Group B was known as the golden age of rally (Insert asian dad meme "Why no group A?"). The cars where bonkers fast, and could have more mods than the immediate family of Joan Rivers. The cars where deemed too dangerous and outlawed faster than NYC's Big Gulps. Now, over 25 years later, we need to look back and see if these cars where really any crazier than the WRC cars we see today.
Group B rally was just an even crazier version of the rally already running, think of rally as Gary Busey and Group B rally as Gary Busey on Meth. Gary Busey on meth, it happened more than once. Group A was basically a production car that was modded to rally, Group B was a rally car that was modded to production because FIA rules and such. At the introduction of Group B fan numbers went up and more manufactures entered the rally game.
The Group B cars where aerodynamic like nothing rally had seen before. Possibly the most famous is the Lancia Stratos that looked as long and flat as women's beach volleyball player while still absolutely beautiful. They could be made almost any lightweight material and there was no restrictions on boost. This means that horsepower in these lightweight, aerodynamic, machines with more boost than the fridge of a retirement home was, in some cases, over 500.
Now lets compare the cars of then to the cars of now. Few auto makers remain in WRC today as compared to the Group B times so that makes this comparison difficult. Adding into the difficulty is the restrictions we see today compared to the "ol' west" of Group B. The WRC cars today are restricted to about 300bhp. These cars have a restrictor placed on them by the FIA to ensure a level playing field. The torque on the cars of today can be very high and a lot of emphasis placed on weight instead of bhp. We all know hp is not the be all and end all of speed, or at least you should.
Power to weight ratio, to me, at least is a more accurate measure. The Ford Focus (I mean come on, its not really a Focus) driven by Mikko Hirvonen weighs about 2932 pounds, as determined by my best Googling. That yields a power to weight ratio of about 9.77 lbs/hp. The Ford RS200 weighed about 2315 pounds and produced 420hp, giving a ratio of 5.51 lbs/hp. That is almost twice as much power to weight as the car of today. The numbers of the Ford RS200 where in an "average" race version, while versions hit over 580hp in some cases.
Now I know there are hundreds of different factors that go in these differences and comparing them is really like beating a dead
Sarah Jessica Parker horse. In comparable rallies the WRC cars are holding their own against the times of their bonkers grandfathers. The WRC will win a twisty rally due to all the new innovations in car tech while rallies with lots of straight would give the Group B an advantage.
Group B was outlawed due to a series of tragic events involving spectators in 1986. The insane speeds of the cars was to blame even though it was not the only factor. Keeping fans farther from the road keeps them safer, but does take away from insanity of people in the Group B days. Cars of WRC today are technically slower than those cars but driving skill and innovations make the races comparable.