Are two twice as good as one?

[9D280]

Originally Posted by The Beard

Mmm, I noticed that as well. I think that torque figures are generally (turbos possibly not included) simply a mathematical calculation based on the hp figure.

Unfortunately not. Torque is related to stroke & a host of other things, whioch can be mucked about with even more with on an EFI engine. Grab any car brochure - the torque curve is always roughtly "hill" shaped (i.e as revs increase, torque will peak & eventually drop off a bit), the power curve is a straight-ish line going up as revs increase.

[gotchiguy]

Originally Posted by The Beard

Mmm, I noticed that as well. I think that torque figures are generally (turbos possibly not included) simply a mathematical calculation based on the hp figure.

AFAIK isn't it the other way round. Ie. KWs are calculated from the torque figure and then converted to hp?

Of course that only works for a set amount of the unit "work done" whereas what you are talking about is the line graph showing output/revs which is completely different. Similar though because on the rolling roads the hp figure is calculated as the car winds to a stop isn't it, using the rolling resistance of the wheels or something like that in an equation with the torque figure produced during the running up of the engine.

[jwalker]

Power and torque are not inseparable. Torque is simply a turning force...so taking the turning force and RPM power can be calculated. Therefore the converse is true, if you know the power and RPM you can calculate the torque.

What is generally meant by a torquey engine is an engine which produces good power at lowish RPM.

[martini]

Originally Posted by jwalker

Power and torque are not inseparable. Torque is simply a turning force...so taking the turning force and RPM power can be calculated.

HP = Torque x RPM ÷ 5252

[gotchiguy]

Originally Posted by jwalker

Power and torque are not inseparable. Torque is simply a turning force...so taking the turning force and RPM power can be calculated. Therefore the converse is true, if you know the power and RPM you can calculate the torque.

Well obviously. You just re-arrange the formula.......Not exactly rocket science!! But what I was saying that on rolling roads and dynos etc they calculate the HP from the torque figure not the other way round.

Originally Posted by martini

HP = Torque x RPM ÷ 5252

So keeping Jeff happy (HPx5252)/RPM = Torque. All I'm saying is that in practical applications Matt's formula is used

Incidentally for that equation what would the units of torque be? Pounds/foot? Surely it would be easier to use the simpler metric equation using NM and KW and negating the need for the 5252 division

[martini]

Originally Posted by gotchiguy

Incidentally for that equation what would the units of torque be? Pounds/foot? Surely it would be easier to use the simpler metric equation using NM and KW and negating the need for the 5252 division

ft/lbs. Only U18's and french people use anything else

[Erin]

I presume these simple formulas only work at the flywheel and not after the effects of the gearbox are added in. The losses must vary at different rpm's and therefore affect torque too.

[jwalker]

1. Gotchiguy chill. My post was nothing to do with your post... so sensitive.

2. My understanding is that outboard power figures are given at the prop.

[Andy B]

Originally Posted by Simon B

2. An engine of the correct size cannot be found i.e. a single 400hp outboard = 2x200hp

the Harrell Engineering 580HP 6.2L Chevy V8 Outboard

[chewy]

Originally Posted by Andy B

the Harrell Engineering 580HP 6.2L Chevy V8 Outboard

Kleenex anyone?

[Searider]

Originally Posted by jwalker

Power and torque are not inseparable. Torque is simply a turning force...so taking the turning force and RPM power can be calculated. Therefore the converse is true, if you know the power and RPM you can calculate the torque.

What is generally meant by a torquey engine is an engine which produces good power at lowish RPM.

Exactly,

But what some of the other posters have missed is that outboard manufacturers don't publish their maximum torque figures and state at what rpm it occurs.

It's not as simple as doing the maths using max hp and max rpm (HPx5252)/RPM = Torque

I guess the best we get from the manufacturers is to see at what minimum engine speed maximum power is obtained:

An engine that produces 50hp at 4500-5500 rpm produces more torque than one that produces 50hp at 5500-6500rpm.

Hmmm

[Pikey Dave]

I've said it before & I'll say it again, torque is everything. We all get hung up on HP, but without torque, HP is nowt. E.g, Bugatti Veyron, over 1000HP & delivers 1500Nm, Scania R500 tractor unit = 500hp delivers 2500Nm@1300rpm. Buggati twice as "powerful" as Scania, but lets see it pull 40tonnes of baked beans uphill from a standing start. With a boat, we don't have the luxury of a multi speed gearbox, so we need torque low down in the rev range, otherwise we'd never get on the plane or pull up a skier. It's all very well saying engine X is this HP compared to engine Y, but what we need to know is where in the rev range the TORQUE is delivered, it's no good at 5000RPM because without lowdown torque you'll never get there. As manufacturers seem unable or unwilling to publish torque curves we can only go on experience. And as everybody knows 2st Etecs have LOADS of lowdown torque

[jwalker]

If you've got the manufacturer's power curve you can plot your own wee graph of torque from that using the formula given in a previous post. Bear in mind that the power graphs will be power at full throttle unless they say otherwise and it's not a common situation to have a boat engine running at full throttle but low revs for a prolonged period. It can occasionally happen eg. climbing the back of a wave or fast acceleration but these are transient. Comparison with a road vehicle is not particularly valid since the sea surface doesn't have prolonged hills for us to climb. Even if there are large waves they would pass under us if the boat was still, unlike a road incline.

PD, in your example at 1300 rpm and high torque that engine will therefore be producing high power too at those revs.

[Pikey Dave]

Originally Posted by jwalker

PD, in your example at 1300 rpm and high torque that engine will therefore be producing high power too at those revs.

Yup! agreed. But it's still 500hp v 1000hp+, which one is going to get you up the back of that wave when you bury the throttles, 2500nm or 1500nm? As you quite rightly say, Manf's figures are usually quoted at WOT, how often do we run at WOT? It's low down grunt that we need, the ability of the engines to spin the props from low rpm to planing speed under load, is more important (IMHO) than the practically useless HP rating at WOT.
High torque at low rpm
(Milk floats & series wound DC motors spring to mind!)

[BogMonster]

Originally Posted by Pikey Dave

(Milk floats & series wound DC motors spring to mind!)

I think the batteries would make your boat too heavy.....

A good judge of torque is the Squeal Factor when you fill the boat up with a bunch of girlies and give it some breakfast. Am I allowed to say "E-TECs make a girl squeal" or will we get bilged?

I've no experience of a modern 4 stroke on a similar sized boat but the 150 does get up and go

[Erin]

Originally Posted by BogMonster

Am I allowed to say "E-TECs make a girl squeal" or will we get bilged?

Yep, cos Gotchi's got one, and all he does is squeal like a girl

[BogMonster]

Originally Posted by Erin

Yep, cos Gotchi's got one, and all he does is squeal like a girl