I/O vs. Outboard HP rating

NMShooter · Nov 21, 2012

OK, I bet this has been beat to death, but I searched and could not find it.

How does I/O rated horsepower compare to outboard rated horsepower?

Is it rated at the flywheel or prop?

The I/O has two direction changes, the outboard only one.

Thanks in advance.....

86 century · Nov 21, 2012

Re: I/O vs. Outboard HP rating

They made the switch from crank to prop shaft rating in the late 80's I believe.

So both are the rated hp at the prop shaft now. The big difference is power to waight ratio.

jkust · Nov 22, 2012

Re: I/O vs. Outboard HP rating

Sure and O/B's rev a lot higher.

smokeonthewater · Nov 22, 2012

Re: I/O vs. Outboard HP rating

jkust said:
Sure and 2 stroke's rev a lot higher.

fixed it

southkogs · Nov 22, 2012

Re: I/O vs. Outboard HP rating

86 century said:
...The big difference is power to waight ratio.

Big time ... I'm amazed at how much heavier the I/O's tend to be over the old OB runabouts.

Campylobacter · Nov 22, 2012

Re: I/O vs. Outboard HP rating

smokeonthewater said:
fixed it

Red line on my Suzuki 4 stroke (DF140) is 6200.

airshot · Nov 22, 2012

Re: I/O vs. Outboard HP rating

While the weight of the I/O is much greater, the added fuel economy of the I/O is worth it in my book. Three yrs ago I bought my first I/O, a used but in great condition 3.0 and I find the 140 hp I/O uses only a little more fuel than my 40 hp outboard. I went from a 16' to a 22' and have only a 15-20% increase in fuel consumption. Yes there is alot more maintence in the I/O but can do that work myself, while it is more work I find it easier to work on than the tight packed outboard.

Airshot

BF · Nov 22, 2012

Re: I/O vs. Outboard HP rating

Where it's rated etc and all that aside... My Dad's old glastron 17' with 120 Mercruiser, handled like I'd expect the same hull would behave with an 80 or 85 hp outboard. My 17' Searay with 135 HP Mercruiser performs about like I'd expect the same hull would do with a 115 hp or so outboard (except burns less gas). Is that really what you're asking?

NMShooter · Nov 22, 2012

Re: I/O vs. Outboard HP rating

Yup... sort of.

I was trying to determine if 175hp of outboard equals 175 hp of I/O. Sounds as though the weight is the big factor, and maybe fuel efficiency too.

NYBo · Nov 22, 2012

Re: I/O vs. Outboard HP rating

Outboards aren't all the same, so "175 HP" outboard doesn't tell the whole story. There are old-school 2-stroke carbed models, 2-stroke FI models, 4-stroke carbed,, and 4-stroke FI.

tboltmike · Nov 23, 2012

Re: I/O vs. Outboard HP rating

Regarding Old Engine Power to New rating system. Had a 1975 115 Johnnie on my 18' TriHull. It pushed it at 39, maybe a little faster if going down hill. Repowered with a 130 ETEC and gained about 7 mph. However, it gets 50% better gas mileage.

smokeonthewater · Nov 23, 2012

Re: I/O vs. Outboard HP rating

tboltmike said:
Regarding Old Engine Power to New rating system. Had a 1975 115 Johnnie on my 18' TriHull. It pushed it at 39, maybe a little faster if going down hill. Repowered with a 130 ETEC and gained about 7 mph. However, it gets 50% better gas mileage.

I would love to see a picture of this hill.......

just razzing ya

86 century · Nov 23, 2012

Re: I/O vs. Outboard HP rating

The older outboards are on the thirsty side.

My '77 Johnson 115 easily used twice as much fuel as a mercruiser 140 of the same vintage.

skibrain · Nov 23, 2012

Re: I/O vs. Outboard HP rating

BF comparison is consistent with my experience. On a smaller boat especially, weight is a big factor in performance.

2-stroke OMC outboards in the 85/88/90/100/115 HP range are around 300 lbs.
Mercruiser (120/140 hp) 3.0 w/ outdrive is around 700 lbs.

cwarends · Nov 24, 2012

Re: I/O vs. Outboard HP rating

For me it is a tough comparison. Fuel consumption differs greatly depending on weight, RPM, model year and motor torque curve.
I had a 1996 26 foot deck boat with a 5.7L mercruiser 220 HP that got about 3 miles per gallon cruising ar 25MPH at 3200RPM
I now have a 2013 Bennington 25 foot tritoon pontoon with a 3.3L Yamaha F250 HP 4 stroke outboard. The pontoon will run 43MPH at 6000RPM but we cruise at 23MPH and get about 4.5 miles per gallon at 3000RPM.

Yamaha makes a 4.2L F250 that while it has 250HP it has a torque curve that gets you out of the hole faster at the expense of fuel consumption.

New outboards are more fuel efficient then older models and I believe 4 strokes are more efficient than two strokes.

We are in Lake of the Ozarks, MIssouri where I was charged $160 in the fall for winterization and $100 in the spring for de-winterization for the I/O. Here for outboards we only add stabil to the gas for winterazation so you can use the boat on nice days that come up. We used the boat 7 days in November when temperatures got above 65 degrees.

After having 4 boats - three with outboards, I prefer the outboard for our style of boating.

jkust · Nov 24, 2012

Re: I/O vs. Outboard HP rating

NMShooter said:
Yup... sort of.

I was trying to determine if 175hp of outboard equals 175 hp of I/O. Sounds as though the weight is the big factor, and maybe fuel efficiency too.

I think last summer, Boating Magazine did a comparison to determine the differences between identical boats, one with an I/O and the other with an O/B. It was a 190hp v6 vs a 115 vtec on an Ebbtide 188. The O/b weighs 390lbs vs the Sterndrive package of 865.
The lower hp does a respectable job with better economy, faster to plane given the massive weight savings. Of course the I/O has a huge torque advantage for lower planing speeds and pulling skiers out of the water. Would have liked to see the test with a 130hp I/O.

cyclops2 · Nov 24, 2012

Re: I/O vs. Outboard HP rating

Power is exactly the same. 200 hp is 200 hp . Airplane submarine or a golf kart.
The couple of hundred pounds difference does not make a big difference in 18' glass boats with people in it.

jkust · Nov 25, 2012

Re: I/O vs. Outboard HP rating

cyclops2 said:
Power is exactly the same. 200 hp is 200 hp . Airplane submarine or a golf kart.
The couple of hundred pounds difference does not make a big difference in 18' glass boats with people in it.

Yeah but in my example above 500lbs makes a big difference. The reason a Glastron and most other boats identically equipped run by my sig boat at WOT...500lbs except its dry weight not engine weight.

NYBo · Nov 25, 2012

Re: I/O vs. Outboard HP rating

Especially since that extra weight is at the stern...

H20Rat · Nov 25, 2012

Re: I/O vs. Outboard HP rating

cyclops2 said:
Power is exactly the same. 200 hp is 200 hp . Airplane submarine or a golf kart.
The couple of hundred pounds difference does not make a big difference in 18' glass boats with people in it.

yes, but not when it comes to planing or pulling people out of the hole. The shape of the torque curve is just as important as the peak HP.

I/O vs. Outboard HP rating

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