Hey guys, I read somewhere that the radial engine has rpm limitations, I dont see why since the site later described it as an easily balanced engine and the crank is small, the only weight is in mainly the master rod and articulating rods and pistons. It is also a lightweight engine in comparison to most other inline types but that may be because most radial engines are air cooled. Really I just want to know if there are rpm limitations due to the design of it that I may not be aware of or if that was just because most radial engines were from like WWII aircraft that dont need or want high rpms to turn the props. Also anyone heard about a car called the Forzano Fulcrum, its some sort of supercar that has a radial engine mated to a TF-727 3 speed auto but thats about all the info ive been able to find out about it...

 

Most aircraft radial engines are enormous. Typically 6-7" bore, 7-8" stroke, times 14 to 18 cylinders, and you have one BIG motor. A single cylinder for a Wright 1820 displaces over 200 cu. in. ! The parts are also robust. They have to be to survive the boost at low altitudes. All this heavy stuff thrashing around really limits RPM, which is OK for aircraft since they don't want to operate at high RPM for durability reasons.

 

So say a smaller one of a normal engine displacement say 5 liters probably would not have any trouble spinning as fast as any other type of engine correct?

 

Radial engines have severe packaging problems in cars, but were ideal for mounting on the wing of an airplane. Sure you could build a radial or maybe an double row X, but if it's a 4-stroke, I see more negatives than positives like high center of gravity, complex intake and exhaust for any sort of tuning, lots of heads, jugs, etc. Air cooling would be a bitch, and liquid cooled would make it heavier than a V8 of equivalent size.

Propellor tip velocity defines the best operating range for aircraft; the prop tips have to stay subsonic (.88 to .92 mach), which means a 12 ft prop gets up to about 1600 rpm. A small private aircraft with a 6 ft. prop would be about 3200 rpm, with the hp of a car engine. That assumes direct drive of the prop. There have been some higher speed geared aircraft engines, but they have been opposed (boxer) types. There have also been car engine conversions, including aluminum big block Chevys which were also geared.

I really don't remember seeing high speed radials other than maybe model airplane engines, and they run at a fraction of the rpm of 2-stroke singles with similar displacement per cylinder. That master/slave rod arrangement might cause some major strength problems at high rpm and high hp/cubic inch levels. High rpm (5000+) on a 5L radial would probably not be practical, IMO.

40 years ago one of the GM car divisions experimented with a 4-cyl X two-stroke, and also a gas turbine engine in a car. Neither was practical nor economical at the time of $.30/ gallon gas and no emission rules. Today would not be easier.

Early (WWI) radials sometimes had fixed cranks with the prop fastened to the block and the cylinders, etc. rotating. It helped the air cooling, I guess. Probably ran pretty slow!

Consider me the wet blanket.