Pud237

Pre-cats in the manifold are usually very small, they are there because they heat up a lot faster than the cat in your exhaust, so work pretty much from the turn of the key. It is important for environmental reasons to have cats in the manifold, as petrol engines run very rich during the start-up phase, so lots of unburnt fuel would otherwise go straight through the main cat without burning off, because it wouldn't be up to temp.