Is there still some love for the good old flathead layout?

[cRiPpLe_rOoStEr]

Though flathead engines are not so likely to make a comeback in automotive applications, there are some recent developments in this field targeting the aircraft market mostly because of an upcoming phaseout of leaded AvGas.

Anyway, an engine like the D-Motor LF26 would be a reasonable option if I were looking for a replacement engine to my dad's Impreza...
D-Motor LF26 aircraft engine specifications, D-Motor USA Fort Pierce Florida.

[Frank Lee]

Flathead might be OK in aircraft where a high premium is placed on lightweight and compact. Also aircraft engines can't be as "optimized" for peak power output as they are run full throttle for hours at a time; try that with an automotive engine and see how long it lasts. (Auto engines converted for aircraft use are often detuned and mated to gear reduction boxes.) The point being since aircraft engines can't use optimal compression ratios and power anyway, it's OK to compromise on those. Unfortunately flatheads have awful combustion chamber shapes vs hemi's or wedges so I'd expect some decrease in fuel efficiency.

They aren't coming back to automotive use.

Even the cheap little lawn and garden end of the ICE spectrum are all converting to OHV, right down to 21cc weed whips.

[user removed]

Flat heads are low compression engines, with serious limitations as to breathing and combustion chamber volume. Hard to get one breathing without lower compression.
Limit seems to be about 8-1.

regards
mech

[Frank Lee]

That's OK because aircraft engines can't be highly stressed anyway. Heh, turbo it to compensate for poor breathing then raise the CR to get some combustion efficiency back; that helps with altitude power loss anyway.

P.S. A direct drive aircraft engine doesn't need to breathe deeply anyway as it will be a low rpm unit.

[cRiPpLe_rOoStEr]

Just a remind: my dad has a Subaru, and its compression is already kinda low, and neither him or I take it above 3000RPM so often.

[Frank Lee]

With a flathead you'd still have the poor chamber shape with it's unfriendly for flame propagation shape and excess surface area to lose heat into thus resulting in some drop of fuel efficiency.

[some_other_dave]

But it keeps the profile of the engine nice and low, which is excellent for drag reduction when you have a very narrow vehicle.

-soD

[cRiPpLe_rOoStEr]

Originally Posted by some_other_dave

But it keeps the profile of the engine nice and low, which is excellent for drag reduction when you have a very narrow vehicle.

That's an excellent point. Well, actually, when it comes to boxer engines, a flathead is gonna be a little taller, though it would still be narrower enough to have an overall smaller physical volume, while an inline would be slightly wider and lower. In a V engine, the compromise between height and width seems to be of a lesser extent, in spite of the Ford flathead V8 being a tea kettle due to its exhaust running through the block.

[freebeard]

Fords ran hot because they didn't always get all the casting sand out of the water jacket. The solution to breathing was the GMC 6-71 supercharger.

Nobody's addressing your question: "Is there still some love for the good old flathead layout?"



Oh, yeah.

[Frank Lee]

Uh huh! Maybe that aircraft application has merit... maybe not. But for almost everything else- automotive, lawn and garden, marine, whatever- flatheads seem to be outclassed by OHVs. The power and fuel economy disadvantages are too big to ignore.