 Nyora kupfuurira kweiyi ndima - Akawedzerawo IMSA GTP zita mukati me1983 muChevrolet nePorsche powered March 83G apo Porsche yakanga isingakwanisi kuita 956 yavo yakakodzera kukwikwidza gore iroro. Musi waFebruary 27, 1983, akahwina Grand Prix yeMiami. Holbert akapedza ari wechina mu 1984 Indianapolis 500, uye akatungamira kuedza kwePorsche IndyCar muna 1987-1988. Akakunda 24 Hours of Le Mans muna 1983, 1986, uye 1987, 24 Hours of Daytona muna 1986 uye 1987 uye 12 Hours of Sebring muna 1976 uye 1981. Holbert akanga ari mukuru wePorsche North Americas Motorsports Division uye akatungamirira boka rake rokumhanya, Holbert Racing. Akakunda mamwe makwikwi maviri eIMSA GTP kumashure kumashure muna 1985 na1986 achityaira Porsche 962 inotsigirwa naLowenbrau. Muna 1988, Holbert akaziva kuti Porsche 962 yakanga yamuunzira kubudirira mumakore ake okutanga yakanga yava yechinyakare nechizvarwa chitsva chemakwikwi kubva kuJaguar XJR-9 uye Electramotives Nissan GTP ZX-Turbo. Urongwa hwake hwaiva hwokuvaka motokari yemakwikwi ine injini yePorsche yakavhurika nokuda kwamakwikwi evatengi. Porsche pakupedzisira yakavaka motokari yakadaro makore gumi gare gare, kunyange zvazvo WSC-95 isingazombogadzirwi kumapoka evatengi sezvo Holbert naPorsche vaida. 