Topic: Octane?

[jameseb444]

I've been using 89 octane in my '74 350F. I don't know if I should or not. I would like to hear what everyone uses and why.

[kghost]

92 - 93 octane with a dash of 100LL aviation fuel

Aviation fuel aint really for rockets but that was the closest choice

[jevfro]

87
CB750 stock
It makes it go and it's cheaper than all the rest.

No problems so far.

[jameseb444]

I wasn't sure if 87 would be good or not. If I ran it in my old '79 Bronco all you heard was valves chattering. I'm glad I don't have to fill that 33 gallon tank anymore.

[fuzzybutt]

it runs without rattling on the 87 and since it's the cheapest at 3.40 a gallon.................

[TwoTired]

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....

[cb650]

At my altitude the best pump you can get it 89 prem. unless you want to get special rare sht. No problems.  With less air they say you dont need he octane but me haznt had the skolin to now difrnt.

[jameseb444]

It looks like I'll be using 87 from now on.

[Gordon]

I use the lowest octane, which in Denver is 85.

Why?  Because these bikes, in stock configuration, were made to use regular.  You'll see no benefit from running high octane unless there's something wrong with your engine, or you've modified your engine for high compression.

[Bob Wessner]

I use 87, always have, cheapest and no problems.

[Ecosse]

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....

Well I was using Sunoco High test (Ultra) until I read this! Reason being not the octane so much as the beneficial additives I understand those fuels have. From now on maybe I'll just add some fuel additive now and then.

[cb650]

Didnt "REGULAR" when these bike came out have lead in it?  That also helped the "octane" some. Yes lead was mostly for the lubrication but it helped the knocks too.  But I kned to stdy mor boks
two.

[Gordon]

But I kned to stdy mor boks
two.

Evidently...

[aptech77]

92oct, cuz I'm running a 400F with 466cc Wiseco pistons, a big cam, and 31mm carbs, electric ign, etc....

[jameseb444]

Didnt "REGULAR" when these bike came out have lead in it?  That also helped the "octane" some. Yes lead was mostly for the lubrication but it helped the knocks too.  But I kned to stdy mor boks
two.

This is why I assumed I should run a higher octane.

[Gordon]

This is why I assumed I should run a higher octane.

Nope, no need to.  When these bikes were made regular was 90 or 91 Research Octane Number.  Today regular is 87 Pump Octane Number, which is just the average of RON and MON (Motor Octane Number).  Same octane, different number. 

[TwoTired]

Didnt "REGULAR" when these bike came out have lead in it?  That also helped the "octane" some. Yes lead was mostly for the lubrication but it helped the knocks too.  But I kned to stdy mor boks
two.

This is why I assumed I should run a higher octane.

As Gordon points out, the industry changed the rating system and pump labeling (as well as removing the lead) so as to further confuse the buying public.
It worked.  And, they've made many extra million$ from a confused public buying more expensive gas than they needed.

[jameseb444]

Very interesting. This is why this forum is so invaluable. I have learned a lot in the last month because of all your input.

[Terry in Australia]

Yeah it's interesting how one engine will happily run on a low octane fuel, but bump the compression ration up some, and it turns to shiit.

I tried to run regular unleaded 92 octane in my BMW R100RS and it "pinged" like a bastaard, and my BMW K1100LT hates it too (it'll run on it ok due to the miracle of computers but it runs a lot "sweeter" on minimum 95 and preferably 98) and my CB750K1 with 12.5:1 pistons prefers 100 Octane Shell "V Power plus".

My CB750K0 and my Suzuki GS1000S prefer regular unleaded, but I've just bought another GS1000 with full Yoshimura top end, so I'm guessing the V Power will get the nod again. Cheers, Terry.

[fuzzybutt]

Terry, whats the 750k with 12.5:1 like on the street? as far as getting too hot. and how hard is it to kick over?

[Terry in Australia]

G'Day Mate, well I haven't done a compression check yet, but it certainly has more "resistance" as measured by my size 11 boot. It starts on the electric starter ok, but I've got a charging problem (which might be fixed once I install Hondaman's diode kit on the Dyna 3 OHM coils) so I tend to use the kicker more than the "electric leg", ha ha!

On the street it's a bit of a pig, but I haven't sorted the Keihin CR29mm carbs completely and the Megacycle 125/75 cam isn't the best for the street either. No biggie though, I've got plenty of bikes for regular use, so this one is for pointing at quiet country back roads for some "less than legal" blasting, ha ha!

[detdrbuzzard]

both of my 750s run better with 92/93 octaine but the mustang and t-bird run best with 89 and the wings and 450 get 89 to cut down on confusion

[Chad]

On my CB360T I run 87.  I haven't had trouble with it for the 20 miles I've owned it.   

[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.

[markjenn]

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