Topic: Regular or hitest gas

[6adan]

   Question on regular or hitest gas. I have allways used hitest gas in my 70 CB750,I bought it new. I have looked at FAQ and am still not sure if I should or could switch to regular gas. Can anyone tell me for sure? Thanks Dannie

[canyon750]

There is no reason what-so-ever to run "hitest" or premium gas.  A lot of times the hi grade gas will have detergents and additives that will hurt your bike more than help it.  That's the word i got from a mechanic that works on nothing but old Honda's like the one's we love so much!    Save your money and the possible damage and run the regular.

My $0.02 USD.

[neil young]

i have run on both and i found my cb 550 runs better on cheap gas .why i dont know.and i'm not going to question it

[rhinoracer]

Hi test or high octane gas won't give you any more power than regular gas.

What high octane does is permit you to run a higher compression ratio which will net you more power. But the simple use of hitest on a stock motor is a waste of your money.

[TwoTired]

Unless you've increased the compression ratio of the motor, regular fuel is all you need or require.  Hi test won't give you any more power.  But, if you feel the oil companies need higher profits, do use Premium whenever you wish.

Cheers,

[Tom Stark]

Octane is resistance to detonation.  Detonation will destroy a motor in short order.  The more power a motor makes, the faster it blows up from detonation.  If your bike does not detonate with regular, that is all you need.  If it DOES detonate, then you should check your timing, because unless you modified the motor, it does not have enough compression to require premium.  Extra octane will NOT make more power.  In fact, octane over what you need may well actually LOSE power for you.  Higher octane gas burns more slowly than gas with more heptane.  (97 octane gas has 97% octane and 3% heptane.  If I recall correctly, the difference between the hydrocarbon molecules of octane and heptane is one carbon atom.)
Do not use more octane than you need to prevent detonation.  This rule applies to cars also, BUT most cars today have detonation sensors, and will retard timing at the first hint of detonation, so you need to know what octane is required, either by manufacturer's recommendation or engine builder's.
Tom

[greenjeans]

Anybody know what effects, if any, that ethanol has on these things ?
Down here in Texas, you can't find gas without it having up to 10-15% ethanol in there.
Wasn't in there back in the 70's....

[luvhonda750]

any motor over 9:1 compession should run premium, it will just run better and last longer

[6adan]

  Thank everyone. I think I will try a tank of regular next fill up and see what happines. I was just afraid it might have some carbon buildup from all the years of using hi octane gas,it has about 34000 miles in it. Thanks again  Dannie

[Gordon]

Highest is always better...

Sorry, but that's just wrong.  As others have pointed out in this thread, and many, many other times in previous discussion, you will only see a benefit from high octane fuel if you have a high compression engine that requires it to keep the fuel from pre-igniting.  High octane fuel does not contain more energy.  The misconception comes from the fact that high power, high compression engines that create more power tend to require the use of high octane fuel, but they don't get their extra power from that fuel. 

[LoopsAndLogic]

Would you say the 400f's should use 93 octane?? There running a 9.4:1 ratio.

I've been using regular for 600miles, but did notice it pre-ignited once when I was revving at 5k for a long time.

Clarification on this would be great,

LL

[TwoTired]

Anybody know what effects, if any, that ethanol has on these things ?
Down here in Texas, you can't find gas without it having up to 10-15% ethanol in there.
Wasn't in there back in the 70's....

Ethanol isn't a good thing for old technology machines like our SOHC4s. It's not the ethanol itself, actually, it is the reaction of ethanol with oxygen and water and the formation of acetic acid.
Ethanol has quite the affinity for water and will draw it right out of the humidity in the air, where oxygen is also found.  A small application of heat and you get acetic acid, which is corrosive to aluminum and other metals.  All the SOHC4's have fuel systems vented to the atmosphere.

One way to combat damage is to keep the machine constantly running. The acids don't have time to settle in and eat at metals in the fuel system (carburetor bits) and head toward the exhaust.  Unless you have stainless steel replacement exhaust, it will corrode away faster, too.

It's going to be harder for preservationists to resurrect a bike that has been sitting a while with ethanol in the fuel system.

The recommendation for long term survival of ethanol using machines, is to have a lining coat of PTFE.  I wonder what that would cost?

Cheers,

[Gordon]

Would you say the 400f's should use 93 octane?? There running a 9.4:1 ratio.

I've been using regular for 600miles, but did notice it pre-ignited once when I was revving at 5k for a long time.

Clarification on this would be great,

LL

None of the sohc4's need high octane fuel unless they've been modified for higher compression or there is a large build-up of carbon in the combustion chambers causing abnormally high compression, in which case it's time for the top end to be cleaned out.

[Tom Stark]

Agree 100% with you Gordon.  Just a quick FYI, you can "steam clean" quite a bit inside the motor by running it at higher RPM, maybe 5500 or so, and shooting water through the intake.  On a bike it would be a little hard with the IR set up.  Also, you have to be sure not to use too much or you could cause REAL damage.  But the water will do a hell of a job cleaning.  Anyone who has ever torn down a motor that had water injection, or a blown head gasket, can tell you how much a little water can do!
Also, so far as compression and octane goes on our bikes, these bikes have fairly large cams.  I think the cam in my buddy's 500 Honda is 234 @ .050  I have no idea what my 750 cam is, (but if I work on the motor this winter, I will measure it for the heck of it), but I am sure it is in the ball park of 234 or 236 @ .050  Those are fairly big cams, and allow a lot of compression bleed off at low speed.  This reduces the octane requirement of the engine.  In my car, I run 10.5:1 measured compression, (on 5 holes, the sixth has about 10.7:1 compression), and am running a cam with 208 @ .050, and it runs just fine on 93 octane.  The car weighs 2800 pounds, and has 3.55 gears with 23.2" tall tires.   Our bikes with ADVERTISED compression of around 9:1 or 9.5:1 do not need anywhere near as much octane as the car described does.  Plus, remember, ADVETISED cr and actual are two different things.  With cars, advertised compression is almost always a full point more than actual.  That way they can have tolerance stack up, all in the wrong direction, and not go over what they advertise.  I would think the bikes are a little closer, but still I bet if they advertise 9:1 it is probably closer to 8.5:1
Tom

[medic09]

Anybody know what effects, if any, that ethanol has on these things ?
Down here in Texas, you can't find gas without it having up to 10-15% ethanol in there.
Wasn't in there back in the 70's....

TT already answered.  I'll  just add, anecdotally, that I accidentally ran a tank of gas with ethanol recently.  My K8 was *not* happy.  Once it was ll gone, bike started running better.  Just a one-time, uncontrolled-for-other-variables observation.

[Tom Stark]

In addition to what TT said about ethanol, depending on the % of ethanol added, your bike will be lean.  Not sure why yours "wasn't happy" but could be due to the lean aspect.  Motors can make more power on ethanol IF they are set up for it.  It is my hope that E85 catches on around here, because my hot rod can make more power.  But you have to burn more fuel.  Methanol is close to 2:1 for gas, one pound of methanol will do the work of one pound of gas.  Ethanol is not quite as bad.  I do not know the exact ratio but it is more like 1.5:1 or so.  (don't quote the ratio, just know it is better than methanol!)  Getting the engines tuned to run with ethanol is not a problem.  You can add compression and or timing, and fatten up the jets.  The real problem comes in all the fuel system parts that will be attacked by the acid formation mentioned by TT
Tom

[gkw120649]

I had been running 87 octane in my 77 K7.  I noticed on the highway that it didn't have any pep.  I changed to BP Amoco 93 octane and it runs a lot better at all RPM ranges.  I'm in the Metro Chicago area.

[Dave K]

I am probably wrong, but if I remember correctly the K2 and later 750's could run no lead and at a lower compression than the earlier 750's. I was told, that is another one of the reasons that the earlier bikes were faster, among other changes as well.

[Gordon]

If an engine that's meant to run on regular gas (like the sohc4's) runs better on premium, then the high octane is just a band-aid fix for some other problem.  There's nothing wrong with that, but I prefer to get to the root of the actual problem.

_{except when it comes to dealing with the wiring harness...}

[kaybee]

well to add to the confusion, what about aviation fuel (for the early 70's machines)? I realize that it is not easy to obtain for most people. The octane is not as high as most people think and it is leaded fuel. I run a tank through on occasion.... It smells so damm sweet!

[XsTatiC]

Straight from the horses mouth.  This is from the CB350F Honda manual though.

[Gordon]

Straight from the horses mouth.  This is from the CB350F Honda manual though.

That horse was speaking from a time when octane ratings were calculated differently, and is the cause of much of the confusion about what type of gas these bikes were meant to run on.  What was called 91 octane in the 70's was the "regular" gas is the equivalent of today's 87 octane. 

If someone wants to spend the extra money (and it's really not that much considering the size of a motorcycle tank) on premium, then that's fine, but not necessary.  The fact is all of the SOHC4's were designed to run on regular gas and will only benefit from high octane if there is some problem that's causing pre-ignition or detonation, or they have been modified to run higher than stock compression ratios.

[TwoTired]

Straight from the horses mouth.  This is from the CB350F Honda manual though.

Yes, but that particular horse's mouth spoke in 1973 when a different octane rating system was mandated by the Gubmint.

Look up RON, MON and AKI.
Research Octane Method.
Motor Octane Method.
Anti Knock Index. (RON + MON/2) or "Pump Octane"

I speculate these were mostly promulgated to confuse the public and has succeed in this endeavor to a great extent, convincing millions of consumers to pay more for gasoline than they are actually required to do.

Welcome to the ranks of the beguiled.

Cheers,

[Geeto67]

Personally if you ride in as much traffic as I do you want to run the highest octane as possible. Why? because the higher octane gas helps stave off the detonation that these bikes expirence when overheating. I am in stop and go all day and after sitting at one light the bike on regular, in the summer, will have that familar rocks in the coffee can sound as I pull away. Once I get moving the bike cools  and no problem...until the next traffic light. The higher octane buys me some extra time. I run it only in the spring and summer months when the temps run above 85, otherwise I run special or regular.

and by the way, more detergents do not get added the higher the octane. Each brand's formula has about the same amount reguardless of octane rating.

[Dave Wyatt]

I run premium in the Hondamatic, in spite of what the owner's manual says.  I tried a few tanks of regular, mid-grade, and premium last summer in an effort to check performance and mileage against fuel cost.  The mileage was the same acoss the board, making regular the way to go.  Unfortunately I had a fair amount of detonation when accerating if the engine lugged at all.  Since they only have 2 speeds, 'matics tend to lug the engines from time to time unless you constantly downshift, and that is not always practical.  Some mid-grade fuel would give me detonation, but none of the premium did.

I am certain if I had it on the drag strip or a dyno I might see minimal gain in performance, but on the road it's about the same.

The few cents spent in fuel costs is better spent than what it would cost if I burned a piston.

[Wheelhorse77]

Did the 77 750F require leaded fuel?

Lead was not only a great, cheap octane improver, but a valve to seat cushion as well.


BTW, up to 10% Eth is your only choice in Chicago and 100 mile surrounding cities. When alcohol was first introduced, all of the rubber swelled up, got soft in the fuel systems which then had to be replaced by alcohol resistant materials.

I bumped the timing up on my SOHC because 9 out of 10 times you get a slight increase in performance. Most cars see a 5-10 rwhp increase so my guess is the Honda would pick up 2-3 crank hp. Now before people get their SOHC's in a bunch...Honda tuned for a balance of EPA and performance. I have statistically proven and written reports on the relationship between timing curves and emissions and the factory timing was always the best comprimise between the two.

Some technology is great to adapt to our old rocket ships, but in order to run a piezo crystal sensor to detect bad ju ju, it needs to be in tune with the detonation and preignition frequencies that your particular mill makes. A trick to semi desensitize the sensor was to wrap pipe tape around the threads and try different depths into the port and tune it by ear.



For anyone who gets detonation, your advance mechanism maybe advancing prematurely, hence the detonation under low-speed high-load.

[TwoTired]

Did the 77 750F require leaded fuel?

No. The Owner's Manual says 91 RON or 86 Pump Octane minimum.

Lead was not only a great, cheap octane improver, but a valve to seat cushion as well.

True.  But, aluminum head engines have hardened steel seats where seat lube isn't strictly required.

Cheers,

[Tom Stark]

Did the 77 750F require leaded fuel?

No. The Owner's Manual says 91 RON or 86 Pump Octane minimum.

Lead was not only a great, cheap octane improver, but a valve to seat cushion as well.

True.  But, aluminum head engines have hardened steel seats where seat lube isn't strictly required.

Cheers,

Contray to popular belief, the seats in heads do not, (in normal engines, fuel engines excluded!), sink.  What happens is they contact weld to the valve, and the valve pulls a tiny bit off when it opens.  After a while, it looks like the seat is pounded into the head.  Cast iron is not real hard, and lends itself to this.  Aluminum heads need seats, and at least now are all good hardened seats.  Lead did help keep this contact weld from happening.  Lead also helped keep the guides lubricated.  And again, aluminum heads have guides in them, many heads using bronze or similar material.
Tom

[SOHC steve]

The other interesting thing about valve seat recession is it seems to be a bigger problem on fresh cut seats.I've personally seen cast iron head engines that have seen a fair amount of miles run quite happily on LPG (propane) without valve probs.It seems to be that the seats tend to work harden with use.
       As far as bike engines go,sure,they have seat inserts,are they hardened? Probably not as tough as todays seats,but if it was an older engine with miles on it I would say you have nothing to worry about due to the work hardening i mentioned.If I had a fresh rebuilt stock standard honda engine,I would probably be tempted to put some miles on it before cutting back to regular unleaded,thats just cheap insurance in my book.
        At the end of the day,it really depends on what you can get away with,do you wring its neck and have healthy amounts of comp?Do you have detonation probs? you need higher octane.None of above and you want to save a little?try regular,if you get away with it great.

[Gordon]

        At the end of the day,it really depends on what you can get away with,do you wring its neck and have healthy amounts of comp?Do you have detonation probs? you need higher octane.None of above and you want to save a little?try regular,if you get away with it great.

But that's exactly the point.  It's not a matter of "getting away" with using regular gas.  These bikes were designed to run on regular, so if you have to run premium to keep a stock sohc4 from knocking or pinging, then your bike is either out of tune or in serious need of a combustion chamber cleaning.  If you've modified the engine to have higher compression, then yes, you will probably need high octane, but if not, then it's just treating the symptoms and not the problem.

[SOHC steve]

I obviously have no idea on the quality and/or quality control of fuels in your part of the world,but,I can tell you the new "standard unleaded" seems to vary in quality over here.I have rebuilt/restored older low comp car engines (8.5/1) that were originally made to run standard.With hardened seats and all the good valves etc and original timing curves (yes it was correct curve and in fine condition) it would detonate occasionally under load with standard unleaded.Premium was not a prob.The owner was happier to stick with the original advance curve and run premium because of its limited use.I also have a early fuel injected car that does not have knock sensing,once again,timing is spot on and engine is fairly fresh,it too will be heard to ping occasionally with certain unleaded fuels.
      I would rather be safe and try standard unleaded and see if "you can get away with it" ...more than likely it will be fine,but I cannot honestly say that my experiences and comparisons between standard leaded vs unleaded has found them exact equals.
       These machines are over 30 years old now,they may have all been equals when they left the factory,but now? Who knows if the PO slipped in a mild cam,maybe you had a leaky head gasket and got the surfaces milled to true them up,maybe valves getting a bit thin...what I'm getting at is they are all no longer identical and I doubt their fuel needs are either