49panhead

[militaryrider]

Does anyone see a problom with running 100 octane low lead gasoline in a 49 panhead ?
The Engine is stock , I have had probloms with pump gas when it get warmer out,with pre ignition.
I put the 100 octane in and what a difference !
I just wonder calculating the stroke and compression ratio of a stock Pan is the 100 octane completely burnt at the bottom of the stroke ? any comments Thank You

[Kojack]

If it stops spark knock than it's good. Higher octane fuels are more resistent to preignition but once they ignite they'll burn evenly and completely so that shouldn't be an issue. The fact that you mention low lead instead of unleaded makes me wonder if you're using avgas. The main problem there is that avgas is illegal to use in landgoing vehicles since there are no road taxes paid on it. Don't get caught using it, some police are real serious about that kind of thing. I read an article a few years ago about some other issues with using avgas but I'll still stick with my opening statement. I use unleaded racing fuel in my shovel when I can and it does indeed make quite a difference. I've tried a few different bottled octane boosters and found them about worthless.

[Lonnie]

Re spark knock in a '49 pan, there could be other factors at play, including excessive compression due to piston design (remember as bore is increased as in a boring job, compression also increases) or in carbon build-up or in incorrect timing. Additionally, if there is an intake leak, as the motor heats up you may find that you develop a pre-ignition rattle due to excessive mixture leanness. Also, the carburetor could be out of adjustment.

Re Avgas, it is a "dry" fuel and can wash down your cylinder walls. In the early '70s at the naval station in Rota, Spain, the military mixed it with the local very low octane Spanish gasoline so that the American cars requiring higher octane would be able to run and did not have this "oil wash" problem. I have used it in a '72 shovel that had the same problem you have and I added a small quantity of Marvel Mystery Oil. I also had good luck mixing it with pump gas. In that particular motor, it had a history of spark knocking since at least '77, though it had only 28K miles. I am inclined to believe that a top end (and resultant decarbonization) would have done wonders!

In the rebuilt motors I have (2 pans, 1 knuckle) and in most of the motors of my friends in our local chapter, we use 87 octane. No spark knock and much easier to crank, so I repeat, you have some other issue to deal with.

Good luck, Lonnie