'00 KTM 640 LC4 Adventure R fuel comparison

Chart Key
	Horsepower
	Torque
	CO%
	closely fitted valve guides, valve job, valve springs upgraded & shimmed, mild port clean up, compression reduced, piston lightened & surface area reduced, wrist pin lightened, specially fitted rings, adapted Remus "Freeride" two-muffler exhaust system, inside of exhaust system ported, airbox lid & snorkel removed, slide stop modified, emulsion tube modified, custom needle and tuning, using 91 octane Chevron fuel (at 100% throttle opening)
	same as above using 89 octane Chevron fuel (at 100% throttle opening)

The loss in HP shows demonstrates that the engine is over the edge (ignition timing too advanced, still too much compression, still not enough exhaust flow etc.) and therefore at risk for detonation when using 89 octane. The delta points to it in fact still possibly being over the edge with 91. One way to test that would be to try more octane yet to see if HP increases. Nowadays we use the NOx content in the exhaust gas to give a feel for detonation risk (we didn't have that capability when this test was performed).

before & after modifications

piston & pin

detonation-induced mechanical failure