Air mass meter on Rallycross cars?

[1975DCS]

Hi there,

I am watching Rallycross for 20 years now and noticed that even the current Div 1 cars don't have air mass meters. Kind of strang because all current road going cars seem to have one.

There must be a big downfall with using an air mass meter if they don't use them? I can understand it restricts flow, but a bigger one could be used right? Like Audi did with there S3 engines?

So what is the reason Rallycross car's don't use them and (modified) road car's (like an EVO FQ340) do? What are the pro's and con's?

Thanks in advance!