Topic: gas

[Sandcast 2356]

Does anyone know if there is some sort of advantage to using a particular grade of fuel over another in my '69 CB 750??

[heffay]

got gas?
i was speaking with an engineer friend of mine who did a study while in college on different grades of gas.  his studies were found to be very minimal in the differences between grades if not any difference at all.  if you would like i could talk with him a little further in depth and get back with you.  but, basically what i can tell you from my studies at mmi is that the octane rating really refers to the cleanliness at which the gas burns.  meaning, the higher the octane the more efficiently the fuel is being consumed rather than how much better or worse it might be for a motor.  as i see it... an internal combustion engine is an internal combustion engine and they are not specifically designed for a certain octane.  i usually stick to 87 or higher in my cb350 simply due to the fact that it MIGHT be better for the environment aside from the fact that it is depleting our fossil fuel resource.  but, in order to produce a higher octane fuel requires more energy anyway which is why this a somewhat controversial topic.

[Bob Wessner]

I've a '69 750 also and it runs and always did run just fine on regular, or 87 octane.

[Bob Wessner]

Unleaded fuel should pose no problem for the early Hondas. The valve seats are hardened..er, or at least they were and still should be unless worn through.

[Bryan Boyle]

Actually, the octane value is a measure of the resistance of the fuel-air mixture to combustion.  Higher octane=slower combustion, which means that the chance of preignition or detonation is reduced.

Higher comression engines typically need it, which is why the gas companies provide the high octane stuff in the
first place.  Interesting point is that in the 30s, the typical octane value was around 60 or so.   Today, in the cage
world, the absolute need for the good stuff is reduced somewhat by the ignition and fuel computers in most cars
that compensate for low octane by changing the power mapping of the engine (that's one reason for the knock
sensor...).  You do pay for it in some respects, since the change to the map will also change the efficiency and power
output of your engine. 

That being said, what should you do?  First off: run the best QUALITY gas you can buy.  While the off-brands may be
a tempting sight as you speed past it with its .05 a gallon advantage, typically those stations are serviced with the
tail end of the refinery runs (after the big guys take their allotted amount).  While it (on paper anyway) meets spec,
it's not the 'first squeezings' by any stretch.  (I used to work for a major oil company, ok???  )

Second: Use the octane specified in the owner's manual if you're stock.  If you've gone high compression or down that
route, you have nothing to go on.  So, you use the octane value that won't cause pinging in your engine.  A good rule
of thumb is that anything over 9:1 compression and normal advance curve is going to start asking for 89 or 92 pump
octane fuel. 

Third: by the middle of the 60s, and certainly by the time the 750 change everything, valve seats were already hardened.
So, unleaded fuel is ok for them. 

That being said, if you have a local small airport around, it may be worthwhile to make friends with the folks there.  Why?
Those planes you see out there run on 100 octane LEADED fuel.  And they're air cooled and carbureted besides.  I can
tell you from experience that my CBs run just fine on avgas...  Side benefit is that the stuff is remarkably clean, free
of debris and water, and even smells like gas used to smell like.    Bad side is that it is in the 3.50-3.75 a gallon range
right now (last time I filled my tanks...).  But, running a tank or two once a year is probably worth it to get some lead
up on the valve seats. 

Just some random thoughts....

[cben750f0]

with the 836 kit i run premium unleaded.. its about 95-98 RON... and its been drinking it for the past 10K km... run the unleaded alls good...peace

[cb750_chris]

I use high test in my Suzi but I'm sure it's really not needed.  I don't race it, and its not a high compression engine.  These early hona engines are not high compression either.  On the other hand they only have two valves per cylinder, and the squash chamber is probably not that good. 

I would try the low test, and move up if you hear any pinging.   As long as you don't red line the tach you'll probably be fine. 

[heffay]

just like to say this is all fantastic info and this is why i keep coming back. 

[Sandcast 2356]

I agree with Heffay, learn something new every time I log on!! Thanks!

[cb650]

I used to deliver fuel and you would be suprised on stations putting reg in all there tanks. Prem should have some tint to it.  Either yellow or red.  Also dont get gas while the truck is there droping off.  Stirs up the #$%* on the bottom which wouldnt be a problem if they maintain the filters in the pumps but you relieing on someone doing there job.



                   Terry

[Salem350F]

Very interesting discussion on the gasoline qualities.  I always run premium in my CB 450 twin  and will be running it in the CB 350F project I will have on the road soon.  For the few penniies more on a fill up, the anti detonation qualities are worth it. 

The Avgas issue brought up by one memeber is interesting and brought back memories.  I have an Airframe and Powerplant license and in the old days when I was bending wrenches at a FBO in the late 70's early 80's all the mnechanics bikes ran on 100%  100 octane all the time.  I was riding a Yamaha RD-350 and my budies had 750F's we always filled our bikes up from the drained fuel from aircraft being workied on, one of the wise guys even had the odd days left wing, even days right wing sign posted ( if you remember the odd-even rationing then)  The bikes ran great the exahust smelled sweet, and the commute was cheap.  We akso got jet fuel for our home heaters in the winter.

[heffay]

in the late 70's early 80's all the mnechanics bikes ran on 100%  100 octane all the time.

when i was motocrossing in AZ several years back i too ran this fuel but, was under the impression the octane was even higher (like 115-120 if i remember right, which is a problem).  now, is that really possible?

[Salem350F]

Avgas used to come in 80, 100, 115 octane before everything went to 100LL ( low lead).

The 115 was mostly military use for the big radial engines.  I was at an air show and a B-24 was there.  I spoke to the crew cheif about what they were doing for fuel, he said they ran fine, but they didn't push things hard,  lower manifold pressures etc. There were no detonation problems , power was Ok.  But then again they were not flying at 24,000 ft over the 3rd Reich with a maximum load

[jsaab2748]

On this same subject, does anybody know the effects, if any, that ethanol will have when blended into
gasoline? Can it make a difference in the way an sohc 4 acts?  A ten percent blend of ethanol ( in all grades) is all that's available at the pump where I live. I've heard it's in there for emissions purposes.
Just curious. Thanks. jim

[heffay]

In order to become old and wise we must first be young and foolish

sorry for the interruption  i just noticed this and would like to say all the old and wise on here have schooled me already. 

[cb650]

Ethanol is made from corn like alcohol I belive.  Heard it isnt nice on rubber but its usually only a 10% blend.



                                 Terry

[Gordon]

Just my $0.02 worth based of what I've learned in the past reading up on this particular subject:

A common misconception is that high octane fuel has more energy per volume than regular gas.  This is actually opposite of the truth.   As has been said before, what the added octane does is allow an engine with a high compression ratio to run without pre-detonating or pinging.  Adding octane to gas actually reduces the total amount of energy (although very slightly), but this is more than made up for by running at a higher compression.  If an engine was not built specifically to take advantage of higher octane gas, you're only spending more money and sacrificing a tiny bit of power. 

Gordon