Quote:
Originally Posted by Joshinator99
A cam helps the engine become more efficient at moving air. So, per Kings post, moving more air after the compressor lowers your boost number. It’s a good thing!
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Bingo bango to these guys, of course. Another way to look at it / more detail:
A belt-driven root-style FI device is spinning at a fixed RPM at a ratio to engine RPM. It's trying to push air at a fixed volume per revolution into and through the engine. A cam and ported heads and better flowing exhaust allow the engine to move that air through the engine without it getting backed up. When the engine is less backed up, it has less "boost."
On the other side, as you increase intake diameters, filter sizes, TB sizes, and blower ratio (through pullies), you can push more air into the heads against the intake valve. If it's more than the engine can ingest by itself based on RPM, valve lift and valve open duration, then you get a backup, measured as "boost."
As you feed an engine more boost, it has more pressure pushing against the intake valve waiting to get let in, and then it can fill the cylinder with more air under boost pressure in the time that's available while the valve is open. Then it's just a matter of balancing cylinder pressure with how much octane you have available.
So, as you get the engine flowing better on the downstream side (heads/cam/exhaust) you likely want to get it flowing more on the upstream side (pullies, blower upgrades, intake stuff) to achieve an acceptable amount of boost to fill the cylinder to max capacity again, while balancing cylinder pressures against available octane.