torque

[jim dozier]

Re: torque

Or what about the same stroke but different connecting rod lengths and rod-length/stroke ratio.

 

[ZmOz]

Re: torque

 

[tee-boy]

Re: torque

Answer this:<br /><br />The longer the stroke, the higher the torque? Seems to me a longer stroke would turn a wider radius at the crank and hence more torque.

 

[ZmOz]

Re: torque

Yes, that's why "stroker" motors are built. More torque.

 

[LubeDude]

Re: torque

Just to through gas on the fire, why did 7,500 RPM short stroke 327s in Cameros out run 6,500 RPM longer stroke 350s in Cameros in around 1968? All the time, it wasnt tourque, it was quicker reving. Id take a modified 327 over a similarly modified 350 for drag racing any day. Been there done that.

 

[kd6nem]

Re: torque

If you want to compare two particular engines just compare them. All the generalizations in the world won't matter. I use real engines with individual characteristics, not generalized generic engines, as if there were such a thing. Your four stroke depends on intake and exhaust tuning (Cam, carb/FI, headers, etc) the two stroke depends on port timing, carb/FI, exhaust tuning. Ignition timing and advance curve are also factors for both, as is compression ratio. Both have many variations within each category depending what they were designed for. My Dad's old tractor had loads of torque (at very low RPM and therefore horsepower), so did the pickup, the big flat bed truck, and his V8 powered car. They all delivered it somewhat differently because of engine design and drive train gear ratio. Same with 2 strokes. Depends on what RPM and how broad an RPM. That rice rocket 4 banger Civic will cream a lot of V8 pickup trucks in a quarter mile race, though don't try to tow your cabin cruiser with one. Of course a crotch rocket motorcycle will positively dust the import car or the V8. SO WHAT? The tractor will outpull any of them. A racing snow mobile 2 stroke engine has unbelievable power but not reliability. An observed trials motorcycle's two stroke has loads of torque but barely gets out of its own way. Nothing really proved here except the need to match the powerplant and vehicle with what you need to use it for. Used to be you could at least say the 4 stroke used less fuel and were cleaner. Can't even generalize about that with the new 2 stroke outboards now. Lifespan of the V8 ought to be a lot longer than any high stressed high rpm motor, but it is certainly possible for the smaller motor to win in some cases. Depends on SO MANY factors... Didn't we just read this in a previous post? Little motors can indeed produce lots of torque if designed to do so at high rpms. I'm not so sure I find this useful except in my weed eater, personally. Some of those Jap engines can wind up and still last a long time though, to my amazement. Still like my American Iron but sometimes I start to wonder why with the quality issues I keep seeing. (Are you listening, Detroit???)<br /><br />Riddle: what has a 1.3 liter displacement engine, makes a reliable 250 horsepower without any forced induction, is oil injected, but is actually a four stroke? This is in production today.

 

[MajBach]

Re: torque

Riddle: what has a 1.3 liter displacement engine, makes a reliable 250 horsepower...

I am gonna take a stab at this and say Wankel twin rotary. RX-8?

 

[kd6nem]

Re: torque

The question was too easy.... Naw, you're just good.

 

[Oldsaltydog]

Re: torque

Will there be an exam on all this?

 

[jim dozier]

Re: torque

As with many things its not always black and white but mostly shades of grey. There is the issue, as Dhadley alluded to, of bore vs stroke as a component of displacement. It not just that increasing stroke may increase low rpm torque but that it also inhibits high rpm torque. It increases piston speed which reduces the max rpm because the pistons have to accelerate and decelerate more quickly for each revolution, which at high rpm becomes a big issue. Many racing automotive engines went to oversquare (large bore smaller stroke) to reach higher rpms but also to allow larger intake and exhaust valves and less combustion chamber shrouding where the valve meet the combustion chamber edges for better flow at high rpms. This (valve shrouding) would not be an issue in a 2-stroke but squish velocities and the ability of the porting to scavenge a cylinder are, I believe, affected by increasing the bore, and limit bore size somewhat.<br /><br />Then there is the issue of connecting rod to stroke ratio. Imagine as an exaggerated example, 2 engines of the same displacement with a stroke of 1 inch. One engine has a connecting rod 36 inches long and the other has connecting rod 1.5 inches long. When the piston is halfway between top dead center and bottom dead center (at approximately max downforce) the long rod has a straighter and more efficient force vector on the crankshaft while the short rod has an angled and inefficient force vector down on the crank.<br /><br />I'm sure its even more complex than this. There are so many variables to total engine performance, and base design is just one, which is why I just crank it up and go and don't worry about my conn-rod ratios anymore.<br /><br />There will be an essay test on this at midnight

 

[MajBach]

Re: torque

jimd and bearcat:<br /><br />So basically, what you two are both saying is that the concept that "toque gets you there and hp keeps you there" and visa versa is basically remedial thinking and has no validity in the real world. In other words, there are just to many factors to consider to make such broad statements?

 

[LubeDude]

Re: torque

Originally posted by MajBach:<br />

Riddle: what has a 1.3 liter displacement engine, makes a reliable 250 horsepower...

I am gonna take a stab at this and say Wankel twin rotary. RX-8?

Ya, But no torque, My wife drives an RX/7, and you have to turn the air conditioner off to take off on a hill. But what a screamer above 5,000.

 

[kd6nem]

Re: torque

LubeDude, MajBach, and everyone else on this thread,<br />In keeping with what I was trying to say before, SURE that RX-7 has torque! Just not as much down low where we wish it was! It couldn't scream above 5K if it didn't have torque! Part of my point was that the characteristics of how & where that torque appears is just as important as how much there is of it, and MANY variables affect how it is delivered. I hear you loud and clear; I talk that way myself, LubeDude. I think part of the above argument may miss the point to some degree, though MajBach and others were pointing it out before I did. We all have our favorite engines be it cars, boats, or whatever. There is more than one way to get the job done, and each may have its own merits. Generalities can be useful at times but how do we make this useful? Only way I can see is if we were trying to find a particular solution for a particular need. (i.e.: no longer general but specific)<br />Has been interesting and thought provoking. Many great points were brought out. Somewhere in all this we may have something of an answer, too.

Given two engines with same HP; what is the advantage from a boating perspective of the one with more torque? It seems that the 4strokes have more torque than the equvivalent 2strokes.

Well, to summarize: horsepower is the rating of how much work can get done (how much force AND how fast). Torque measures how much twisting force is available to the propeller. More torque can handle a more aggressive propeller. More torque is GOOD! It takes torque to make horsepower. But is the torque available WHERE we want it (all across the RPM band or only at higher RPM?), where it will be most useful the way WE use the boat? DEPENDS on WHICH four stroke and WHICH two stroke. Both can get the job done. Both need to be optimally propped for the given boat for a fair comparison- (sounds like some 4 strokes are too often under propped?). The four stroke may or may not drink much less fuel, take more or less maintenance and/or last longer, cost a bit more, and probably be heavier. DEPENDS on how you will use it, right? "Which has more torque?" is not really such a simple question after all then, is it? Is it for a race boat or a barge? Or somewhere in between? Cruising or skiing? Seems to me that we need to ask enough of the right questions about what YOUR particular need is first off. One size does NOT fit all! I doubt if all outboards are as dissimilar as I make it sound, but the point is there are too many variables to say one is better for someone than another without knowing a whole bunch of details. Then there is still the matter of personal preference. Maybe more than my 2 cents worth- it is HARD to stop typing once I get started.... Better hide the computer again!

 

[tee-boy]

Re: torque

Thanks. I definitely received my monies worth plus some interest. So torque does help on an outboard. And it really depends where the torque shows up over the range of rpm. If hp is a function of torque, then my hp will vary with the torque (as well as engine rpm). So do they tend to wash one another out (torque and rpm) i.e. 60 hp always be produced at whatever throttle I want to run, or should I have just stopped at the previous sentence? Torque is good! It seems that torque is very similar to voltage in a wire and rpm is similar to amperage in the same wire. The product of the two which is watts or power. Stated another way, torque is similar to pressure in a pipe and rpm is similar to velocity of the fluid within the pipe. The product of the two is the power of the discharge. Hey that's pretty good for a 27 yr old accountant (probably should go back for the engineering degree-would be a heck of a lot more interesting).

 

[MajBach]

Re: torque

tee-boy:<br />Pretty good analogy. I too often compare electricity or torque and hp to water in a garden hose. This is why HP IS measured in watts or kilojoules. If I recall, 1 hp is 746 watts.<br />Careful though as it is not a perfect comparison. <br />You're a little off though about hp and throttle settings. Peak hp is almost always only at full throttle (but not necassarily high rpm). You could have two scenarios where an engine is running at say 4000 rpm, one at half throttle and the other at full throttle. Only the latter one is max hp being produced.<br /><br />jimd:<br /><br />

Then there is the issue of connecting rod to stroke ratio. Imagine as an exaggerated example, 2 engines of the same displacement with a stroke of 1 inch. One engine has a connecting rod 36 inches long and the other has connecting rod 1.5 inches long. When the piston is halfway between top dead center and bottom dead center (at approximately max downforce) the long rod has a straighter and more efficient force vector on the crankshaft while the short rod has an angled and inefficient force vector down on the crank.

I meant to ask you about this a few days ago. I wanted to contemplate it a bit first. I think it's an interesting theory (quite legit I am sure but overlooked by all but engineers).<br /><br />I am having difficulty trying to validate it my head however and perhaps you can shed some light. It makes perfect sense, what you said but the logic behind it seems to work against it as well. What about on the compression stroke? When the piston is on it's way back up, it's uses either/both momentum to compress the mixture or the power from the remaining cyclinders to do so. ANy advantage gained on the power stroke would be lost on the compression stroke, no?<br /><br />I found this much easier to comprehend if you imagine two cylinders - one with an exaggerated stroke and small connecting rod. Halfway down the power stroke, you can imagine easily the connecting rod being on an extreme angle as it passes the neutral point on the crankshaft. Like this, you can easily compare the moment arm of the connecting rod/crankshaft to say the length of the tire iron on a wheel bolt. Obviously, with the tire iron in the horizontal position waiting for force to be applied, much greater torque is produced if you were to suddenly make the tire iron twice as long. Makes sense, especially when you consider that generally, longer stroke engines are 'torquey'. But what about on the compression stroke? In both cases, the force on the piston is still 'downwards' - away from TDC. But when the piston is travelling 'upwards', the compressing mixture has a much greater 'lever' opposing this motion becasue of the offset to the connecting rod to center of axis on the crankshaft.<br />Still foggy in my head - to many years out of university I reckon.<br />Your thoughts?

 

[jim dozier]

Re: torque

MajBach, my point was that stroke is not the only issue, because as in my example with 2 engines of the same bore and stroke (so neither bore nor stroke is a variable) you can throw in another issue of connecting rod length (and as a result a connecting rod to stroke ratio). Longer rod lengths give a better angle on the piston thrust. I used an exaggerated example for clarity (apparently not clear enough!). Of course at some point the lengthening of the conn rod causes increased weight and excessive engine size. But engines with higher connecting rod length to stroke ratios (other things being equal which they never are) may have the potential for higher power outputs.<br /><br />I guess my point that got lost was that there is more to the issue of engine power than just one thing such as stroke. Or torque. Or horsepower. There are always extenuating circumstances and there is more than one way to skin a cat.

 

[MajBach]

Re: torque

jimd:<br />Just to clarify, I wasn't disputing your point in any way and I agree with all that you said - I did get the point that there are many variables etc. <br />I merely found your example intriguing in that it brought other issues in my mind that I do not clearly understand the cause/effects. Just thought you could shed some light.

 

[tee-boy]

Re: torque

Majbach:<br /><br />"Peak hp is almost always only at full throttle (but not necassarily high rpm). You could have two scenarios where an engine is running at say 4000 rpm, one at half throttle and the other at full throttle. Only the latter one is max hp being produced."<br /><br />Are you referring to a car engine with a multi speed transmission? It seems that if I put my car in 1st gear and smoothly pressed on the accelerator, the needle on my tach would continuously move to the right (probably at a decreasing rate-more or less logarithmically).

 

[jim dozier]

Re: torque

Tee-boy (and everyone else) is probably tired of this by now but so what! As to the issue of losing the gain on the upstroke. Downstroke piston force is large and an increase in efficiency here results in a large increase in force. Upstroke compression force is relatively small and an equal loss in efficiency will result in a small difference in acutal compression force. Keep in mind that the issue of changing stroke changes displacement unless you modify bore in the other direction. A lot of people forget that, so that some of the change comes from change in displacement not just the effects of the change in stroke on lever arm. Similarly, you can change the conn-rod length by moving the piston wrist pin higher without necessarily changing the stroke. But if you change the stroke you will be changing the conn-rod/stroke ratio even if the conn-rod length is unchanged. So at some point, in an absurd exaggerated sample, if you increase the stroke a huge amount and decrease the bore (to keep displacement the same) and keep the same length conn-rod, (low conn-rod/stroke ratio) you will get a really awkward angle on the crankshaft and the rod is going to whack the side of the bore and your design engineer will get fired.

 

[tee-boy]

Re: torque

Jimd,<br /><br />I think hp is what it all boils down to. ft-lbs/second. Or how much work something can do in a given amount of time. All the things we are talking about (compression ratios, this ratio, this stroke, this bore, this angle...) all determine the final outcome which is hp (which happens to be a function of torque).<br /><br />In summary:<br /><br />TORQUE IS GOOD!!