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Agree with AC. There's no magic number. Depending on RPM, # of cylinders, same displacement engines will have different HP. Some may have more torque and less HP (compare a Deere to say a Farmall). At risk of opening up the post to EVERYONE's opinion, my advice would be to lay out what you have stock -- bore, stroke, displacement, cylinders, type of piston (Cast iron or aluminum), compression ratio in a spreadsheet. 1. If you go from iron to aluminum pistons you will gain some umph (some will disagree, and that's fine) 2. Changing displacement without changing anything else will increase HP about in direct relationship (not quite, but close enough) 3. Changing compression ratio - by the math it is supposed to be a direct increase - for example if you go from 4:1 to 8:1, it should double. Just doesn't work that way. Someone posted a good chart a while back that showed the % gains at each CR. If you can't find that chart, you could probably estimate that it is about 50-60% effective. 4. Changing stroke seems to do more than just increase displacement - There seems to be a bonus factor (10% or something, not on total HP, just on the increase). IE if you go from 4" to 5" stoke, that is a 25% increase in stroke and cubic inches - tractor pulling is all about torque (except maybe for you Div V guys) and the bigger lever arms seems to help with that. The increase in stroke was 25% and 10% of that is 2.5% -- add that on. 5. Fuel - for the most part racing gas doesn't have any significant amount of extra energy in it. It just lets you run the higher octane. The problem is, since we are all pushing these things real hard and making all the parts smaller than factory and in most cases making them move faster, we don't want them to detonate, so we default to the high octane gas - detonation will KILL these engines. 6. Breathing (volumetric efficiency) - Usually lots of room for changes here. Make sure you know what you are doing though. Too much air flow and you will lose your bottom end. To little and you won't get top end. Too smooth on the intake and you won't get mixing. Too rough/eratic on the intake and you won't get flow. Find someone a LOT smarter than I am for that. Bigger is not always better. Sorry to ramble on...
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