Topic: Octane rating? 91 or 95

[Hush]

Seeing as how the whole world has different octane ratings for their petrol, what is the general consensus on what our SOHC 4's should be running on?
My old Honda trained mechanic says try 91 and if the bike "pinks" then use the 95.
Originally these bikes would have been running on leaded fuel, many countries like New Zealand have gone green and stopped using any form of leaded gasoline, I wonder how that has affected these 30 year old machines?

[MoTo-BunnY]

No offense, but just so you know, this has been addressed many times already - might want to do a search for the (numerous) other threads - plus I think it's also in the FAQ section, too?

(UPDATE: the above and below statements were regarding the U.S. - I didn't realize Hush was in NZ - sawry! (bad bunny . . .BAD!)

These bikes are fine for regular, unleaded - like as in 87 octane. The OEM owner's manual for my '73 CB500 says "91" octane but that was the old rating system - with today's rating system the equivalent would be about 87.

Unless your engine is all modded out with higher compression, any higher octane is a waste of $$$. There is a LOT of misinformation out there regarding this, with people thinking "higher # = better" or something. Attendants at the gas station here, tell me to put the highest octane in my bike all the time, but that's false info. The valve seats are hardened or whatever, so 87 is fine.

(Unlike my old '73 Dodge RV with a 360 V8 in it, where it was like the last year without the hardened valve seats in the heads, and I have to put a lead replacement additive (for valve lubrication - what lead used to do, among other things) in every tank).   

[Dunstall_74]

I've always used mid-grade 89-90 I guess, Until recently.  I've started putting regular in the tank because that's all I can afford.  Bike runs just as good.  Actually seemed like it ran better, but I'm sure that it was probably just me.

[Hush]

Appreciate what you say about this subject being thrashed out Motobunny but as only a handfull of us here in NZ are forum members and as I understand it petrol grades vary so much across the world it is probably worth raising again.
My Nissan Sylvia is a 2 litre car and is supposed to run on 91 but since they changed the grades on us again from 93 to 95 it turns out my car runs better and more ecconomcally on 95?
The cost difference works out in favour of the more expensive gas!

[MoTo-BunnY]

Appreciate what you say about this subject being thrashed out Motobunny but as only a handfull of us here in NZ are forum members and as I understand it petrol grades vary so much across the world it is probably worth raising again.
My Nissan Sylvia is a 2 litre car and is supposed to run on 91 but since they changed the grades on us again from 93 to 95 it turns out my car runs better and more ecconomcally on 95?
The cost difference works out in favour of the more expensive gas!

*doh* I was being the typical Yank and not even thinking in a worldview. . . sorry!  lol    Yeah, there are definitely different octane system ratings in use, world wide, as well as different gas formulas. I imagine NZ is kinda strict on it all too, given the respect for the environment there (which I think is awesome. . .here in Portland, it's real "green" or whatever, for the US anyways. . . lol)

Oh, also, I remember when I did a cross-country trip in my '73 VW van with a freshly rebuilt engine and I was keeping track of mileage, experimenting with different timing settings and octanes, that it got better mileage on some of the higher octanes. . .. seems like I remember that 89 was the sweet spot for best mileage for the money, even though it was supposed to not need it that high.

[TwoTired]

If your bike still has the stock compression ratio,  Honda made it run on regular, or lowest grade gas you can find.
When Honda made the bikes, the RON octane rating method was commonly used world wide.  Now different rating methods are used in different parts of the world.  See:
http://en.wikipedia.org/wiki/Octane_rating

However, some countries used to the conversion to unleaded gasoline as an opportunity to further confused the buying public towards buying a higher octane than needed, by changing the rating method.
The US, for example, uses an AKI ,PdON, or PON method so as to add even more confusion. (Consumer confusion leads to higher margin profitability on sales.)

Lower grade fuels combust at a lower temperature than higher grades.  A high compression engine heats the A/F mixture simply by compressing it. Add the heat from the engine and the previous combustion cycle, and the mixture could ignite prior to the spark stimulus.  This is called pre-ignition, and the engine makes "pinging" noises when doing so.  Extreme cases are called detonation where all the A/F mixture ignites and explodes before the piston has reached TDC.  Such occurances may bend rods, hole pistons, and generally make your engine into a boat anchor.

Higher octane fuels are blended to be more difficult to ignite with higher operating temperatures, thus making them suitable for high compression engines, and keeps the A/F mixture ignited where the spark timing is set.

Whatever the octane rating of gasoline, the energy content is very nearly the same among the grades.  Note that diesel fuel has a significantly higher energy content, yet a very much lower octane rating.  Further, alcohol has a higher octane rating (requires higher temps to ignite) but has a much lower energy content.  So, if your gas is blended with alcohol, you need more of it to do the same amount of "work" than just straight gasoline.

Modern cars often have an anti-knock sensor.  This informs the computer, so it can fatten the mixture (for extra cooling) and retard the spark.  Both of these actions can reduce the power output capability of the motor, and reduce fuel mileage.  For example, my 89 Camaro has the recommendation in the owners manual to use premium fuel for best power.  If premium is unavailable, lower grade fuels down to regular grade can be used, but the engine will have reduced power output (due to the knock sensor ant the computer controlling timing and the fuel injectors.)  This also noticeably reduces the fuel economy.

The computer, anti-knock sensor, and timing adjuster on the SOHC4 is located just above where the operator sits.

Cheers,

[Dunstall_74]

TwoTired- when you go to the pumps, what (grade) do you put in your bike?

[KB02]

Oh, also, I remember when I did a cross-country trip in my '73 VW van with a freshly rebuilt engine and I was keeping track of mileage, experimenting with different timing settings and octanes, that it got better mileage on some of the higher octanes. . .. seems like I remember that 89 was the sweet spot for best mileage for the money, even though it was supposed to not need it that high.

My first car, 1985 Jeep Cherokee with an 85hp inline 2.5 liter four banger (the poorest of the Poor-Fours  ) ran the best on 89 octane. It ran like crap on 91. 

On my Ducati I run high tes as that is what the manual recommends. On my F1 I ran regular. On my K* I will be running high tes, but that is because of all the work the engine is getting (higher CR, cam... etc...)

[OldSchool_IsCool]

The computer, anti-knock sensor, and timing adjuster on the SOHC4 is located just above where the operator sits.

My computer and timing adjuster seems to be on the fritz, but the knock detector is working fine! 

[TwoTired]

TwoTired- when you go to the pumps, what (grade) do you put in your bike?

Lowest grade regular.  I think it says 87 on the pump.  California, US.  All my bikes are right at or near 9:1 compression ratio.  Usually, you have to be above 10 or 10.5:1 Comp ratio to need higher octane.  But, here are tricks with cam grinds and timing that can change that.

I suppose it is worth noting that carbon deposits in the motor can build up and change your compression ratio (upwards).  The additives in higher grade gasoline (to prevent pre-ignition) sorta need the extra heat made in high compression engines to burn off and exit the exhaust.  The lower temps of low compression engines don't burn off the extra additives in higher grade fuels as well, and can lead to carbon/deposit build-up, which ups the comp ratio.

This is a long winded way of saying that if you use premium fuels in low compression engines, they can develop an addiction to the fuel, when the deposits build up to change their compression ratio.

Cheers,

[Really?]

TwoTired- when you go to the pumps, what (grade) do you put in your bike?

Lowest grade regular.  I think it says 87 on the pump.  California, US.  All my bikes are right at or near 9:1 compression ratio.  Usually, you have to be above 10 or 10.5:1 Comp ratio to need higher octane.  But, here are tricks with cam grinds and timing that can change that.

I suppose it is worth noting that carbon deposits in the motor can build up and change your compression ratio (upwards).  The additives in higher grade gasoline (to prevent pre-ignition) sorta need the extra heat made in high compression engines to burn off and exit the exhaust.  The lower temps of low compression engines don't burn off the extra additives in higher grade fuels as well, and can lead to carbon/deposit build-up, which ups the comp ratio.

This is a long winded way of saying that if you use premium fuels in low compression engines, they can develop an addiction to the fuel, when the deposits build up to change their compression ratio.

Cheers,

Sounds like a poormans way to a Hi Perf engine, lol.

[Dunstall_74]

TwoTired- when you go to the pumps, what (grade) do you put in your bike?

Lowest grade regular.  I think it says 87 on the pump.  California, US.  All my bikes are right at or near 9:1 compression ratio.  Usually, you have to be above 10 or 10.5:1 Comp ratio to need higher octane.  But, here are tricks with cam grinds and timing that can change that.

I suppose it is worth noting that carbon deposits in the motor can build up and change your compression ratio (upwards).  The additives in higher grade gasoline (to prevent pre-ignition) sorta need the extra heat made in high compression engines to burn off and exit the exhaust.  The lower temps of low compression engines don't burn off the extra additives in higher grade fuels as well, and can lead to carbon/deposit build-up, which ups the comp ratio.

This is a long winded way of saying that if you use premium fuels in low compression engines, they can develop an addiction to the fuel, when the deposits build up to change their compression ratio.

Cheers,

Damn, hope my bike isn't hooked.  Can't afford expensive Rehab, nor do I have time for weekly Fuel Anonymous Meetings