Pondering about flathead potentials

[Nikolas Ojala]

Excuse me because I start this with a disclaimer: I am not an engine specialist. I don't have my fingers on flathead engines. I have only recently researched these on my leisure time.

I guess that I don't need to explain why flathead ie. side-valve engines are so rare nowadays. So, I'll leave the history for historians.

I see potentials in flathead engines. Some of them are very unusual.

• Pop-up pistons. These may be used to increase compression ratio without damaging breathing.
• Roller cams and lifters. These reduce friction losses.
• The turbulence grooves on combustion chamber's squish area, by Somender Singh. These are used for improved fuel mixture.
• Variable valve timing (VVT). Due to concentric cam shafts (by Mahle and Mechadyne) variable timing is possible also in cam-in-block engines such as OHV and side-valve engines.
• OmniValves special intake valves to improve low rpm torque.
• Cup valves, especially exhaust valves, could be literally cool exhaust valves. The cup valve design enables higher compression ratio and valve cooling with lubricating oil. See the valve patent by John Hutchinson and later patents by Michael L. Clements.
• Combining VVT and OmniValves could lead to a throttleless design, because adjusting intake valve timing radically in a flathead engine does not cause a collision between valve and piston. Why use a throttle if you could do that with the intake valve timing?
• Using all those possibilities could lead to incredibly high compression rate. Sounds like a case for laser ignition.

I recommend you to glance these patents:

• US 6,237,579
• US 6,598,577
• US 1,250,463
• US 6,328,012
• US 6,601,558

As you see, these are not my own original ideas, except perhaps combination of VVT and OmniValves. Probably someone else has thought about that too.

[Kevin Johnson]

Hi Nikolas,

There are a lot of interesting ideas presented. I know that a number of these topics have been discussed over the years on Speedtalk. You should use the search function to bring up some of the older discussions. Start with Omnivalves.

[PackardV8]

None of the above address the myriad inherent mechanical and thermodynamic shortcomings; there are few-to-no flathead engines being sold for a reason; for the past few years even most cheap lawnmowers are OHV/OHC.

While I'm an afficianado of the Packard straight eight flatheads and will talk them as long as anyone wants, the flathead as a performance engine is just bench racing. That horse is not only dead, we can't beat it; long ago it was ground into pet food, eaten and shat out by the junkyard dog.

1. Breathing. The charge has to be pulled through that double-U-shaped loop down/up/down. Then it has to be compressed up and back across to the far corners. Then the flame/pressure has to traverse that same route. Too much restriction and wasted motion.

2. Inherently large area combustion chamber. Todays most efficient combustion chambers are small in surface area.

3. Running the exhaust out through the side of the block creates an inherent hot spot and the longer path rejects more heat into the cooling system.

So yes, the flathead can be made less bad. It just can't ever be made good. I wasted some of the best years of my young life trying. To paraphrase one of our members. "Just because one hasn't studied thermodynamics doesn't mean it won't kick one's ass."

[SchmidtMotorWorks]

If you want to get a good understanding of the potential of flat head engines, get a look at an Outlaw Jr Dragster (not the lower performance Jr Dragster engines used in bracket style racing.)

http://www.youtube.com/watch?v=j4DCKBU-ZMs

[140Air]

Excuse me because I start this with a disclaimer: I am not an engine specialist. I don't have my fingers on flathead engines. I have only recently researched these on my leisure time.

I guess that I don't need to explain why flathead ie. side-valve engines are so rare nowadays. So, I'll leave the history for historians.

In this case "history" is not the history of ladies hats where old fashions can come back. It is a history of painstaking scientific and technological development. You should not expect to see steam locomotives and wooden sailing ships come back because they are technologically inferior. PackardV8's 3 points are overwhelming disadvantages of the flathead design. They will ALWAYS be disadvantages.

Some of the ideas you list are interesting, some are not. Those that hold promise would be better used if applied to more advanced engine designs.

[Nikolas Ojala]

None of the above address the myriad inherent mechanical and thermodynamic shortcomings; there are few-to-no flathead engines being sold for a reason; for the past few years even most cheap lawnmowers are OHV/OHC.

Briggs & Stratton still produce flatheads. Yes, they have also OHV engines.

1. Breathing. The charge has to be pulled through that double-U-shaped loop down/up/down. Then it has to be compressed up and back across to the far corners. Then the flame/pressure has to traverse that same route. Too much restriction and wasted motion.

Yes, although the cup valves would reduce that traveling inside the combustion chamber during compression and expansion strokes.

2. Inherently large area combustion chamber. Todays most efficient combustion chambers are small in surface area.

Yes, and this is a good argument for using both pop-up piston and cup valves. Also the cool exhaust valve would reduce knocking tendency.

3. Running the exhaust out through the side of the block creates an inherent hot spot and the longer path rejects more heat into the cooling system.

This one seems like an argument against the old Ford flathead V8. If we want to talk about flathead engines generally, let's not limit ourselves to some specific design.

This could be my favorite flathead: http://www.d-motor.eu/nl/specifications-38.htm

[Olefud]

Check Ricardo’s work on L-head engines. But time and engine design marches on. You can no doubt now build a better side-valve engine. But it’s still not going to breath like an OHV.

http://en.wikipedia.org/wiki/Harry_Ricardo

[nickmckinney]

Now what other drawbacks are there to any pop up piston that can make decent compression with a flatty. The flow is simply horrible anyway, there is a reason OHV is done by all the OEM. Do some flow testing on one (I never have I will admit) and lets see whats required to get even 200CFM on the intake side. On another similar thought, I would love to see a picture of a flatty in a seat and guide machine.

[Ken_Parkman]

Not terribly relevant, but interesting history is what some of the manufacturers did to get away from side valve. I find what Willys did (maybe others?) fascinating; they moved the intake valve into the head and kept the exhaust valve in the block when they converted the L134 and L161 to F heads for about 20% more power each, then in 62 they converted the old L226 into a SOHC Hemi for about 30% more power.

[140Air]

Not terribly relevant, but interesting history is what some of the manufacturers did to get away from side valve. I find what Willys did (maybe others?) fascinating; they moved the intake valve into the head and kept the exhaust valve in the block when they converted the L134 and L161 to F heads for about 20% more power each, then in 62 they converted the old L226 into a SOHC Hemi for about 30% more power.

OHV, OHC, DOHC and 4-valve DOHC, F-head and strange-head designs go back to well before 1920. They all are more complex than the flathead and would not have been used if the flathead had been satisfactory, but the flathead was never considered to be an efficient design. It was the simplest and the cheapest.

[dwilliams]

the flathead was never considered to be an efficient design.

Sure it was. That's one of the reasons the flathead came about.

Back when "motor fuel" was anything from naptha to lamp oil, compression ratios had to be extremely conservative to prevent detonation. Eliminating hot spots in the chamber was just as important.

By moving the valves to the sides where they could be better cooled, flatheads were able to run higher compression ratios than comparable OHV engines, therefore getting better fuel efficiency. Since almost nobody cared about displacement, BSFC was the order of the day.

Yes, racing and aircraft engines stayed with overhead valves. Because they could generally know what sort of flammable liquid they were going to burn, and could work up against the limits of the fuels they would be running. Things like, oh, mixtures of toluene and nitrobenzine in the 1930s...

The first Otto and Diesel engines were OHV. Flatheads, particularly the Ricardo-style "high turbulence" desgns, were an improvement to allow better performance with wildly variable fuel quality. And in the days when your spark advance and mixture control were on the steering wheel, that meant a lot.

It was the simplest and the cheapest.

Not so. Casting and machining a flathead block was more expensive than an OHV. But the flathead allowed operation on cheaper/unknown grades of fuel, at an operational cost savings, so was often found in economy cars.

You also found flatheads in some luxury cars. Back when an operator was expected to lift the hood and oil the camshaft, followers, rockers, pivots, valve tips, and whatnot with an oil can, thereby making an oil/dirt abrasive mess as the car bumped down dusty dirt roads, the flathead wrapped all its naughty bits on the inside, protected from water and dust, and continuously oiled. Well, most of them, anyway. Also, the enclosed valvetrain meant that, as well as not flinging excess lubricant all over the engine compartment and passengers, the flatheads were noticeably quieter during operation.

As fuel quality improved and became standardized, many countries and racing organizations imposed displacement limits or taxes, so the better breathing potential of an OHV engine became more important than fuel flexibility.


Those old designers weren't stupid, they were just working with a different set of constraints. Same for aircraft, marine, and OTR Diesel engine designs. "Volumetric efficiency" is meaningless, BSFC and MTBF are what count.

[140Air]

the flathead was never considered to be an efficient design.

Sure it was. That's one of the reasons the flathead came about.

Back when "motor fuel" was anything from naptha to lamp oil, compression ratios had to be extremely conservative to prevent detonation. Eliminating hot spots in the chamber was just as important.

By moving the valves to the sides where they could be better cooled, flatheads were able to run higher compression ratios than comparable OHV engines, therefore getting better fuel efficiency. Since almost nobody cared about displacement, BSFC was the order of the day.

Yes, racing and aircraft engines stayed with overhead valves. Because they could generally know what sort of flammable liquid they were going to burn, and could work up against the limits of the fuels they would be running. Things like, oh, mixtures of toluene and nitrobenzine in the 1930s...

The first Otto and Diesel engines were OHV. Flatheads, particularly the Ricardo-style "high turbulence" desgns, were an improvement to allow better performance with wildly variable fuel quality. And in the days when your spark advance and mixture control were on the steering wheel, that meant a lot.

It was the simplest and the cheapest.

Not so. Casting and machining a flathead block was more expensive than an OHV. But the flathead allowed operation on cheaper/unknown grades of fuel, at an operational cost savings, so was often found in economy cars.

You also found flatheads in some luxury cars. Back when an operator was expected to lift the hood and oil the camshaft, followers, rockers, pivots, valve tips, and whatnot with an oil can, thereby making an oil/dirt abrasive mess as the car bumped down dusty dirt roads, the flathead wrapped all its naughty bits on the inside, protected from water and dust, and continuously oiled. Well, most of them, anyway. Also, the enclosed valvetrain meant that, as well as not flinging excess lubricant all over the engine compartment and passengers, the flatheads were noticeably quieter during operation.

As fuel quality improved and became standardized, many countries and racing organizations imposed displacement limits or taxes, so the better breathing potential of an OHV engine became more important than fuel flexibility.


Those old designers weren't stupid, they were just working with a different set of constraints. Same for aircraft, marine, and OTR Diesel engine designs. "Volumetric efficiency" is meaningless, BSFC and MTBF are what count.

I think you are wrong. For one, by parts count alone it is obvious a flathead is inherently a cheaper design than any OHV. Add to that the ultra simplicity of a cylinder head that is a pancake with a water jacket. The additional complexity of the block casting is more than made up for by the simplification of the head casting and machining and the elimination of a drive for the valves be it pushrods or chain or gear driven OHCs. The flathead was the base engine for the cheapest cars from before the Model T as well as the only way to make a lawnmower engine because it was cheap, NOT because it was more efficient and more expensive.
True, Ricardo's high turbulence squish head brought flathead CRs up from 4:1 to 7:1 while still using the cheapest fuels while OHV designs were needing higher grade gas, but that supports the point that the OHV engine was the premium engine. Once the OHV wedge was developed allowing CRs beyond 8:1 the flathead was left far behind on CR to add to its poorer breathing.
The flathead was a good work horse for tractors and Fords (no offense). You wouldn't look for them in high power, high efficiency or high classed applications.

[Ron E]

the flathead was never considered to be an efficient design.

Sure it was. That's one of the reasons the flathead came about.

Back when "motor fuel" was anything from naptha to lamp oil, compression ratios had to be extremely conservative to prevent detonation. Eliminating hot spots in the chamber was just as important.

By moving the valves to the sides where they could be better cooled, flatheads were able to run higher compression ratios than comparable OHV engines, therefore getting better fuel efficiency. Since almost nobody cared about displacement, BSFC was the order of the day.

Yes, racing and aircraft engines stayed with overhead valves. Because they could generally know what sort of flammable liquid they were going to burn, and could work up against the limits of the fuels they would be running. Things like, oh, mixtures of toluene and nitrobenzine in the 1930s...

The first Otto and Diesel engines were OHV. Flatheads, particularly the Ricardo-style "high turbulence" desgns, were an improvement to allow better performance with wildly variable fuel quality. And in the days when your spark advance and mixture control were on the steering wheel, that meant a lot.

It was the simplest and the cheapest.

Not so. Casting and machining a flathead block was more expensive than an OHV. But the flathead allowed operation on cheaper/unknown grades of fuel, at an operational cost savings, so was often found in economy cars.

You also found flatheads in some luxury cars. Back when an operator was expected to lift the hood and oil the camshaft, followers, rockers, pivots, valve tips, and whatnot with an oil can, thereby making an oil/dirt abrasive mess as the car bumped down dusty dirt roads, the flathead wrapped all its naughty bits on the inside, protected from water and dust, and continuously oiled. Well, most of them, anyway. Also, the enclosed valvetrain meant that, as well as not flinging excess lubricant all over the engine compartment and passengers, the flatheads were noticeably quieter during operation.

As fuel quality improved and became standardized, many countries and racing organizations imposed displacement limits or taxes, so the better breathing potential of an OHV engine became more important than fuel flexibility.


Those old designers weren't stupid, they were just working with a different set of constraints. Same for aircraft, marine, and OTR Diesel engine designs. "Volumetric efficiency" is meaningless, BSFC and MTBF are what count.

May, or may not have been cheaper. But, considering the fuels of the day, it makes a lot of sense. Great points that I've never thought about. Thanks

[Nikolas Ojala]

I did a small comparison between three different engines:

• Mazda Renesis 13B-MSP (modern 2 rotor Wankel engine)
  Max power output: 210 hp (160 kW)
  Weight of engine: 247 lbs (122 kg)
  Power to weight ratio: 0.85 hp/lbs (1.31 kW/kg)
  https://en.wikipedia.org/wiki/Mazda_RX- ... and_design
  https://en.wikipedia.org/wiki/Renesis_% ... SP_Renesis
• D-motor LF26 (modern flathead engine for aeroplanes, 4 cylinder, boxer)
  Max power output: 88.8 hp (65.3 kW)
  Weight of engine: 128 lbs (58 kg)
  Power to weight ratio: 0.69 hp/lbs (1.13 kW/kg)
  http://www.d-motor.eu/nl/specifications-38.htm
• GM 7.0 L LS7 (modern high performance OHV V8)
  Max power output: 505 hp (377 kW)
  Weight of engine: 454 lbs (206 kg)
  Power to weight ratio: 1.11 hp/lbs (1.83 kW/kg)
  http://gmauthority.com/blog/gm/gm-engines/ls7/#tab-2

As we see, the high performance OHV engine wins and beats also the rotary engine. The flathead is not bad either. All these engines are quite new. There are few things that make comparison difficult. The biggest engine is almost six times more powerful than the smallest. These engines were intended for different use (performance car vs aeroplane). The winner in this comparison was built by one of the world's biggest motor company while the loser was produced by an almost unknown small company.

[Kevin Johnson]

I did a small comparison between three different engines:

• Mazda Renesis 13B-MSP (modern 2 rotor Wankel engine)
  Max power output: 210 hp (160 kW)
  Weight of engine: 247 lbs (122 kg)
  Power to weight ratio: 0.85 hp/lbs (1.31 kW/kg)
  https://en.wikipedia.org/wiki/Mazda_RX- ... and_design
  https://en.wikipedia.org/wiki/Renesis_% ... SP_Renesis
• D-motor LF26 (modern flathead engine for aeroplanes, 4 cylinder, boxer)
  Max power output: 88.8 hp (65.3 kW)
  Weight of engine: 128 lbs (58 kg)
  Power to weight ratio: 0.69 hp/lbs (1.13 kW/kg)
  http://www.d-motor.eu/nl/specifications-38.htm
• GM 7.0 L LS7 (modern high performance OHV V8)
  Max power output: 505 hp (377 kW)
  Weight of engine: 454 lbs (206 kg)
  Power to weight ratio: 1.11 hp/lbs (1.83 kW/kg)
  http://gmauthority.com/blog/gm/gm-engines/ls7/#tab-2

As we see, the high performance OHV engine wins and beats also the rotary engine. The flathead is not bad either. All these engines are quite new. There are few things that make comparison difficult. The biggest engine is almost six times more powerful than the smallest. These engines were intended for different use (performance car vs aeroplane). The winner in this comparison was built by one of the world's biggest motor company while the loser was produced by an almost unknown small company.

The winner was also proleptically judged by an aircraft engine expert as a poor choice for an aviation powerplant. http://www.epi-eng.com/aircraft_engine_ ... ngines.htm