Back in the days, when engines ingested fuel via carburettors, there were Bakelite or phenol spacers installed between the intake manifold and carburettor. The role, for those thermal spacers, was to prevent the gas from boiling and evaporating in the carb. Today, the thermal spacers, sometimes called phenolic spacers, are one of the ways to reduce the intake temperature. When coupled together with a cold air intake and/or an intake manifold wrapping in a heat reflective material, they can greatly help in reducing IAT (Intake Air Temperature). Heat may be transferred to the intake manifold by one of three modes: conduction, thermal radiation and convection. The phenolic spacer is blocking conductive heat transfer. You can observe the IAT on your Stand Alone Engine controller (like. Motec, MS3Pro, AEM, Ecumaster, VEMS and many others) or by diagnostic interface to your OEM ECU. Before logging the IAT, please check where IAT sensor is located, if it's located BEFORE the intake manifold, then it's pointless to use those readouts. When idling or driving on light loads you can notice how high the IAT can be (it varies from car to car obviously), when the engine is on higher load, the IAT is slowly lowering, it's simply because the air flowing to the combustion chambers is cooling of the intake manifold. However, on some cars when the engine is kept for an extended period of time on high load, the amount of heat generated under the hood is starting to heat up the intake manifold. Every car (with different engines) has its own perks. The thermal spacer will help to keep the intake manifold cooler, especially on idling and light loads, preventing the anti-knock feature to step in. Also from a standstill start it will help to maintain torque output (you don't have to wait a dozen or hundreds of seconds for the rushing air to cool the intake manifold). As mentioned earlier, it is recommended to use also other thermal protection against thermal radiation and convection, thus wrapping or shielding the intake manifold in a heat reflective material, for example an aluminium reflective heat shield.