Several years ago, I set my XS1B ignition timing a bit retarded to improve my idle stability, reduce vibrations, and give a smoother and more docile power output to match my sedate 'old man' riding style. Then, after building my LED Ignition Timing Light, I reset my retarded ignition timing back to the factory spec of 40° BTDC full advance.
This new performance of the engine was awful. The idle became unstable again, but more importantly, the highway performance was alarming. To maintain 60-70 mph required much more throttle, power was down, vibration was way up, and my typical hot oil temps rose from 230°F to around 250°F.
Whut's goin' on?
Warning: *Ramble mode engaged*
The XS1 was developed in the late '60s, and held a respectable position in the early '70s. Fuels, oils, and sparkplugs of that time have changed, and we've had to adapt. I'm still catching up on this new world of low-octane gasohol, and its effects on vintage engines. Back in the late '60s - early '70s, gasoline was easily available in 98-108 octane, enhanced with tetra-ethyl-lead. My primitive 'oldschool' understanding of octane ratings was that the higher octanes had a higher flash temperature and a slower, controlled burn, a requirement of the high-compression muscle cars of that time. Folks thought that higher octane gave you more power, but the reverse was true. To get more power, you increased the compression ratio (C/R), which then required the higher octane rating. On a lower C/R engine, more power was realized on the lower octane (faster burning) fuels.
Fast forward to our modern gasohols, with octane ratings of 87-91. Which makes me think that these modern fuels burn faster, and need LESS spark advance.
A Google search of "ignition timing curve" and "spark advance curve" shows numerous charts where the vast majority of modern engines limit max spark advance to around 30° BTDC, with very few going to 35° BTDC, and rarely to 40° BTDC (for vintage engines).
Thanks to advancements in engine design technology, and associated analysis tools, a new perspective exists that may be applied to our vintage engine. For example, this pictorial of combustion chamber designs shows associated max spark advance values. Note the vintage side-plug hemi-chamber value of 40°-42°, versus the newer pentroof at 30° and less. Our combustion chamber is somewhat between the two, with its higher mounted plug position. This would seem to support reducing our full advance timing.
I'm not having much luck finding good comparisons of fuel burn rates, fuels of 45 years ago versus modern gasohols.
http://performancetrends.com/Definitions/Burn-Rate.htm
But, this somewhat summarizes my thinking of what's going on:
Energy density of gasohol is about 3% less than gasoline, requiring more throttle.
More throttle requires less spark advance.
Faster burn rate of lower octane rated fuels requires less spark advance.
A search thru this forum found recommendations from XSJohn (rip) of retarding timing about 5°, from this thread:
http://www.xs650.com/forum/showthread.php?t=2426
View attachment 2243
And, I found a post from grizld1 of his running a max of 38° BTDC. There may be more, like the recommended ignition timing setups for the track, but those would probably apply to the old days and vintage fuels.
For now, until I can do more exhaustive testing, these are the ignition timing positions I'm running:
This represents my modified current ignition timing curve:
So, I'm thinking that if you're experiencing poor highway performance, excessive vibration, hot engine, poor gas mileage, an ignition timing 'retard' of about 5° may help your condition.
Comments, guys...?
This new performance of the engine was awful. The idle became unstable again, but more importantly, the highway performance was alarming. To maintain 60-70 mph required much more throttle, power was down, vibration was way up, and my typical hot oil temps rose from 230°F to around 250°F.
Whut's goin' on?
Warning: *Ramble mode engaged*
The XS1 was developed in the late '60s, and held a respectable position in the early '70s. Fuels, oils, and sparkplugs of that time have changed, and we've had to adapt. I'm still catching up on this new world of low-octane gasohol, and its effects on vintage engines. Back in the late '60s - early '70s, gasoline was easily available in 98-108 octane, enhanced with tetra-ethyl-lead. My primitive 'oldschool' understanding of octane ratings was that the higher octanes had a higher flash temperature and a slower, controlled burn, a requirement of the high-compression muscle cars of that time. Folks thought that higher octane gave you more power, but the reverse was true. To get more power, you increased the compression ratio (C/R), which then required the higher octane rating. On a lower C/R engine, more power was realized on the lower octane (faster burning) fuels.
Fast forward to our modern gasohols, with octane ratings of 87-91. Which makes me think that these modern fuels burn faster, and need LESS spark advance.
A Google search of "ignition timing curve" and "spark advance curve" shows numerous charts where the vast majority of modern engines limit max spark advance to around 30° BTDC, with very few going to 35° BTDC, and rarely to 40° BTDC (for vintage engines).
Thanks to advancements in engine design technology, and associated analysis tools, a new perspective exists that may be applied to our vintage engine. For example, this pictorial of combustion chamber designs shows associated max spark advance values. Note the vintage side-plug hemi-chamber value of 40°-42°, versus the newer pentroof at 30° and less. Our combustion chamber is somewhat between the two, with its higher mounted plug position. This would seem to support reducing our full advance timing.
I'm not having much luck finding good comparisons of fuel burn rates, fuels of 45 years ago versus modern gasohols.
http://performancetrends.com/Definitions/Burn-Rate.htm
But, this somewhat summarizes my thinking of what's going on:
Energy density of gasohol is about 3% less than gasoline, requiring more throttle.
More throttle requires less spark advance.
Faster burn rate of lower octane rated fuels requires less spark advance.
A search thru this forum found recommendations from XSJohn (rip) of retarding timing about 5°, from this thread:
http://www.xs650.com/forum/showthread.php?t=2426
View attachment 2243
And, I found a post from grizld1 of his running a max of 38° BTDC. There may be more, like the recommended ignition timing setups for the track, but those would probably apply to the old days and vintage fuels.
For now, until I can do more exhaustive testing, these are the ignition timing positions I'm running:
This represents my modified current ignition timing curve:
So, I'm thinking that if you're experiencing poor highway performance, excessive vibration, hot engine, poor gas mileage, an ignition timing 'retard' of about 5° may help your condition.
Comments, guys...?