, the system in the 2016 and newer RDX is probably much better than it has been. (It is either all 6 or just 3)
That said, age and miles will soon show if these have the same issues some of the older systems have had. Honda used some experimental piston rings in specific years,
those would "line up" and cause a lot of oil to bypass. This also caused scoring of cylinder walls. Honda has settled a class action suit on these and have been
rebuilding the bottom of the engines. Some are scored so bad they have to just replace a short block. In the Odysseys I am familiar with, those rings were no longer
used after 2013. (That is the cut off on the warranty extension from the lawsuit) However, folks with newer models are now having spark plug fouling issues.
A misfire code is usually how this shows up. Honda has quietly offered some assistance for owners with less than 100k. The advise for those who get no offer,
or a very low offer, is to change the plugs, muzzle, add some sea foam, drive the shit out of it, and change the oil early.

A Muzzler is a simple devise that plugs between the engine coolant sensor and the wiring harness. It offsets the reported temp just enough to keep the VCM from
activating. These are available with fixed resisters, swapable ones, a dial, and electronic ones that adjust themselves. The auto ones shut themselves off if the engine
overheats and shows the actual temp on the dash gauge. The VCMTunerII Advanced even shuts itself off when idling for a while to allow for any idle relearn diagnostics.

Shutting off VCM can cost about 1-2 MPG. Some notice no change or even better mileage. VCM is really for Honda's CAFE rating. For the whole fleet, it adds up, for individuals,
not so much.

There is a service bulletin for the RDX involving VCM. Rocker arms stick. Service bulletins are in a sticky at the top of the forum.

 

Let's first, if the engine is not worn out, the piston rings are doing their job and nothing is clogged, then a lot of physically cannot enter the combustion chamber even if the rings become "line up".The thermal gap of one upper ring on the RDX is 0.2-0.35 mm, which means that on a running heated engine the gap will be 10-150 microns (remember the honing depth on a new engine reaches 10 microns) and if the rings of all pistons become "line up", then with such a gap the engine will consume no more than 100 grams per 3000-4000 miles. The spark plugs become dirty only when the oil burns, if the oil burns, then there is some kind of malfunction in the engine. Scratches on the cylinder walls appear only during dry friction, that is, if there is physically no oil in the engine or the catalytic converter is destroyed. There is no RECALLS for the second generation RDX on the https://www.carcomplaints.com/Acura/RDX/2013/recalls/, which means there are no problems with VCM. The RDX does not need any Muzzler. On the highway, disabling the VCM can increase consumption by up to 5MPG.

 

I think that settles it. Those of us with decades of experience with Honda 6-cylinder engines appreciate the lessons, as well as the tones. Thanks.


RFT!!!
Dave Kelsen
--
"Wisdom too often never comes, and so one ought not to reject it merely because it comes late." -- Felix Frankfurter

 

I am not a fan of deactivating VCM. When there is no load or just normal driving, the VCM saves gas by engaging fewer cylinders. Deactivating VCM defies the whole purpose of this thread's goal of saving on gas.

When we are saying "use top tier" gas, are we saying that there are gas produced that are sub-standard? How the Feds allow that? I think the chemical content of gas is pretty much regulated.

 

Usually the gas in the U.S. all meets the same standards, but some companies claim they put more stuff in their gas to help achieve cleaner and better performance.

 

Same on my 2017 till about a year ago. I used mid 89 and now regular 87. I cannot see any difference.

The Owners Manual states "recommended" not cast in stone.

 

I'm using 88 Octane, Top Tier, 15% Ethanol (E15). The gas mileage would be about 2% lower than non-ethanol gas due to the ethanol lower amount of energy than gasoline. So if I'm getting 20 mpg on non-ethanol gas, I would get 19.6 mpg on E15. The E15 is $.90 per gallon less than the 91 Octane premium here in Iowa. The State of Iowa exempts E15 (and E10 and E85) from the state fuel tax to encourage it's use. I have had no problems in using the E15 and the reduction in gas mileage is minuscule compared to the reduction in price!

 

But they also charge higher prices for those additives. Now, whether that's a marketing scam or they actually clean your engine, I do not know and have hard time to believe it.

 

So you don't see any difference in MPG or engine performance at all?

 

An issue with Ethanol mixed with gas is that when burning it produces water or at least moisture. That reduces the engine power and performance. I guess, it won't damage the internal parts of the cylinder or they would have banned it already. For many years I bought Costco gas which I think has either 10% or 15% Ethanol in it.

 

All fuels, when combusted, produce water vapor, including gasoline.

 

Yes but the mixed Ethanol produces even more.

 

Premium and regular fuel combust “as cleanly” as the other. No difference based on octane.

Gasoline is refined in a refinery. From there, it usually travels through a pipeline to a central hub or depot. From that depot, all gas distributors (Costco, Exxon, Shell, Texaco, etc.) buy the same gasoline. However once in their trucks, each gas distributor adds their own additives. Those additive packages can be VERY different. Top Tier fuels exceed the minimum requirements for an additive package.



As for lower octane, it will not hurt the engine. According to Ktuner, the Honda V6 ignition map is based on 96-octane. Technically, if it runs on anything less it will knock. It’s the ECU’s job is to endlessly monitor knock and to adjust or hold the ignition timing just below the threshold that the engine knocks. On 93-octane, the ECU will pull timing, just less than it pulls on 87-octane. But the ECU can easily manage 87-octane. The engine will not knock on 87-octane. However, the more the ECU retards the timing, the less efficiently the engine operates. That is why you get slightly less power and lower fuel economy on 87-octane than on 93-octane. But it is barely noticeable in daily driving.

 

Yes. To be more exact, ethanol does not in itself produce more water vapor. It increases the hydroscopic nature of gasoline. Water is more dense than gasoline. For that reason water sinks in gasoline. In winter, we know that can create a problem. The answer? We add dry-gas. Dry-gas is a type of alcohol. Ethanol and dry-gas have the same basic effect in gasoline. In short, the alcohol suspends the water in the gasoline, kind of like a fog. Instead of the water settling on the bottom of the gas tank, the alcohol makes the gasoline cloudy with water. That is why E10 (or higher) can “hold” more water than E0. The gasoline can still combust. And as it does, that extra concentration of water becomes part of the exhaust. Because ethanol can allow gasoline to absorb more water, it contributes to more water vapor in the exhaust.

 

As the manual says: "Premium Fuel Recommended", using lower Octane fuel would cause engine inefficiency. I personally believe in the long term it may cause some damage to the engine too.
If it were OK to use regular gas, I don't believe Honda would recommend premium.

 

Shell octane 93 0-60 time = 6.5 seconds.
Costco regular 87 0-60 time = 6.6 seconds.
Try for yourself and post some results 😘

 

The term “recommended” implies “if you want the advertised HP, you must run premium fuel.” Honda recommends premium to “bring out the best in their car.” However, it is absolutely not required. The engine will operate perfectly well on 87-octane. It will not knock or ping. It will not develop any more or less carbon. It will only produce less power.

In the car industry the term “required” means you must use premium fuel. If you operate the engine on anything less than premium, the ECU cannot pull ignition timing enough to prevent knock and the engine will develop internal damage. The only way to prevent knock is to use high octane fuel. That is not as prevalent today as years ago. Even engines with very high cylinder pressures, like the Civic Type R, can safely operate on regular fuel. It is totally OK to run regular in your RDX. It will cause zero damage. Honda’s recommendation for premium has nothing to do with engine safety or longevity. It is only for power output.

If you want to split hairs, an engine should technically last longer on regular fuel than on premium fuel. The reason is slightly less cylinder pressure. Premium fuel allows the spark to ignite the fuel air mixture a little bit earlier in the compression stroke. The result is that more of that heat expansion occurs closer to top dead center and therefore provides more downward push on the piston – that’s what creates torque. Depending on the exact engine, premium fuel creates about 2% to 4% more pressure in the cylinder. That is partially why premium makes more power. But that extra cylinder pressure technically puts a tiny bit more wear-n-tear on all the moving parts. This is more of an academic discussion than anything measurable in real life. But just know that regular fuel will in no way damage your engine. For what its worth, I only ran premium in both my V6-6MT Accords and in my current RDX. But the only reason is I want that sliver of extra acceleration when I hit the gas pedal. I want that advertised power.

 

I was trying to determine the same answer in regards to the 2013-2018 RDX. I can't say my conclusion applies to any years newer. My research involved me reading nearly 300 pages of posts regarding this exact question. What I concluded for the 2013-2018 is that the engine was exactly the identical j35Z2-VCM used in a particular year/years Honda Accord. The Accord recommends "Regular 87 Octane Fuel"
The 2013-2018 Acura RDX Recommends but does not require 91 or was it 92 or 93 Octane Fuel? I can't recall the exact octane that was recommended.
My findings;
It is MUCH More important to use a "TOP TIER" gas.
Therefore I concluded that Top Tier 87 is sufficient for the 2013-2018 RDX.
This was for my daughter's 2015 and I believe I told her if she wanted to play it safe, she could go with the Mid-Grade Top Tier.

 

Honda/Acura recommends premium be used in the gen 2 RDX, but the owners manual also states that 87 octane is acceptable for use. Do you believe Honda would print that information knowing that it will damage the engine?

 

Honda warns of knocking noise, knock will destroy the engine, but after a warranty. If you drive 150k miles with detonation, then the engine will definitely die, but Honda is no longer responsible for such millage)

 

Strange in that the 2017 Honda Accord V6 (same engine as my 2017 RDX, same compression, etc) recommends Regular.

Go figure! And now that I have used Premium before and Regular now, I get the same gas mileage under the same conditions (same highway at the same cruising speed). And Premium by itself does not increase HP in an engine that takes Regular.

 

Nothing strange, the software of the ECU for the RDX has a late ignition angle, for the Accord for an early ignition angle. Therefore, on RDX on a hot engine, after restarting, there will be knock until the knock sensor detects them and the ECU change the ignition angle.

 

Funny, I've never heard the knock you describe and I use 87 octane frequently. Actually I've never heard what I would consider a knock in any engine unless there was something seriously wrong with it. The so-called "spark knock" is more of a light ping to my ears. (I've never heard that sound in an RDX either).

 

The knocking occurs only for a couple of seconds during acceleration on a hot engine, and disappears until the next restart, it is not so loud that it can be heard well, especially on RDX.

 

These threads are going to live forever, and everyone will have an opinion. I've never read an account of actual engine damage. I'm actually not sure if any engine damage has been documented on any of the 2G RDX on this forum, for any reason, and lots of people run 87 octane gas. If there is a risk it is very long term and very minimal. These engines are pretty sturdy.

 

Well I am not "loading" my engine by pulling a trailer, flooring it to get to the next red light first and I live on level ground instead of mountainous terrain.

I am not the least bit worried.

More of a concern is the ethanol in fuel. I just did an experiment on regular 87 gas and sure enough, 10% ethanol and that is what their web site claimed after I checked. I'll be doing that test on the 91 and 94 that I also use.

 

It may also be acceptable to pump cooking oil in your RDX if there's no other choice. But aside from that, we are interested in the long term effect of using lower octane gas. So far we have got different opinions as some people have posted knocking and possible long term damage (in terms of carbon build up) and some others say nothing would happen. For the past 7 years I have used only premium and afetr reading all of these posts, I'm thinking whether it's worth of any possible risk to switch over to regular.

 

You didn't mention the sentence that says "use of gasoline with a pump octane of less than 87 can lead to engine damage" in the "Fuel Information" page of the owner's manual that you posted. I'm sure that was unintentional on your part. In case you don't know, 87 octane is regular grade gasoline in the US. Gasoline with less than 87 Octane isn't sold in most parts of the US.

 

Exactly. The engine manufacturer tells us that anything lower than 91 octane *can* (not *will*) cause knocking. They also tell us that anything lower than 87 octane *can* lead to engine damage. I would tend to avoid that, myself, but as you point out, one would have to go out of one's way to put gasoline with an octane rating lower than 87 in the vehicle, so it's not a big worry.

There are things Honda did not say; specifically, they neither said nor implied that fuel with an octane rating higher than 86 but lower than 91 can lead to engine damage.

For myself, I have tended to alternate tanks of 91 and 87 octane fuel - but I have never thought that 87 octane fuel would damage my engine, because of the whole 'being able to read' thing. As I am about to stop working and live off my savings and SS, I will likely use 91 (or higher) octane fuel less often. My own lengthy experience with Honda engines, along with the information in Honda owner's manuals like the above, and the fact that I have never heard anyone who even claimed that they had a Honda engine damaged or destroyed by using 87 octane gas, makes me feel pretty safe about using 87 octane fuel more often. Other's feelings (and in this case, their actual mileage) may vary.


RFT!!!
Dave Kelsen
--
Long night, I woke up in some Japanese family's rec room... and they... would... NOT... stop... screaming.