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Need help w/new setup

lt133

lt133

Member
Long time member/lurker(2003)-new to posting. Long story short I purchased a 1988 251 Convincor in 02. Boat had an unknown replacement motor (454). I had no specs on the motor. Boat has since been completely updated (paint, graphics, interior, gauges, etc.), engine finally blew last year. I was running 63-64mph best with a 23 pitch mirage plus @4800. Just found out the engine was some time of auto replacement, probably 330hp(?). New engine is a 502 which should be right at 500hp. I need to start thinking about a new prop. I'm leaning towards a 24 or 26 pitch Bravo 1 and looking to be running in the mid 70's. Any suggestions would be helpful. Thanks
 
In order to prop correctly, you would have to know what your target rpm would be. Do you have a dyno sheet? Can you be more specific on the internals of the engine?

If I had to make a general rule guess, assuming a 1.5 ratio drive, I would say that a labbed 26" Bravo 1 would be a good choice. You can also go with a labbed 24" Hydromotive. That should get you 5200 rpm or so. That should be about 70 mph. I dont think you will see mid 70's with 500 h.p.
 
New engine started as a 435hp marine base 502. Cam and heads were changed (cam is a custom ground Howard, I don't have the spec sheet in front of me) engine has not been dyno'ed but last engine with same changes dyno'ed at 514hp. Drive is 1.5 Bravo 1 and my target rpm is 5200. Assuming my last motor was only 330hp spinning a 23 mirage @4800/61mph 13% slip I figured I should be able to spin a 26 Bravo 1 @5200 with a 12% for 75mph. Maybe I'm off here but those were my quick calculations.
 
The formula you are using is correct, but you are not inputting the correct information. A 26" Bravo 1 4 blade prop actually measures 24 1/2" of pitch. For some reason, Merc rates that prop at the maximum pitch, while all other props are rated at average pitch. There is a 3" pitch change from the leading edge of the blade to the trailing edge. In order to use the speed formula, you have to input average pitch. Thats why you have to subtract 1 1/2" with that prop. With that said, a 26" Bravo1 @ 5200 rpm with 12% slip, nets you 70.7 mph.
 
Thank you Vinny, I'm always learning something. Any thoughts on an intake for this setup? I'll be running a Holley 800 and am considering an Edlebrock Performer RPM or a Hardin "XS" Extreme....right now I am leaning heavily towards the Hardin.
 
Be very careful when choosing an intake manifold. The wrong choice will kill your low end or top end power. You need something that will be a compromise of the 2. The Hardin seems too big, it may kill you low end. The Edelbrock may be too small , choking the top end, but that depends on the engine. I dont think that your engine would be too radical, so, I would say the Edelbrock would be the better of the 2 choices.

Just to give you an idea of what I did for my 540. I wanted to go with a Dart intake. I couldnt decide whether to use a rectangular port or oval port. My cylinder heads are AFR 315 CNC, rectangular port. The advantage of the oval intake is that it keeps the torque curve down low where we boaters want it to be. The advantage of rectangular port is the extra flow at top end. An uncommon trick is to use an oval port intake manifold on rectangular port heads. Then port match the last 1 1/2" of the intake runners to the heads. Not an easy job, but it can be done.
 
Thanks again for the advice. I like the idea of port matching, the old intake is an oval port Edlebrock. I start spending any more money the wife is going to kill me!
 
Thanks Vinny, I got the photos. I've decided not to decide-the guy who is installing my motor picked up the Hardin for me, real short money new. I will port match the old Performer RPM and try both and let you know which works best. Thanks for the help.
 
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