Long Stroke vs Short Stroke 4x4 Engine, Why?

[nicholastanguma]

The Suzuki SJ (Jimny, Samurai, Gypsy, Drover, has different names in different markets) is my favorite old 4x4 of all time. Likewise, the Suzuki G13 and G16 that powers the SJ and various SJ cousins like the Vitara, Tracker, S90, etc is also one of my favorite old engine groups, too. All of them were SOHC mills, with either 8 or 16 valve heads.

But why did Suzuki build the G mills like they did? All of them in 4x4 usage are either of 74 or 75 mm bore, with strokes ranging from 77 to 90 mm. In fact even the performance standout of the bunch, which was equipped with DOHC and 16 valve heads, is a long stroke unit and capped at about only 9.5 thou rpm.

Obviously having a long stroke engine makes sense for a 4x4 application, where low rpm torque is the rule of the day.

But wouldn't a big bore, short stroke mill with a 16 valve head have been a better choice nonetheless, since the largest of the G engines was only 1.6 liters of displacement? I mean, if you're working with such small engines why not make them capable of not only low end torque but also spinning up fast to redline, so as to take advantage of high end horsepower as well?

For instance, the G16 has a 75mm bore coupled to a huge 90mm stroke; some were 8 valvers, others 16 valve. The G13 has a 74mm bore with a 77 mm stroke, some 8 valvers, others 16.

Okay, so why not just make the G13 with a 75mm bore and a 71mm stroke and give it the 16 valve head--wouldn't it then be capable of a flatter power curve up to a higher redline even though it's 300cc lower in displacment?

 

[Pskhaat]

I don't know the engines that you've stated, but as I did study this stuff back in Uni, allow me to respond to maybe a couple of your questions?

As you've stated an "overbored" engine where the bore diameter ratio is longer than the stroke (values arbitrary) are of course able to rotate faster due less distance on the stroke. Of course have less reciprocating forces at TDC (and bottom). This creates a really nice street power curve, because it allows RPMs to get high and keep one's transmission in the same gear longer.

You said "where low rpm torque is the rule of the day". Well, it's actually not just that but how steep that torque falloff is. Anything off-road IMNSHO should be focused on an overstroked engine. This design allows for a larger crank throw and crank arms. Actually cart before the horse, as the crank throw ultimately determines the stroke. The force acting from ignition stroke against a longer arm will of course has greater moment and torque potential. Maximize all of these geometries for a stroker and you'll get a steep & deep torque curve, which if put low enough in the RPM range can alllow for the very best operating RPMs. Best off-road is between torque peak and HP peak because you're operating in the torque rise arena. You do not want a flat torque/power curve, unless you're racing (again, my biased opinion)

I'm not sure that 1.6 liters of displacement in an overbored engine design operating consistently at high RPMs would be an answer for Suzuki's range of all-terrain vehicle products. You need a low torque peak to get the torque rise, balanced with something that can smoothly get to some level of higher RPMs, mate with proper tranny ratios, and in a small engine, to do that you need to make it a stroker.

 

[nicholastanguma]

I don't know the engines that you've stated, but as I did study this stuff back in Uni, allow me to respond to maybe a couple of your questions?

As you've stated an "overbored" engine where the bore diameter ratio is longer than the stroke (values arbitrary) are of course able to rotate faster due less distance on the stroke. Of course have less reciprocating forces at TDC (and bottom). This creates a really nice street power curve, because it allows RPMs to get high and keep one's transmission in the same gear longer.

You said "where low rpm torque is the rule of the day". Well, it's actually not just that but how steep that torque falloff is. Anything off-road IMNSHO should be focused on an overstroked engine. This design allows for a larger crank throw and crank arms. Actually cart before the horse, as the crank throw ultimately determines the stroke. The force acting from ignition stroke against a longer arm will of course has greater moment and torque potential. Maximize all of these geometries for a stroker and you'll get a steep & deep torque curve, which if put low enough in the RPM range can alllow for the very best operating RPMs. Best off-road is between torque peak and HP peak because you're operating in the torque rise arena. You do not want a flat torque/power curve, unless you're racing (again, my biased opinion)

I'm not sure that 1.6 liters of displacement in an overbored engine design operating consistently at high RPMs would be an answer for Suzuki's range of all-terrain vehicle products. You need a low torque peak to get the torque rise, balanced with something that can smoothly get to some level of higher RPMs, mate with proper tranny ratios, and in a small engine, to do that you need to make it a stroker.

Torque rise vs flat curve, hmmm, I think I understand. Thanks so much for use of your education! ?

 

[nickw]

Lots of tradeoffs with engine design. For instance, looking at the new Fords, the Ecoboost in the Ranger (2.3L) is over-stroked turbo engine that has excellent low end power. But we don't have to look much further to see another Ford engine taking a different approach, the new 7.3 "godzilla" is over-bored and is focused on industrial/commercial use and low end power as well. The 3.5L ecoboost is the same as the 7.3, over-bored.

My un-scientific opinion, just from being around this stuff for a few years is that with current technology, most engines have plenty of power, it really comes down to fine tuning how the power is delivered, balanced with engine dimensions balanced with fuel efficiency.

I'm squarely in the camp of wanting a wide torque spread, which generally means modest HP and a lot of "area under the curve" to get work done.....HP is what we care about and the lower it's usable, the wider it is, the more area under the curve, the easier we can put the power to work. Of course gearing needs to be available to make use of it, but that is generally not a constraint.

It's also why "max" torque (and HP to a certain extend) is meaningless on engine spec sheets. Engines are designed based on HP requirements and having good a good spread of it given the intended use, which means understanding what the HP values are (or calcing it from torque) at certain RPM's.

 

[Pskhaat]

Torque rise vs flat curve, hmmm,

Imagine a torque curve that peaks early (say 1600rpm) and falls off nicely as the RPMs go up. Now let us say you are driving along very happily at 2200rpm and you encounter a hill or an obstacle or whatever. Your speed decreases but what happens to the potential torque as the RPMs drop? Available torque actually rises.

Contrast to a flat overbored torque curve or one that has peak torque much higher. Smae scenario: your tires encounter that big rock and your torque? Well it doesn't rise or doesn't rise nearly as much, not allowing the engine to easily lug over it, and stalling can occur (unless you drive a slushbox).

This doesn't matter much on most highways, but a high torque rise engine is a delight to drive off highway.

 

[leeleatherwood]

Valve timing (duration and lift), valve springs, intake runner length, intake runner diameter, plenum size, exhaust primary length, compression ratio and ignition timing have more to do with a gasoline engines power curve and "revability" than bore x stroke ratio.

If you were to order them somehow as to how much they affect the torque curve of an engine, valve timing and ignition time would be at the top, probably followed by intake runner diameter and length. Bore and stroke would be either at the bottom or near the bottom.

This is not to say that bore x stroke ratio is not important, it's more to say that you can have wildly different engine torque curves without changing the bore and stroke.

I think the reason OEMs choose the bore and stroke ratios they do has more to do with packaging, durability, etc than power curve, because it's easier to change the power profile of an engine elsewhere using other parts of the engine.

Now, when it comes down to engines built for motorsports use, it is very common to see bore and stroke played with in order to align with the other engine parts to maximize the gains. For instance, put the 4.8L crank into a 6.2L LS block with appropriate camshaft, valve springs, etc and that thing will love to rev to 8 to 10k rpm if your connecting rods can handle it.

 

[nickw]

Imagine a torque curve that peaks early (say 1600rpm) and falls off nicely as the RPMs go up. Now let us say you are driving along very happily at 2200rpm and you encounter a hill or an obstacle or whatever. Your speed decreases but what happens to the potential torque as the RPMs drop? Available torque actually rises.

Contrast to a flat overbored torque curve or one that has peak torque much higher. Smae scenario: your tires encounter that big rock and your torque? Well it doesn't rise or doesn't rise nearly as much, not allowing the engine to easily lug over it, and stalling can occur (unless you drive a slushbox).

This doesn't matter much on most highways, but a high torque rise engine is a delight to drive off highway.

"Torque rise" or "torque reserve" are really only used in the tractor and heavy machine world and it makes sense when thinking about a tractor plowing using max rated HP and gets lugged down traversing a difficult section....but less so when wheeling with clutches, multiple gears and auto-transmissions in most modern vehicles. The reality is all it's doing is keeping HP static (or increasing maybe) as it drags down over the 1600-2200 RPM. Contrary to your example, if a rig has peak torque higher in the RPM range (and less torque rise), it's going to have more HP and would be less likely to drag down or fall off to begin with and be much more driveable.

 

[Pskhaat]

...if a rig has peak torque higher in the RPM range (and less torque rise), it's going to have more HP and would be less likely to drag down or fall off to begin with and be much more driveable.

As to the drivability, we very well may be arguing subjective viewpoints For all off-highway driving at least that I do, my need isn't power and the RPMs needed to get it, it is indeed torque (not that I don't want HP when needed).

 

[nickw]

As to the drivability, we very well may be arguing subjective viewpoints For all off-highway driving at least that I do, my need isn't power and the RPMs needed to get it, it is indeed torque (not that I don't want HP when needed).

The problem you run into is if you want a high "torque reserve" engine, or want to design one with a super long stroke to the OP's question, it's (generally speaking) going to be a HD diesel engine with huge torque numbers at very low RPM, not conducive to variable 4x4 travel. Your LC's are good examples of this, they are well balanced V8's that travel well offroad....they are not high torque reserve engines but have a wide usable powerband.

I think you want a wide HP "curve". If you swapped in a low RPM diesel into your cruiser with the same torque value, which will have more torque rise, I highly doubt you'd be happy with it's overall performance and it certainly wouldn't be any more capable...although you may be able to skip a gas station here or there

 

[billiebob]

Imagine a torque curve that peaks early (say 1600rpm) and falls off nicely as the RPMs go up. Now let us say you are driving along very happily at 2200rpm and you encounter a hill or an obstacle or whatever. Your speed decreases but what happens to the potential torque as the RPMs drop? Available torque actually rises.

Contrast to a flat overbored torque curve or one that has peak torque much higher. Smae scenario: your tires encounter that big rock and your torque? Well it doesn't rise or doesn't rise nearly as much, not allowing the engine to easily lug over it, and stalling can occur (unless you drive a slushbox).

This doesn't matter much on most highways, but a high torque rise engine is a delight to drive off highway.

You have described Class 8 heavy truck diesels perfectly. And a fully loaded SuperBee is a dream to drive over mountain passes.
They climb better at 1600rpm than 2200rpm.

I'm curious when overstroked vs overbored became terms. I always used oversquare, square or undersquare.

 

[85_Ranger4x4]

Torque vs horsepower vs rpm are a tricky thing to talk about and even trickier to visually display it.

It is a either ya got it or ya don't thing. Bore and stroke are only part of the picture. Heck even the engine itself is only part of the picture.

 

[nickw]

Torque vs horsepower vs rpm are a tricky thing to talk about and even trickier to visually display it.

It is a either ya got it or ya don't thing. Bore and stroke are only part of the picture. Heck even the engine itself is only part of the picture.

Judging by your name....you get it....along with Marlin Crawler

 

[luthj]

I will take a wide linear powerband over peak torque any day. Typically thats an engine near square proportions. This allows for good valve size and reasonable RPM limits. Torque is a spurious measurement at best. Horsepower (kw) is the true measurement, and is a combination of torque times RPM. RPM limits in diesel engines are a function of of the flame front speed of diesel fuel. The larger the combustion chamber, the longer it takes to combust fully, thus limiting the effective top RPM.

As long as the engine makes good power starting just off idle through most of its usable RPM range, then its just a matter of correct gearing for the desired speed. This is why big trucks have so many gears. The narrow powerband of the big diesels combined with heavy loads. Also note that large commercial diesel engines are designed for maximum fuel efficiency at a specific RPM/Load combination. Placing the engine at that combination is important to reduce fuel costs. Engines in light passenger vehicles are a bit different, as they operate at very low loads the vast majority of the time.

As far as off road operation, the low range gearing provided is important, as it allows the engine to operate at a significant portion of its rated power, even at very low speeds.

 

[nickw]

You have described Class 8 heavy truck diesels perfectly. And a fully loaded SuperBee is a dream to drive over mountain passes.
They climb better at 1600rpm than 2200rpm.

I'm curious when overstroked vs overbored became terms. I always used oversquare, square or undersquare.

It pulls better at 1600 because it has more HP at that RPM and/or it's gearing is optimized for that RPM.

Good question on the terminology....I'd agree that the 'square' terminology probably more accurately reflects the design.

 

[nickw]

I will take a wide linear powerband over peak torque any day. Typically thats an engine near square proportions. This allows for good valve size and reasonable RPM limits. Torque is a spurious measurement at best. Horsepower (kw) is the true measurement, and is a combination of torque times RPM. RPM limits in diesel engines are a function of of the flame front speed of diesel fuel. The larger the combustion chamber, the longer it takes to combust fully, thus limiting the effective top RPM.

As long as the engine makes good power starting just off idle through most of its usable RPM range, then its just a matter of correct gearing for the desired speed. This is why big trucks have so many gears. The narrow powerband of the big diesels combined with heavy loads. Also note that large commercial diesel engines are designed for maximum fuel efficiency at a specific RPM/Load combination. Placing the engine at that combination is important to reduce fuel costs. Engines in light passenger vehicles are a bit different, as they operate at very low loads the vast majority of the time.

As far as off road operation, the low range gearing provided is important, as it allows the engine to operate at a significant portion of its rated power, even at very low speeds.

Yup!