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He doesn’t want to use an 8.1L, that is just my example engine I mentioned, he wants to custom build a 6.3L by stroking a 5.7L.
If you knew the basic torque curve of your old motor, your rpms at your old crusing speeds, and roughly how much throttle you were giving it (e.g. ¼ throttle) to maintain speed you could probably make an educated guess as to the HP. HP=Torque*RPM/5252=[(Max torque @ RPM)*(% Throttle)]*RPM/5252. For example, if you were going 80 at 2500 RPM using 25% throttle and look up that you motor has 300 ft-lbs torque @ 2500RPM, you could estimate HP=[(300)*(25%)]*2500/5252=35.7 HP.
To be clear, ecky’s chart is just an example. Every engine would have a different chart. Most people do not have BSFC charts for their engines, they are fairly rare. Ideally you would be in the lowest region of the chart, which for this example is in the “eye” at 2000-2500 RPM and 120-140 ft-lbs. Absent this information, you end up having to make educated guesses such as lower RPM and over 50% throttle is probably where you want to be.
To custom build an engine such as this, since you can’t find a build to copy, you would probably need to work with an experienced builder or purchase simulation software. Guys I know that raced always modeled engines on software that would spit out torque curves based off of all the components/specs.
Ecky’s idea of building it and then dialing in the gears would work as well.
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