454 vs 7.4
 

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?

I thought the 330's were crank rated and were a little weaker with approximate 290 hp at prop. No? Read it on oso in several threads. Don't know for sure.

LC

The 500EFI was also 470 prop and 500 crank.
540 bulldog 500 crank and 470 prop
525sc 525 crank 490 prop
800sc 800 crank 700 prop
575sci 575 crank 550 prop
just to name a few.

About the 7,4's and 454's

This is the 1997 line-up, and the last year for the carbed 7,4. I think one of the misunderstandings was, that the carbed 7,4 was 330 at the crank and 300 at the prop, and at the same time there was the 7,4LX MPI with 330 prop hp. Before the 454MAG MPI 385 hp came out, the mag was 365 and carbed. In 1998 the 7,4L carb and 7,4 LX EFI was replaced by the 7,4MPI with 310 prop hp. This lineup also applies to the 2 previous years. Haven't looked in the earlier catalogs, have to dig them out.

7,4L Carb 300 hp
7,4LX EFI 300 hp
7,4LX MPI 330 hp
454Mag 350 hp
454Mag MPI 385 hp
All are prop hp.

If you go back far enough, the oldest 454 Magnum was carbed and 350hp prop, then 365hp a few years later, then 385hp with EFI. The 502 Mag was carbed and 390hp prop, then 415hp prop with EFI. The 330 from the 70s and 80s was a 454 and 330hp at the crank, as was the 260hp small block etc. All crank until others started measuring at the prop and then so did merc.

Yep

The 7,4l carb 330 crank hp motor stayed more or less the same thru the years. When measured @ the prop it was rated @ 300 hp. I don't remember when they started prop hp, but my guess is around 85-87. When I find the old spec's I can post them. In the 90's. They actually gave both prop and crank ho, but the norm had become the prop hp as you stated.
The mag efi (tbi) was a 350 hp. It was only the mag mpi that was 385.

OK, just quoting as I go and have not read everything here yet. The 330HP 7.4L was at the crank 10% loss would be 300 at the prop.

Engines are 1989 7.4 Some told me they are 330 hp.

They are 330 crank and 300 prop.