Why is displacement-specific output such a big deal?

Recently had a discussion with a friend on the merits of OHV vs sidevalve in a lawnmower application, me taking the “flathead” side.

“Look,” I said, “it costs more, opens up the chance of eating a valve, and lends itself poorly to splash lube. Yeah it breaths better, but…in an engine goverened to under 3000 RPM, you really aren’t utilizing what you’ve paid for.”

“But it uses more fuel!”

“Hey, I get 40 minutes out of a pint. Cutting fuel consumption 20% isn’t going to pay for the price premium this decade.”

“But it produces more power for the same sized engine. A Honda GCV160 and a Briggs Quantum (190cc) have the same rating”

“yeah, but the flathead is smaller above the top of the stroke to make up.”

That had me thinking: outside of racing specs, what difference does “hp per liter” make? I don’t think I’ve ever cared about “how much power per cc” my engine produced. Per $ spent, per lb weight, possibly even per displacement of the entire engine body, yes…but having difficulty imagining caring about hp/cm^3.

So, why such a BFD? Is it indeed relevant to non-racers? What, if anything, would be a better measure?

(As a thought experiment, what engine design would you use if you were told “build me the most powerful engine you can, subject to the criteria that the volume of the engine in total, plus all accessories necessary to allow it to function, must not exceed 10,000cc.”)

There were these lawn tractors built in the 60s that were called 10 horse. They had flat heads and were twice as powerful as the 20 horse engines of today. They were also bigger and better than anything you can buy today for $1,600. It seems as if todays machinery is all based on big lies.

Within racing it only has meaning if there is a displacement cap in the rules of whatever race you’re in. If you can only have a 2L engine, then making more hp per liter than the next guy = making more horsepower than the next guy.

On the street, it gives people something to brag about. The implication is “Well if Honda made the Civic engine as big as a Viper engine it’d be faster!” Well, yes, maybe it would. But they didn’t, and it isn’t.

This kind of talk smacks of the old, equally BS, “ain’t no replacement for displacement” crap the V8 guys used to say when someone with a turbo 4 or 6 showed up. It’s dumb. Sure there’s a replacement for displacement. You might have a bigger engine in that GTO, but a tiny-engined turbo Porsche is still faster. End of story.

If you want a real world measure of a car’s “power” performance, look at its 0-60 times. If I want to beat that Porsche 911 Turbo S then I need to go 0-60 in less than 2.9 seconds. I don’t care if I do it with 1 or 1,000 horsepower. At the end of the day, I either did it or not, no matter what the numbers on my engine say.

After all, horsepower means nothing without taking into account other factors such as torque, vehicle weight, gearing, etc. Failing to consider that means you know absolutely nothing about the vehicle’s performance. “Hey, wanna drive my 450 hp vehicle?” “Yeah! That’s fast!” “Erm, no, it’s a Kenworth.”

Men will make a contest out of anything. Seriously. You could be sitting next to the lake enjoying the view and sooner or later, if there are at least two men, a game of who can throw the rock farthest, most accurate, skip the most is likely to ensue.

HP/volume is important when you’re comparing apples to apples. If we both have 454 cu in motors and I have been able to get, for example, 590 hp out of my naturally aspirated motor, it will be a point of bragging rights until you can better it.

If the goal is to start at the same point and time and cross a finish line first, it has little if any meaning. Too many other factors to consider that have significant contribution to success.

No, I do not believe hp:cc is relevant to non-racers. Or to lawnmowers.
The valve position is only relevant IMHO if you’re discussing antique engines. Clearly, OHV engines produce more power per displacement than side valve engines. That was recognized sometime early in the last century.

What would I design given your criteriia?
First of all, what does “the volume of the engine in total, plus all accessories necessary to allow it to function, must not exceed 10,000cc” mean? Are we talking total volume (or mass) calculations including the alternator, water pump, supercharger (I’d use one), battery, starter, etc.? That’s very, very different from a discussion of displacement. Displacement does not include accessories. The total volume calculations would be more relevant to fitment, currently the forte of “packaging engineers”.

Bottom line: I’d use OHV…and since your’re seeking maximum power to displacement ratio, it’d be a two cycle. You said nothing about emissions or drivability, only power to displacement.

@TSM: I meant what I said: if you tossed the engine, and everything attached to it that’s necesssary for it to function into a bathtub, how high would the water rise?

Meaning that “how big” an engine is isn’t really a function of cylinder displacement alone. Look at the (liquid-cooled, OHC) H-D V-Rod: engine so big as to be out of proportion to the rest of bike; yet, at 1250cc, it’s around the same displacement as a standard pshrod Evolution engine.

As for what engine I’d go with: probably an air-cooled, 2 stroke, engine with glow-plug ignition (like a scaled-up RC airplane). No required accessories means a smaller overall package!

Displacement specific output has been the golden ring that every engine designer EVER, has been shooting for since the inception of the internal combustion engine.
And everyone has a different idea and theory to try.
This also answers the other thread about why auto manufacturers design and build their own engines.
Trying, with every new design, to get hte best ratio between piston size, piston travel , connecting rod length, crankshaft throw, fuel delivery , power AND fuel economy.

A NEVER ENDING quest for the perfect equation.

Now that we’re talking lawn mowers, I bought my twin cylinder because of the way it sounds. I like the rumble. Don’t know what cc it is but its a flat head and enough. Something nice though about having a mower that sounds like a tractor. Just kidding but I do like the sound.

A big reason to go to OHV from side valve is emissions, easier to meet the current and coming emissions laws for small engines with OHV.

Going to the OHV configuration for lawn mower engines adds to the complexity and the decrease in fuel consumption is probably negligible for most people. However, it does reduce emissions. I have a smoke detector in the ceiling of my garage. I can start my cars in the garage when the wind is blowing into my garage and the smoke detector does not go off. However, if I start my lawnmower outside the garage with the wind blowing the exhaust into the garage, it will set off the smoke alarm. These little side valve engines really have a dirty exhaust. The amount of pollution from my lawnmower is insignificant, as is the amount of fuel consumed for the hour to mow the yard, but multiplied over millions of mowers, then the difference between a side valve engine and an OHV engine may be significant.

Back in 1961, Studebaker adapted its 169 cubic inch side valve 6 to an OHV engine. The block remained the same, but the adaptation to overhead valves raised the horsepower from 90 to 112 with an equivalent jump in mpg. All of this while the displacement remained at 169.
AMC did the same thing with its 196 cubic inch side valve engine. In 1956, the engine was converted to OHV with the same displacement. The horsepower increased from 90 to 120. Both the 196 side valve engine and the 196 OHV version were available in the Rambler American from 1960 through 1965. The mileage and acceleration was significantly better with the OHV engine. In 1966, the flathead 6 disappeared for good with a redesigned OHV 7 bearing 6 was the only engine offered in the Rambler American. This new engine was available with 199 cubic inch displacement or with 232 cubic inch displacement.

Well they actually produce a small L-head or flathead for a small motorcycle now,dont know the name or the reason-Flatheads can be made better,but whats the point now?-Kevin

Our sky haze blue 54 Ford V8 had OHV. I was only 6 at the time so didn’t know what it meant but now remember the emblem being there someplace. Guess Ford was proud of it.

@Bing–Ford replaced its flathead V-8 in 1954 with a new overhead valve V-8. The Ford flathead V-8 made quite a hit when it arrived in 1932. Clyde Barrow, of Bonnie and Clyde was so impressed that he wrote to Henry Ford praising the car. He said that in his line of work (bank robbery), nothing could beat the Ford V-8. The 1954 Ford V-8 was a complete break from the past. In fact, Ford called it the Y-block V-8.
Ford brought out a 6 cylinder engine for its 1941 models. It was a flathead design. In 1952, this engine was replaced with an OHV 6 engine. The story was that the 1952 OHV 6 could accelerate and outrun the old flathead Ford V-8 that was an option. This may have sped up the development of the Ford OHV V-8.

An additional reason for replacing the flathead V8 was that the machinery (1932-1953) to make these engines was worn out. Ford started with a clean slate with the OHV V8.


So, when they put OHV on a pre-existing engine, the HP bumps up 25% or so. What happens to the dimensions if the engine assembly, what with all that stuff perched atop the combustion chamber? I remember my first head gasket job, taking off the cams, head, etc and being wowed at how much smaller what remained was. I’ll bet a 3.5L flathead could occupy the same space as a 2.0L DOHC.

What I like about valve-in-block for OPE is that you’re dealing with a class if engine built to a price point where you’ll likely junk the engine vs ever crack the case on a repair. That makes the timing belt/chain the limiting factor on OHV longevity. Also, a head gasket job is economically feasible on a flathead (remove 8 bolts, replace gasket, reassemble) but possibly not on an OHV, due to the labor-intensive nature of getting the valvetrain out of the way.

@meanjoe75fan–I agree with you about the simplicity of the flathead inline engine. When AMC and Studebaker converted the flathead engines to OHV, the engine grew a lot taller. You almost needed a magnifying glass to see the flathead engine down in the engine well. Valve jobs were simple on the flathead engine–remove the side panels from the block, remove the valve keepers remove the head and push the valves right on out of the block. The Ford flathead V-8 engine took quite a bit more work for a valve job.
One engine I find fascinating was the Stearns-Knight sleeve valve engine. There were problems, including oil consumption, but it was a unique design.

One incentive back when to switch to OHV was the availability of higher octane fuel.
Higher compression ratios could be reached with OHV.

One bragging point back in the 1940s and early 1950s that Ford and Plymouth owners had was that the engine was quieter than the Chevrolet engines, especially when idling. The Chevrolet engines back then had solid tappets and the overhead valve lash was often out of adjustment that made these engines seem noisier than the flathead engines where the engine block shielded much of the noise. I suspect that this was the reason hydraulic tappets came along for OHV engines. With the hydraulic tappets came the need for detergent oil to keep the oil passages clear so that the hydraulic tappets would work.

As long as the engine fits on the mower or under the hood, produces the amount of power needed for the application, is reliable, and isn’t excessively heavy, noisy, etc., who cares? You might as well argue that you don’t like that it’s painted purple.

Sometimes a larger-displacement motor not making as much horsepower as a screaming engine that has had every last pony extracted with high compression, forced induction, etc. is more desirable for longevity. Maybe the best example of this I’ve seen was in a police chase where a little turbo 4-banger was being chased by a Crown Vic. The rice rocket easily pulled away from the cop car a half dozen times, but eventually couldn’t take the constant abuse and blew up. I’m sure you could beat on the slower Crown Vic all day long without mishap, and it probably had 200K on the clock. Same with pretty much any car or pickup with an older-tech V8. Now on a twisty road with a driver that wasn’t just an idiot running scared, the story probably would have been different…