1986 Mercury 175hp Black max - RPM Question?

[K.Smith]

I have a 1986 Mercury 175hp Black Max on my 1985 Ranger 370V. I currently have a High Five prop (I believe either 22 or 24 pitch, I know without the exact pitch it might be hard to answer the question. I will post once I have a chance to look). I was just wondering what the max rpm's I should be running on that motor? I typically run it about 5000rpm's / 50-52mph on my GPS. I have been reading guys that are running 65-70mph with the same year boat / motors. Sounds a little high, and I am sure they are running three blade props. not fives. My motor is older and I just don't want to run it to hard, if you know what I mean.

 

[CharlieB]

Re: 1986 Mercury 175hp Black max - RPM Question?

There are a great many interpretations of 'run it hard', many have NO relationship to a Marine motor.

Marine motors work HARD, all the time, the faster they run, the harder they work, IF, and this is a great big IF, IF the motor is propped correctly.

The factory engineers have determined through rigorous testing, the recommend RPM range to operate your engine, prop your motor to run WOT at or near the top of this range is working your motor to the best OEM standards.

While a bit more prop pitch, the motor may run at or near the bottom of the OEM RPM range, the motor is then loaded, (and working a bit harder) more than with the slightly less pitch prop.

Slam the throttle to quickly get the boat up on plane, then pull the throttle back to simply maintain a comfortable cruising speed. Regular maintenance, quality oil, clean fuel, and your motor should provide many years of reliable service.

Most of the V6's run well near 5600 WOT, boat normally loaded, 1/2 tank of fuel.

 

[j_martin]

Re: 1986 Mercury 175hp Black max - RPM Question?

Coupla things. If you have an XR2, then the high five prop that fits would be even pitches, with 24 being the steepest. If it's a standard 4 3/4 inch LU then the pitches would odd, and I think 25 is the steepest. I suspect you have the large LU and a 25 pitch.

I have an 88 Tracker with an XR4 and a 24 pitch high five on it. It runs in the low 60's when light at about 6200 rpm, and loaded right at 60 at 5900 rpm. When I'm towing a water skier, or camping, family low throttle stuff, I put on a 22 inch high five. If I find a cheap one, I'll go to a 20. If I shaved the heads and upped the compression a little bit, and threw out the trolling motor batteries, 70 would be easily achievable.

5000 rpm at WOT will beat that engine to death. It needs to run at 5500, and 6000 won't hurt it. 6500 is about the limit for the VRO oil injection system.

Top performance is achieved by running the engine set back 6 inches and high enough so the prop is surfacing. Mine is high enough so that I have to watch the water pressure. It is set so that just before I lose water pressure, the engine is trimmed about level with the lake, and throws a roost that's only a few feet high. The prop is surfacing about 2 inches at that point. That gives me both best speed and drivability at speed.

When I bought my boat, I gained almost 5 mph just by going through the engine and carefully detailing all adjustments, mostly float level and timing.

I find that with the high five, I can get enough bite to launch like a slingshot, and still run it surfacing. A 4 1/4 inch LU usually will fail when surfacing a 3 blade prop, but seems to stand up fine with a 5 blade. Stands to reason as the tortional vibration would be less, and that's the killer. With a 3 blade, I'd have to run deeper, and would not gain any top end and would lose hole shot.
I ramble a lot
hope it helps
John