Topic: Octane?

[Duke McDukiedook]

87 octane on my 750 K3, runs great with it.

[Inigo Montoya]

Timing is another reason to use different octanes. If you have knock, you can increase octane to stop the knock.

[medic09]

It works fine.  '78 750Four.

89 Octane in the '01 Sprint 955i.

[BeSeeingYou]

None of the stock SOHC4 Hondas require anything but the lowest octane pump gas.  Their compression ratios are too low to require anything higher.
The octane rating has nothing to do with the energy content of the gasoline.  It does have to do with the temperature and pressure for it to self ignite.  Higher octane gas burns slower and cooler than regular grade. If you use it in engines that don't require it, it leaves deposit build ups over time that will increase compression ratio and form hot embers to light off the next charge prematurely, necessitating a slower burning, more difficult to ignite fuel, which IS high octane gasoline.  Engines sort of get addicted to the high octane, without receiving much (if any) benefit from it.

I have to wonder, why in today's era of high gas prices, would anyone bolster the coffers of oil companies by paying even more for fuel than is necessary to run their engines safely and properly?

Mysteries of the human psyche....

I got shot down on another forum for making an identical reply to a similar question.  But I have always thought it true.  You can make an octane junkie by doing this.  I use 87 in my XS650 and CB750F.

[Gordon]

None of the stock SOHC4 Hondas require anything but the lowest octane pump gas.  Their compression ratios are too low to require anything higher.
The octane rating has nothing to do with the energy content of the gasoline.  It does have to do with the temperature and pressure for it to self ignite.  Higher octane gas burns slower and cooler than regular grade. If you use it in engines that don't require it, it leaves deposit build ups over time that will increase compression ratio and form hot embers to light off the next charge prematurely, necessitating a slower burning, more difficult to ignite fuel, which IS high octane gasoline.  Engines sort of get addicted to the high octane, without receiving much (if any) benefit from it.

I have to wonder, why in today's era of high gas prices, would anyone bolster the coffers of oil companies by paying even more for fuel than is necessary to run their engines safely and properly?

Mysteries of the human psyche....

I got shot down on another forum for making an identical reply to a similar question.  But I have always thought it true.  You can make an octane junkie by doing this.  I use 87 in my XS650 and CB750F.

A lot of people aren't too happy about admitting that they've been duped by slick marketing into spending more money than they needed to, and would rather just continue to waste their money and try to convince others that they should waste theirs, too.

I'm sure I do the same thing fairly often in other areas of my life, but at least gasoline isn't one of them.  

[Terry in Australia]

Interestingly enough, my new Triumph Rocket 3 only runs 8.9:1 compression (lower than a CB750) but requires a minimum of 95 RON fuel?

I certainly haven't tried anything lower yet as it hasn't had it's first service, (tomorrow) but I'm wondering why a relatively "low compression" engine needs top grade (and top dollar) fuel? Do any experts out there have a clue? Cheers, Terry.

Because there is more to required octane than just compression ratio.   My old Yamaha R6 with 12:5 compression that revved to 15K RPM required only regular fuel (87 pump octane in the US), but many very low compression air-cooled twins require premium and still tend to ping in hot weather.

Big pistons and low RPM are the reasons in this case.  The Rocket 3 has huge 765cc pistons and runs at ultra-low RPM.  Despite liquid cooling it's very difficult to ignite such large volumes of mixture at slow speeds without getting big variations in mixture across the combustion volume which causes pinging.

Everything else being equal, high CR, low RPM, air-cooling, and carbs tend to ping more than low CR, high RPM, liquid-cooling, and fuel injection.  And that's just the big factors - there's also combustion chamber design, 2-valve vs. 4-valve, types of valves, bore vs. stroke, etc. etc. etc.    There's a lot more to an engine's octane requirement than just CR.

- Mark

Thanks Mark, that makes sense to me! Cheers, Terry.

[manjisann]

Interestingly enough, my new Triumph Rocket 3 only runs 8.9:1 compression (lower than a CB750) but requires a minimum of 95 RON fuel?

I certainly haven't tried anything lower yet as it hasn't had it's first service, (tomorrow) but I'm wondering why a relatively "low compression" engine needs top grade (and top dollar) fuel? Do any experts out there have a clue? Cheers, Terry.

Ahh come on, the simplest answer is because Rockets require Rocket fuel! 

I use the lowest I can find, 85 or 87 I can't remember which. I read one of the previous posts explaining that octane had nothing to do with power etc, and that our stock bikes were originally configured for lower anyhow. Seemed pointless to spend money on something that wasn't gonna do any good anyhow.

Brandon

[Terry in Australia]

Interestingly enough, my new Triumph Rocket 3 only runs 8.9:1 compression (lower than a CB750) but requires a minimum of 95 RON fuel?

I certainly haven't tried anything lower yet as it hasn't had it's first service, (tomorrow) but I'm wondering why a relatively "low compression" engine needs top grade (and top dollar) fuel? Do any experts out there have a clue? Cheers, Terry.

Because there is more to required octane than just compression ratio.   My old Yamaha R6 with 12:5 compression that revved to 15K RPM required only regular fuel (87 pump octane in the US), but many very low compression air-cooled twins require premium and still tend to ping in hot weather.

Big pistons and low RPM are the reasons in this case.  The Rocket 3 has huge 765cc pistons and runs at ultra-low RPM.  Despite liquid cooling it's very difficult to ignite such large volumes of mixture at slow speeds without getting big variations in mixture across the combustion volume which causes pinging.

Everything else being equal, high CR, low RPM, air-cooling, and carbs tend to ping more than low CR, high RPM, liquid-cooling, and fuel injection.  And that's just the big factors - there's also combustion chamber design, 2-valve vs. 4-valve, types of valves, bore vs. stroke, etc. etc. etc.    There's a lot more to an engine's octane requirement than just CR.

- Mark

Well after deciding that I undersood all that, I thought about my old Ford car sitting in the driveway. It's a big old water-cooled 6 with about the same bore and stroke as the Rocket (albeit with twice as many cylinders) but 2 valves per cylinder compared to 4 valves in the Rocket, 1 spark plug per cylinder compared to 2 for the Rocket, and runs a fairly inefficient ignition system and twin throat carb, but it only needs low octane fuel?

By my slightly skewed reckoning, the Rocket, with modern electronic fuel injection and engine management, 4 valves per cylinder and 2 spark plugs across those massive combustion chambers would operate much more efficiently than my 23 year old Ford, so once again, I wonder why the Rocket needs high octane fuel when my old Ford doesn't? Cheers, Terry.

[markjenn]

By my slightly skewed reckoning, the Rocket, with modern electronic fuel injection and engine management, 4 valves per cylinder and 2 spark plugs across those massive combustion chambers would operate much more efficiently than my 23 year old Ford, so once again, I wonder why the Rocket needs high octane fuel when my old Ford doesn't? Cheers, Terry.

Hard to say for sure, but I'd guess specific power output (engine tuning) is the reason.  A Rocket 3 makes about 140 hp from 2300cc, or about 57 hp/liter.  My guess is that your old Ford is about 4100cc and makes about 150 hp, or about 37 hp/liter.

And maybe Triumph is just being more conservative or even has decided to make the Rocket 3 a premium fuel bike for marketing reasons.  Customers tend to associate higher performance machines with the use of premium fuel.

- Mark

[Terry in Australia]

Thanks Mark, you were "spot on" regarding the Ford's capacity and BHP figures, I'm not sure about the "performance marketing" theory though, Britain is about the most expensive country in the world to buy gasoline, so if the Rocket didn't need it, I'm sure Triumph would be more than happy to tell potential "Rocketeers", that they can use the cheaper fuel to power their bikes, as British car manufacturers do with their marketing, no-one in the UK wants to spend more money on fuel than they have to. Cheers, Terry.

[cudjo]

I just filled my tank up, i used 93 just to see if there was any difference.
by the time i got home ( about a mile from the gas stop) the bike was smoking a bit if i revved it while it was in neutral?
the smoke was pretty white (dont think that I saw any blue) and it smelled like gas.
would the change in gas, or a mix of whatever was in there have caused this?
should I not have stopped riding so soon after a fill up?

i have a 78 750F
thanks

[HondaMan]

Interestingly enough, my new Triumph Rocket 3 only runs 8.9:1 compression (lower than a CB750) but requires a minimum of 95 RON fuel?

I certainly haven't tried anything lower yet as it hasn't had it's first service, (tomorrow) but I'm wondering why a relatively "low compression" engine needs top grade (and top dollar) fuel? Do any experts out there have a clue? Cheers, Terry.

In the "old days (i.e. 1960s, when I was a younger so-and-so)" the Brits used to use a LOT of spark advance on their hopped-up versions of the Trumpet twins (like the TR750, etc.). That required lots of octane as the result. I also found that it fouled a lot of sparkplugs and made for short bore life, with high oil consumption, too. By backing off on the advance a few degrees, cutting half a turn off the advancer springs, and leaning out the carbs about 5% to 8%, they ran BETTER than with the stock "hi-perf" settings, and started easier. And ran on normal gas (like regular), with cleaner plugs....maybe the "3" is like that? I never got to do much with those engines, myself. 

[Gorms]

I use 87.  550's have a CR of 9:1 which is low.  Anything higher is a waste of money as the higher octane just protects against preignition in high compression engines.  I actually got asked at the pump last week why I was putting 87 in my '72 Porsche 911S.  I was gonna say that its CR is only 8.5:1 so preignition is not a problem but i just said "It doesnt need it"   Most people just think higher is better.

[Markcb750]

I run 89 in my CB 750 and my 1500 cc Vulcan.  The CB does not seem to run any better, but someone here who I trust tells us the current 89 is close to the octane recommended by Honda.


Kawasaki says regular is OK for the Vulcan, But I found it knocked a little under acceleration and in the hills. so I started running 89, because I am a cheap SOB.  

[Grnrngr]

I run regular in everything, on the few occasions I ran premium in my 750, didn't really notice any difference in performance, don't remember now if there was improved mileage but I dont think so, and yeah..I'm in the "don't give them Ba$^&#s anymore than you absolutely need to" crowd...

[HondaMan]

Adjusted for altitude, the 89 in my vote is 87 octane here. 

With the alcohol content in most of our fuels, a slightly lower octane will help keep the plugs cleaner in city traffic. On the open road, it's premium, though.

[Retro Rocket]

So at 10.5 to 1 compression what would you guys recommend..?

[Terry in Australia]

So at 10.5 to 1 compression what would you guys recommend..?

Shell Optimax 98 RON fuel is what I run all my bikes on Mick. I couldn't give a crap about wasting my money, I drink and smoke and pay taxes to make that greedy red-headed witch happy, so another buck or two per tank-full doesn't bother me, especially since I drive a company car, so my fuel bill is next to nothing anyway.

All of my CB750's with CR ranging from 9:1 to 12.5:1 seem to run very well on Optimax, and they "feel" (using my recently calibrated "seat of the pants O meter") smoother than when I run regular (91 RON) fuel. The last thing you want is damage caused by detonation as a result of you going cheap and running low octane fuel in your new engine. Cheers, Terry.  

[Retro Rocket]

Thanks mate, thats what i was thinking, just wanted some opinions.....

[anthony]

i run 94 the reason is that it has little to no additives that could be harmful on old engines meant to run on leaded fuel.

[mrbreeze]

i run 94 the reason is that it has little to no additives that could be harmful on old engines meant to run on leaded fuel.

These bikes don't need leaded gas. If they did.....there would be lots of people pulling out engines for head work (hardened valve seats). Our bikes are fine with unleaded. I drove a 69' Chevy truck with a 65' Vette 327 for about 1 1/2 years after the last trace of leaded gas in Wyoming was depleted . It slowly developed power lose until finally dropping a couple of valves (350 heads to the rescue!!!).

[anthony]

sorry someone told me they ran a lot better on low lead fuel then the fuels of today.

[TwoTired]

sorry someone told me they ran a lot better on low lead fuel then the fuels of today.

They ran a lot better on fuel without ethanol dilution.

[Pinhead]

There are a TON of variables that effect an engine's propensity to experience detonation... In-chamber turbulence, air/fuel mixture quality, intake air temperature, ignition advance requirements (which differs between engines according to the first three variables), dynamic ompression ratio (cylinder pressure), combustion chamber size... That's why engines with drastically different compression ratios can require seemingly strange octane ratings.

Which is why the previously noted 8.9:1 CR engine requires higher octane rated fuel than my 11:1 CR CB650 engine.

[Danno]

I always use premium or the highest octane possible since as pointed out they were designed to run on regular leaded gasoline and without the lead now higher octane is prudent my slightly modified cb550 runs much better on it but does not ping without it the only real worry is lead did two things it increased the duration of the burn and burned lead or lead oxide is also known as teflon like in the cookware it lubed the top of the cylinder and combustion chamber so you would not carbon up the combustion chamber as much i do not run additives to replace lead and have had no problems but i still would not run it on low grade gas