From Flemming's website (courtesy of Google Translate
The Dane Flemming Graversen, currently one of the most sought-after tuners in the speedway scene, is now coming onto the market with an in-house design: a DOHC cylinder head for the short-stroke GM engine. The aim of this new design was to achieve a higher torque in the lower speed range in order to improve the acceleration after the start. Therefore, the maximum speed is only 11,000 rpm. but an increase in performance between 2 and 3 hp compared to the series was achieved. The camshafts are driven by 2 chains, whereby the chain coming from the crankshaft drives the inlet camshaft and a second chain then branches off to the exhaust camshaft. The camshafts are mounted on one side in needle bearings and on the other side in ball bearings, which keeps friction losses low. By using light metal materials with very low weight and machining with the latest CNC milling technology, the weight of the The complete engine, as offered by Graversen, weighs 27.2 kg, only about 1.2 kg more than the production GM. As is usual with four-valve engines, the spark plug is placed exactly in the middle of the combustion chamber. The inlet valves have a plate diameter of 34 mm and a shaft diameter of 5 mm. The plate diameter of the outlet valves is 33 mm and the shaft diameter is also 5 mm. The valves face the combustion chamber at an angle of 22 °.
So he has built a completely different cylinder head (double overhead camshaft ) completely changing the characteristics of the original GM, but re uses the the GM "bottom end" ? Lots of tuners have done his in the past ... adding new top ends to existing bottom ends. So don't see an issue. Sounds like GM need to step up and offer thier own dohc motor ?