old vs new and torque

[dafox99]

I have a bit of a bet going with dad. He says the "old" motors that revved lower, produced more torque .. especially at low end than the higher revvers (say 6000 rpm). Since outboards don't have the advantage of multiple-gear trannies, they don't get to stay in their hp sweetspot while going through the gears to get up to top speed. So, while they are trying to push the boat onto a plane, they don't get to "drop a gear", but rather have to make due with whatever hp and torque they produce at that particular rpm .. which is substantially less than max. Because of this, he thinks the older motors could accelerate better and pull skiers better. I see his point, but only if his assumption about higher torque is true. I have never seen torque curves for any outboards to substantiate this. Is he right?

 

[TOHATSU GURU]

Re: old vs new and torque

You win the bet...At least in most comparisions.<br />You will have to look at individual engines, but you can use the following formula and run the numbers yourself for powerhead numbers:<br /><br />HP X 5252 ÷ RPM = Torque<br /><br />This formula does not take into account lower unit gear ratios's to see what deliverable HP at the prop would be.<br /><br />Keep in mind that deliverable HP at the prop is the only important thing to worry about. Engine torque is just one factor used in deterning that HP. Usually when people use the term "torque" in an outboard it is because they have heard someone else use the term to describe the benefits of a 4-stroke or lower gear ratio outboard. It's basically a voodoo word that should be left out of the outboard vocabulary.

 

[Paul Moir]

Re: old vs new and torque

That's not a voodoo word at all, the question is how many HP is being produced at say 2000 RPM?<br />It's both quantifiable and knowable.<br /><br />EDIT: Or are you saying that since marine engines strive for a flat torque "curve" that it doesn't really matter? IE, they're all generally flat?

 

[dafox99]

Re: old vs new and torque

Cool. That's what I thought. It seems the only metric of importance was the force required to move the boat a certain speed (force X speed = lbs X ft/sec = (ft-lbs/sec) = horsepower). So, if the "old" motor develops 60 hp (at the prop) at 4500 and is propped to go 30 mph, maybe it develops 30 hp at 3000 and goes 20 mph, and 10 hp at 1200 rpm goes 5 mph. The "new" motor develops 60 hp (again, at the prop)at 6000 rpm, and still goes 30 mph. It develops the 30 hp at 4500 rpm, and still goes 30 mph, and maybe goes 5 mph at 10 hp and 1600 rpm. Regardless, as long as you prop them properly, it shouldn't matter.<br /><br />EDIT: It seems Paul and I type at the same time alot. Paul .. do you buy the above argument?

 

[dafox99]

Re: old vs new and torque

BTW, every physics book or even motor article clearly states the 5250/rpm relationship between torque and hp, so I have to accept it, but I still don't quite get it since a long stroke motor has the longer journal, so the force on the piston is applied to a longer moment arm, thus more torque than a short throw motor of the same displacement (shorter throw, larger bore). And it does prove true that the longer throw motor will develop more torque at low rpm than the shorter throw, larger bore motor. So, to this feeble brain, is just seems like motor design should affect this 5250/rpm ratio. Good thing I'm not in engine design .. might accidentally break some physical laws

 

[dafox99]

Re: old vs new and torque

Just found a silly flaw in my logic above. Think I'll quit typing while I'm behind.

 

[rodbolt]

Re: old vs new and torque

you will also find a relationship with bearing speeds and piston speeds due to the rod/stroke ratio. thats why a short stroke small bore motor tends to rev higher faster than say my .060" over 455 with a 4.25 inch stroke. it will rip cast pistons apart at the wrist pin above about 5500, dont ask me how I finally learned it .<br /> just remember the difference in piston speed from dead stop to dead stop is way diffrent at 6K between a motor with a 3" stroke compared to one with a 4.25 inch stroke. both shafts are at the same RPM which one has the greater piston speed ?<br /> piston speed will play a part in availble power .

 

[dafox99]

Re: old vs new and torque

I never really tried to analyze it as I think the math quickly overcame me, but I always thought our current internal combustion engine was horribly inefficient as we have all this mass constanty changing direction. It seemed like a turbine or jet engine was much more effient in that all the converted fuel energy was always pushing something (the turbine) in one directon, and we didn't waste energy moving it one way, and then stopping it to move it the opposite direction as the piston does. So, in my mind, that we were losing energy like crazy in constantly changing the direction of these fast-moving pistons and rods. An engineer designer buddy of mine said that wasn't true, that the energy to slow down that piston, turn it around and send it back was used to move the crank, and wasn't "lost". Beyond me. So, rodbolt, are you saying the longer stroke / faster moving piston has more internal energy (both momentum in mass X Velocity and Force in mass X deceleration), and contributes more power?

 

[rodbolt]

Re: old vs new and torque

partly and partly due to the longer"lever" of the crank throw. the balance and arguements of HP VS torque are funny. occasionally a motor will bend the rules. just depends on what it was desigened to do and if it was modified from the original design. most 2 stroke outboards with above the prop exhaust tend to have a bit more low end pulling power, most through hub systems tend to have a bit more top end power. its all due the the exhaust scavenging characteristics.<br /> if ya wanna see some wildly different numbers compare the T and HP curve of an early seventies 500 CID cadillac motor to the DZ 302 z28 camero engine.

 

[TOHATSU GURU]

Re: old vs new and torque

Paul,<br /><br />"Voodoo", the way I used it, referred to the rather constant usage by advocates of four-stroke power of the "torque" in describing the way four-stroke engines develop their power. The curve is typically different than that of a two-stroke with the same CID. The two-stroke generally suffers in the mid range while a four-stroke continues to climb in a fairly steady fashion. IE. Engine A is a 4-stroke 90 HP. From 750 RPM to 1800 RPM the initial power curve does not provide as much available HP as engine B, a 2-stroke of the same HP. But, around that 1800 RPM the 4-stroke begins to develop more HP than a 2-stroke. Then somewhere...say 3000 RPM the 4-stroke looses it's advantage and the 2-stroke catch's back up. From there it's probably a dead heat up through the top RPM for both. There are a lot of exceptions to this and there are a lot of other factor's that come into play, Rodbolt named some, but that's the general rule.<br /><br />For me it's not that "torque" is not a real measurable measurment. Because it is. It's just that propshaft HP is a better measurement to use when describing an outboard for most people. PS HP is what the government/industry decided was the most accurate single measurement to use. That was to avoid confusion and eliminate other measurements(real, but focused on different areas that often did not contribute to an overall picture).<br /><br />I have a lot people who volunteer their knowledge at boatshows about how they need a "torquey" motor for their sailboat or pontoon boat. They often have a preconceived notion that a 4-stroke is going to to magically propel them through the water with greater force than a 2-stroke of the same HP. And at some RPM's they are right. But, they don't know what the word means...I tell them to loose that word and think in terms of "better mid range" for a 4-stroke. If that meets their needs great. But, a lot of those people turn out to need an engine that has more initial power than a 4-stroke offers. Again, it's the misuse of the word that causes my problem with it.

 

[itstippy]

Re: old vs new and torque

The formula: <br />HP X 5252 ÷ RPM = Torque<br />determines torque rating at a given Horsepower and RPM. You cannot say that because a motor produces 60HP @ 5000RPM (63 lbs/ft torque) that it will produce 63 lbs/ft torque at 2500RPM, unless you KNOW it produces 30HP at 2500 RPM. That's the whole point of a horsepower curve and a torque curve.<br />Dad is saying that the older motors have a torque curve that spikes upward at lower RPM's than the newer outboards. If this is true then he wins the "pop up the skier" bet. Superior low-end torque gives you superior acceleration. The torque-monsters rule the 1/4 mile strip. <br />I would LOVE to see a T and HP curve for a 1963 40HP Bigtwin compared to a modern motor of similar HP rating. How about it, you guys? Please post them!

 

[dafox99]

Re: old vs new and torque

stippy .. I'm in total agreement. I made up those numbers to make a point. If we knew the curves, we'd know the answer .. I've just never seen them.

 

[TOHATSU GURU]

Re: old vs new and torque

Unfortunately it's rather hard to find any published data on individual outboard models. Manufacturers don't release information on performance curves all that often. And when they do it's very selective as to model and very fuzzy as to the actual values. Usually they will only provide a curve, without values, comparing one model to a rivals model. From a motorhead standpoint power curves and detailed performance data can be interesting. But, for the average boater, you loose them at "The formu...."

 

[Paul Moir]

Re: old vs new and torque

Thanks for the explanation Elvin. I'm with you now!

 

[rodbolt]

Re: old vs new and torque

most of my observations came from playing with a IDC 2300 series dyno. seemed I could make more pressure at a lower RPM with an older through the leg type motor but it fell flat near the upper RPM band and the loopers and engines with through hub seemed to be flat until the upper 1/3 of the RPM range. was not real scientic just something I was playing with.<br /> the 33 I run on my 14 jon will almost toss ya off if you suddenly nail it, the 79 35 I run is much faster on the top end.<br /> the 33 suffers from cavitaion burns on the prop, I run a stainless on the 35 but both are close in pitch and diameter, I dont remember the gear ratios off hand.

 

[lark2004]

Re: old vs new and torque

I think the 33 has a 12:21 ratio.<br /><br />no idea on the 35.<br /><br />I do think the older motors are stronger down low than the new motors, look at where they are given their hp rating. My 68 Evinrude 40hp says 40hp @ 4500rpm.<br /><br />A friends 89 yammy says 40hp @ 5500rpm

 

[rodbolt]

Re: old vs new and torque

the other thing I can tell is the 33 uses almost 6 gallons an hour and the 35 about 3. man that 33 loves fuel <br />but I have a 83 25 on the boat now that does very well on fuel, I hope to have my 9.9 yam converted to a 15 running by thanksgiving, bought a brand new powerhead for it. cant afford to run my 120 jonny or the 115 yam anymore. I hope to have the 225 johnson ready for the 18 hydrosports sometime after christmas. man I think I got to many projects. I guess I should sell a few <br /> but I did get the block down down for my 19 seaswirl

 

[rodbolt]

Re: old vs new and torque

speaking of projects. I have a 26ft wood boat thats never been over since it was constructed about 93, I set a 460 foerd marinized by chysler with a 1.95/1 borgwarner, freshwater cooled. it ran 2 years ago when I set it in the hull. have not been in the hull since. bring a trailer and its yours<br /> <br /> gotta get it outta my horse pasture.

 

[orca]

Re: old vs new and torque

If I can add another side to the discussion, Many motors on the market have a certain horsepower rating but are considerably differant in cubic capacity. eg. the the Mercury 3 Cyl 40 HP 2 stroke is 968cc, However many other 40HP motors are around 600cc. This variation is all over the horsepower range and is not just Mercury.<br />I have been under the assumption that the 40HP Merc has more torque than other motors but I have no figures to back this claim. Am I wrong? Is there any corrolation between torque and capacity.

 

[dafox99]

Re: old vs new and torque

I'll venture on that one. My answer is, "if your compression ratio is similar, in theory, yes." So, to leverage our earlier discussion, horsepower is measureable via a dyno. The work the dyno pump is doing is displayed as a function of fluid velocity (flow)and pressure over time (Power = Work/time). If you look at the units, horsepower can be expressed as (distance X Force)/time. A common unit is 1 hp = 550 ft-lbs/sec or 6600 in-lbs/sec. Now, pressure is in Force/area. A common one is lbs/sq in. Flow is in volume/time. Let's use (cu in/sec). A prop shaft spins a pump that creates both pressure and flow(controlled by the operator) Take pressure X flow, and you get (lbs/sq in) X (cu in/sec) .. you get lbs -in/sec (horsepower!). So, if you create 2000 psi and a flow of 66 cu in/sec, you have 132,000 lb-in/sec. Divide by 6600 and you get 20 hp. Long way to say torque is not measured (sorry for the long diatribe).<br />So, torque (as I understand it), cannot be measured, but is the cumulative perpendicular force applied to the journals times the journal length. It's constantly changing over time, as the pistons move. displacement is the area of the cylinder X the piston travel. Your greater-displacement motor must either have a greater cylinder area (thus more gas/air and more force on the piston), or greater travel distance, which is related to the journal length. If either increases, the torque MUST increase. Whew .. hope that wasn't too boring .. and it's just my guess. I'm thinking the only caveat is if greater cylinder area does not create greater force on the piston due to a heavier piston, lower compression, etc.