Peter, this is easy. Surely NRE did an engine dyno. So just strap your ride down to a chassis dyno and let us know the result
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Seriously though I've put some thought into this lately as well. Primarily because I can't figure out why the LS setups put down so much rwhp compared to some pretty strong big block combos. I would generally think flywheel hp was flywheel hp regardless of it being LS, BBC or even a Ferd FE
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However, I then read a few examples where some solid 600+hp BBC builds were only getting high 400/low 500 rwhp numbers. That would support the closer to 20% drivetrain loss figure.
On the other hand, it seems fairly common/easy to get 500 rwhp out of an LS3. I know they are efficient and all but geez. Similar rwhp and 100 less cubes???? I also can't see a basic LS3 putting out 600 flywheel HP to start with.
That just doesn't seem to add up so I gotta think drivetrain differences maybe. Most certainly any BBC rwhp number you'll find is likely through an auto trans so that may explain some of it.