Any harm in using higher octane?

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Originally Posted By: Kuato
Originally Posted By: OVERKILL
Originally Posted By: Kuato
My brother ran a vehicle requiring 87 on 93 for several years. Deposits formed and eventually it started lightly knocking on 93. The vehicle was stepped down gradually from 93 to 87 after I bought it from him. Any heavy acceleration would cause.detonation until it had run on 87 for 5 or 6 tanks and had a spark plug change. YMMV but in your case I would stick with 87.

I also had a vehicle that began to knock lightly in the late fall. Turned out that there was water in the tank, either from a questionable station or condensation. 3 consecutive tanks with Drigas cleared it up.

Hope that helps.


I ran my Mustang (called for '87) on 91 (because I advanced the base timing to 14 degrees) for years. When I took the heads off it the pistons looked fantastic.

Mind you the engine had the living tar beat out of it regularly (it was like one constant ITU) but I don't think that higher octane gas causes deposits. It is more resistant to preignition and burns a bit slower, which allows you to run more ignition advance.


I can only report what I experienced, and surmise that without advancing the timing, you'd probably have had deposits too.


I highly doubt it. I've seen numerous engines run on 91 because the owner thought it was "better" and they were also clean.

Some engines are prone to carboning up due to chamber design, which, in conjunction with the top of the piston shape, dictates flame front propagation. Driven lightly (think lower load, which means slower inlet air flow, poorer air/fuel blending and lower cylinder pressures), certain engine designs will also carbon up. An "issue" that can be alleviated simply by running them hard periodically. Other engines, like the Ford 302, don't seem to care one way or the other, the same goes for the 2V Modular.

As I said, the higher octane rating means the fuel is less prone to pre-ignition and detonation/pinging and also burns slightly slower, allowing the flame front to fully propagate across the bore in situations that require it (the higher octane fuel). On an engine that does not require it what this means is that you have some still burning/expanding combustion gasses exiting the exhaust port. This does not create deposits. Remember, combustion is not an "explosion" but rather a controlled burn of the air fuel mixture that rapidly expands outward from the ignition point. Proper octane fuel for a given application means that the combustion is complete; the air/fuel mixture burns evenly across the top of the piston providing even downforce and producing no piston "rock" and that the air/fuel mixture is not ignited prematurely by deposits (pre-ignition) or during the combustion event at another point in the chamber (detonation) which can cause damage and results in an incomplete combustion event.

Higher octane fuel allows for the advancement of ignition timing, meaning that the air/fuel mixture can be ignited earlier (as the piston approaches TDC) so that you get more effective gas expansion before the exhaust valve opens; the mixture is "more useful" and this results in a gain in power. Of course this is only to a point and varies wildly with engine/chamber design and camshaft timing. Going much beyond 14BTDC on a 302 Ford for example does not yield any additional power. This is probably due to the short 3" stroke.

A higher static compression ratio also requires increased resistance to pre-ignition and with the mixture more compressed, this promotes a faster burn, requiring a fuel that burns a little longer in order to allow for complete flame front propagation.

Ultimately the fuel being more resistant be being ignited by a source other than the spark plug as either pre-ignition or detonation does not lend itself to producing deposits. But it does mean that you usually accomplish nothing by using it in an application that doesn't require it. That said, on some cars, with knock sensors, they constantly "learn" how much ignition timing they can get away with and will in fact make more power on higher octane fuel, even if they don't call for it. Others, due to the reasons I already mentioned, won't. My HEMI is an example of the former, requiring a minimum octane of 87 with 89 "recommended".

The usual sources for deposits are:

1. compromised injector spray pattern resulting in a less than optimal mixture entering the chamber (think fuel puddling and the fuel not blended perfectly with the air that it was sprayed into as the intake stroke ingested it) which will in turn screw up the readings that the oxygen sensors see, further exasperating the issue.

2. Worn plugs causing periodic misfires or partial/improper ignition (you stated you also changed your plugs.....) or plugs with carbon deposits built-up on them, which can cause detonation or pre-ignition (or both). This also results in mixed readings from the O2's and can result in improper mixture optimization by the ECM.

3. Faulty EGR

4. Faulty PCV plumbing resulting in oil burning, which reduces the effective octane of the fuel and will also create pinging


Using high octane fuel in a car that doesn't require it is a bit like using synthetic oil at 3,000 mile OCI's. It doesn't hurt anything but it isn't helping really either
smile.gif
 
Originally Posted By: Bamaro
Originally Posted By: aquariuscsm
I've often wondered when we buy top tier gas,are we REALLY getting top tier,or getting ripped off?

When you buy non top tier are you REALLY getting EPA regulated gas or are you getting ripped off?
The EPA doesn't "regulate" all motor fuel sold here? There's a Federal government standard for regular grade motor fuel. The military used to call it "MOGAS".
 
All of this discussion raises the question of for WHAT application is 89 octane designed to be used. I don't recall seeing a recent owner's manual specifing it. It would seem to me only TWO grades are really needed with modern EFI/knock sensor technology.
 
My 2012 Ram 1500 5.7L owner's manual specifies 89 octane recommended, 87 octane minimum acceptable.

I have seen numerous retail dispensers no longer offering 89 octane at locations like Sam's Club for a number of years now.
 
Originally Posted By: HerrStig
All of this discussion raises the question of for WHAT application is 89 octane designed to be used. I don't recall seeing a recent owner's manual specifing it. It would seem to me only TWO grades are really needed with modern EFI/knock sensor technology.


A number of cars call for 89 octane...it's a SMALL number, but they're out there. A number of Chryslers call for 89 octane. I think the older 3.5L V-6 engines wanted 89 octane. Possibly some of the Hemis, too, but I could be mistaken on that.
 
Originally Posted By: Wolf359
You don't have to go all super. You could just try a few gallons of super with regular to bring up the average octane in the tank. That's how they make plus anyway, they mix regular and super. Mix would be 1/3 super to 2/3 regular with 93 super.


My wife requires super in her 2005 Legacy GT(turbo) and does this nuttiness to make the 91 octane she requires!
 
Makes one wonder how this discussion fits in with E85's 100+ octane rating. Of course, the makeup of the fuel itself is different than premium gasoline, but it still got my curiosity piqued.
 
May I offer an alternate view point? Are you sure it is pinging? Or are you hearing converter heat shield rattling? The age of the vehicle is about right to have one of the weld to rust way on the shield and then you get the rattle at specific rpm and load.
 
I have never had a problem running the manufactures specified octane rating. An Accord is a basic low compression motor, their are literally millions running millions of miles on the cheapest 87 they sell with no problems.

If your having problems their is something wrong with it.
 
Considering any OEM engine is going to find service in a broad range of locations, including other, lower of standard living, countries. It would seem logical that they build the engines to handle 87 octane quite well. Now that idea does exclude the higher end performance autos. And given that fuel quality is generally better for most of us than what we would buy in those other countries, that would also support the idea that 87 is just fine for the majority of folks here. Not so sure that many actually take a look at the big picture. Sometimes it pays to step back and take a broader look at the issue.
 
There may be exceptions. '07 Prius manual stated that using higher than required octane may cause unwanted engine deposits.
 
Originally Posted By: lenjack
There may be exceptions. '07 Prius manual stated that using higher than required octane may cause unwanted engine deposits.


Where?

http://www.toyota.com/t3Portal/document/om/OM47568U/pdf/03.pdf

Originally Posted By: Toyota

FUEL TYPE
Your vehicle must use only unleaded
gasoline.
To help prevent gas station mixups, your
Toyota has a smaller fuel tank opening.
The special nozzle on pumps with unleaded
fuel will fit it, but the larger standard
nozzle on pumps with leaded gas will
not.
At a minimum, the gasoline you use
should meet the specifications of ASTM
D4814 in the U.S.A. and CGSB 3.5−M93
in Canada.

NOTICE
Do not use leaded gasoline. Use of
leaded gasoline will cause the three−
way catalytic converter to lose its effectiveness
and the emission control
system to function improperly. Also,
this can increase maintenance costs.

OCTANE RATING
Select unleaded gasoline with an Octane
Rating of 87 (Research Octane
Number 91) or higher.


Use of unleaded fuel with an octane rating
lower than 87 may result in engine knocking.
Persistent knocking can lead to engine
damage.
If your engine knocks...
If you detect heavy knocking even when
using the recommended fuel, or if you
hear steady knocking while holding a
steady speed on level roads, consult your
Toyota dealer.
However, occasionally, you may notice
light knocking for a short time while accelerating
or driving up hills. This is normal
and there is no need for concern.
GASOLINE CONTAINING DETERGENT
ADDITIVES
Toyota recommends the use of gasoline
that contains detergent additives to
avoid build−up of engine deposits.
However, all gasoline sold in the U.S.
contains detergent additives to keep clean
and/or clean intake systems.


That is from here:
http://www.toyota.com/owners/web/pages/r...#Owners-Manuals

Which is the 2007 Prius owners manual.
 
Originally Posted By: Hokiefyd
Originally Posted By: HerrStig
All of this discussion raises the question of for WHAT application is 89 octane designed to be used. I don't recall seeing a recent owner's manual specifing it. It would seem to me only TWO grades are really needed with modern EFI/knock sensor technology.


A number of cars call for 89 octane...it's a SMALL number, but they're out there. A number of Chryslers call for 89 octane. I think the older 3.5L V-6 engines wanted 89 octane. Possibly some of the Hemis, too, but I could be mistaken on that.


The 5.7 Hemi asks for 89. The 6.1 asks for 91 minimum. The 6.4's ask for 89 or 91 depending on application.

It should be noted that my new 14 5.7 Hemi runs excellent on 87 octane...
 
My owners manual for the Charger, which I dug into last night in a quest for tire pressures for the steel wheels (spoiler, it isn't in there) mentions that in the 5.7L you need to run 89 for maximum performance, but 87 is acceptable.
 
Originally Posted By: aquariuscsm
I've often wondered when we buy top tier gas,are we REALLY getting top tier,or getting ripped off?


I'm guessing they get very good compliance on that. For one thing, the detergents probably aren't very expensive. I have to assume they are a little more expensive than the gas, but probably not that much. The 'Top Tier' licensing people have a very big incentive to force compliance. They have high-end manufacturers recommending their product and they charge the refiners the long bucks for the certification. If they don't enforce it, everything falls apart, sooner, rather than later.
 
IIRC the difference in cost to the selling company is fractions of a cent per gallon.
 
I can feel the difference in my vehicles when downgrading to regular,and my fuel consumption takes a hit.

Octane means resistance to pre-ignition due to compression or heat.
In a spark ignition does more octane equal a slower burn?
If so why?
I thought more octane just mean't more resistance to detonation/pre-ignition,so if that's the case when the spark fires and the flame propagates why would it be slower,if that is indeed the case.
I've read it here at bitog that more octane burns slower,I'm just trying to understand why since once the spark event takes place wouldn't octane be irrelevant now and flame propagation be similar with both fuels.
Thanks in advance.
 
As you correctly pointed out Clevy, octane equivalent is the resistance to the air/fuel mixture igniting of it's own volition. Burn rate is due to flame propagation rate which is about the same regardless.

What's strange (logical) is that the part of the burn where the octane counts is towards the end, when the remaining air and fuel has been compressed by the expanding, heating, burned gasses and can "diesel", a phenomonon called "end gas auto-ignition"
 
The top tier thing, while laudable, is not really a big deal from my view. Around metro areas and such, it is more available, but there are many, many rural areas where top tier gas is non-existent option for the consumer, unless they travel excessive distances to get it. Yet, their motors are lasting as long or longer, with similar maintenance issues, than any other auto/pickup owner that live in metro areas and is religious about top tier fuel. Marketing and OEM nonsense makes all of this top tier thing an issue. Gives great peace of mind, I guess, but I lose absolutely no sleep over the fact that I have never had top tier rated gas in my pickup, except maybe the first tank from the dealer. And even then, I am skeptical that they filled it with a top tier rated fuel. They drove it over to the local farmer's coop gas station and filled it up. I know that fuel there is not top tier.
 
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