Kw vs Cubes Observation

[Splash]

Team,

I have made an interesting observation in KW vs cubes between following donks:


Merc115EFI - 1732CC with 86kW output
DF115 - 1950CC with 84.6kW outputWhy/how does less CC result in more kW for the Merc when compared to the DF115?

Splash

[FNQCairns]

Hey splash add what is the recommended RPM range for each and also the bore and stroke.
Then recompare....

cheers fnq

[Splash]

Merc 115 EFI
Bore/Stroke: 82mm x 82mm
RPM Range : 5,800-6,400
CC : 1732
KW : 86

DF115
Bore x Stroke: 84 x 88 mm
RPM Range : 5,000-6,000
CC : 1950
Kw : 84.6

[Splash]

Why/how does less CC result in more kW for the Merc when compared to the DF115?

Splash

[FNQCairns]

Cheers Splash can you see the trend? Can you guess which one had the most torque and would therefore spin the biggest pitch prop for diameter? and as a result has the option to either be setup to travel long distances or be setup to slingshot the same boat through a bar relative to each other and without compromise in comparison?.

Basically speaking the zuke is lumping along at every rpm compared to the merc. The zuke is a 'lower performance' motor that will return better performance:) :)

cheers fnq

[FNQCairns]

But specificly the Merc engine is 'square' in bore and stroke with less CC (generically the mother of all comparison).

Engines with longer strokes than bore produce more torque and approach max theoretical piston speed faster than more square engines, an engine that is sq or over sq are ring ting ting engines and the longer stroked engines are more like the big block lets tow something engines.

All very basic with caveats all over the place but always in first assumption the above will tell you the engine you are getting....then something else must be added (like a blower, variable valve timeing etc etc) to unseat the assumption and even then it may not sway the entire equation.

cheers fnq

[Blaster Bretty]

But specificly the Merc engine is 'square' in bore and stroke with less CC (generically the mother of all comparison).

Engines with longer strokes than bore produce more torque and approach max theoretical piston speed faster than more square engines, an engine that is sq or over sq are ring ting ting engines and the longer stroked engines are more like the big block lets tow something engines.

All very basic with caveats all over the place but always in first assumption the above will tell you the engine you are getting....then something else must be added (like a blower, variable valve timeing etc etc) to unseat the assumption and even then it may not sway the entire equation.

cheers fnq

Put quite simply..... you can get two motors of equal power, ie, kilowatts, but one motor will put its power to the ground at eg, 3000rpm as opposed to the other at 4000 rpm therefore the beforementioned motor has more torque.
its the same in car motors and bike motors. Add into the equation the weight of the actual motor you are looking at and then look at the price!!! what do you really want?
At the end of the day they are all very close in performance value, ie, bang for buck, it just depends on what you want out of the motor, fishing or wake boarding/ski'ing or even both there are motors designed for all three type's of usage.

Bretty

[ozbee]

merc on a light boat the df will before better on a heavier boat . remember brands tend to use the same piston on a variety of HP MOTORS . THIS IS WHY some motors perform better than others in the same brand . by adjusting the length of stroke you can alter compression and cubic capacity without changing block tooling saving massive costs at factory . look carefully e tec have the same piston size for a heap of motors. search the other brands you will see the same effects.

[Splash]

But specificly the Merc engine is 'square' in bore and stroke with less CC (generically the mother of all comparison).

Engines with longer strokes than bore produce more torque and approach max theoretical piston speed faster than more square engines, an engine that is sq or over sq are ring ting ting engines and the longer stroked engines are more like the big block lets tow something engines.

All very basic with caveats all over the place but always in first assumption the above will tell you the engine you are getting....then something else must be added (like a blower, variable valve timeing etc etc) to unseat the assumption and even then it may not sway the entire equation.

cheers fnq

FNQ - Your thoughts on these specs for BF90 :
Bore/Stroke: 73 x 89.4mm
Displacement: 1,496cc

Usage : Pure fishing, no skiing or towing.

SPlash

[Splash]

FNQ -

Here is what I have learnt from you and recetn internet search:

An "oversquare" cylinder has a shorter stroke and larger bore and usually creates smaller torque numbers at lower RPMs.

"Undersquare" cylinders have longer strokes than bore dimensions and create lower horsepower but larger torque numbers in the lower RPM range.

Which option is best for a regular fishing boat - Over or Under Square?

Splash

[black runner]

Horsepower = Torque x RPM/5252

To get kilowatts multiply the answer by .75 (approx 750watts = 1hp)

Power is a measurement of work done over time. That's why you might see a lower cubic capacity motor with a higher hp rating than a higher cubic capacity motor and invariably it is beacause it has a higher full throttle rpm - all other things being equal - same fuel deliver, normally aspirated etc. If the two motors in your example were subject to the same load conditions at their repective max wot the merc would have done more work in 1 minute ie turned the crank 400 revolution more. If selecting a motor HP or KW should be used as a guide to get you in the ball park. Then look at CC bore, stroke, number of cylinders to ensure that the motor is matched to the task.

Take a formula one 2.5 litre motor delivering 700hp. Sounds a bit high until you realize they spin at 20,000 rpm!! Drop the rpms back down to what we are used to and you see the hp back around 'normal' figures. Not a very useful motor for pulling the boat out at the ramp. In fact it couldn't do it unless it was geared down enormously. The stroke is about half the diameter of the bore and therefor not very torquey at low rpm. Thats why they are easy to stall.

Another way to think of torque/stroke is to think of the crank pin on the crank shaft. The further the crankpin is from the axis of the mains , the longer the piston stroke has to be and the greater the turning force for a given piston diameter.

Cheers

[Splash]

Excellent post Runner.

Based on this info, is it better to have a longer stroke or shorter stroke for a regular fishing baot?

SPlash

[black runner]

That all depends on the type of boat, hull's minimum planing speed, weight, your expectations of WOT speed, cruising speed at say 4,000 rpm etc, operational area (are you likely to be in a situation where you need extra grunt in heavy weather at low speed). Don't forget that the choice of propellor is just as critical in the whole setup so it's not just a matter of longer or shorter stroke. Brand X 90 hp might have a bore of 73mm/stroke 89mm and brand Y 90 might have a bore 81/stroke 78. These are both under square and would probably perform similarly after correct proping.

I would be inclined to research your requirements by reading reviews and talking to dealers and other owners about your application.

Cheers

[skipalong]

would you even feel those figures

[FNQCairns]

Splash that bf90 looks good but I am looking at it in isolation, comparison with other like engines make the best comparisons. Very long stroke it must have some cutting edge tuning for the higher rpm ranges, to read those specs fuel economy was the aim first then they made it work from that point, would be interesting to see if it runs out of puff early in a real world environment on the water.
Also suspect that prop pitch will be governed by the lack of push up high ie it has the punch to push an extra pitch lower in rpm range but the prop that must be used in one lower because of the torque at the higher ranges, little bit like my 3cyl 90 yam on the water.

Cheers fnq

[FNQCairns]

FNQ -

Here is what I have learnt from you and recetn internet search:

An "oversquare" cylinder has a shorter stroke and larger bore and usually creates smaller torque numbers at lower RPMs.

"Undersquare" cylinders have longer strokes than bore dimensions and create lower horsepower but larger torque numbers in the lower RPM range.

Which option is best for a regular fishing boat - Over or Under Square?

Splash

For me undersquare but with enough CC to cover the cover all rpms like the zuk 115 or 90, beaut numbers for a 90hp engine that one on paper, gotta have balls everywhere for a 4st engine.
But really only in comparison can they be assumed, it's not like we can nip out the back and design our own. Ya take what you are given, they are all good although I believe one will be happier with one over another if they had the opportunity.

cheers fnq

[Splash]

Many thanks FNQ!

[dec0guy]

Simply put, torque is a function of the piston area (ie bore) and stroke. Power is function of torque and rpm, or how quickly you can make torque. If you put a turbo or supercharger on, and run everything at say 2 bar, its the same as running a piston with twice the area at 1 bar (ie not supercharged)

Unfortunately the smaller you make the stroke (ie less torque), the quicker the engine can spin, so you quickly gain power at the expense of torque. In the end, so in the end its all a compromise, and with current technology (material strength,combustion technology, reliability etc) the optimum maximum speed is around 5000-6000rpm.

If a 10m plank (or stroke) is supported at one, and a 100kg fish (or combustion pressure on a piston) is hanging off the far end you would generate nearly 10,000Nm of torque. Because the plank is not turning no power is produced, but the the plank were to rotate at 1000rpm, you would generate 695hp, or 1400hp if turning at 2000rpm, or 5200hp if turning at 10,000 rpm etc.

[FNQCairns]

Simply put, torque is a function of the piston area (ie bore) and stroke. Power is function of torque and rpm, or how quickly you can make torque. If you put a turbo or supercharger on, and run everything at say 2 bar, its the same as running a piston with twice the area at 1 bar (ie not supercharged)

Unfortunately the smaller you make the stroke (ie less torque), the quicker the engine can spin, so you quickly gain power at the expense of torque. In the end, so in the end its all a compromise, and with current technology (material strength,combustion technology, reliability etc) the optimum maximum speed is around 5000-6000rpm.

If a 10m plank (or stroke) is supported at one, and a 100kg fish (or combustion pressure on a piston) is hanging off the far end you would generate nearly 10,000Nm of torque. Because the plank is not turning no power is produced, but the the plank were to rotate at 1000rpm, you would generate 695hp, or 1400hp if turning at 2000rpm, or 5200hp if turning at 10,000 rpm etc.

Where can I buy the 10,000rpm plank please!;D

cheers fnq

[Noelm]

did someone say "KILLER WASPS"???