The idea that the head is the limiter to the rev range would be correct except for one thing. The cam can add enough lift to compensate, and even if there was, say, only 140 cfm flow at 28 inches of water, and 400 thou valve lift through the intake, you could add almost proportional increases to that by raising lift to a greater level. Considering the cost of internal mods, it's better to consider all full-house cam with moderate duration and high lift before getting into the basement and throwing your monkey wrench around! :smash:
Back in Australia in the late 1960's an engineer at Ford Oz did considerable develpment work on the new 250 Ford motors. Unlike the US 250, the Oz one carried most of the block set out from the smaller 200, while your US 250 gained stronger pan rail, better oil pump, new timing gears and water pump. Any way, the guy found that even with a turbo, triple carbs, four barrels or fuel injection, the old lump would flat line at around 4800 rpm.
Camming up the motor would raise power, but not raise the rev range significantly above that 4800 prm point. The engineer concluded, in an Aussie Street Machine magazine in 1990, that the reason was the crappy L;R ratio which mated a 5.885 inch rod to a 3.91 inch stroke. To make the engine rev longer and sronger, a signifcantly longer rod with shallower deck piston was needed. The L:R ratio with both US and Oz 250/4.1 and even latter Oz 3.9/4.0 SOHC sixes has always stayed at 1.505 :1, and Ford has focused its attention to raising torque and optimising gearing, not power.
The solution to the problem is an expensive rod and piston change. Ford Australia did this in 1999, 19 years after the advent of the 250, with 6 inch rods in there 4.0 AU Ford Six, which had OHC, but the same bore spacings and block dimensions as the US 250. This only made a minor improvement, to a 1.53:1 ratio. Retro -fitting these to the 250's is not a common swap. In Australia, anyone idealistic and rich enough simply use shot blasted Oz 3.3 con-rods with 6.275 inch centre spacings, and use either Omega pistons for stroker 4.6 Rover V8's(3.7 or 3.725 inch) or Chev 305 style pistons (56 thou over) with 1.25 inch compression height. These increase rev range with a small loss in efective compression.
There is always a power increase with these 3.3 items as the frictional hp losses due to side loading the block is far less with these rods. The L:R ratio is 1.61:1 with these rods. In the States, the 6.244 inch 300 Ford rod, narrowed to suit the 250 crank pin, would do the same thing, but add signifcant strength. The information on frictional hp loses was covered by Englishman David Vizard in a book on Modified Minis. In that book ther was an article on long stroke Minis which, he found, never produced proportional power increases as the stroke was increased unless a longer rod was included. The guy also had similar misgivings on the 5 inch rod used on 2000 OHC Pinto engines, even though its L:R ratio here was 1.65:1. The fact that Ford Australia spent bulk dollars on adding only slightly longer rods to slightly improve the L:R ratio confirms that this is where the secrete to raising the rev range is. The AU Falcons have a red line of 5900rpm, while the earlier 250 roded engines had 4500 to 5000 rpm rev limits. Even DOHC 258 cube 4.2 Jag XK motors have 7 inch long rods to go with that massive 4.17 inch stroke. The L:R ratio for that is 1.67:1!
Back in Australia in the late 1960's an engineer at Ford Oz did considerable develpment work on the new 250 Ford motors. Unlike the US 250, the Oz one carried most of the block set out from the smaller 200, while your US 250 gained stronger pan rail, better oil pump, new timing gears and water pump. Any way, the guy found that even with a turbo, triple carbs, four barrels or fuel injection, the old lump would flat line at around 4800 rpm.
Camming up the motor would raise power, but not raise the rev range significantly above that 4800 prm point. The engineer concluded, in an Aussie Street Machine magazine in 1990, that the reason was the crappy L;R ratio which mated a 5.885 inch rod to a 3.91 inch stroke. To make the engine rev longer and sronger, a signifcantly longer rod with shallower deck piston was needed. The L:R ratio with both US and Oz 250/4.1 and even latter Oz 3.9/4.0 SOHC sixes has always stayed at 1.505 :1, and Ford has focused its attention to raising torque and optimising gearing, not power.
The solution to the problem is an expensive rod and piston change. Ford Australia did this in 1999, 19 years after the advent of the 250, with 6 inch rods in there 4.0 AU Ford Six, which had OHC, but the same bore spacings and block dimensions as the US 250. This only made a minor improvement, to a 1.53:1 ratio. Retro -fitting these to the 250's is not a common swap. In Australia, anyone idealistic and rich enough simply use shot blasted Oz 3.3 con-rods with 6.275 inch centre spacings, and use either Omega pistons for stroker 4.6 Rover V8's(3.7 or 3.725 inch) or Chev 305 style pistons (56 thou over) with 1.25 inch compression height. These increase rev range with a small loss in efective compression.
There is always a power increase with these 3.3 items as the frictional hp losses due to side loading the block is far less with these rods. The L:R ratio is 1.61:1 with these rods. In the States, the 6.244 inch 300 Ford rod, narrowed to suit the 250 crank pin, would do the same thing, but add signifcant strength. The information on frictional hp loses was covered by Englishman David Vizard in a book on Modified Minis. In that book ther was an article on long stroke Minis which, he found, never produced proportional power increases as the stroke was increased unless a longer rod was included. The guy also had similar misgivings on the 5 inch rod used on 2000 OHC Pinto engines, even though its L:R ratio here was 1.65:1. The fact that Ford Australia spent bulk dollars on adding only slightly longer rods to slightly improve the L:R ratio confirms that this is where the secrete to raising the rev range is. The AU Falcons have a red line of 5900rpm, while the earlier 250 roded engines had 4500 to 5000 rpm rev limits. Even DOHC 258 cube 4.2 Jag XK motors have 7 inch long rods to go with that massive 4.17 inch stroke. The L:R ratio for that is 1.67:1!
