# Relationship between Torque and Engine displacement?



## Green (Dec 24, 2017)

In all of my research on mower engines this past year, I realized that engine displacement and torque aren't necessarily related...or so it seems.

As an example, my mower has a Briggs and Stratton 190cc 675ex series engine. The torque is rated at 6.75 ft-lbs. Toro used to use the same engine series in their mowers, too.

Then a year or two ago, I noticed manufacturers were replacing these with the 675exi and 725exi series in their latest mowers. I noticed that the 675exi is a 163cc engine rated for 6.75 ft-lbs of torque. And the 725exi is also a 163cc engine and rated for 7.25 ft-lbs of torque.

My question is, how are they getting equal or higher torque in the newer engines, with the same or smaller engine displacement than the older engines? The repair guy I went to also commented that the newer model isn't made as well or as reliable, but that's another topic.


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## J_nick (Jan 28, 2017)

How do the new v6's in cars have the same hp/tq as my 2006 5.3L v8? Over time they improve the design of combustion chambers, cam profiles, etc...


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## g-man (Jun 15, 2017)

If I recall correctly, displacement is really just the volume of space to fill with the air/gas mixture when the piston is at the top of the stroke. Stroke length, piston diameter, compression ratio and others will all affect the torque/rpm output. All of these are design choices when making the engine. The stated max torque could also be at a different room and might not even be a useful rpm for a mower.

In short, it's complicated.


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## J_nick (Jan 28, 2017)

Displacment is bore area x stroke. You can have 2 engines with the same displacement but completely different characteristics depending on bore size and stroke length. Finding the perfect combo for the particular need is the hard part. If all else fails the old saying is "there's no replacement for displacement"


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## g-man (Jun 15, 2017)

Yup. I had to Google it. It is the total travel distance x bore of all piston. Wikipedia has a nice gift. https://en.m.wikipedia.org/wiki/Engine_displacement

The space at tdc is the combustion chamber.


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## MasterMech (Sep 24, 2017)

There can be changes to ignition timing, fuel delivery, and air flow but the most direct way to up the power that significantly would be to bump the compression ratio. If the displacement has not increased, then the size of the combustion chamber has almost certainly decreased unless there were some intentional design limitations built into the previous series.


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## Green (Dec 24, 2017)

I'll have to delve more into this topic over time and ask more questions. I know I'm oversimplifying everything you guys said above, but...

So, chances are, the 190cc 675ex is more powerful than the newer 163cc 675exi, right? Again, both are rated for 6.75 torque.
I think we'd all agree that's likely.

But, what about the 163cc 725exi vs. the 190cc 675ex? How the heck do you evaluate the specs and figure out which will do better in thick grass?

What's the true measure of cutting power when it comes to these small engines? Is it the torque, or something else?

Where does compression ratio fit into the given specs?


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## MasterMech (Sep 24, 2017)

Green said:


> I'll have to delve more into this topic over time and ask more questions. I know I'm oversimplifying everything you guys said above, but...
> 
> So, chances are, the 190cc 675ex is more powerful than the newer 163cc 675exi, right? Again, both are rated for 6.75 torque.
> I think we'd all agree that's likely.
> ...


I'd disagree that the older 675ex (190cc) engine is more powerful than it's replacement. More durable perhaps, but no guarantees. -IF- the torque specs are accurate, and both engines are running at the same rpm when that measurement is taken, then the power available from either must be equal.

Work done is pretty simple to calculate. In the US, we like to use horsepower (HP) for units. So, since our torque specs are in ft/lbs:

P(hp) = T(lb/ft) x N (rpm)
---------------
5252

P(hp) = 6.75 x 3000
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5252

P(hp) = 3.855674028941356

So unless our 675exi is making it's 6.75lb/ft of torque at a different rpm (possible but unlikely since most mowers are designed to run at the same rpm) than the previous 675ex, it is indeed a far more efficient engine.

As far as explaining the torque increase of the 725exi over the 625exi, I've seen that done most commonly with a different throttle shaft. The wide open stop on the throttle shaft is likely moved to allow 100% WOT on the 725 where the the 625 is likely not quite fully open. There could also be differences in compression or carburetor venturi size. If you really want the answer, I'd start by comparing the parts breakdowns. Use the breakdowns for two off-the-shelf service replacement engines, NOT OEM supplied units. Then compare the part numbers for the "shortblock", cylinder head, and carburetor. I'm guessing the carbs will be different part #'s and if individual breakdowns of each carb assembly are available, you can repeat the process for each carburetor. Compare #'s for the bodies, jets, the throttle plates, and the throttle shafts for likely culprits.


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## g-man (Jun 15, 2017)

B&S has the torque curves in their website.

https://www.briggsandstratton.com/na/en_us/product-catalog/engines/push-mower-engines/exi-engine-series.html


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## Ridgerunner (May 16, 2017)

The term "torque" represents a rotational force. Akin to a lever and a weight placed on the end of the lever, measured as a distance-weight (e.g. foot-pounds). Using a box wrench on the bolt attaching the blade to the lawn mower engine shaft results in applying torque (a twisting force) to the bolt. The wrench is the lever and applying your weight on the end of the wrench (lever) applies a rotational force (torque) to the bolt. The more weight applied, the more torque (rotational force). In the alternative the longer the wrench, the less amount of weight needed to achieve the same amount of torque. Applying this to an internal combustion engine: The lever is the distance from the center of the crank shaft out to the journal where the piston rod attaches to the crank shaft (the stroke). the "weight" applied is derived from the explosive force of the air/gas mixture in the ignition chamber. Over simplifying, the bigger the chamber (displacement), the more "weight" applied to the end of the lever. Torque can be increased by either increasing the explosive force, "weight" (one way to do this is to increase displacement- e.g increase the bore) or by increasing the stroke (length of the lever) or both. In the alternative, if you increase the stroke, you would need to decrease the explosive power (e.g. decreasing the displacement) to end up with the same amount of torque.


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## GrassFarmer (Sep 21, 2017)

Buy a diesel and you will have plenty of torque.


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## Green (Dec 24, 2017)

Great stuff here. I'm just scratching the surface so far...a lot to learn.

Here is the link to my mower's engine: http://sears.briggsandstratton.com/walk/gold_series_675ex.asp
Any idea where I can the in depth information it online, like the power curve? The guy who worked on it felt it was better than the newer engines. Note, we are also comparing the newer OHV engines to this...not sure what technology it had or what it's called.


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## MasterMech (Sep 24, 2017)

Green said:


> Great stuff here. I'm just scratching the surface so far...a lot to learn.
> 
> Here is the link to my mower's engine: http://sears.briggsandstratton.com/walk/gold_series_675ex.asp
> Any idea where I can the in depth information it online, like the power curve? The guy who worked on it felt it was better than the newer engines. Note, we are also comparing the newer OHV engines to this...not sure what technology it had or what it's called.


Well there's your answer! :lol: Your engine is an L-Head or flathead design. Compression ratio on those is going to be significantly lower than an OHV design due to the relatively large combustion chamber. Also, the combustion chamber is far from the optimum shape so there are further inefficiencies there. So I would certainly expect a 165 cc OHV to at least match the performance of a 190cc flathead.

Your local repair mech is right about those flathead Briggs motors being stone reliable. The OHV motors have been good too, but they have several decades to go before they catch up to flathead's length of service.


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## Green (Dec 24, 2017)

So, OHV is not as reliable...as in, it just won't last as long as a flathead design on average? Or less "durable" too? Or is there a certain part of it that will need maintenance more often? Sounds like it's a relatively new technology. But they're pushing OHV on everything sold today. I can't find a mower for sale without an OHV. How much shorter is the life in your experience? Are you already seeing dead OHVs that can't be fixed? I'm not sure how long they've been around. Makes me wonder why they switched to OHV.


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## MasterMech (Sep 24, 2017)

OHV is nothing new. At all. It's been around since, well, a long time! (WWI era and even older). It's been on mowers since at least the late 80s. Flatheads were so prevalent for one reason only, they are cheaper to manufacture. There certainly are not piles of OHV engines suffering premature death and while there may be one additional maintenance point (valve lash) compared to flatheads, it's fairly infrequent. In fact, most owners never fool with it. Come to think of it, valve lash is a maintenance item on flatheads too, that hardly anybody ever needed to mess with. And it's much more difficult on a flathead to adjust that clearance.

Why the switch? Emissions regulations. Yup, the E.P.A. OHV and OHC engines run much more efficiently so cleaner exhaust comes naturally. These engines make more power and use less fuel than their flathead ancestors, while remaining simple to maintain. What's not to like?


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## Green (Dec 24, 2017)

MasterMech said:


> OHV is nothing new.


Any opinions on how the current Briggs (140, 150, 163, and 190CC), Kohler (149cc 6.5 and 6.75 torque), and Honda (160 and 190 GCV) stack up? Or are any of them about equally good?

Also, how about the current 190cc Briggs vs. Honda mower engines specifically? I see the Hondas are "OHC" rather than OHV.

Finally, is something like the Kawasaki FJ180V a definite step up over the others?


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## MasterMech (Sep 24, 2017)

I would say most of those are equally good. Out of all of the ones you mentioned, Kawasaki is the only one offering a pressure lube system with an oil filter. That's a significant step up for sure.

It's important to remember that most of these engine brands, especially Briggs, Kohler, and Honda, offer multiple levels of quality within their line. Just because your neighbors $99 k-mart special mower grenaded it's entry-level Briggs, does not mean all Briggs engines will perform similarly.


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## Green (Dec 24, 2017)

I was just reading something on another site in which a bunch of people complained about the Kawasaki, saying that it was powerful and truly commercial grade, but nowhere near as reliable in their experience as the Hondas.

Do you prefer any specific brand, personally? If so, why?


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## high leverage (Jun 11, 2017)

Green said:


> I was just reading something on another site in which a bunch of people complained about the Kawasaki, saying that it was powerful and truly commercial grade, but nowhere near as reliable in their experience as the Hondas.
> 
> Do you prefer any specific brand, personally? If so, why?


The Honda GX series engines are very popular in commercial application and have a proven track record. I own two pieces of equipment with the Kawi FJ180 with well over 750+ hours with no problems.


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## Green (Dec 24, 2017)

high leverage said:


> Green said:
> 
> 
> > I was just reading something on another site in which a bunch of people complained about the Kawasaki, saying that it was powerful and truly commercial grade, but nowhere near as reliable in their experience as the Hondas.
> ...


Just to be clear, do you mean this design? 
http://www.kawasakienginesusa.com/engines/fj/fj180v

Or this one?
http://www.kawasakienginesusa.com/engines/fj/fj180v-kai

I'm not sure about the differences. They seem similar in specs, but the design differs.


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## Green (Dec 24, 2017)

One more thing...I was referring to the Kohler XTX engines above, not the HD series.


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## high leverage (Jun 11, 2017)

Green said:


> high leverage said:
> 
> 
> > Green said:
> ...


I should have been more specific. The KAI version


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