More HP = more efficent cruise?

[newbster]

Looking at 350 based SBC's, they come in common builds from 240-350+ HP depending on builds. Given that the weight is the same, would I be safe in assuming that a higher HP engine would be more efficient in the same boat at a given cruise speed?

 

[HT32BSX115]

Re: More HP = more efficent cruise?

Howdy,


A 350 hp SB engine might be little more efficient than a similar 240 hp engine only because the compression ratio is a lot higher. That would increase the efficiency of the higher HP engine a little.

You might not notice a huge difference in fuel flow though.

 

[QC]

Re: More HP = more efficent cruise?

I am with Rick on this. Maybe a build that results in more HP would be more efficient at some points, but maybe worse. Depends on what mods and what RPM you consider cruise, and what the resulting load is at that point. There is really no way to gain that info without a back to back test with all other things being equal. Could be done in a test cell and could result in reasonably trustworthy data. Buuuuut, even two of their 240's could show different fuel rates at the same RPM and load if their individual tolerances were swung in opposite directions. My recommendation is to believe that you will get better efficiency with either decision you make and then you can smile while you're cruising along. The alternative is not knowing which is of course what you will have so you might as well be happy . . .

 

[HT32BSX115]

Re: More HP = more efficent cruise?

I will add however, if you're in need of an engine rebuild, go for the higher HP engine just because all boats can usually use a little more HP!!

 

[Black as]

Re: More HP = more efficent cruise?

Howdy,


A 350 hp SB engine might be little more efficient than a similar 240 hp engine only because the compression ratio is a lot higher. That would increase the efficiency of the higher HP engine a little.

You might not notice a huge difference in fuel flow though.

Honesrly more HP means more fuel, it all depends how you use it and I must say a lot more fun.
I have 2 cars one with a 5.7 litre and one with a 6 litre, they are both exactly the same engine but the 6 litre does uses more fuel

 

[newbster]

Re: More HP = more efficent cruise?

Some things I think sound right are a higher HP engine should breathe better at mid range RPM, lets say 3000 though better porting, roller rockers etc. A higher HP engine could swing a bigger prop better thus giving a better cruise efficiency.

Basically, if a higher output engine provides more efficiency at a cruise speed, maybe it makes economic sense over the long run to go that route.

The assumption is the engine needs a rebuild anyway and is another 100 HP worth 500 bucks? If the boat sees 10 150 mile trips per year, another 10% efficiency seems to make sense with the added plus of more reserve HP.

I don't really know the reality, hence, I ask.

 

[newbster]

Re: More HP = more efficent cruise?

Honesrly more HP means more fuel, it all depends how you use it and I must say a lot more fun.
I have 2 cars one with a 5.7 litre and one with a 6 litre, they are both exactly the same engine but the 6 litre does uses more fuel

At the same power output?

 

[HT32BSX115]

Re: More HP = more efficent cruise?

Honesrly more HP means more fuel,

Well, not always. very low compression engines do not produce as much HP as similarly configured higher compression engines.

A great example is the Franklin 165hp aircraft engine. A 335cu-in 8:1 comp carbed engine, it burned about 10 gal/hr at a 75%(of 165hp) "cruise" power setting.

The Franklin 220hp engine which was approx the same "size" (and weight)..... only 350 cu-in and 10:1 compression ratio, burned about the same 10 gal per hour at the same 75%(of 220hp) power setting. more HP same fuel burn.


A 200hp Lycoming IO-360 can also be run at 75% power but because it's also high compression AND fuel injected it burns around 10 gallons an hour.

My point it that efficiency is directly related to compression ratio and injection, vs carb and much lower compression ratio for similar cu-in engines.


I don't think there's a huge difference in compression ratios in marine engines. maybe 8.5-9.1 etc..... (the hot rod stuff excluded of course)

The RPM ranges for cruise settings don't vary much either. 3000-3500 rpm......


But if you were to put a SBC carbed 240hp engine in a boat and run it at 30 mph..............

And run a SBC 383 MPI in an identical boat at the same speed, the one with the 383 would probably do it at a slightly lower fuel flow.

 

[wca_tim]

Re: More HP = more efficent cruise?

Do a search for a series of posts by 45auto. they include some plots of fuel consumption vs speed for direct comparison of different engines... a very interesting read on exactly this topic...

 

[QC]

Re: More HP = more efficent cruise?

A higher HP engine could swing a bigger prop better thus giving a better cruise efficiency.

This is probably the most usable statement here and is factual.

a 5.7 litre and one with a 6 litre, they are both exactly the same engine but the 6 litre does uses more fuel

Well I would not call a 5.7 and 6.O "exactly the same engine" as a 6.0 is of course larger displacement. This translates to more throttling (less open) for the same power output and should result in poorer efficiency than the 5.7 at the same output. The opposite is true with the bigger prop, less throttling (more open throttle) and lower RPM for the same speed = better efficiency. All on paper of course and what I am guessing the OP wants is better efficiency on the water . . . Finicky isn't he

 

[achris]

Re: More HP = more efficent cruise?

Here's my input...

Same boat (the one in my signature). Started its life with a 1972 '165' (250cid, inline 6) and at 30 knots burned 55 litres per hour (I have a fuel flow meter). Next engine was a 1994 4.3LX (carbed) V6, at 30 knots burned 45 litres per hour. Current engine is 4.3MPI and at 30 knots burns 40 litres per hour.

Power outputs

'165' -> 165 hp (crank, about 110 prop)
4.3LX -> 180 hp (prop)
4.3MPI -> 220 (prop)

Unless you're looking for straight out top speed, the figure you need to be looking at is torque, maximum horsepower is less significant.

As the power output increases, for a given engine, usually the mid-range torque suffers, and if you're cruising, that's what you want.

Although the 4.3MPI is a little better on fuel, the 4.3LX (carbed) engine was way better for mid-range cruising, because the torque was there. I still miss that engine.

Chris.........

 

[HT32BSX115]

Re: More HP = more efficent cruise?

what's a litre?

 

[180shabah]

Re: More HP = more efficent cruise?

what's a litre?

It's about a quart low

 

[bigskiohio]

Re: More HP = more efficent cruise?

not a follower.

 

[180shabah]

Re: More HP = more efficent cruise?

newbster - Guessing you want some real world gains and I think chris gave you the best data for comparison. Original engine, was thirsty, 4.3LX was much better, and modern 4.3EFI still a little better on gas. Each lso produced progressivly higher horsepower.

BUT

What you saw was a progression in engine efficiency that also netted more power. The increased power was a "side effect" and not the reason for lower fuel burn. There are major differences in airflow between his three engines and that last one also had the added benefit of EFI, which also helps a little.

On the water, in any particular boat, 30mph requires X hp to maintain. X hp requires Y fuel to produce. Maximum hp possible from said engine is irrelevent in cruising. Maximise engine efficiency at your cruising speed, and lower fuel consumption will follow.

I will add one more thing, best dollar you can spend on your new engine will be vortec heads, you could spend more (alot more) for better heads but at less than 5k rpm, you'll be waisting your money.

If we are talking about an engine build there are many more questions you need to answer. First, what kind of boat is this, what are your power/performance goals and what is the budget?

 

[SeanMcl]

Re: More HP = more efficent cruise?

As I understand it, if you are looking for increased fuel efficiency at a given power output (cruise speed), you need to increase Volumetric Efficiency (V.E.) of the engine. Achris, HT32 and 180shabah up there illustrated it nicely. Increasing prop efficiency would also help get you where you want to go.

Each engine is going to operate at peak efficiency at a given RPM. You could get the same/better fuel efficiency out of a bigger motor at a given cruise speed if it can output the needed power at it's peak V.E. rpm, while a smaller motor would suffer if it has to operate above it's peak V.E. RPM to output the needed power. Modern motors with 4 valve or high flow heads, fuel injection and HEI also have overall higher VE ratings regardless of displacement and RPM.

All that said, it seems to be a very difficult thing for the weekend warrior to actually measure the V.E. curve of the engine in his/her boat to know what the best cruise RPM is. The best thing to do seems to be installing a fuel flow meter, and using a GPS for accurate speed measurements, find max speed/gph ratio. That's your effective max V.E. with your current prop, outdrive, etc. Use that as your cruise speed and RPM until you change props, boat loads, etc.

If you are going to be putting in a crate motor, you may be able to get a hold of the techs and have them tell you what the max V.E. RPM is, if they know, and then prop accordingly to get your desired cruise speed. Keep in mind that your WOT speed, holeshot performance, etc. may suffer if you optimize for a given cruise speed, but if that's where you spend all your time then it is a worth while trade off.

 

[HVAC Cruiser]

Re: More HP = more efficent cruise?

Here is a term Rick can probably explain better than I I think is really what your looking for/ at. Judging from his references to aircraft "Volumetric Efficiency" . Just don't start talking about jet engine efficiencies or you'll really have them confused lol

EDIT: OOps, guess I had the browser open to long while having dinner didn't see that Sean was outlining it ( V.E.) Sorry

 

[achris]

Re: More HP = more efficent cruise?

.... The best thing to do seems to be installing a fuel flow meter, and using a GPS for accurate speed measurements, find max speed/gph ratio. ....

You mean like this???

Blue line is speed (knots), Pink line is fuel flow (litres per hour), Yellow line is economy (litres per nautical mile.

Best speed, for me is 3100 to 3700 rpm (21 to 30 knots)....

Chris......

 

[Black as]

Re: More HP = more efficent cruise?

Honestly guys, if we have more horsepower we will use it! we just reach that optimum speed we want a lot faster, hence you will use more fuel. Yes change the prop but again you have just up-graded horse power against propulsion and again its all about using a bit more fuel and power to weight ratio

If you want to increase horsepower and really want to save money on fuel fit a supercharger

And if you stepped into a Ferrari would you drive it like your SUV

 

[SeanMcl]

Re: More HP = more efficent cruise?

You mean like this???

Nice. Way nice. Is that for the 72 Bertram or the 06 Merc?