High compression ratio: is higher octane better?

Joined
Apr 4, 2012
Messages
14,785
Location
Kendall, FL
Our Hyundai vehicles have very high compression ratios but the only fuel required is an 87 octane, although the OM adds, or higher. I have read on various Forums that a higher octane fuel would 'burn cleaner' and is better for direct injection and other high CR engines. Why is this? P.S. the Turbo 2.0T gets high octane but the OM also states that 87 is all that's needed. My area: 87, 89 and 93 octane are available. Your thoughts.
 
Joined
Feb 1, 2011
Messages
1,004
Location
MA
I'd run them on 87 octane if that's what the owner manual says is all it requires. Most modern vehicles will adjust the timing if there is any pinging. Why spend the extra money on gas?
 
Joined
Oct 8, 2014
Messages
1,648
Location
SANTOS, BR
As a Turbo you should be on the safe side and use at least 89. Unless you drive very light foot, then, the least octane will be the most efficiente fuel, if it doesn't tend to ping, of course.
 
Last edited:
Joined
Oct 8, 2009
Messages
1,154
Location
Richmond, VA
Your answer: test it. I had a 1999 Chrysler 300M. With 2004-6 era gas prices the MPG were such an improvement using 93 that it was cheaper per mile- so that's why I used. I drove on 93 for two tanks, averaged the next two, switched to 87 for two tanks then averaged the next two then did the math. Allowed the car to learn the new fuel and was pleasantly surprised. Car was much happier on 93 as well but I was going for a numbers-only comparison.
 
Joined
Jan 25, 2008
Messages
4,452
Location
Guilford, CT
Pretty much EVERY vehicle's owners manual says to use "xx octane or higher." That's because using higher octane than necessary will not damage anything, however it has no benefit and is a waste of money.
 
Joined
Jun 24, 2004
Messages
14,505
Location
Top of Virginia
My Camry ran much better on 93 octane than on 87 octane. The least expensive fuel will be recommended, but that doesn't mean that the powertrain management software can't adjust for more. The better fuel economy I got out of my Camry on 93 pretty much paid for the cost difference. It was dramatic. On the other hand, while I can tell a slight difference in our Honda with 93 octane, and I can tell that the engine can sense it too (watching timing maps via Torque), there is no measureable difference in fuel economy and I don't use it regularly. So I agree with those who say that you just have to try it and see how it runs in your car. You may or may not be able to tell a difference.
 
Joined
Oct 16, 2010
Messages
2,095
Location
.
High compression ratios and 87 octane fuel are possible in a DI engine because the evaporation in the air/fuel mixture in the combustion chamber cools things sufficiently to avoid pre-ignition. Some DI owners believe their engines are more responsive with 89/91/93 octane fuels, presumably because timing is not retarded to prevent detonation in some circumstances. And in some models, it's believed pre-ignition is also prevented by richening the air/fuel mixture. Avoiding this richening may provide better fuel economy and may also reduce fuel diuttion of engine oil. You are almost surely safe going by the manufacturer's recommendation, but you may want to try a tank or two of higher octane fuel to see if you notice a difference. I've had a DI engine for 3 years and have settled on 91/93 octane during warm months and 89 during cool.
 
Joined
Feb 25, 2012
Messages
534
Location
North Carolina
I thought the same thing about my Subaru with its 10.5:1 compression ratio but your Sonata is even higher @ 11.3:1 I just follow the OM and buy the 87 octane. Car runs fine and I like paying around 2.70/gal now.
 
Joined
Aug 15, 2005
Messages
2,329
Location
Lexington, KY
Originally Posted By: Hokiefyd
My Camry ran much better on 93 octane than on 87 octane. ...
Which Camry was that? The old 1MZ V6 engines were well known for running better on premium, but I seem to recall you had a much more recent Camry. Are you referring to the newer 4 cylinder 2AR or 2AZ engines.
 

CT8

Joined
Oct 9, 2014
Messages
15,387
Location
Idaho
Run several tanks and see if you notice a difference. The combustion chamber designs have improved over the years. Stuff is kinda the same but different.
 
Joined
May 6, 2005
Messages
7,351
Location
San Francisco Bay Area
Originally Posted By: BearZDefect
Originally Posted By: Hokiefyd
My Camry ran much better on 93 octane than on 87 octane. ...
Which Camry was that? The old 1MZ V6 engines were well known for running better on premium, but I seem to recall you had a much more recent Camry. Are you referring to the newer 4 cylinder 2AR or 2AZ engines.
It depends on what the marketing types are trying to get across. Many performance cars are marketed as "Premium unleaded required. May run temporarily on regular without damage but with reduced performance." The Camry V6 was an interesting case. Most drivers don't even try to push it or would even be able to tell there are any performance gains. In that case they'd say "Regular unleaded required, but higher performance with premium." It's basically saying the same thing, but trying to reach different audiences. My parents own a Camry V6 but to my knowledge they've never, ever bought premium for it.
 
Joined
Nov 20, 2006
Messages
27,028
Location
MA, Mittelfranken.de
The engine management can compensate for it for the most part but higher octane usually runs better and allows to for higher timings to be pulled making a few extra ponies. The last thing the marketing dept want is "must" use high octane in the owners manual. Like you say try it. Some 87 engines run so good on 93 that the fuel mileage goes up substantially and more than pays for the difference. My late fathers 3.4 Chevy venture 3.4 of all things was one of them.
 

gathermewool

Site Donor 2022
Joined
Jan 9, 2009
Messages
9,341
Location
New England
I've always considered the worst thing to do to an engine that can advance timing to take advantage of higher octane fuel is to continue to switch back and forth. When switching from low to high octane, the ECM has to unlearn knock correction and add timing before it's taking full advantage of the change to higher octane fuel. I never tested this out in any of my cars, so it may be that the ECM compensates very quickly and the time wasted at retarded timing is miminal. When from high to low octane, the ECM has to quickly compensate for and feedback timing correction to compensate for what the knock sensor perceives as knock. This feedback isn't learned fully until each RPM/load cell is populated with new timing values. The risk here could be damage, be it negligible or possibly harmful. For me, I'd rather allow for a buffer, so that the ECM is never detecting knock, no matter what I throw at it. Hot days, hill climb, high load and relatively low RPM, whatever. Even if the risk is minimal, it's not worth saving a few bucks per fill-up, even if lower octane is suitable under most or nearly all scenarios.
 
Joined
Nov 11, 2010
Messages
9,783
Location
Saskatoon canada
Originally Posted By: exranger06
Pretty much EVERY vehicle's owners manual says to use "xx octane or higher." That's because using higher octane than necessary will not damage anything, however it has no benefit and is a waste of money.
Nope. Modern vehicles "learn". The ecu is constantly adjusting fuel mapping and timing to maximize output. So more octane will benefit with more power. If you lighten up on the pedal that translates to better fuel economy. My 01 C3 has a 6.0. I have been using premium since I bought it however this month regular has been just over a buck so I thought I'd try it. So 3 tanks now of regular. My highway fuel consumption has been 20+ mpg. It's now 16. And it was an immediate loss. My charger is the same way. I'll pay the extra few bucks a tank for premium. Easier on fuel means less fuel burned which equals extended oil change intervals. Winning
 

wemay

Thread starter
Joined
Apr 4, 2012
Messages
14,785
Location
Kendall, FL
Originally Posted By: Clevy
...I'll pay the extra few bucks a tank for premium. Easier on fuel means less fuel burned which equals extended oil change intervals. Winning
Hi Clevy, could you expound on this?
 
Last edited:
Joined
Oct 12, 2005
Messages
5,762
Location
Da Swamp
I've tried it, with E10 gasoline (the few places that still have E0 are pricing it out of the stratosphere). The Regal's DI non-turbo engine seemed a bit more responsive on 92-93, yes. But the mileage increase was only about 5%, while premium costs >10% over 87 regular.
 
Joined
Nov 7, 2009
Messages
2,434
Location
Oconomowoc Wi
I have recently used 3 tanks of 93 fuel in my wife's 2011 Santa FE V6 and it runs MUCH smoother on the higher octane gas. I always got a slightly rough idle at a red light/stop and now it idles smooth as silk. I have no idea if the MPG are better but will stay with a higher octane fuel...
 
Joined
Jan 5, 2008
Messages
3,561
Location
Central Iowa
Originally Posted By: wemay
Originally Posted By: Clevy
...I'll pay the extra few bucks a tank for premium. Easier on fuel means less fuel burned which equals extended oil change intervals. Winning
Hi Clevy, could you expound on this?
I will. Let's take a look at commercial trucks. Cat engines or example. In the lubrication manual for Cat engines, if the engine is getting less than 6 mpg (I know this sounds low, but we are talking about heavy semi truck engines here), the oil change interval is recommended at 15,000 miles. If the engine is getting over 6 mpg, it is recommended at 25,000 miles. This is for the Cat C-15. The oil is not being subjected to as many particulates, combustion acids, etc and possible fuel dilution issues when getting above a certain mpg threshold. Hence, the OEM recommends extending the drain if the fuel economy is above their determined threshold. Detroit Diesel, International, etc all have similar directives. The auto/pickup OEM's don't elaborate on such details. Most folks either would find a way to mess it up, or ignore it altogether. But, they have factored it into may Oil Life Monitor algorithms. That is one reason the Oil Life Monitors are not all that bad when it comes to oil life.
 
Top