legacycentral bbs

Depends on the grind. Airflow characteristics at low rpm are different than those at high rpm and require suitable valve timing/lift to match. Without head work to increase high speed air flow your heads are going to be a restriction no matter what, so I'd recommend something for more low end torque (at the expense of high end horsepower which those stock heads aren't designed for anyway) and maximize the engine's good characteristics rather than hobble it at the expense of a marginal increase in a weak area. Low end torque will be better for street driveability. What those approximate gains would be would be dependent on the aggressiveness of the grind for the particular rpm range you seek to use them in. But the more you help one end, the more you'll likely hurt the other. Everything's a compromise.

like said, it all depends on how mild or agressive you get it
you might get like 20hp from a moderate cam, dont know about torque
you should get a little longer duration so it will sound like a V8 at idle
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a little FYI
if your getting your cam reground, you need to put in new springs as well, ask what they recomend because it will vary with the cam profile

The lumping idle comes from increased overlap which is better for high rpm air flow.

so can you put the 2.5RS heads on the EJ22 block or is it just the EJ22T?
if you could put those heads on, then you could just get some COBB stage heads, might be a bit more expensive but your getting the whole thing redone, not just cams