Merc 7.4/Bravo III Prop Question

[greg82255]

Hi All,

Today I test drove/agreed to purchase a '94 Formula 252 with a Merc 7.4/Bravo III setup. I've never had a Bravo III before, and it has 26p props on it. The boat hit 51 mph at 5000 rpm. I am pretty sure this is higher than the recommended RPM range for a 7.4 (42-4600?). I am not familiar with the RPM gain/loss with different pitches on bravo III and am looking to see what I should try next. Any recommendation will be appreciated.

 

[Fun Times]

Hi there, if everything is stock than you should be able to find/confirm the RPM range on one of the stickers on top of the engine.... If not, try finding the engine serial number to help confirm the exact engine model and find the engine specifications inside the service manual as the RPM range varies between models. If you'd like a helping hand with that info, go ahead post the serial numbers.

Also on the side of the upper stern drive there should be the gear ratio that you may need to know at some point.

Where to Find Mercruiser Stern Drive and Engine serial numbers.
Drive units
In-line Engines
V6/V8 Engines

 

[tpenfield]

The RPM is high for a stock 7.4L but if it is a "454 Magnum", it is on the upper range.

The prop pitch seems fine for that boat, and the top speed is about right, so I would be thinking that maybe the drive ratio is a little out of whack. I do not believe the F-252 came with the BIII outdrive, (Formula typically used Bravo 1's on those boats) So, it may have been replaced with a Bravo III and the gear ratio is not quite the right match.

You could go with the 28 inch prop set, but that may be too much pitch on the low end. Another option is just don't take the engine to WOT.

 

[greg82255]

I don't think it is a 454 Magnum. The boat needs some glass work on the engine hatch before I take delivery, so I can't get the serial #s just yet, but I'll come back to the thread as soon as I have them.