Fuel Octane

Status
Not open for further replies.

highdesert

Joined
Jun 17, 2009
Messages
642
Help me out with my reasoning?

Take a standard 750 or 850 with a compression ratio of around 8.5 or so, certainly under 9.

Normal street usage.

My understanding is that a high octane like US premium grade at our gas stations is just a waste of money.

That the middle grade 87 stuff is just fine for these standard motors. Can see higher comp Combats needing premium.
 
My bike has run fine on everything from 82 to 95 octane. The mileage is a little worse with the low octane stuff. Every spring I fill the tank from my tractor gas fuel barrel and it gets me by until I get to the gas station.

Greg
 
A standard 750 is around 9.5 to 1 and needs high octain. A standard 850 is around 8.5 to one and runs happily on midgrade. A combat motor is around 10.5 to one and needs the best you can get. Jim
 
comnoz said:
A standard 750 is around 9.5 to 1 and needs high octain. A standard 850 is around 8.5 to one and runs happily on midgrade. A combat motor is around 10.5 to one and needs the best you can get. Jim
Click to expand...

the ratios for 750 might be a bit optimistic. Std 750 is listed at 9.0:1, combat 10:1, '73 high compression 750 9.3:1, low compression 8.9:1. Even the combat at 10:1 requires a thin head gasket. Regarding octane raing, I was having serious problems a few months ago with my combat pinging. It turned out to be the carbon build up. After a de-coke it runs fine on local 92 Octane unleaded premium. That indicates the 10:1 ratio is probably the limit for current pump gas. I've tried 112 octane leaded race gas...works very nice even with the carbon build up. It's about $6 a gallon.
 
Depends on cam and CR. My two Combats ran fine in normal sane use on 87 octane in a pinch but stayed aware not to push my luck in summer flings as on 93 octane.
i did have to make sure timing was no higher than 28' total, which Combats liked best anyway, no back fire starts no hesitation to throttle no mater the octane.
Oil weeps into chamber lower detonation conditions. Rpm raises detonation threshold on same octane. So a sliding scale in reality to adapt too.

Conservative Rule of thumb is CR 1/10'th the octane rating plus .50 CR - if Al head and mixture and gearing proper along with cam valve events.
 
illf8ed said:
comnoz said:
A standard 750 is around 9.5 to 1 and needs high octain. A standard 850 is around 8.5 to one and runs happily on midgrade. A combat motor is around 10.5 to one and needs the best you can get. Jim
Click to expand...

the ratios for 750 might be a bit optimistic. Std 750 is listed at 9.0:1, combat 10:1, '73 high compression 750 9.3:1, low compression 8.9:1. Even the combat at 10:1 requires a thin head gasket. Regarding octane raing, I was having serious problems a few months ago with my combat pinging. It turned out to be the carbon build up. After a de-coke it runs fine on local 92 Octane unleaded premium. That indicates the 10:1 ratio is probably the limit for current pump gas. I've tried 112 octane leaded race gas...works very nice even with the carbon build up. It's about $6 a gallon.
Click to expand...


Yes I was probably being optomistic although I have checked a few 750s in the past and seen anywhere from 9.3 to 9.7. Chances are most heads have been straightened a time or two though. Jim
 
Got ya Jim, so didn't change avatar to another death deer either. hobot
 
Steve, how bout this one

Fuel Octane
 
Another guy who's a regular on the xlforum (HD Sportster forum) uses that avatar, Hobot, so this has potential to get confusing.

Unless that guy is you...?
 
No desire to re-stimulate post traumatic stress having to deal with my own and others up close. At one time, for a long time, I was that mad scientist, Dr Jekyll was my model and got to live it out till it mostly killed me with some electricity thrown into the mix. I can talk shop with the crystal cookers here. Glass ware just on display collecting dust, but can still tell ya exactly what temp a particular sand grain crystal melts against its neighbors diluting it. But impure white powders and brain shocks don't do it for me no more. Will dig up more gleeful picture of thrill.
 
Steve,
You know I was only kidding about your avatar, actually I thought it was kind of a neat picture even if it bring back a couple memories. Jim
 
Ok Jim I know your as tough as they get. Flame image bothers me too actually in more than one way. Putting electric arc gizmo below the fuel system is one reason, other is riding risks no behavior except not riding can prevent. I'm thinking to trade back hoe work rather than money/parts for goat downed SV650. I've 5 non DOT race only fatso rear tires stacked up at office for a scale of my shattered out look. Twice this week, mentally composing email to Ken Canaga on how silly I am to worry, seeing everyone else riding here, deer shoot by so close it takes my breath away, knowing if I was on bike I'd be 5-10 mph faster just there, SPLAT!. Last scare, I'd of been at 35 mph so deer would have hit from behind as much as the side. I have stopped 4 x's this week to take fall photo's I'd be seeing if on Commando dream machine, but didn't as I wasn't and wonder if i ever can - so I didn't take new ones. Current avatar was my 7th day
on 1st Combat-pre-Peel in summer '99, when I got the jump on a wheelie bar 1200 Sportster cammed too aggressive for public use. Combat torque on tire did in my R knee just beyond the X-mass tree. I had tank of track fuel that day.

I think leaness of alcohol fuel is more risk of heat seizure than detonation of low octane - especially if pilot knows to just lighten up loads till better gas gotten.

I've gave up on octane and fuel cooling in my dream machine, expect not to top 17:1 if I'm careful. If I never slow up no deer can touch me in my dreams.
 
I have a mid 1972 combat.
I use 93 Sunoco, lead additive, and some snake oil( octane booster)
It works nicely all local from auto zone
Marshal
 
MarshalNorton said:
I have a mid 1972 combat.
I use 93 Sunoco, lead additive, and some snake oil( octane booster)
It works nicely all local from auto zone
Marshal
Click to expand...

Kind of the same mix I do for my 72 Combat. Instead of snake oil, I mix some 110 Sunoco Racing gas. Try to get it around 97 Octane. After 17,000+ miles doing this, the Norton never complained. :D
 
Reading various engine tech sites while staying in cheap motels, there is some extra torque to be had if engine set up can tolerate lower octane's faster pressure/heat punch. This can be reflected in the ignition advance too, less advance has bragging rights over more advanced spark to get flame going, as implies a faster burn, but on other hand it could more often mean too detonation prone on that octane and must delay pressure rise giving up most the power might have gained higher CR etc.
Over all a dang dull lack of detonation detected let alone damaging.

A few years before I re-upped here i asked for examples of Commando's detonating.
There were very few and mostly on hi altitude home based Cdo's coming down to valley desert then climbing back up loaded in summer in 4th with a dry tail wind.
There were some with oil leaks involved to solve. There were some with too advanced timing. There were some putting standard cam in 10 CR Combats.
I'm sure there are 11+ CR out there that must have special fuel to have fun on.
 
MarshalNorton said:
Jerry,
Where do I find 110 Sunoco? :oops:
Marshal
Click to expand...

Marshall, I did a search for your state.

Bazell Oil Co
http://www.baxellracefuel.com
1-800-521-5420
or
Insinger Performance Inc.
Dushore,PA
http://www.insingerfuels.com
570-924-4323

Lucked out as I have a local distributor that's about 10 miles from me that supplies for racing events. Have a 6 gallon gasoline jug and I think I paid around $6 a gallon. I'll put in 4 and half gallons of Sunoco 93 and a half gallon of 110 racing fuel and mix 1oz. of Marvel Mystery Oil to it. Get's it around 97. I talked to one club member awhile ago and he does half 87 oct & half 110 oct. (98.5 oct) For my amount of driving, the 6 gallons get's me by for the summer. There is already lead in the Sunoco 110 Racing Gas. Hope you find one nearby. Jerry
 
Jerry
Thanks for the great info.
I think I'll track down the fuel and try the mix like you suggested.
Marshal
 
Status
Not open for further replies.
Back
Top