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This will start a fight , but octane isn't a measure of how fast or slow gasoline burns . Octane is only a measure of how resistant gasoline is to ignition . Gasoline can be blended to burn slightly slower or faster at any octane rating , although the differences are tiny . What chemists and IC engine designers are looking for are the conditions where the combustion process is complete at a certain degree of crankshaft rotation , low octane fuel ignites more readily , so in a high compression engine or in an old aircooled design that runs hot it can ignite too soon , meaning higher octane is needed to resist pre-ignition . Ethanol does tend to burn a tiny bit slower than gasoline , but that is a different discussion . Dusty
Never start a fight before you do your research.Octane is *not* 'resistance to ignition'. Octane is resistance to *detonation*. It is a critical distinction. Gasoline can be blended for *great* difference in burn rate, and also for energy density. 'Pre-Ignition' is caused by hot spots in the combustion chamber, and is not very sensitive to octane. 'Detonation' is the *un-ignited* spontaneous combustion or explosion of the fuel mixture, caused by heat and pressure beyond the combustion threshold.
if I ever so slightly let off the throttle or go down a freeway slope at 4000+, the noise goes away.
Rod bearings are usually noisy on the over run…..Tom says he applied the choke and no change? Fattening the mixture that much should eliminate or at least lessen part throttle detonation… if you have not tried high octane fuel yet now is the time to do it…..And what you describe ,regardless of new parts ,sounds like loose piston pins….
So your saying that the rod bearing can be quiet at 3500rpm and make noise at 4000rpm on a steady throttle? Would more or less advance change this?Thanks!Tom