Ace wroteYou need to purchase a Dynojet Power Commander for the ECU. This permits programming of the fuel and ignition. The user soft ware allows increase of the rev limit to 6000 rpm. To increase it further requires requesting a code from Dynojet, which you can put in to the program.
Some Ace GT Billet Head owners have done this, and reported on the US forum. In general, increasing the limit to 6000 rpm gives enough revs to get over The Ton.
The factory valve springs are known to become unstable at rpms above 5800.
I changed the valve springs ... I forgot to specify. You are right ACE.
ACE, what does the ECU limiter do? On the spark (ignition)? Or on the fuel?