My bike is stock as we speak,but `im fixing to put on Roadhouse
Dooleys and a Thunder TearDrop Air Cleaner so should I still switch to
regular Gas ????

You should always use low octane fuel (87) for an unmodified engine ..... no high compression pistons.

I may not run as well if you put in higher octane fuel.

Doc

Hi Doc,

Don't intend to start a controversy here, just inspire some thought.

And what you said is commonly said on the RSR forum, but think about this possible exception:

High octane fuel isn't just about "flashpoint" (decomposition), which does address compression ratios. It's also about "burn rate", which is slower in high octane fuel.

Those who run Dyna ignition systems and want to run greater ignition advance to achieve greater power, might benefit with high octane fuel. In the same way that newer cars "sense" lower octane and retard timing (with a loss of power), timing can be advanced for more power with a slower burning fuel (premium)

Modern engines strive to reduce timing advance and therefor "negative workload" (the sooner you start the spark, the more/longer you're pushing a piston against a combustion process) through very sophisticated intake systems.

Consider the 2005 Corvette with ~400Hp running regular gas (premium recommended, but not required). With 10.9-to-1 compression, this motor will accept regular by "sensing" and switching to a "map" that retards timing. I also believe that it achieves a lot of horsepower through modern intake technology, as much or more than high compression, minimizing negative workload.

Our R*s are very low tech motors by those standards and like old chevy V8s, they seem to respond very well to more timing advance. If you want more and own a Dyna, I think you can get more by running higher octane fuel WITH greater timing advance. And, the "carbon-up" forum logic is not valid if you're starting the spark sooner to burn all or most of your fuel.

With a stock ignition system, you very well could cause a lot of un-burnt fuel to carbon-up the motor, running premium. But it's not due to a lower compression ratio, it's due to ignition timing.

Here's a link to Gas FAQs. Section 7.5 discusses ignition timing and octane
http://www.faqs.org/faqs/autos/gasoline-faq/part3/

The strongest running stock motored R*s I've played with have Dyna 3000s running aggressive advance (34A), aftermarket carbs/intakes/exhaust and a VOES siwtch, using vacuum to control ignition timing. The best overall 1600 dyno'd 75HP (internally stock) running an S&S and belongs to Jimaray on the RSR forum. The best overall running R*s IMHO (not necessarily the most powerful at wide open throttle, but the most crisp and responsive at part throttle openings) had Mikuni HSRs with aggressive timing, intake, exhaust and a VOES.

It's hard to determine just how much difference each modification contributes and certainly "un-corking the intake" is the best contributor. But, for those that spent ~$250 for a Dyna (not just for the rev limiter), it can help "wring out a litle more power", coupled with premium fuel and more timing advance

Respectfully submitted for technical inspiration,

Jim<br><br>Post edited by: Bucardo, at: 2006/04/19 10:55

Well said Jim!

Thanks for providing the detailed explanation.

I was only trying to address running low octane in an 'unmodified' engine. Guess I'll need to be more precise with you around

I encourage everyone to read the info in the link provide in the previous message (Bucardo). It's rather loooooong, and doesn't specifically talk about timing/octane but there's a lot of useful info in the article, including the other Parts.

Doc

Oops, sorry

got the paragraph reference wrong in the link. Para 7.4 and 6.17 discuss timing and octane along with tuning (timing) to run higher octane. There's a table that correlates different timing advance to required octane levels.

Jim

The page must have loaded incompletely the time I went there as the article ended at &quot;mothballs&quot; (Section 6.20).

Section 6.17 talks in generalities; Section 7.4 is the 'meat'.

Thanks,

Doc