My engines are mercruiser 7.4. I see listings for 7.4 and 454 on same sites. I always thought a 7.4 was the same as 454?

There are the same.

Magnum is the 454 HP motor!

No..

7.4 litre is a 454 cubic inch motor but there were diff versions of the 454, the 7.4 mpi was 310hp and the 454 mag was 385, there was also a 330hp version which i beleive was the carbed 7.4's?

True, but earlier versions of the mag were 365 HP. The quesion should be more specific to what year since merc made so many changes over the years. They also had a 420HP 454 with TRS drives.

I thought the 330 was measured a the crank, and the 310 was prop?

if memory serves me right, 310 was rated hp and actual prop hp was around 300

The 310 hp was prop hp. It was a carb engine known as the 7,4L that went out with the 502 mag and 454 mag when the 496 came in and replaced by the 6,2. The 330 was also prop hp. It was not untill later years (around when the 575sci went out) that merc racing rated the engines at the crank.

Black mercs are still on the prop. If anyone disagrees with this, look at mercs website.

An exampel of this change was:
HP525EFI. 500 prop hp, then changed to 525 crank hp, when the 600sci was introduced. Untill then everything was prop hp, and the name of the merc racing engines the supposed crank hp. Ie, 900 sc/dry sump six = 850 prop hp and 900 shaft hp or 600 sc = 550 prop and 600 shaft ect.

Hope this helps.

i follow what your saying about shaft hp and prop hp, so basically all blue motors are shaft hp and all black motors are prop hp, correct? the hp500 was actually 470hp so what was the 500efi? the 310hp 7.4 was fuel injected not carbed so was i right about the carbed 7.4 being 330hp?