thirdchildhood

iTrader: (6)

So, why is a 470 hp 500 EFI over-rated and a 525 EFI underrated since they make between 540-570 hp depending on who you talk too. Is the 525 really almost 100 hp more than the 500?

Knot 4 Me

Marine engine manufactures can have up to I believe a +/- 10% difference in stated vs. actual HP.

thirdchildhood

iTrader: (6)

I realize that Mercury has under-rated outboard hp in order to gain an advantage in racing but I don't see why they would call a 470 hp engine a 500 EFI and then turn around and under rate it's successor, the 525 EFI. Until recently I thought the 500 was 500 hp.

Knot 4 Me

I agree it is confusing.

Uncle Dave

From an I/O perpective after looking into this earlier it seems the 525 EFI is the only engine in the lineup to consistently outperform its sticker rating

According to guys that test them regularly the 600 & 700 consistently pull well under their rated #'s when stock.

Merc does not dyno each unit until you get to the very upper end of their engines.

Ilmor insures each 700 series engine (710/725) delivers exactly that by dynoing each one.

Other guys like Teague have been underrating their engines so long that they have bumped the rating a bit and still clear the # handily.


Uncle Dave

Raylar

You need to understand that Mercury Racing has been for a long time establishing for them selves an engine horsepower rating on the Racing engines only that represent approximate horsepower at the prop or less drive losses. When others look at Mercruiser engines ,ie the MPI's, EFI's, 496's and now 502HO's etc. they are using engine crankshaft horsepower as their advertised horsepower rating, no drive losses or horsepower at the prop.
On the Mercury Racing HP500efi the engine really makes about 505-510 HP at the Crankshaft and about 470-480 at the prop with its Bravo Drive losses.
The Mercury Racing hp525efi makes about 550-560HP at the crankshaft and this results in about 525HP at the prop in a Bravo XR ( bigger drive losses in an XR)
When Mercruiser 496HO's for example are measured at the prop they have shown to be about 390-400HP at the prop in a standard Bravo 1 or 1X drive.
The differences here are mostly due to the way two different divisions of Mercury Marine choose to rate and measure their engine offerings horsepower and this has obviously created a lot of confusion for customers who view them all as Mercury Marine engines.
Kind of a simple way to remember:
If its Mercruiser black its at the crank
If its Mercury Racing Blue its at the prop

Also keep in mind that most aftermarket marine engine manufacturers like ourselves measure our horsepower at the crankshaft on dyno's with most using corrected outputs with all accessories running on the dyno.

Hope this info helps with some of the confusion.

Best Regards,
Ray @ Raylar

thirdchildhood

iTrader: (6)

Yes, it does help. Thanks, Ray.

Uncle Dave

Sort of- the 6 & 700 series are part of the "blue" line and they are rated at the crank. (and fall short)


UD

Thunderstruck

To add some detail to Raylar's post. The dyno correction factor procedure/method used makes a big difference in hp on the dyno. Merc may use the SAE 1349 procedure like the big, big engine builders (Ford, GM) use. Smaller shops sometimes use the SAE607 correction factor which gives more hp due to less account for friction losses from accessories, etc. The factors can be 5% different so if Merc gets 500 hp the SAE 607 correction factor could give 525 hp for example. I don't know exactly what Merc does but I have seen writeups along these lines that help explain inconsistencies in hp ratings.