Have yourself a chat with an engine builder, and take this with him.
Basically, programs like Engine Anylsier et al will work it out if you have the head flow figures acurately defined. What follows is from my raided supply of information from the 1982 Grand Prix and 1987 SVO Falcon publicity related info, and words from its developer Dick Johnston, Mick Web, Kevin Bartlett, and other people external to the project such as ex Repco and Ford SVE engineer Bill Santocicione. Information was published from 1982 to 1990 in Street Machine, and it's specialist offshoots, Hot Sixes and Hot 302/351's. They provide benchmarks from people who have raced and blown up Ford engines for a living.
First off, you'll see Nothing. The XF head is a high swirl head designed to be electronically ignition managed to run at 8.7:1. It'll detonate like a closed chamber GT 351 on 91 octane even on 95 octane fuel.
Any canted valve Ford engine is detonation limited, and won't give you the power you seek unless you use a racing fuel with 10.7:1 C/R. All you'll suceed in doing is holeing a piston.
If you'd like to reduce the compression to 9.5:1, and run 98 octane with an additive, you might get the close to the 206 hp that is possible with a stock XF head with 490 thou lift. Your ignition needs to be tuned for mean best torque, and the total avance held back to ensure it never detonates.
Second issue is the intake. I'm not knocking the local Redline product, it serves the purpose for getting 350, 500 or 650 cfm of Holley 2-bbl on the 250. It's priced well and is a huge step up on the stock Weber ADM set-up, but its flow numbers and cylinder to cylinder efficiency are not as good as the Ultraflow. Hence an extensively reworked engine won't see 500 cfm of Holley 4412 if its lost through ports which are robbed of flow compared to a more modern, flow bench developed intake.
Thirdly. The XF head has very small intake ports, shroaded and restricted valve sizes, and you going to flat-line at about 205 hp unless you spend 2000 dollars gas-flowing the head and then going for a better quality Ultraflow 2-bbl intake and a modified Barry Grant or Demon 2-bbl with 1.75" throttles and 1.4375" venturis. The specific power on a big canted valve Ford (V8 302 or 351) with big ports and a 280 degree cam is 50 to 57 hp per litre, with perfectly flowing carb and intake giving the higher figure. This is about 205 to 235 hp on a 4.1 Liter I6. The power increases with after market Chevy Big Block style 1.8:1 rockers over the 1.73 :1 items too, because the heads cfm rate drops of above 430 thou, and there is no proportional increase in cfm, so you have to lift the valve really high to get extra power. The costs of increasing the cfm at 430 lift onwards is very high if you do it by increasing the port sizes...there is no extra metal to highe port the heads and create a preferabel D or oval port, about 35 mm is it and then you start to hit oil in the rocker cover. Thats way good modifyiers of these heads weld them up, and savagely elogate the intkae ports to get some good high lift cfm's happening. Going to more aggressive rocker helps a lot, gas flowing helps to an extent, increasing port sizes is mega expensive!
Dick Johnston in the extensive work done by the Brisbane Engine centre from 1983 to 1987 on there proposed SVO Falcon said 196 hp with 280 degree cam, bigger carb, better intake was what you could expect before.
Forthly, the exhast is wrong. Copy the stock EB XR6 item, and stop with this 2.5" drain pipe stuff the indusry forces on the unthinking I6 boys. Anyone who has driven one would be sick of I6's in half a days driving, they drone, they loose torque. Dick Johnston mentioned that a conventional exhast must provide low end torque without restriction to wide open throttle operation in an old Street Machine 1984 magazine. Copy the 1992 Tickford exhast, which is bascially an XR8 set-up modified to tee into the XR6 exhast, and just add a good aftermarket extractor/header.
Fifth. There are no apples meets apples comparisons with rod ratios except from Engine Anaylser, and internal papers on Mopar, Chevy Ford and Mini engines. The following results are determined from a 1988 Mini A-series article, a Popular Hot Rodding Mopar LA article, and a special Ford roded Chev 350 engine article. Indications are that the 6.27" rod only liberates 3% more top end power in this range of L/R. An improvement in L/R ratio from 1.5:1 to 1.61:1 is equal to 6% improvement in ratio, but less than half the percentage increase in power. You'll be very lucky to see 6 hp more.
It is valid only when a 1.5 ratio is upscaled to 1.61, and its not valid for a 302 Z28 with 6.0 inch rods verses stock 5.7" rods (L/R goes form 1.9 to 2.0, and sees no increase)
