Old mercruiser 470 safe cruising rpm?

Hello, I have an old 1979 mercruiser 470. It's the 170 hp version. The manual says that the rpm range should be between 3800 and 4200, and my max rpm is 4300 (43 mph) there are 2 types of 470's. the 170 hp and the 190. The 190 hp is the same motor with a bigger carb, and that has a rpm range up to 4600 I think, so I know I'm not doing damage by maxing out at 4300. I generally cruise my boat at 3500 rpm which is 81% throttle. I am wondering if this is bad to cruise long periods of time at considering how old the motor is. I just don't want it to blow up. Also it is on a 1979 formula thunderbird boat that I completely restored. (Took me 600 of my own hours) and it would be terrible if it blew up after all the work I have done to the boat. So my question is, is cruising at 81% throttle bad on a 35 year old motor?

Re: Old mercruiser 470 safe cruising rpm?

Hi Will, Welcome aboard! I Don't know that it would make that much of a difference but Mercruiser recommends 75% of WOT for extended cruising. At 35 years old you probably have a little loss of compression, a valve or two that's not quite at 100% etc., so maybe play it a little safer and cruise a little slower. It would be a shame to throw a piston through the wall of the motor after all the work you've put into the boat. Also be sure to keep an eye on your temp gauge as overheating seems to be a problem with the 470's. I have a 1976 Formula F-233c that I just finished fully restoring. They're great boats, Enjoy!

Re: Old mercruiser 470 safe cruising rpm?

Thanks for the input! My motor does seem to be running hot, even though I put the bigger 4 inch heat exchanger in. After long periods of running, or when I am pulling a tube, it can heat up to 200 degrees. Whenever it gets that hot or sometimes a little hotter (don't see gauge until to late) :facepalm: I immediately shift into neutral and rev it to about 1,500 rpm until it cools down. Anyone else care to comment on this thread? Lots of input would be greatly appreciated!

Re: Old mercruiser 470 safe cruising rpm?

You changed subjects, and the title says you want to to the correct cruising rpm, now you have overheat problems. People may not notice that.

The engine should be able to be run at WOT (wide open throttle) all day without overheating.
You NEVER want to overheat a 470, that is the one things those engines don't like.

Some things to look for.

1. When was the last time the impeller in the drive was checked?
2. What's your WOT rpm
3. Do a compression test and possibly a leakdown test with the cap off the expansion tank and see if you perhaps have a blown head gasket. That would give you the overheat.

Re: Old mercruiser 470 safe cruising rpm?

Sorry for changing subjects. I would still like to stick to the safe cruisin rpm thread though. So I assume that I won't do damage by cruising at 80% throttle.

Re: Old mercruiser 470 safe cruising rpm?

willmartens said:

Sorry for changing subjects. I would still like to stick to the safe cruisin rpm thread though. So I assume that I won't do damage by cruising at 80% throttle.

Click to expand...

You will do damage if it's overheating.

Re: Old mercruiser 470 safe cruising rpm?

It never gets above 200 degrees. It really only starts heating up to almost 200 when I am pulling a tube, or running it hard. It also doesn't help that the water is almost 80 degrees.
Also do you think that 80% (3500 rpm) is to hard to run it consistently at?

Re: Old mercruiser 470 safe cruising rpm?

It rarely gets that hot and whenever it does I bring it back down. There's no Crack in the head or head gasket, also the impeller is 1 year old. I don't really think that it is a overheating problem because It only does that when it is run really hard in warm water. I also just replaced the thermostat so I think that everything is ok on the engine. I also have another question. Would it be bad to run at wot or 5 minutes on the engine? (It's 35 years old) if that is bad, what's the longest I can go wot for?

Re: Old mercruiser 470 safe cruising rpm?

I was going to start a new thread about the overheating issue. I really wanted this thread to be about a good cruising rpm

Re: Old mercruiser 470 safe cruising rpm?

Anyone else have any opinions?

Re: Old mercruiser 470 safe cruising rpm?

Yeah, your temperature should not creep up, no matter how hard you are running the engine, and agree 200 is too hot! Something is wrong there and you need to figure it out and get it fixed. First thing to do is change the impeller (1 year old or not), and the housing. Replace with OEM Mercruiser parts just IMHO. Good luck, treat that old 470 well and it should last for a while more.

Re: Old mercruiser 470 safe cruising rpm?

willmartens said:

Anyone else have any opinions?

Click to expand...

Yep... After you fix the overheating problem, you can run it at any throttle setting you need for as long as you need..... As long as everything else is in good order....

Do a compression test, and a leak-down test. That will give you a good idea of the block/pistons, rings and valves condition. Next check the initial ignition timing and the timing advance curve. Check you points/condenser/rotor/cap and leads. Throw in a new set of spark plugs. Ignition system sorted.

Change all the fuel filters. I would even pull the carb off and check all the settings inside... Only about 2 hours work, tops. That will be the fuel system sorted out.

Next pull the exhaust elbow and check that, thoroughly! Any doubts, trash-can it and put a new one on.

With all that done (and the reason for the overheating sorted, and don't be mistaken into thinking if it's only getting to 200, which is WAY too much, when you're pulling a tube it doesn't have an overheating problem.... It does... Fix it!!!)

After all that, run hard and fast all day long...

Chris.........