Originally Posted by barrylnv
What he said, but not as much as 30 if you keep all variables the same. I installed DT shorties, Hennessey CAI, and Borla S-type cat-back. Prior to install, filled it up with 93 octane (we only have 91 readily available out here), did baseline pulls, got about 360 RWHP corrected. It was eddy-current type dyno, not DynoJet, hence the lower numbers. Installed shorties, CAI, cat-back and drove for about 300 miles on 93 octane, did battery pull, and re-dyno'd. Got 378 RWHP. Did 4 pulls, all were within 2-3 HP, 378 was the average.
In talking with several builders and doing lots of reading, seems you can realistically expect 8-10 HP with shorties, 5-8 HP with CAI, and 0-4 HP with cat-back (if that, usually nothing). You can also gain 10-12 with just a tune and changing no bolt-ons. My thoughts are that the 18-22 that manufacturers claim with shorties are about an equal split between the shorties themselves and a tune. They just don't tell you about the tune. Or, and I've seen this before, they mess with correction factors, smoothing, etc. and you can always sway things quite a bit that way.
Long story short, I've talked to a lot of people about this since I was greatly curious when buying my shorties back 7 months ago, but there is no way you're getting 18+ RWHP with just shorties and nothing else. With CAI, possibly. With tune, possible. With both, yeah, but probably no more than 22 or so total. You can't just add all the numbers. Doesn't work that way.
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