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My guess is that they are running a crazy low compression ratio. Probably something close to what the old Mitsubishi 4G63's ran in the early 90's. They ran a 8.0:1 from 89 - 94 and about a 8.5:1 from 95-99. With a small turbo and 19 or so psi they would put out similar numbers.
The reason we get such high HP numbers with low boost levels is because our compression ratio is already high. Adding a turbo or s/c effectively raises the compression ratio. The reason you do not want to run high compression with forced induction is pre-detonation. The lower the compression ratio the more air you can pump in without igniting the gas from hot spots on the piston. The reason some people here can get away with it without blowing their engine to smitherines is because (and I hope) they use good gasoline.
Factories have issues putting boost onto high compression engines because they cannot control what kind of gas you put into your engine. For a good safety margin they can put in a computer controlled boost controller and low compression pistons. Then if a consumer puts in cheap gas they can retard the timing and cut boost to keep the engine from crapping out. Thus you end up with 21psi, low compression, decent on gas, and relatively safe engines.
I don't know what the actual compression ratio is but my guess is around the 8.0:1. It may be a place to start though.
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