Currently the exhaust glow scales with engine RPM. I suppose the idea was to tie it to engine load. Typically the metal takes a while to heat up, and once the load from the engine goes, it takes a while to cool down. You should probably put a lowpass filter somewhere to emulate the slow heating and cooling.

Thanks for considering.

--edit: In principle it should be tied to throttle position, lowpassed, but that'd probably be a bigger change.