Well, the Group C prototype regs were pretty tight - but in the glory days in the late '80s when Porsche, Jaguar and Mercedes were going head to head the cars were quite different.
What I loved about the formula was that the manufacturers had to use a stock engine block from their car line-up (you were free to turbo-charge it or whatever, but the block had to be stock). Porsche of course used the flat six (with a couple of turbos), Jag had the V12 (with much massaging from Cosworth) and Mercedes used their 5 litre V8 with twin turbos.
That was before the 3.5 litre NA regs came in (the Peugeot you see above was built to those regs). We saw the absurdity of the Jag running around with the Ford F1 V8 in it - ended up killing the formula.