It's because of the already "softened" chamber<<< I dislike that terminology--It's scalloped IMO. The HEMI has a factory scalloped chamber that widens the tuning window significantly under power adder applications--Basically the HEMI chamber is a slow burning chamber which is why when you add a second plug--things change so much. Hate to say it but a single plug wide open 170cc chamber is inefficient. J.Rob