flathead w/ forced induction?

[Deleted User]

I'm not too familiar with flathead motors, so forgive my ignorance. I know the valves are in the block and the head in essentially a cap for the cylinders. Every flathead I've heard of has a low compression ratio. Is that due to the fuels available at the time or is it the flathead design? What about using custom pistons to take up more space in the cylinder? I'm ASSuming that the pistons never reach the level of the valves.



If CR is left stock, how well do flathead engines respond to forced induction (turbo/supercharger)? Can they tolerate similar boost levels as OHV/OHC motors if built properly? I'm still trying to figure out what I'd like to do with the Jet someday.

Geoff

You are pretty much limited by the combustion chamber space as far as compression ratio is concerned. You can shave off the head to raise the ratio, within the limits of valve lift - if you take too much off the valves will hit the spark plugs! Within certain parameters a side valve is quite efficient due to the turbulence generated. However, tuning is restricted to cam modifiications, porting and polishing, manifolding and carburetion rather than increases to compression ratio. You can't fit higher top pistons because the head is usually flat above the piston, and the only space you have in there is the thickness of the head gasket. Going way back - the first "Power dome" head was in the '29 Essex, which increased the compression ratio by flattening off the area above the piston, and introducing a "Waisted" area between the valves and the cylinder, plus a recess above this area to increase turbulence. Along with an extra 7 cubic inches, improved inlet porting and carburettion, and higher valve lift, they increased horespower from 44 to 60. And the developments just kept on happening from there, in the Terraplanes and Hudsons, an culminating in the Hornet of course. Look at a '36 and a '37 Terraplane engine, the only difference is in the porting and manifolding, but they gained an extra 12 h.p.. Railton used Hudson 6 and 8 engines in their cars, and increased the horsepower of the 6 from 101 to 125 by fitting a supercharger. The standard Jet motor is 104 b.h.p @ 4000 r.p.m., and with improvements such as twin carburettors and alloy head is boosted to 114. I'm sure with another head shave, modified cam, and headers, this could be tweaked to around 130 or more. However, you cannot bore Jet oversize more than .040, due to the very small distance between the cylinders, unlike the Hornet which had lots of meat there. Bore a Jet out more than that and you have severe distortion and head gasket problems.

Geoff.