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Gas octane ratings

5K views 32 replies 14 participants last post by  cmitch 
#1 ·
I would like to see what you are running in your new ML's. Please let us know what ML you have, and gas octane ratings you run.

I have a ML350 and run high octane, super. Most of the time I run Shell V-power.

Has anyone seen a difference using middle grade vs. super?
 
#3 ·
And I second that... The engine computer just retards the timing to compensate for crappy fuel. (Which is why Montana can run a Benz on 94 Octane, and that same car runs in California on 91 Octane.)
 
#4 ·
Mercedes engines are built to very close tolerances and are designed to run on high octane fuel. It makes it clear in most all owner manuals that running low octane fuel can damage your engine.

While opinions on this issue may vary, I had a conversation with my mechanic about this one time. He said that engines designed to run high octane should never be fueled with anthing less than 91 octane unless you live in higher elevations. He further explained that poor fuel can damage the catylitic converter(s) and cause excessive carbon build up on the tops of the pistons. When this happens, he said that the carbon will glow red hot at optimum engine temperature and can cause a variety of problems such as burning a hole in the top of your piston or pieces of it breaking off and causing internal engine damage. He has seen a lot of damage caused by those who wanted to cheap out and buy low grade fuel. You'll pay now or most likely, pay later!

His opinion was: If you're going to drive an expensive vehicle that you paid a lot of money for, you should be safe and spend the money to buy the premium fuel. I totally agree with him.

OTOH, if you run high octane fuel in an engine designed to run regular, you're most likely wasting your money.
 
#6 ·
From what I understand about this, high octane fuel (91 and up) burns slower and cleaner while lower octane fuel burns faster and dirtier. The dirtier burning fuel lends to the carbon build up on pistons. The resulting effect seems like an oxymoron, purer burning cleaner but slower and cheaper burning faster but not clean (the faster the explosion, the less efficient the combustion of fuel). This results in minute amounts of carbon and raw gas being forced into the catalytic converter which clogs and damages it (a bad converter many times will smell like sulfur burning). This is also why engines will knock with cheaper fuel. There are actually 2 explosions taking place in the chamber which results in the effect of a hammer hitting the top of the piston giving it the metallic sound.

While the engine timing is adjusted to compensate for knock, hence allowing the fuel to burn cleaner, buying cheap fuel isn't in no way good for a car designed to run high octane. While many folks run cheap fuel in their vehicles, when 91 or higher is required, without noticeable damage, there are the many who are just as unlucky and damage their engine.

If running cheap fuel, I would definitely be more concerned with what it might do to my engine more than the converter, since the computer does compensate somewhat for low octane.

There are many other things that can damage your catalytic converter as well such as:
1. Driving with a bad O2 sensor
2. Plugged injectors (causes unburned gas to ignite in the converter, thereby damaging it)
3. Piston Blow by
4. Leaking head gasket
5. Fouled or bad plugs
6. Bad intake gasket

I am by no means an expert, but I hope this helps! [:)]
 
#8 ·
BenzofBend - 8/17/2005 8:33 PM

Knock is actually the fuel exploding before the spark plug ignites it, if my understanding is correct.
You are absolutely correct! The 2 explosions I was referring to is the one that takes place prematurely causing the knock and the correct one a nanosecond later as the result of the spark.
 
#9 ·
And the loss of power resulting is not from the premature explosion, but from the car's ECU pushing back the timing so far to compensate for the premature detonation, that the car actually never has ignition close to TDC in the compression stroke, but rather, on the downstroke of the piston.

(Sorry if this is the car tech nerd thread all of the sudden, I just want to make sure that my knowledge of the tech is correct.)
 
#10 ·
BenzofBend - 8/18/2005 7:56 PM

And the loss of power resulting is not from the premature explosion, but from the car's ECU pushing back the timing so far to compensate for the premature detonation, that the car actually never has ignition close to TDC in the compression stroke, but rather, on the downstroke of the piston.

(Sorry if this is the car tech nerd thread all of the sudden, I just want to make sure that my knowledge of the tech is correct.)
Thus why the knocking goes away! My Dodge Ram truck has this problem and the dealer cannot seem to fix it. While it does okay on regular driving, if you have to get on it, it just doesn't have the power it should for a 5.2 liter. My truck is rated for 87 but no matter what octane I use, it runs crappy. The last time I had it in, they adjusted the settings in my computer and I lost 2 mpg and I'm now getting the sulpfur smell we previously discussed (rich fuel). I know it has got to be destroying my cat! OTOH, I run 91 in my ML430 and it runs great!

BTW, I love Car Tech Nerd (CTN) Threads![:D]
 
#12 ·
beumpie - 8/19/2005 12:11 AM

"
"He said that engines designed to run high octane should never be fueled with anthing less than 91 octane unless you live in higher elevations"http://

I live in near Denver also known as the Mile Hi city so what octane level should I use?
91 or higher. You might be able to get away with lower, due to the lack of 21% oxygen in the air, but I wouldn't try it, you might leave the mountain and then be pinging. I believe an engine looses 3% power every 1,000 feet due to oxygen loss. Altho if you have a superchaged engine it is less effected to the fact that it can turn with lower resistance.
 
#13 ·
Here's Mercedes Official Recommendations:
To maintain the engine's durability and performance, Premium Unleaded Gasoline must be used. The octane number (posted at the pump) must be 91 min. It is an average of both the research (R) octane number and the Motor (M) octane number: [(R+M)/2]. This is also known as the ANTI-KNOCK INDEX.

If premium unleaded is not available and low octane is used, follow these precautions:
1. have the fuel tank filled only partially with unleaded regular and fill up with premium unleaded as soon as possible,
2. avoid full throttle driving and abrubt acceleration,
3. do not exceed 2/3 of maximum acceleration position, if the vehicle is fully loaded or operating in mountainous terrain.

This can be viewed on MBUSA's owners online under the question: What is the recommended fuel octane rating for my vehicle?
 
#14 ·
I think the real question is whether or not Shell's V-Power (extra detergent) is better than anyone else's 93 octane gasoline. Last I heard, no one had been able to prove that the extra detergent (and extra couple of cents per gallon) had any effect on performance or longevity.

Here's another good one:

Refineries determine octane by varying the amount of ignition retardent in gasoline. Does increasing the ignition retardend actually cost the refineries more? And before anyone mentions detergents, keep in mind that detergents are added to all grades of fuel, not just midgrade or premium (with the exception of Shell's V-Power).
 
#17 ·
dahappybanana is correct - -

ALL automotive gasoline starts off the same . . .the greatest variable is octane. What you buy at your local Shell station may well not have been produced at a Shell/Equiva refinery, but it had Shell developed additives put in at the terminal rack in your city where the pipelines come out of the ground.

For example, more than 70% of the automotive gasoline here in Phoenix comes via pipeline from the LA basin in California - -through San Bernardino and along I-10 to the Phoenix tank terminal on the west side of the city.

Every reseller in the Phoenix area-- 76, Chevron, Shell, ExxonMobil, QuickTrip, pull their fuel from that terminal-- when their tanker trucks are filled, the "V-Power", "Pro-Power", etc additives go into the truck.

Independents get the same gas on the spot market- most get whatever is cheapest or surplus to the market that day. And most if not all have additives--they just don't always know whose.

I'm certainly not an expert, but have a bit of experience in this arena, and simply scratch my head over the wonders of effective advertising !

Regards-
 
G
#19 ·
finally some people who are smart enough to know that the only fuel worth buying, ANYWHERE, is the best u can buy, PERIOD!!!

reading this thread was uplifing.....finally, someone agreeing with me, that i'm "right"! everyone always trys to say regular is "better' and i waste money, and will ruin cars, etc., but its physics, the better the octan, the better the performance. period.!
 
#20 ·
wadman - 8/21/2005 2:13 PM

Perhaps I'll simplify the question:

Does higher octane gasoline actually cost more to produce?
Again, I'm not an engineer, but from everything I've learned working for a large oil company-- YES.. it costs slightly more because of the additional refining processes. Low sulfer, low pollution fuels (ie CA+AZ) also cost more to produce.

Regards-
 
#23 ·
AMSOIL300CDT - 8/21/2005 12:46 AM
reading this thread was uplifing.....finally, someone agreeing with me, that i'm "right"! everyone always trys to say regular is "better' and i waste money, and will ruin cars, etc., but its physics, the better the octan, the better the performance. period.!
There is one caveat, the higher octane is only beneficial to cars that need it. That is because the energy density is basically the same for all of the octane ratings (or AKIs) we are talking about. Higher octane lets you cram more fuel and more air into the cylinder under higher compression so you can get more power.

With that, I definitely agree with you, don't use lower octane fuel than your car says it needs.

The other day I was watching the news and they had the guy from UCAN and also interviewed the guy from the regulatory agency that controls gasoline (but I can't think of the name right now). As someone else said, all fuel pretty much starts the same and then each brand puts in the additives. Those fuels considered top tier like Chevron and Mobile (as identified by car mfg's lik BMW, GM, etc.) have more detergents. When asked the question which should you buy, both eventually said go for the best price you can find. But then, they said if you go with the "economy" brands, you should use a fuel additive every 6 months.
 
#24 ·
I use MB of Littleton. They recommend 87 Octane for altitude driving. Ask for the Technical Service Bulletin from MB. At our altitude, the higher octane gases do not burn clean. They leave deposits on the injectors and pistons.
I have used 85/87/and 91. I find I get the same performance from 87 as I do from 91. But when I go to lower elevations 5K and less, I use the higher octane fuels.
 
#25 ·
SledDog - 8/27/2005 4:51 PM

AMSOIL300CDT - 8/21/2005 12:46 AM
reading this thread was uplifing.....finally, someone agreeing with me, that i'm "right"! everyone always trys to say regular is "better' and i waste money, and will ruin cars, etc., but its physics, the better the octan, the better the performance. period.!
There is one caveat, the higher octane is only beneficial to cars that need it. That is because the energy density is basically the same for all of the octane ratings (or AKIs) we are talking about. Higher octane lets you cram more fuel and more air into the cylinder under higher compression so you can get more power.

With that, I definitely agree with you, don't use lower octane fuel than your car says it needs.
Right on Am and Sled![:)]

I decided to bump this thread up since it seems some guys are just not getting it that high octane is recommended for a reason.
 
#26 ·
[8D]
Actually (putting on my engineer hat), the "knock" or rapping sound you hear is metal meeting metal when the rod bearing hits the crankshaft with almost no oil cushion. The oil is displaced by the extreme pressure of the premature (pre-TDC) ignition. Of course, it is possible to design engines to handle this sort of pressure, as in a diesel engine where the rapping sound is not an indication of immanent bearing failure.

Because the anti-knock software in your engine control computer is fast-acting, most drivers never hear the one or two raps that trigger it. That is why many drivers have the impression they can use low-octane fuel in a high compression engine. Nonetheless, the damage done by those two raps every time the engine is required to pull is cumulative. What you don't hear will eventually cause a bearing failure.

Cheers!
Dave
 
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