Originally Posted by
Force Fed Mopar
I've only ever run swirl heads, and stock ones at that with stock Garretts. But, I've had good luck with them so far, and have helped a couple others tune theirs with good results.
I've discussed it with 5DIGITS at length before, and my consensus is that when you start getting higher than 14 psi on a FB head, the swirl it induces is so fast that it starts becoming counterproductive. You end up having to pull more and more timing. Whether it's because the efficiency goes up and needs less timing, or the sharp edges of the chamber cause knock, or the heat of the charge goes up, or whatever, I'm not sure. I know that at around 20psi and up on my best tune, the timing drops to 1 degree total timing at spool-up and ramps back up pretty quickly after 4500, which is where the pumping efficiency starts to drop off. I have run up to 25psi on a stock FB with a stock Garrett and a awic system, but that is pretty much the useful limit of pump 93 octane IMO. I ended up going just as fast at 20psi as it did at 25 with some tuning, so 20-22 is pretty much where I'd stop on that combo.
Also, mileage was about 26-28mpg in normal country/town driving, and 34-35mpg on the interstate on a 2.5 in the Lebaron.