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Different Octane question

Jim B.

Jim B.

Junior Member
The water cooled engine on my 05 Goldwing is a low compression engine and runs fine on 87 octane gas. The water cooled engine on my 05 Honda Shadow Sabre was also low compression and also ran fine on 87 octane gas. So, my question is, why does the low compression Harley engine require higher octane like 91 or 93? Is it because it is air cooled or is there some other reason? Just curious.
 
Bigger pistons in the harley, slower flame front propagation in a bigger cylinder and air cooled runs hotter too.
 
It is also important to remember that the actual cylinder PSI obtained with a compression tester has little to do with the stated compression ratio. If it did, given a stated compression ratio, a person would be able to calculate the PSI even before using a compression gauge.. But you can't . Cam profiles (lift , overlap etc) will and can change the actual PSI readings from computed compression ratio numbers.. Also if you look at spark advance numbers on simple stage 1 maps, you will see spark advance numbers you would thingk would not work. Screaming Eagle stage 1 spark advance numbers in the High 30's, low and mid 40's are considered Ok by HD ! Those are numbers you rarely see anywhere. Those numbers require 93 octane.

Hoop!
 
I love this forum. Post a question, come back a few minutes later, and there are several answers from people who know what they are talking about. Thanks, all.

And, I can't wait to use Glider's phrase, "slower flame front propagation."
 
Jim B. said:
I love this forum. Post a question, come back a few minutes later, and there are several answers from people who know what they are talking about. Thanks, all.

And, I can't wait to use Glider's phrase, "slower flame front propagation."
Click to expand...

Don't include me in that group of people. I make this stuff up on the Fly!!:p
 
What do you consider low compression? My Evo has a CR of 8.5-1 and runs fine on 87 octane. The current 96ci engines are 9.2-1, so they're iffy for 87 octane and H-D recommends 91.
 
:D
Jim B. said:
The water cooled engine on my 05 Goldwing is a low compression engine and runs fine on 87 octane gas. The water cooled engine on my 05 Honda Shadow Sabre was also low compression and also ran fine on 87 octane gas. So, my question is, why does the low compression Harley engine require higher octane like 91 or 93? Is it because it is air cooled or is there some other reason? Just curious.
Click to expand...

MY harleys have all been upgraded to HIGH comp, sooner or later.. MY New FLHR 09 is probably staying stock.. but i still put the slower burning fuel into it. (high octane)

ONE reason HARLEY bikes NEED higher octane is because lower RPM torque means LUGGING... some really like to make them GRUNT....

YOU wouldn't lug a honda like some do their HARLEYS...

Me, I like to keep my rpm's up above 2300 and never twist a full throttle unless i shift down to obtain easier power band to allow engine (torque or not) to enjoy the ride also....

Get my drift?
signed....BUBBIE
 
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Axel said:
What do you consider low compression? My Evo has a CR of 8.5-1 and runs fine on 87 octane. The current 96ci engines are 9.2-1, so they're iffy for 87 octane and H-D recommends 91.
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Low is usually below 10-1... 9.6 is getting into the mid to upper section and really should use high test.
 
You can write a book on the subject. but basically, higher compression requires higher octane. What we are really looking at is cylinder pressure rise time, and lower RPM motors have the pressure rise at an earlier degree of rotation, given the same timing.
Another major factor is combustion effeciency, which is a combination of chamber shape &volume, and piston shape. You can basically measure the combustion efficiency by the total timing needed to make max power. The more timing needed, the lower the combustion efficiency and the poorer the chamber design. The most effecient chambers are the top hat on 2 strokes, that make max power at only 25* timing. Designs that need over 40* timing for max power need improved.

One other thing, if you run higher octane than is needed, you will make less HP, not more.
 
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