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Understand maximum effective compression. This is when you have almost no manifold vacuum. It means your engine is creating very close to the mechanical potential compression. Maximum effective compression is the main reason why you have best BSFC when the difference between the compression created and the pressure increase when the fuel is ignited you have the best "leverage" and creation of power. Maximum effective compression can occur at very low throttle positions, which a vacuum gauge will confirm. When you combine best compression and lower engine speeds, you have loaded the engine so it produces the best power for the least fuel.
The Prius runs close to 13.5 to 1 compression ratios, so that level of EGR would be possibly an essential component of that compression level. The same situation will not apply to any engine that has significantly lower compression. The reason why the early 70s engines were so gutless was they dropped the compression ratios and added EGR. When people tried to disconnect the EGR pre ignition was the consequence.
When oxygen sensors appeared and computer controls made engine management much more precise, compression ratios rose back to pre emission premium fuel levels. This happened beginning in the early 1980s. Nissan bumped the compression in the Z Car from 160 gauge to 190 gauge and power levels climbed.
My Echo runs 10.5 to 1, same engine as the Prius (up to 09). I doubt that you would see that percentage of EGR on the same engine with 10.5 to 1. Not sure if the Echo uses the Atkinson cycle. Atkinson developed his cycle to try to get around Otto's patents. (think that is right might be Benz).
regards
Mech
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