I race the car in IHRA Stock where they have a list of factory available crate engines that can be substituted for OEM combos. The crates have their own blueprint specs and rules, similar to "regular" Stock class cars in IHRA and NHRA. Open headers are allowed, but in talking to TTI and doing my own research, it appeared that I "might" be able to pick up a little horsepower with an X pipe. I'm always looking for ways to go faster within the strict rules.
I couldn't do a same-day back-to-back comparison, but have kept detailed records over the years. Every indication was that the car picked up between .05 and .10 in the 1/4 mile in my particular combination. My theory is that the X-pipe helped and the DynoMax UltraFlo mufflers didn't hurt. It worked best with a jet number leaner all around in the carb and the extra 60+ lbs that the pipes, mufflers and hangers added was removed from elsewhere in the car to keep the before/after weight the same. Your results may vary. You would have to experiment with your particular combo and it would depend how close and on what side of optimum jetting you would start out at.
If I wasn't convinced of the benefit, I wouldn't have kept it on the car. I have to drop the system to change the trans and it's pretty cramped when removing the center section. I also plan on having the same exhaust on my next project, which is an Aspen wagon with an OEM spec 360, so I can race it in both, NHRA and IHRA Stock. But I will be using DynoMax Race Bullet mufflers for more room around the center section. I'm anxious as heck to get it going, but time and $$$ have been limiting factors.
As a side note, I kinda got use to the quieter exhaust & like it. It actually scared me the first few times I went down the track because, despite still being loud, I heard things I never heard before! But that can be a good thing too in case a problem does develop. The Bullet mufflers will likely be a bit louder than the UltraFlos. But both are straight-through designs and I don't expect any difference in flow or performance. Also, the X-pipe ended up around the driveshaft loop and I had to "ding" it for clearance.
