The GT40 had a 4.7l engine which produced 380hp and ran a compression ratio of 10.5:1 http://www.carfolio.com/specifications/models/car/?car=2035
The MkII and MkIV came with a 427 with 485 - 550bhp and 475 - 535lb-ft torque and could reach a top speed of around 205. http://www.gt40spf.com/gt40-specifications.aspx The MkI and MkIII all come with 289s.
The Gulf cars, the most successful of the GT40's, Mk1's used the Gurney-Weslake 302cc, with about 540 horsepower, top speed of about 211.
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