Restrictor plates are inserted in (or on) the engines to keep the speeds down...Lowers the max speed of the vehical. Claims to "make it safer and a fairer race" but really...it's crap.
If we wanted safer, we'd not have them racing round in the first place....Fairer....then you'd need to have more dosh or a better mechanic to make the BEST engine function you can....
Often times we have these on vehicals for hire to keep us from maxing them out....(usually set just above max speed limit for the area). Sux to rent a nice Mustang, find a great streach of open road only to find out they've got a restrictor in place and you can only go 75 max

__________________
You can't wield supreme executive power just because some watery tart threw a sword at you - Look, if I went 'round saying I was an Emperor, just because some moistened bint had lobbed a scimitar at me, they'd put me away!