black max 175 rpm questions (1750)

[ohioriverrat]

alright so i just got this engine - i know the black max is a bit a bit of an older design but my dad has the 150hp and it runs great. this is the late 70s early 80 v6 175 hp 2700 something cc's that they called a 3 liter.

the motor runs well, has 120psi on all cylinders and the only problem with it is that it needs an impeller- i got the entire boat for 500 and i think i made out like a bandit just because of the engine.

one of my friends just told me that his director at work has a boat with 2 of these engines on it and he just had them both rebuilt to the tune of 6000 dollars. he said the director told him that the mechanic doing the rebuilding said the motors blew up because they were designed to only run over 4000 rpms only , and running them slower will blow them up.
my first instinct here is not believe him obviously - i mean how can you blast through no wake zones at 4000 rpm? i figure this is just another case of misinformation and over exageration.
after all my dads 150 black max rarely gets run over 3500 rpm and its been going strong strong strong for 5 or 6 years now.

i do however want to know if there is any truth to this at all. perhaps there is a system failure more prone at lower rpm such as oil injection - which is null and void because i will be mixing my own - i have had the misfortune of dealing with an oil pump failure on 2 strokes before.

 

[j_martin]

Re: black max 175 rpm questions (1750)

You need to prop them so that they can turn freely. There is a WOT rpm specification for them, and if the prop is so steep it cannot get into that range, it will be overloaded throughout it's range. If in doubt, go a little high rather than a little low on WOT rpms.

It's common knowledge that over-propping will kill them a lot quicker than over-speed.

They'll run happily at half throttle. They don't like long periods of idle.

 

[ohioriverrat]

Re: black max 175 rpm questions (1750)

thanks for the reply, the prop was something i thought of to remedy the need for higher rpms if i indeed needed to run at high rpms but the question was if there was an inherant flaw to this engine that required it to be ran at WOT all the time.

-- ive owned several boats, and run the gambit with different props - i just wanted to know if there was something particular about this engine that would cause a mechanic to say that.
the engine is on an 18ft bass boat and the reason i am curious is that the boat runs about 72 mph at wide open throttle -

i can just imagine the coast guard when i tell them - sorry but i have to run the ole girl wide open or she will blow!!

 

[sschefer]

Re: black max 175 rpm questions (1750)

The mechanic may have seen the prop and boat and figured the prop was way too steep (too much pitch) for the boat weight. From that he probabaly did some quick math and came up with 4000 RPM's as the max the engine could turn with that setup. If that's the case, (and I suspect it was), he was exactly right.

Your challenge now is to find the right prop for your engine and boat so the engine can turn 5800 rpm at WOT. When you do you'll be able to run at any RPM without over loading the engine and causing it to fail.

 

[ohioriverrat]

Re: black max 175 rpm questions (1750)

correct - i think the boat is already propped pretty good the guy just put a brand new SS prop and it now has 2 trips on it - as soon as it warms up ill know for sure.
i already have a myriad of props to choose from at home from different boats i have owned in the past, i but i think the one that is on it is correct.

the issue was that the guy is under the impression that if you run the boat at half throttle it will blow it up. like there was something magical inside the motor that required it to run at 4000 rpms at all times.

so seeing as my boat goes about 30 to 35 mph at half throttle and about 70 at full throttle that would be ridiculous.

 

[j_martin]

Re: black max 175 rpm questions (1750)

If it's set up right, it should be able to run at any speed without damage. With a bass boat, there's a certain speed where it just falls off plane, and you either have to slow down, or make a huge wake and spend a lot of power, lugging the motor to keep the speed. The motor don't like that for long. On mine, that's between 30 and 40 depending on where the weight is in the boat, either passenger forward or ballast forward allows me to slow down. There's some places I have to no-wake idle for miles, and it's OK, but it tends to load up a bit. I think most big 2 cycle carb'd outboards would do that.

If yer set up for speed, you'll find something like that to be the case. Welcome to performance boating.

John