The action of crevices in an operating SI engine has been studied with a fast-FID. A single-cylinder Ricardo E6 research engine was fuelled with propane and operated at 1300 RPM. FID measurements in the exhaust port have shown that advancing the ignition timing from 30°BTDC (MBT) to 60°BTDC raises the HC concentration by 25% during the first 120°CA of the exhaust stroke and by 20% for the remainder of the stroke. A static “artificial” crevice of known volume, mounted inside the engine cylinder was used to study the differing HC outgassing characteristics at the two ignition timings. When sampling in-cylinder at the mouth of this crevice, the opposite effect of a 50% reduction in outgas HC concentration occurred when the ignition was advanced to 60°BTDC. It is argued that advancing the ignition causes earlier enflamement of the static crevice and induces burned as well as unburned gas to enter the crevice thereby diluting the HCs from this source. In contrast to this, it has been shown that displacement of the piston down the cylinder bore causes later enflamement of the top-land with advanced ignition and results in higher HC concentrations in the exhaust.