Because the CVH head is about as efficient as 2 legged police dog, at least compared with a zetec/YB head unless it has had serious (and good) work done to it. On a more serious note the CVH (yours is a large capacity CVH) has a high BSFC which means it uses more fuel to make the same power as a more modern engine. So you need bigger injectors than say a cosworth/zetec etc for the same power.

Buy a set of Siemens deka 630cc/min injectors and never have to upgrade again. Better and cheaper than a large set of old Bosch injectors.

Rob,