Carbon buildup and octane rating

[nicholas]

Higher octane prevents detonation - not compression. There should be no reason that high octane fuel would cause carbon.

Those that do not know what detonation is please google it.

High octane fuel 89-91 in a car spec for 87 would ignite just as easily as 87 when the spark plug fires.

There is a lot of misinformation in this thread!

The car in question, is probably driven in high traffic situations, ie. low engine rpm and traffic and possibly cold weather.

 

[crazyoildude]

GM sends engine techs to our engine seminars and on more than one time i heard them say this exact thing and a lot of engine rebuilders in the association disagreed but think about it they put millions into r&d i think they may be on to something in the meantime i knew this for many years im not a chemist but i have been around engines a long long time with over 30 years hands on cutting cranks boring and assembling them all during rebuilds i was doing heads at 15 we were one of the first shops that rebuilt the northstar and we do most of the aluminum BMW's that some shops won't touch. if i am wrong then GM is wrong and there was also a tech from another engine company i dont remember which one that also agreed. years back i always used premium in everything because it really did give engines more power and we could advance the timing that was in the late 70's when i just started driving. Dont get me wrong i would use a higher octane rather than have a pinging noise but today the knock sensors take care of that pretty well

 

[mechtech2]

Like you said, you are not a chemist...

 

[SteveSRT8]

Originally Posted By: crazyoildude
same with the older northstars that required high test some of them sounded like they were going to blow any minute we would clean the carbon and they sounded like new.


We have seen tons of carbon buildup in our vans that are almost all 6.0 engines and run only 87 octane fuel. I have spent 2 hours grinding it all off a pair of heads more than once!

In some duty cycles you will get carbon build up, and in some engines it can make TERRIBLE noises!

 

[Trav]

Originally Posted By: Clevy
Originally Posted By: crazyoildude
higher octane usually means more carbon. I have seen it many times

Ditto. More octane equals harder to burn and can lend to incomplete burn. Could be why there is more detergent in premium?

If there was incomplete burn HC would be through the roof, MPG would drop and Cats would be destroyed from unburned fuel.

Fuel delivery issues seem to be the major culprit in carbon formation.
Long but interesting.

http://web.anl.gov/PCS/acsfuel/preprint archive/Files/49_2_Philadelphia_10-04_1143.pdf

 

[SteveSRT8]

I think that one of the reasons folks have developed an opinion about high carbon and high test gas is because many higher performance engines that run high octane fuel use fuel enrichment liberally to cool temps in the combustion chamber.

Many engines run 'pig rich' during a lot of their operation. Especially under heavy load.

 

[Trav]

+1

 

[crazyoildude]

everyone has an opinion i gave mine everybody can do what they want and all is good.

 

[OilFool]

Originally Posted By: Clevy

Ditto. More octane equals harder to burn and can lend to incomplete burn. Could be why there is more detergent in premium?


Bingo!!!

 

[Quest]

whenever this said subject RE: higher octane gasoline burns "slower" thus leaves more carbon comes around, there's always a trial of housewifey's tales and urban legends...

funny not a lot of these similar urban legends come along when hanging out on other countries's automotive bulletin boards.

Makes me wonder where the intelligence/common sense lies these days.

Q.

 

[mechtech2]

Originally Posted By: SteveSRT8
I think that one of the reasons folks have developed an opinion about high carbon and high test gas is because many higher performance engines that run high octane fuel use fuel enrichment liberally to cool temps in the combustion chamber.

Many engines run 'pig rich' during a lot of their operation. Especially under heavy load.


Very true. Esp. with turbos and strong tuning. That fuel is for cooling, and you can get a bunch more boost and power .

 

[SteveSRT8]

Originally Posted By: crazyoildude
everyone has an opinion i gave mine everybody can do what they want and all is good.


Bravo, but you are dead right about carbon causing a ton of mechanical noise. It makes a noise in our service vans on startup that is virtually impossible to distinguish from piston slap!

 

[ARB1977]

Can using higher octane than specified lead to higher exhaust temps under load? I towed 1500 miles in my Tacoma and used 91-93 depending what state I was in. When I got to my destination one of the exhaust hangers melted and the exhaust pipe was discolored.

 

[SteveSRT8]

Originally Posted By: ARB1977
Can using higher octane than specified lead to higher exhaust temps under load? I towed 1500 miles in my Tacoma and used 91-93 depending what state I was in. When I got to my destination one of the exhaust hangers melted and the exhaust pipe was discolored.


The fuel enrichment under consistent loading was what got you. If you have no codes odds are that the PCM was simply richening the mixture to keep cylinder temps within range, the added fuel works the converter causing VERY high exhaust temps.

What were you towing?

 

[Clevy]

Originally Posted By: ARB1977
Can using higher octane than specified lead to higher exhaust temps under load? I towed 1500 miles in my Tacoma and used 91-93 depending what state I was in. When I got to my destination one of the exhaust hangers melted and the exhaust pipe was discolored.

Wow. Heat like that leads me to assume a lean condition.

 

[SteveSRT8]

Not when it's on a modern catalytic converter equipped vehicle.

Plus if he was running lean his engine would have likely holed a piston had it continued for 1500 miles under heavy load!

 

[Clevy]

Originally Posted By: SteveSRT8
Not when it's on a modern catalytic converter equipped vehicle.

Plus if he was running lean his engine would have likely holed a piston had it continued for 1500 miles under heavy load!

Alright. That makes sense. Thank you.
That had to be some serious heat to melt a hanger.

 

[SteveSRT8]

Originally Posted By: Clevy
Originally Posted By: SteveSRT8
Not when it's on a modern catalytic converter equipped vehicle.

Plus if he was running lean his engine would have likely holed a piston had it continued for 1500 miles under heavy load!

Alright. That makes sense. Thank you.
That had to be some serious heat to melt a hanger.


Indeed it was. But I have seen similar damage on a car with a dead cylinder and a catalytic converter. The dead cylinder pumped enough fuel into the cat to make it burn the undercoating off the car! Must have been unbelievably hot.

 

[meep]

Originally Posted By: Shannow
Originally Posted By: Clevy
Originally Posted By: crazyoildude
higher octane usually means more carbon. I have seen it many times

Ditto. More octane equals harder to burn and can lend to incomplete burn. Could be why there is more detergent in premium?


Nope, not even close.

It's resistance to auto-ignition, not difficulty in burning.


i don't think it's as simple on either side. Higher octane = resistance to detonation/auto-ignition. ALSO has slower flame speed. Flame speed will be influenced by density / compression. THAT behavior will be influenced by the engine type, compression, RPM, fuel stratification. Compression could be influenced by.... presence of other carbon in the CC. Lots of variables here.

Since I don't have an opinion other than thinking there is more here than a simple "this does that" rule, I'll go back under my rock now.

 

[UNC017]

That's hard to say. It could be due to driving pattern (short/city/cold vs more long/hot/highway). could e that the other one burns more oil. Could be the other one has a less effective catcon for some reason. Could be additive differences in the fuel, not related to octane.

Many, many factors to consider.