just boil the darn thing
If time is not a constraint, just boil up a big pot of water.
Stick the pipe/pipes/manifolds in the pot of boiling water for about 15 minutes or so (depending on how thick the layer of grot is), put on thick and long rubber gloves and scrub the grot off with a brush. The varnish/tire grot should have softened enough for it to come off very cleanly.
Heat is the key because it softens the hardened varnish that's holding the grot to the pipe/manifold. Do not use naked flame though (such as blowtorches) as naked flames may cause spot hardening on parts of the pipe which will affect performance/durability, also... it softens the varnish only very temporarily... then immediately hardens up again, making removal super challenging.
Have fun cleaning! bright n shiny!
During heats, i use durex for both front and rear, with Hard inserts.
Sorex, i meant Sorex.