Topic: Gas - What octane do you use?

[bjbuchanan]

Higher octane fuel resists detonation for longer than lower octane. The whole point for using it right? When you use higher than necessary there can be excess carbon buildup due to an incomplete burn. You get an incomplete burn because that higher octane resists burning longer and your combination of spark and compression just isn't enough to expend all of what's in the chamber.

I don't know where you learned all this but I can't follow the logic.

What are you missing? You don't burn all the fuel, it becomes a carbon deposit. You don't believe me look it up. I know I'm right, I'm not gonna waste time posting links that you will not follow, I.e. edwardmorris

[Deltarider]

Aren't explosion and combustion different phenomena? I just wonder why I've never heard of this 'excessive' carbon buildup before. Believe me, I have only read about this in this forum. That's why I asked. I am sorry I waisted your time by my dumb asking.

[dave500]

in the old days of high octane leaded fuel you would find lead deposits on plugs and exhaust valves etc if you overdosed on using too high octane fuel for your engine,removing lead has been one of the best things for petrol engines,exhaust systems last longer,they run cleaner off course,less fouled plugs.

[Bodi]

Higher octane fuel resists preignition better than low octane fuel. That's all: it needs a higher temperature to ignite.
Compression increases a gas's temperature, compression ratios above 14:1 will cause preignition with almost any gasoline - over that and you're into diesel territory where the engine relies on compression heat for ignition.
Once ignited by the extremely hot spark, high octane fuel is going to completely burn. Using it does not increase carbon deposits. Using it in an engine that doesn't need it only lightens the wallet.
Carbon deposits are either from incomplete combustion due to an overly rich mixture, or oil getting into the combustion chamber. With carbureted engines the mixture can not be tightly controlled (even barometer changes affect mixture) so to avoid overheating and seizing from a too-lean condition the mixture is set slightly rich. Engines with closed loop EFI should not have the carbon buildup problem of carb engines. I have opened high mileage EFI engines that look pretty clean but a normal high mileage carbureted engine always has some carbon on the piston head and chamber.

[edwardmorris]

Higher octane fuel resists preignition better than low octane fuel. That's all: it needs a higher temperature to ignite.
Compression increases a gas's temperature, compression ratios above 14:1 will cause preignition with almost any gasoline - over that and you're into diesel territory where the engine relies on compression heat for ignition.
Once ignited by the extremely hot spark, high octane fuel is going to completely burn. Using it does not increase carbon deposits. Using it in an engine that doesn't need it only lightens the wallet.
Carbon deposits are either from incomplete combustion due to an overly rich mixture, or oil getting into the combustion chamber. With carbureted engines the mixture can not be tightly controlled (even barometer changes affect mixture) so to avoid overheating and seizing from a too-lean condition the mixture is set slightly rich. Engines with closed loop EFI should not have the carbon buildup problem of carb engines. I have opened high mileage EFI engines that look pretty clean but a normal high mileage carbureted engine always has some carbon on the piston head and chamber.

This was in my head in the most absurdly jumbled manner, all straightened out now, thanks!

[DaveBarbier]

Although, as is stated in other texts and maybe above but didn't see it, is that a motor that has been designed for 87 octane still might require 91 or higher later in life due to natural carbon build-up increasing compression.


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1978 Honda CB550K

[Mooshie]

 I use 91 octane as when I tried 85 and 89 my gas mileage went down.  I like tracking MPG's.  ;)Once back on 91 it sprung to life again (50 mpg on 91regardless of type of riding and on the others between 43-47 depending on type of riding)

[Deltarider]

Although, as is stated in other texts and maybe above but didn't see it, is that a motor that has been designed for 87 octane still might require 91 or higher later in life due to natural carbon build-up increasing compression.

Yep, so it is. That's maybe why Honda The Netherlands back then - both the importer as well at the dealers - have always advised me to tank what we call 'super' to stay on the safe side. Mechanics that have worked on hundreds of 500s or 550s (and some of them owned one) had best results with super and a few extra degrees advance (just outside the full advance marks). Who am I to question their expertise?
The last few years I've been using designer fuels like Shell's V-Power and BP Ultimate or Aral who claim to clean very well and contain friction modifiers for the top rings. Good stuff but it's a bit more expensive. There's no downside of using a higher octane than strictly needed. Maybe in your wallet. Those who believe there is a disadvantage fell victim to cognitive dissonance I'm afraid.
If you suspect carbon build up, do yourself and your bike a favor and go for a blast ride. Everytime I leave the highway after an hour of 7000 rpm in 5th my bike seems younger and bitier. Unfortunately those that don't know the difference between detonation/explosion and combustion will probably continue to (re)produce myths. Thank you for your attention.

[TwoTired]

That's maybe why Honda The Netherlands back then - both the importer as well at the dealers - have always advised me to tank what we call 'super'. Mechanics that have worked on hundreds of 500s or 550s (and some of them owned one) had best results with super and a few extra degrees advance (just outside the full advance marks). Who was I to question their expertise?

Isn't this what you like to call "mist"?  Or at least, testimonials based on absolutely no reproducible, verifiable facts, or supportive data.

Of course, I'm sure you believe your selected "experiences" are worth far more than any actual facts to be discovered.  It's amazing how much smarter you are than the people who produced the bike in the first place and provided a manual for the owners to read.

Truly astonishing.

But, here are couple of facts to point out....
Has anybody noticed there is a de-coking procedure in the Honda shop manual?  (I know some feel that the Honda manuals are irrelevant, particularly in preference to personal opinion.) 
These engines run rich as a matter of routine.  It's an artifact of the air cooled nature of the beast, the fuel's heat absorption is part of the cooling method.  It doesn't all get burned and what isn't chucked out the exhaust (heat and fuel), gets deposited in the combustion chamber as coke.
This does change the compression ratio.  And, if not decoked as recommended, can lead to the engine requiring a higher octane fuel to curtail detonation.
There was an earlier thread about decoking using water injection.  But, that is a tricky process that is just too risky to put into a shop manual.  Because, if not done in a sane manner and under the proper conditions, will severely damage the engine.


P.S.  Doesn't surprise me that the Dutch variants behave differently than the rest of the world's Hondas.  If similar to Delta's bike, it has a permanent restrictive choking device ahead of the air filter.  Gonna run rich pretty much most of the time, despite only #78 main jets.  Not hard to understand why it would coke up really fast.

[Steve_K]

I usually don't care what octane I use.  I will use ethanol if it is there.  I do use 93 octane with out ethanol on the last 2 or 3 tanks to rid the tank of ethanol for long term storage.  Am I wrong?  I don't care, this has worked for me for years.
Steve

[DaveBarbier]

Am I wrong?  I don't care, this has worked for me for years.
Steve

Haha, that is a hilarious group of sentences.


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1978 Honda CB550K

[BobbyR]

I will throw another Turd in this punchbowl. My car is turbocharged so the Manufacturer tells me I can run regular 87. If I want higher HP I can run higher Octane. The engine control system adapts.

Having said that, I have run regular in my 750 and I have run Premium this summer. I can tell no difference using my seat pf the pants dyno.

[seanbarney41]

Bobby, that is because high octane fuel adds no power even in an engine that requires it.  The extra power is from higher compression, higher cylinder pressure (such as forced induction i.e. a turbo or supercharger, or nitrous), or more advanced ignition timing(also results in higher cylinder pressure)

[BobbyR]

Yes I know. The engine control processors control the turbo, timing, and direct injection. This is the reason I lease these cars, no way am I owning all the techno #$%* down the line. 

[Dunk]

I run 89 octane in my CB750. Stock pistons, chambers and piston tops polished, ported head, stock cam. Never was sure on what these engines need and 89 doesn't detonate so I just keep using it. May run fine on 87, don't know. More octane won't hurt, lesss octane than required will destroy pistons, rings, bearings, etc... If these engines really only need 87 then I'd be fine running that.

[dave500]

if you've ever watched a few documentaries when they interview old ww2 pilots or tank drivers or something,saying how they have high octane fuel onboard they don't want shot to catch fire,wouldnt matter what fuel it was?

[my79650]

My comment earlier didn't just apply to our motorcycles. I had the issue with the cleaning with my 2 year old lawn mower which the owner manual claimed could operate on 10% ethanol. Which after a couple of tanks had to be cleaned several times in order to get it to run on supreme gas after using regular, which is why i only run the regular pump gas if i don't have a choice with my cb650.
Take my point of view how it messed up a single carburetor, it would do the same to a multi carb setup, i should have been more clear on how and why i am dead against using it in anything other than my car. Plus my bike only sits winterized from now til April.

[dave500]

man were lucky here,i can leave fuel in chainsaws etc for near two years and they'll fire up third or fourth pull on the rope,im not #$%*ting you its true!

[DaveBarbier]

My comment earlier didn't just apply to our motorcycles. I had the issue with the cleaning with my 2 year old lawn mower which the owner manual claimed could operate on 10% ethanol. Which after a couple of tanks had to be cleaned several times in order to get it to run on supreme gas after using regular, which is why i only run the regular pump gas if i don't have a choice with my cb650.
Take my point of view how it messed up a single carburetor, it would do the same to a multi carb setup, i should have been more clear on how and why i am dead against using it in anything other than my car. Plus my bike only sits winterized from now til April.

I disagree that your issue was related to regular pump gas. If that's the case, how come so many of us run it without issue? Are we all riding around with layers of caked on varnish in our carbs? Basing your irrational fear on regular pump gas just from a lawn mower that didn't work well to me is just being obtuse. Show me any evidence that regular pump gas is as detrimental as you think.

Haha, I feel like I'm defending 87 octane like it's a person who has feelings.


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1978 Honda CB550K

[HondanutRider]

man were lucky here,i can leave fuel in chainsaws etc for near two years and they'll fire up third or fourth pull on the rope,im not #$%*ting you its true!

I've also had some experience with chainsaws that sit for quite a period, and then get started with no problems.  Mine sit for sometimes 6 m-9 months, but the fuel might go longer.  Because they are 2-stroke, the gas mixture I use has always been a premium synthetic oil that seems to be better at reducing deposits and has a fuel stabilizer added that keeps the fuel from deteriorating.

At any rate, this year after more than 10 years of this use, I had to get my chainsaw overhauled as it seems the regular grade gas with ethanol had rotted the fuel line to the carb and it was leaking so bad it wouldn't run   I've switched to high-test that doesn't contain ethanol for this reason.

[ekpent]

Not to add fuel to the fire-HA HA- but small two stroke engines like chainsaws, weed whips etc should use premium from the research I have read. I use just the regular in my 750's and some other bikes.

[PeWe]

If my CB750 engine was stock I might use 95 octane as we have here in Sweden (with 5% ethanol)
I have 836 kit and I have no reason to downgrade the power (retard ignition). Therefore Shell V-power 98+ octane

EU 95 octane = US 91 octane
EU 98 octane = US 93 octane.

Same with my chip tuned Audi turbo car that has the program for 98 octane or more. Even stock engine need 98 according to the manual, 95 possible with less power. I have read about guys that tried lower octane with result of an engine drinking more fuel/10km.
Why buy the stronger engine and cheap out on the fuel? I buy the extra HP for using them against sleeping people on the road and a quick passage, not showing off with the emblem.

It's never wrong with higher octane. Cost some more though.

[bender01]

I luckily live near the ocean. We have 89 ethanol free at many places. I use it in my toys and tools 25 year old car! I have no problem using 87 with ethanol on a trip since it likely will be gone in 130 miles! I have noticed no real change in mpgs unless im putting 70 mph on the interstate! Usually im passing everything I can to know my surroundings! I use my tools more than some of my friends but im sure they buy a weed whacker every two years cause they use it once a year! I had a friends torn apart that just wouldn't prime any more and it had to be cheap gas and cheap carb parts! I think it was an actual "weed whacker brand" .Ive got a crap leaf blower Poulan that keeps on going! It will be replaced with a Stihl someday but it keeps going!