Commandos and Higher Octane Gas

1up3down · Mar 28, 2014

I can understand the argument against Ethanol in gas.

But why do some, or many, seek out high octane gas for our bikes?

Way back I was taught that the minimum octane to run a motor without pinging had a fairly simple formula,
and that was to take the compression and sort of match it with the correct octane gas.

For example, my standard 850 has a compression ratio of around 8.5/1, therefore at least 86 octane.

And sure enough, my motor runs just fine on "regular" or 86 as it is called here in the states.

So isn't it a waste of money, for no real discernible benefit, to buy significantly higher octane gas?

Why do some people seek out air craft high octane for their Commandos, what is that, 100 octane?

I can accept that many just plain feel the motor runs better on a bit higher, but why go for 100?

ILLF8ED · Mar 28, 2014

How about my 750 combat at 10:1? I use 91 octane pump gas, seems to be OK. It does run completely without pinging on 105 octane race gas, though.

B+Bogus · Mar 28, 2014

I understand your thinking, even though I've never seen it in those terms before.

In my Hooligan youth I had a T140 with 10.5:1 powermax pistons, and on leaded '4-star' (possibly around 98 Octane?) it would pink quite readily at low revs/large throttle.
It now runs 7.6:1 pistons in nikasil lined alloy barrels - it doesn't pink any more, but I don't ride it much any more either :roll:

My Combat runs at (I believe), around 10:1, I run similar Octane unleaded fuel, and although I'm still being reasonably gentle with it, there's no sign of pinking, possibly down to a better combustion chamber shape?

Johnnymac · Mar 28, 2014

I put racing leaded 105 in my combat and it ran like crap. Way too rich. I didn't feel the need to rejet so I went back to pump gas and it's fine. (I'm 10:1 Combat). I used the race gas in my lawnmower...

concours · Mar 28, 2014

1up3down said:
I can understand the argument against Ethanol in gas.

But why do some, or many, seek out high octane gas for our bikes?

Way back I was taught that the minimum octane to run a motor without pinging had a fairly simple formula,
and that was to take the compression and sort of match it with the correct octane gas.

For example, my standard 850 has a compression ratio of around 8.5/1, therefore at least 86 octane.

And sure enough, my motor runs just fine on "regular" or 86 as it is called here in the states.

So isn't it a waste of money, for no real discernible benefit, to buy significantly higher octane gas?

Why do some people seek out air craft high octane for their Commandos, what is that, 100 octane?

I can accept that many just plain feel the motor runs better on a bit higher, but why go for 100?

Quite a lot more to it... combustion chamber shape, lowish revs, and most importantly, ignition timing. You can retard the timing and run rubbish gas... or run higher octane and enjoy the benefit of snappy responding engine using all the advance it'll tolerate. My plodder 850 bumped up to 9:1 (estimated) will ping on SOME crappier pump premium, if you lug it. (the timing is)Right where I want it. :mrgreen:

Hobot · Mar 28, 2014

Hehe Iup/3dn conveys similar understanding as me. Best builders like Master Engine contesters know the score too but at least there's not much down side but extra expense and power loss by over dosing on some excess octane. I run 87-no ethanol in my factory Trixie Combat and Wes his normal '71 no problem though we don't do summer time plug chops up Mt steeps full torque top outs until mostly 91 in tank, or risk a comnoz style detonation damage. You can also find some references and forums saying aviation fuel ain't that great to run either though we ain't heard nothing but loving it from listee's so far.

The day I decided to try to break Ms Peel one way or another to know for sure after 1000 miles of timid run in, I happened upon folks just outside my village square under a walnut tree so pulled over to show off my clunker before risking life and bike, was giving nods of approve and a jar of 'shine offered so popped Peel's lid for a glunk-glunk of 180+ proof and one glunk for me for extra ping and knock tolerance. I have not been the same since thankgoodness.

https://www.google.com/search?q=octane+ ... 68&bih=440

texasSlick · Mar 28, 2014

Paying for more octane than the minimum to prevent pinging is a waste of money. There is no other benefit to be derived from a higher octane rating. Concours points out timing is as important as CR to determine minimum octane necessary.

The advantage of aviation 100LL is principally the non ethanol factor. I would not pay the premium price for it if pump fuel meets your octane needs, although I am becoming more adamant in using non ethanol fuel in all of my vehicles and power equipment.

Slick

L.A.B. · Mar 28, 2014

The original handbook & manual recommendations were given using RON octane numbers.

However, US/Canadian octane rating is (RON+MON)/2. So the equivalent US/Can. pump fuel has an (AKI) octane rating approximately 4-5 points lower than RON.

http://en.wikipedia.org/wiki/Octane_rating

Hobot · Mar 28, 2014

We sure want enough octane reserves or octane rating, as other molecules besides octane help tolerate ping pang boom too. Here's a search I've not gone though so maybe someone can convey the vital data points to guide us. Many lists and forums have covered detonation and seems many high tuned engines run in and out of slight detonation phases w/o detection or harm. If economy mattered a whitworth to Commando owners they be kept inside all the time.
https://www.google.com/#q=too%20much%20octane

pete.v · Mar 28, 2014

I water down 110 leaded with pump premium 50/50, generally the best i can find. I have done this from day one and have had I tank full of straight premium once. One reason is the great smell but really I like the lead. It averages out to about 101 or so. Seems like a good compromise to me.

It is just a procedure that has become habit. I do have Combat compression, a cam, pipe and carb all above stock.

I also get some machismo form pulling from the racing fuel pump. I have 2- 5 gal. containers that I fill half way with the 110, then drive over and top off at the other pump. No big whoop. It's my bike, I do what I want.

Hobot · Mar 28, 2014

hehe a newly wed wife was cutting the ends off the ham when her husband asked why and was told that's how my mother did it so he called the mother and was told "Oh yeah our pan was too small back then".

There are octane calculators galore online to mix up this and that octane to know about what ya actually end up with.

3 factoids around octane subject not completely iso-ctane/hectane related.
1. resistance to self ignite
2. speed of combustion oxidation reactions
3. heat released by major type of molecules

Octane is tested in a single cylinder octane test engine. The MoN is a measure of the gasoline's ability to resist knock under sever operating conditions. MoN affects high speed, part throttle and performance (under load such as in passing). The RoN on the other hand, is a measure of gasoline's ability to resist knock under less sever conditions. RoN affects low to medium speed knock and engine run-on (dieseling). For a given AKI, RoN is typically 8-10 points higher than the MoN. As an example, 87 AKI (pump octane) fuel would have a MoN of 82 and a RoN of 92.

DogT · Mar 28, 2014

I use the BO 110LL because that's the only e free fuel I can find locally. I find it runs, smells and stores great. And it's my favorite color. Only disadvantage is if you leave the top off, it evaporates quickly. Sure doesn't ping. If I get the opportunity to get a metal tank I may go to another fuel type, but I won't be happy about it. The few miles I put on the bike doesn't mean much to the fuel costs to me. I've never had a motor seem so happy on a particular fuel, but that may be my subjective opinion. Only reason I don't use it in the lawnmower or chainsaw/weedeater is they recommend 'regular'. Might be interesting to try it though, I can go through some fuel in the mowing season. Now if I could get that JD2020 turboed with diesel, I could pull some stumps.

Dave
69S

Snorton74 · Mar 29, 2014

Clear gas seems to work great in everything. Not sure why anyone would use leaded gas nowadays though??

olChris · Mar 29, 2014

tomspro · Mar 29, 2014

High Octane is overrated. :wink:
More concerned with incompatibility of E10 gas with older engines and carbs, high engine temps from 'octane boosters' or ethanol (much debate here).
I am lucky to have a station nearby that sells E-free gas.
http://pure-gas.org/

pete.v · Mar 29, 2014

ludwig said:
pete.v said:

I water down 110 leaded with pump premium 50/50, generally the best i can find. I have done this from day one ..

Click to expand...

So that means you have never ridden your bike more than half a tankfull away from home ? ..

Sad, isn't it. No hills or winding roads. Just straight and flat around here. I should be so luck to live it the Alps, Colorado, California or even somewhere along the Appalachian trail. Poor Poor me.

I have gone distances with support vehicles with my mix on board. But due to my crippling condition it's hard to more than 100 miles or so.
Not really, just kidding, I'm healthy.

Like I said, I have had straight Premium in there before and I'm sure that if I get out on some good rides, finally, I will see it in there again.

Sorry for your disappointment in me, ludwig. Just like my dad. :cry: :cry:

gtsun · Mar 29, 2014

I looked on the pure gas web site & am surprised that there are very few listings for Southern California. I thought I would find at least one or two. Bummer, when are we going to be rid of this E fuel crap?

Hobot · Mar 29, 2014

Hehhehe>LOL pete.v you nailed ole Ludwig claims down good. His featherweight wonder would be a loaded down fuel tanker if what he stated was so over the distances he's proven to do in wonderful regions. Funnier yet is if Ludwig has tuned up engine to take advantage of extra octane/detonation resistance in low oxygen altitudes he'd experience same as comnoz did with his EFI dialed in just right to point that one event of questionable 87 octane slapped his engine apart on a tour trip last year. Peel will be immune to boozed/watered or poor fuel quality as can just select spark timing and throttle ease to get to better fuel station. If ya look up the octane boosters beyond their own advertising you'll see at best they can raise octane a point or two.

http://www.wallaceracing.com/octane-mix-calc.php

http://www.bazellracefuels.com/Calcs/OC1.htm

worntorn · Mar 29, 2014

Common thinking is, run high octane, big advance, go like gangbusters. Fact is, regular low octane gas has more energy per unit than high octane does. Octane is a necessary evil to prevent pinging, but due to the power lost in high Octane fuels( they burn slower to avoid ping) best to only use as high as needed.
Same with ignition timing. We used to advance to ping then back off a little, so run all the advance possible. This was wrong way round, now it is retard until power loss is seen on dyno, then advance back a touch.
So run the lowest octane gas you can safely get away with and run the least advance you can get away with, that is where Max power (and safety with the timing) is with those two variables.

Glen

acotrel · Mar 29, 2014

When you buy petrol from a bowser, you often don't know what you are getting. My theory is that if you buy the most commonly used gas, from the most prevalent service station, the supply is more likely to be consistent. In the end you end up optimizing several things in your motor to suit the fuel - comp. ratio, ignition advance, cam timing and jetting. What I usually do it fix the first two, adjust the cam timing to suit the exhaust, and then jet the carbs for max. performance (midrange torque) without detonation, without changing the fuel. If your jetting is near critical, and you get bodgie fuel, you can get motor damage. I believe that is the main reason racers use Elf racing fuel - it is a quality issue. If you want consistent lap times and no engine damage without rejetting every time you refuel, you need consistent fuel quality. If the major usage in your area is high octane, it might be an advantage to use it, however I would not expect a power advantage unless your jetting for the lower grade was previously wrong. Whatever fuel you use the ignition timing and jetting should be optimized to it.

Commandos and Higher Octane Gas

1up3down

ILLF8ED

B+Bogus

Johnnymac

concours

Hobot

texasSlick

L.A.B.

Moderator

Hobot

pete.v

Hobot

DogT

Snorton74

olChris

tomspro

pete.v

gtsun

Hobot

worntorn

acotrel

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