6.0" SBC rods in a 250
- Thread starter mike1157
- Start date
xrwagon
Well-known member
I have myself thought about rod changes in my 200 mega build, but the cost i think outweighs the result, unless your using forced induction or going to pill it. I have considered myself getting the 2.3 pistons as you can get decent pistons, i have read about a combo on an alloy headed 200, its on youtube, a dyno pull anyway, he states diff rods and pistons yet the thing only made just over 200 on a engine dyno, seems like a whole lotta expense for just over 200 hp
8) for a 250, if you use the chevy 6" rods, you dont really need a custom piston, except for the pin diameter, and that difference isnt a problem, as i recall the chevy pin is larger than the ford pin, so you just need to open up the pin hole in the piston.as you noted the crank journals need to be cut down a bit, again no issue for a machine shop, especially if you are having the cranl ground anyway. the other nice thing is it gives you a slightly better rod length/stroke ratio.
Becasue the US log is difference to deck hight by about 100 thou to the Aussie 250, it has an apalling 100thou piston short fall. So adding six inch rods also gives a compression ratio improvement with much better detonation resistance which will make more than just a 1% difference in peak power. So it is a postive step. Ford Australia changed to long rods and shallow pistons in late 1997 in the last EL Falcon, and it was to improve smoothness, peak rpm range, and reduce side thrust in an end to make power. It was a part factor in changing from 5000 red line sixes to 5750 and 6000 rpm red line engines that the later twin cam heads made. Underneath, the early 4.0 ohc's were just Aussie 250's, so they relate to you as the factory 4.0 rod is now 6.06". I'd use tht before I'd usethe Chevy 6 inch rod, as its lighter, specificly fits the 23.1 mm Ford wrist pin and piston.
Don't limit yourself to just 6 inches, though, as there are more and more low deck pistons around which will allow a 5% power increase. If there was just a 1% power gain from a set of 6" rods alone, that isn't going really going to be noticable. Factory Ford Australia and factory Ford Australia aftermarket pistons exist to get 1.163" of piston to wrist pin height, down from the massive 1.629" clearance distance on the US 250, so your rod on that particular engine can be 466 thou taller, or 6.36" without even having to add a boat style thick copper head gasket. An 8% L/R ratio increase makes a 4% power increase.
In the range from 1.4 to 2.2 rod lengths L to stoke throw R , the usefull horsepower gain is half the percentage difference in L/R ratio. or the full amount of what mechanical engineers call the lamda difference. As the L/R ratio goes up, less side thrust, as it goes down, side thrust goes up. As engine speed increases, the gain in power through reduced side thrust is uniform. Torque increase is small and varies, but its always an increase.
On this board, the Atlantic TRD Toyota 1600 cc twin cam 16 vlave engines, when the rod ratio was altered, plotted to this above stricture at 9000rpm. So did David Vizards 1549 verses 1435 Mini A-series OHV engines at 5500 to 6500rpm. In the case of Vizards, specific dyno calculations determined frictional hp loss, and the rod ratio resulted in a frictional hp loss or gain, proportional to HALF the algebraic difference in L/R ratio.
So if you take a 1.505:1 rod ratio 250 with 5.885" rods and 3.91 inch stroke, and then just add a set of 6" rods, the L/R (rod ratio) improves from 1.535, an algeraic difference of 2%. The hp gain is then uniformaly 1% throughout the rev range.
Other things happen, the residence time changes, the load casuing the rings to spark and create detonation changes, the firctional hp drops and the ideal cam timing at maximum piston speed changes, but thats the gain.
On a 250, you can really use a longer Australian 1971 to 1993 long 3.3 Log or X-flow or 3.2 OHC rod, Ford Corsair/Nissan Z24 or the Big Six 240/300, 2.0 or 2.6 liter Silent Shaft Astron 80 or Ford SOHC/DOHC modular 5.4 or 5.8 conrod, and get a 5% power gain from a 10% L/R ratio difference. The results just get better as the L/R improvement goes up.
On a little low deck US 200, there isn't space to fit anything other than a 1.163" inch piston, so your rod length is only able to be 368 thou, making a 302 rod just about the longest you can get, and thats a 4% power increase, tops.
Don't limit yourself to just 6 inches, though, as there are more and more low deck pistons around which will allow a 5% power increase. If there was just a 1% power gain from a set of 6" rods alone, that isn't going really going to be noticable. Factory Ford Australia and factory Ford Australia aftermarket pistons exist to get 1.163" of piston to wrist pin height, down from the massive 1.629" clearance distance on the US 250, so your rod on that particular engine can be 466 thou taller, or 6.36" without even having to add a boat style thick copper head gasket. An 8% L/R ratio increase makes a 4% power increase.
In the range from 1.4 to 2.2 rod lengths L to stoke throw R , the usefull horsepower gain is half the percentage difference in L/R ratio. or the full amount of what mechanical engineers call the lamda difference. As the L/R ratio goes up, less side thrust, as it goes down, side thrust goes up. As engine speed increases, the gain in power through reduced side thrust is uniform. Torque increase is small and varies, but its always an increase.
On this board, the Atlantic TRD Toyota 1600 cc twin cam 16 vlave engines, when the rod ratio was altered, plotted to this above stricture at 9000rpm. So did David Vizards 1549 verses 1435 Mini A-series OHV engines at 5500 to 6500rpm. In the case of Vizards, specific dyno calculations determined frictional hp loss, and the rod ratio resulted in a frictional hp loss or gain, proportional to HALF the algebraic difference in L/R ratio.
So if you take a 1.505:1 rod ratio 250 with 5.885" rods and 3.91 inch stroke, and then just add a set of 6" rods, the L/R (rod ratio) improves from 1.535, an algeraic difference of 2%. The hp gain is then uniformaly 1% throughout the rev range.
Other things happen, the residence time changes, the load casuing the rings to spark and create detonation changes, the firctional hp drops and the ideal cam timing at maximum piston speed changes, but thats the gain.
On a 250, you can really use a longer Australian 1971 to 1993 long 3.3 Log or X-flow or 3.2 OHC rod, Ford Corsair/Nissan Z24 or the Big Six 240/300, 2.0 or 2.6 liter Silent Shaft Astron 80 or Ford SOHC/DOHC modular 5.4 or 5.8 conrod, and get a 5% power gain from a 10% L/R ratio difference. The results just get better as the L/R improvement goes up.
On a little low deck US 200, there isn't space to fit anything other than a 1.163" inch piston, so your rod length is only able to be 368 thou, making a 302 rod just about the longest you can get, and thats a 4% power increase, tops.
8) excellent information X, the nice thing about using the chevy 6" rod though is cost. you can pick them up fairly cheaply, and use a stock 200/250 piston. i also would prefer a longer rod, but custom pistons rather prevent that as it doesmt make economic sense in our sixes, unless you are building something for the track.
This will be a big torque, big power, turbo build.
As mentioned by R bohm, the use of a SBC 6.0 rod completely satisfies the need for a superior forged I beam rod w/ a 3/8" rod bolt/capscrew fastener as a perk, at a price starting at 30.00 ea.
While all this would then be a no brainer for me,..the rod width at the big end becomes a problem. (I think)
Stock 250 rod is around .985 wide at the big end,....SBC rod is .940
I have never "mixed, and matched" engine parts before, and I know just enough to ask,....but I thought that the resulting .090 side gap once the two rods are bolted on the journal would be a bad thing,....and therefore mean that I cannot use them?
:
As mentioned by R bohm, the use of a SBC 6.0 rod completely satisfies the need for a superior forged I beam rod w/ a 3/8" rod bolt/capscrew fastener as a perk, at a price starting at 30.00 ea.
While all this would then be a no brainer for me,..the rod width at the big end becomes a problem. (I think)
Stock 250 rod is around .985 wide at the big end,....SBC rod is .940
I have never "mixed, and matched" engine parts before, and I know just enough to ask,....but I thought that the resulting .090 side gap once the two rods are bolted on the journal would be a bad thing,....and therefore mean that I cannot use them?
:
8) the ford small six does not share rod journals between cylinders, each is on its own journal, thus the side clearance is increased by.045". that can be corrected by shop that machines your crank by welding the crank and grinding the journal sides to give proper clearance between the rod and the crank sides with the chevy rod. just remember to have the install a generous fillet radius in the rod journals to prevent any possibility of cracking.
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