Why do you want more horsepower?

[achris]

Hi all,

I just ran across this brilliant little graph... It shows the same engine with different cam/head and intake configurations to change the engine from 375hp to 400hp. I don't know what engine it is, or how the configuration is set up, so please don't ask, or denigrate me for that.

What it does point out, very starkly, is how trading off just 25hp at the very top end has huge benefits in the mid-range. Not only does the mid-range benefit between 50 and 100 hp, but the maximum power is reached at a lower engine speed. In fact, the 400hp engine doesn't even catch up to the 375hp engine until almost 5,500rpm!

Now, truthfully, how many of us run our engines at 5,500rpm or higher for most of the day? (And anybody who does say they do, I'll call BS!) Most of the time, most boat engines are running between 2,500 and 4,000rpm. And in that range, the 375hp engine is WAY better. And on this engine, at 2,000rpm (probably the speed you'd be using to get out of a harbour, or when the weather kicks up) the 375 is producing a whopping 150hp more. And given that when we 'prop' an engine, we do it for maximum power, this boat with the 400hp engine will be difficult to drive in the mid-range due to be 'over-propped' for the revs.

Here's the graph....



Something to think about when we next see a thread with someone wanting 'more power'... A case of less is more? (maybe the engine suppliers, Merc and Volvo et al, do know a thing or two about what they're doing? )

Chris..........

 

[dingbat]

Horsepower is a useless measure of power/performance. Simply HP = Torque x RPM/ 5252

Throw a gearbox into the mix and all odds are off...

The real story is determined by the motors torque curve and the gearbox to which it is bolted.

 

[Scott Danforth]

Chris....... Always look a the curve, never just compare numbers.

in the motor you show the graph on, its starving for port velocity in the cylinders to keep mid-range and low end. the intake runner volume is now most likely too large

Here is where I was with the warmed over SBC...... and where I am.....

And yes, I fully understand prop curves vs torque curves.

 

[Lou C]

It's something I recall reading a lot about in road tests of classic musclecars when the engines started to get tuned more for racing than for street performance. What you got on the top end you lost on the bottom end it seemed. I think the engine mariniziers have tried to get the most useable power for the least cost and least amount of modifications to the stock GM marine engines they both used. On the other hand the technology used in modern 4 stroke outboards is much more advanced.

 

[jimmbo]

When I want more power, it depends what I am dealing with in the first place. If the induction is restricted, as in a 2 bbl carb, a 4bbl with small primaries will swapped. If it going to take a Camshaft swap to make more power, it will probably result in a Torque Curve difference like shown in the original post. So, I will then opt for more Displacement. An example, my boat was offered with two different 5 litre engines. One was a carbed 220 hp, the other was a FI making 260 hp. My boat came from the factory, for some reason,(that was never an Option listed for it) with a carbed 5.7 rated at 260 hp. I have raced other boats like mine that have the FI 5 litre, and mine has much better Holeshot, and higher top end, even though both are rated at 260 hp. Now Volvo used a 2 bbl Holley that a bit too small for the 5.7, though a perfect match for a 5 litre. It however use a very nice flowing Intake that a 4 bbl could be mounted. I have ran a Holley 650 cfm Spreadbore carb and gained an addition 500 - 600 rpm, which required a prop change. The very small Primaries helped maintain the lowend torque.

 

[Scott Danforth]

It's something I recall reading a lot about in road tests of classic musclecars when the engines started to get tuned more for racing than for street performance. What you got on the top end you lost on the bottom end it seemed. I think the engine mariniziers have tried to get the most useable power for the least cost and least amount of modifications to the stock GM marine engines they both used. On the other hand the technology used in modern 4 stroke outboards is much more advanced.

you give them too much credit...... take a production truck long block and sell it to the marine guys. same rotating assembly, same block and heads, slightly different cam (primarily lobe separation. Most Marine cams are considered "RV" cams)

 

[Lou C]

True and outboard tech on the other hand is nearly as advanced, or as advanced in some cases as modern auto engines, but that has the downside of greatly increased repair costs if and when it breaks!
To me the beauty of inboard and I/Os is the simplicity and low repair cost of the old engine designs.

 

[harringtondav]

I shop for torque with a vehicle. HP is cool, if you plan on street racing close to red line before gear shifts. HP tends to max close to red line, torque in the mid 3Ks.

I'm dialing in my next boat. Closer to reality, farther from a dream. I could power it with a Mercruiser 4.5 225 hp or a Merc 6 cyl O/B same hp. + and - for either from a maintainability view and other considerations . But I want to see the torque curves on each before I decide. The 4.5 red lines at 4800 rpm. The O/B is close to 6K. I rarely boat above 4K, mostly 3 -3.5K plus hole shots. So for me torque is king.

 

[Scott Danforth]

........ An example, my boat was offered with two different 5 litre engines. One was a carbed 220 hp, the other was a FI making 260 hp. My boat came from the factory, for some reason,(that was never an Option listed for it) with a carbed 5.7 rated at 260 hp. I have raced other boats like mine that have the FI 5 litre, and mine has much better Holeshot, and higher top end, even though both are rated at 260 hp.......

The torque curve of the 5.7 is much fatter than the 5.0....... The extra 15% displacement is about 20% more midrange

 

[Bondo]

I shop for torque with a vehicle.

Ayuh,..... Me too,....... Just bought myself another 4 ton toolbox in much nicer shape than my current 4 ton rust-bucket toolbox, with another 7.3l diesel,.....

The torque curve of the 5.7 is much fatter than the 5.0....... The extra 15% displacement is about 20% more midrange

Yep, the reason I still Dream of a 383 SBC,......

I believe it's torque to weight ratio is 'bout the best possible power-plant,....
Short of a diesel anyways,....

 

[Scott Danforth]

Yep, the reason I still Dream of a 383 SBC,......

I believe it's torque to weight ratio is 'bout the best possible power-plant,....
Short of a diesel anyways,....

or a stroked BBC

 

[dingbat]

I believe it's torque to weight ratio is 'bout the best possible power-plant,....
Short of a diesel anyways,....

You may find this interesting.
Local builder.....know the guy and the boat

https://www.boatingmag.com/outboard-vs-inboard/

 

[Lou C]

Interesting article, and you can see the cost and other advantages of the outboards vs inboard diesels, I wonder what the results would have been like with Mercrusier 6.2 liter Bravo III sterndrives if they were ever built that way? The plus of them vs outboards are the availability of closed cooling for longer engine life and not in the way for fishing, but the outboards have so many other advantages no one would choose that option. I would think they'd have more torque than the twin outboards but there's more weight (approx. 800 more vs the OBs for the 2 engines). With the outboards, routine service is easier, you can trim up in shallow water, you can't have a cooling system failure flood the bilge with seawater like inboards, no fuel vapors in the bilge if comparing to gas inboards, etc. My only reservation with outboards is raw water cooling in salt water + extremely high cost for parts, makes rebuilds impractical. So they become a throw away engine after a set time. Whereas the common gas inboards if closed cooled for sure can be rebuilt and the parts are cheap enough that it makes sense.

 

[Lou C]

I shop for torque with a vehicle. HP is cool, if you plan on street racing close to red line before gear shifts. HP tends to max close to red line, torque in the mid 3Ks.

I'm dialing in my next boat. Closer to reality, farther from a dream. I could power it with a Mercruiser 4.5 225 hp or a Merc 6 cyl O/B same hp. + and - for either from a maintainability view and other considerations . But I want to see the torque curves on each before I decide. The 4.5 red lines at 4800 rpm. The O/B is close to 6K. I rarely boat above 4K, mostly 3 -3.5K plus hole shots. So for me torque is king.

The OB has many advantages here are just a few:
essentially self draining, can be used earlier or later in the season with no worries
no cat converter exhaust and the related nuisance problems of check engine lights (and for use in the salt pond, very expensive exhaust replacement, that alone will stop me from buying any more inboards if for a new boat)
cooling system failures can't flood the bilge with sea water
easier engine access for most repairs, but depends on boat design
no water leaks through the transom mount because of an un-maintained bellows, or the classic Merc leaks around the steering arm
no concerns with fuel vapors in the bilge as with gas inboards, all of which should have a gas vapor detector

OB down sides are very high parts cost and complex designs taking major engine repairs out of the do it youselfer realm.

As much as I like the simple Chevrolet small block the only way I'd get another I/O boat is if it was old enough to not have cat converter exhaust. A friend of mine got an estimate of $4500 to do the manifolds on a late model Merc boat with cat converter exhaust. And here in the salt pond either you do that on schedule or risk severe engine damage. Its really not an option, to not replace when its time.

While torque is important, modern OBs are like modern Japanese car engines, they are built to rev high (look at the differences in valve train design vs inboards) and that's why they can provide better performance with less weight than conventional larger displacement cast iron engine lower revving I/Os. Right off the bat you save approx. 400 lbs with an OB.
And some boat builders I swear are sadistic the way they make the rear of the boat in such a way, that servicing the engine becomes a horror show, God forbid you have to change a starter, or something on the rear of the engine one day, even some makers are too cheap to install a remote oil filter when its obviously needed. For shame!
For this and other reasons, OBs have really gotten a hold on the sport boat market.

 

[harringtondav]

Lou C You touched on all of my considerations. Maintainability. My kids will inherit the boat. Son is a Masters Structural engineer, but could care less about checking his oil. Asking daughter is a bit much. So the O/B gets the nod here. Change the lower unit lube, probably an occasional fuel filter, and run it.

I'm fully special tooled and knowledgeable with the Alpha II. And a 4.5 Merc 225 boat is about $8K less than a 225 Merc O/B. I'd be stuck with expensive parts either way since it will be a long time before the after marketeers clone the 4.5 parts.

The premium for a O/B would really be less for me. If I went for the 4.5L I'd get the full closed cooling system so only the PS cooler hose would require winter draining. I'm 100% fresh water, but this would simplify winterization for my heirs.

So it's basically a wash for me. My concern is any engine has a lifetime rev limit - accumulated rpm wear before it needs open heart surgery. Logic tells me a higher revving O/B would need it sooner. But I'll probably be long gone by then.

 

[Lou C]

I would not worry about it. I owned a 1989 Toyota Corolla 1600 GTS twin cam from 1989-1998. Ran it 99,000 miles and spun it up to 6500 rpm every day I drove it. Cruised at 75 mph and 4500 rpm in 5th gear due to the short gearing. When it got to 90K miles I took off the cam covers to check the valve adjustment (just like a Yamaha outboard with bucket tappets and shims, because Yamaha designed the Twin Cam head for Toyota). The valves were all in spec and the cam lobes looked new. I would now and then spin it up to the redline of 7600 rpm. So they are built do to that. Small block GM marine engines as good as they are, were not, at least with a stock valve train. Now Merc may have made some improvements in this regard, who knows.
But at my age, getting another sterndrive is not in the cards. They are just too much work, every year. I'm leaning more toward an Evinrude Etec, I want low low maintenance for my next water craft. It self-winterizes!

 

[Leardriver]

Lingenfelter always said that to build a street engine that is enjoyable to drive, maximize torque in the RPM range that you will spend the most time in.
My diesel truck makes a measly 400 hp. Tongue in cheek with measly, but my BMW 135 makes more than that.
It's the 800 foot pounds of torque that make it fun.

 

[Stinnett21]

Boating Magazine's recent article comparing two 21' Chaparrals, one with 150 Yamaha and the other with 200 Yamaha, cited a statistic that said pleasure boats spend 60% of their time off plane. In my estimation that's about right. Comes into play considering fuel usage. The 200 may cost more than you think.

 

[dingbat]

Boating Magazine's recent article comparing two 21' Chaparrals, one with 150 Yamaha and the other with 200 Yamaha, cited a statistic that said pleasure boats spend 60% of their time off plane. In my estimation that's about right. Comes into play considering fuel usage. The 200 may cost more than you think.

How is that?

It takes X horsepower to move a boat at a given speed

One (1) horsepower = 2,544.43 btu per hour
One (1) gallon of E10 = 111,836 Btu per gallon

Assuming same combustion technologies, both engines will consume the same amount of fuel to move the boat at a given speed.

 

[achris]

How is that?

It takes X horsepower to move a boat at a given speed

One (1) horsepower = 2,544.43 btu per hour
One (1) gallon of E10 = 111,836 Btu per gallon

Assuming same combustion technologies, both engines will consume the same amount of fuel to move the boat at a given speed.

Given that the higher HP engine will be spinning a higher pitch propeller, then for a given boat speed, that engine will be turning slower, and that will push it even further DOWN the torque curve. That will make that engine less efficient.. Therefore needing more throttle to generate the same power. More throttle equals more fuel going down the hole, for the same boat speed. Quod Erat Demonstrandum.

Chris......