Question about flahead engine and detonation

posted a question a few days ago about a flathead engine vs overhead valve. Thank you all for the responses. I would like to ask a follow up question to one of the answers that intrigued me. Someone posted that flathead engines are more prone to detonation. Why is that? Are there hot spots on the head, the shape of the combustion, deposits that get hot, or something else?

Flatheads, Lheads, sidevalves--most common like the old Fords, B&S have offset combustion chambers, slanted Quence areas at top of 2/3 of piston, not compact combustion chamber with even burn from spark plug point. this means a sudden jump in pressure at far end of combustion path sometimes and that means a bit of compression ignition/knock/detonation with higher compression. Harley WR 45 inch side valve with 35 to 40 hp as racers with 7.5 to 8.5 compression ratios while WLA had about 5.5 to one and 25 hp. Detonation was sort of acceptable for a racer running higher RPMs- 6000 rpm sometimes more but the airflow rate was bad then, the S curve of intake path with similar exhaust. WLA was about 4000 rpm, happiest at 1500 to 3000 rpm. Advantage of side valve wasn't performance , was size and economy in building engine with the one comparitive advantage over the OHV designs of lighter valve train mass. Some experiments 1980s with slightly higher compression and dual spark plugs- attempt to use existing tooling with minimal cost imporovements to meet California propose emmissions. Kohler LP gas had with second plug toward center, 1st plug at side of head next to gap between valves, 7.5:1 compression on K241 and K301 engines- 10% increase in power, even heat from the double spark source using ignition system first used on flathead Continental engine used in Piper Cubs. Ragged ignition path means odd heat points and sometimes hot spots at edge of valves. RN

Indeed, they are.

Consider the combustion chamber of a side valve (flathead) engine. The combustion chamber includes the valve pockets and area between the valves and the cylinder. After ignition, the flame must travel the distance between the spark plug and the furthest extent of the combustion chamber, i.e., the valve pockets, inches away. This limits CR and RPM.

That said, I like flatheads. Once in the late 1960s I saw a drag race between a SHP 409 powered altered with Hilborn injection and an old school rail dragster with a blown and injected flathead Ford mill. Of course the rail was lighter and the 409 altered was spotted more than a second but it wasn't even close. Standing by the rail I watched the old flathead overhaul the altered and win going away. I'm not sure if the rail was burning gasoline, alcohol or nitromethane but it was a sight to see. As they once said in the 1950s: "Flatheads forever."

Relatedly, flathead aficionados might want to again listen to the Beach Boys recording of Little Deuce Coupe. Unlike most of the bands that recorded car songs in the mid 1960s, the Beach Boys knew what they were talking about. Pay attention to the line at about 0:35: "She's ported and RELIEVED and she's stroked and bored...." Who knows what "relieved" means?

Unlike an OHV engine, both intake and exhaust gasses must travel between the cylinder and the side valves in a flathead engine. The cylinder head provides an area for this. This area, of course, limits CR and compromises combustion chamber design. It also limits high RPM performance as breathing is limited.

Relieving the block, i.e., machining the block to provide increased area for intake and exhaust gasses to travel between the cylinder and side valves, helped breathing and improved high RPM performance, but it was not enough.

By the mid 1950s, the handwriting was on the wall, and even the "flathead forever" folks accepted the reality that there was no way to make flathead Ford engines competitive with the OHV Oldsmobiles and Cadillacs in the automotive market.

I have digressed. There is nothing wrong with inexpensive side valve engines in relatively low speed industrial applications, especially in the throw away market, e.g., walk behind lawn mowers, etc. Your small, 1800 RPM Kohler generator engine is one such example.

Dean
Little Deuce Coupe

Sorry, folks, but I'm also a W bock (348/409/Z11 427) fan and I just could not help myself.

Enjoy the Beach Boys from 1962.

Dean
Shes Real Fine

Thanks again for the replies. With this Kohler and it's 6.5/1 compression ratio, is detonation going to be a problem with 87 octane fuel? Is it possible to advance the timing a little more than the 16 degrees in the specs?

No.

You may be able to advance the ignition timing beyond the factory spec, but why would you want to do so?

Dean

Big CI, big ports, big valves, very high CR, lots of carburetion, and huge cam.

Old tech but ground pounding SOBs. They would make HP like a small block and would run you down.

BTDT.

Dean

Gotta love the beach boys. RB

IF I want to do WORK I go to my 16 HP flathead Kohler not my 21HP OHV B&S !!

They're torquey, Sarge.

Dean