Lycoming 4 cyl cruising rpm

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Lil Wayne
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Lycoming 4 cyl cruising rpm

Postby Lil Wayne » Mon Apr 17, 2017 7:44 pm

Hi all, I was hoping someone could clue me in about appropriate cruising rpm for the boats I use. A little background, I work out of two company owned boats, one is a 14'x~6' alumitech with an o360, and the other is a 14'x8' palm beach style hull with an o320. The o360 was rebuilt about 300 hours ago, and the o320 was rebuilt less than 100 hours ago with low compression cylinders. Both have composite whirlwind props.

About a week ago, I was told by our mechanic/engine builder that they shouldn't be run above 2300 rpm for more than a minute or two, or they'll overheat and start burning up the engines. The o360 boat will barely stay on a plane at 2300 rpm with about 500lbs of gear and driver, and the o320 will start to come off plane at around 2100 rpm. And that's on flat water and calm winds. If I'm heading into any chop or headwind, I can barely stay on a plane at 2500 rpm. I haven't thought much of it in the past, I've been running the o360 and similar boats for years at 2500 rpm and the temp gauge has never gotten above 200*.

I was curious if 2500 rpm runs for 20-30 minutes at a time is really that unreasonable? Any insight would be appreciated.
Last edited by Lil Wayne on Mon Apr 17, 2017 7:44 pm, edited 1 time in total.
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fletchster76
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Re: Lycoming 4 cyl cruising rpm

Postby fletchster76 » Mon Apr 17, 2017 9:30 pm

Aircraft engines are designed to run at a continuous RPM of 2500-3000 depending on motor. Your 360 can run at 2750 all day. With that said though both boats are pretty big for those engines hauling tools or gear. JMO
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mojoe
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Re: Lycoming 4 cyl cruising rpm

Postby mojoe » Mon Apr 17, 2017 10:16 pm

Sounds to me like you are trying to get advice from here to disagree with what the mechanic who works on you're motors said.

Sounds like trouble to me. He knows those particular motors better than anyone and the guidance of even a moderate to poor mechanic who knows those particular motors is 1000 X better than any advice you will get here.

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Sarahj
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Re: Lycoming 4 cyl cruising rpm

Postby Sarahj » Tue Apr 18, 2017 12:25 am

It sounds like a power to weight ratio issue. Most setups with those particular engines are on 12ft and smaller boats. I run a 0-320 on a 10ft hull and it planes out at around 1200 to 1300 rpm with two people. I cruise between 1300 and 1500 with no wind at a speed of 20+mph. Running with a load to the camp I have to push the boat to 2000 to get it to plane off and cruise at 1800 rpm and 15 mph pushing around 1000 lbs. Red line in an aircraft is 2700. Cruise rpm at 1000 ft agl is 2350 rpm in an aircraft. There is tons of data available from the aviation community and engine manufacturers. My opinion is your boats are under powered but you have to run the boats like pilots run airplanes. Check your temp and if you are really worried install a CHT gauge so you have a very accurate picture of what you engines are doing when you push them hard.
Old School 10ft Taylor 0-320 63" Sensenich H Series

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Lil Wayne
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Re: RE: Re: Lycoming 4 cyl cruising rpm

Postby Lil Wayne » Tue Apr 18, 2017 6:34 am

mojoe wrote:Sounds to me like you are trying to get advice from here to disagree with what the mechanic who works on you're motors said.

Sounds like trouble to me. He knows those particular motors better than anyone and the guidance of even a moderate to poor mechanic who knows those particular motors is 1000 X better than any advice you will get here.

Not necessarily, just wanted to do some research so I'm not reliant on one source of information. The lycoming operator manuals I found spoke more in terms of percent power versus rpm.
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Lil Wayne
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Re: Lycoming 4 cyl cruising rpm

Postby Lil Wayne » Tue Apr 18, 2017 6:45 am

We spend 80-90% of our time in these boats at a fast idle, so I guess I hadn't realized how underpowered these boats were, I knew they wouldn't run dry but I figured staying on a plane between the ramps and our work areas wouldn't be an issue. Ill just have to be aware of headwind and engine speed.

Thanks everyone!

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mojoe
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Re: Lycoming 4 cyl cruising rpm

Postby mojoe » Tue Apr 18, 2017 7:11 am

What I'm trying to get at is that not all 0360s are set up the same. Your mechanic will know yours best. My builder set my motor for wide-open throttle. The closer I get to 2700 the closer to perfect my mixture is. It runs rich at lower RPMs. And the oil temp won't tick till I run it 2/3 throttle for a few minutes. He is trying to protect my motor from damage of hard running.

Since 90% of your running is at idle, it is very likely he set your motors up differently so that you would not constantly foul plugs.

I don't know this for sure, but HE does.

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Lil Wayne
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Re: RE: Re: Lycoming 4 cyl cruising rpm

Postby Lil Wayne » Tue Apr 18, 2017 11:05 am

mojoe wrote:What I'm trying to get at is that not all 0360s are set up the same. Your mechanic will know yours best. My builder set my motor for wide-open throttle. The closer I get to 2700 the closer to perfect my mixture is. It runs rich at lower RPMs. And the oil temp won't tick till I run it 2/3 throttle for a few minutes. He is trying to protect my motor from damage of hard running.

Since 90% of your running is at idle, it is very likely he set your motors up differently so that you would not constantly foul plugs.

I don't know this for sure, but HE does.

Thanks mojoe, I'm pickin up what you're puttin down, it sounds like these engines were built for being good at what we need 90% of the time, and the other 10% is the compromise, ill just have to keep that in mind when I'm trying to cruise between work areas.

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