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2012 Mustang Boss 302 - Heartthrob Flowpack Axle Back Exhaust




A few weeks ago, I started the car up and realized that I forgot something so I ran back in to grab it and when I walked out, I noticed that there's a lot more vapour pouring out of the side pipes than the rears (as the car was warming up). I read many articles that said Ford designed the side pipes for noise and they flow very little compared to the rears. It certainly seemed like there's more flow through the sides than the rear (although I do have the side baffles completely removed). After that, I decided that I wanted an axle back exhaust. I'm obviously not expecting any power gains (nothing more than a couple hp anyway) but I was ready to turn up the decibels!

I decided on the Heartthrob Flowpack kit (the picture above is of a Mustang GT from Heartthrob's website, not mine) and bought it from American Muscle. I was very pleased with shipping. It shipped on the same day despite ordering in the afternoon and it only took one week to deliver despite being in Canada. I wasn't too pleased with the packaging though and its condition. I have ordered my fair share of aftermarket parts and most parts are packaged to not allow movement inside the box and provide support for the shipping box to keep it rigid. The box was damaged; sort of bulging where the ends of the exhaust pipes where and one of the tips was very, very slightly bent. The deflection was less than 2 mm (around 1/16") and can't be seen unless you are under the car. The quality of the packaging is probably Heartthrob's fault but for $279.99, it was just about the cheapest available with "mufflers." The condition of packaging though is either American Muscle's fault or UPS. There is a chance it could have come that way from Heartthrob but American Muscle would still be partially to blame because they shouldn't have let it ship that way if that were the case. Anyway, it didn't bother me that much so I'll move to the important bit: what they're like.


Pros:
  • The noise is phenomenal. It adds a nice subtle rumble at idle that is very nearly as quiet as stock at idle and slightly louder under light load. It makes the exhaust note immensely deep and much louder under load just off idle, though, and all the way to redline. It is intoxicating. After installing it, I tried to find excuses to go out for a drive.
  • The noise out of the side exhausts is barely changed. I was worried the car would sound better overall but the noise would sound a bit distant while driving since more flow would come out the back and less out the sides compared to stock. That wasn't the case.
  • It is lighter than stock. Much lighter. According to my bathroom scale, it weighs 22 lb. less. While that isn't much on its own, it adds up quickly if you have plans to put the car on a diet.


Cons:
  • It isn't the nicest looking. If looks are a priority over noise and budget, you're better off buying a different one. It won't surprise you, though. It looks like it does on the website, not better or worse. You can go to any online store that carries it and compare it to nicer looking ones and see if you are satisfied with the looks. I will post pictures in another post as well.
  • It is made of aluminized steel as opposed to stainless.
  • Drone. There is a range of about 200 rpms between the 1,750 and 2,250 rpm markers on the tachometer where there is drone. The bad news is: it is bad. The good news is: it is a very narrow range that is very easy to avoid. It is really nice and quiet above and below that range in any condition. It is even very quiet in that range on the highway on flat, straight roads but if you are in 6th gear and you want to accelerate in that range or the road changes and you start going uphill, it gets bad. I simply downshift to 5th if I find myself in that position.

As for power, Heartthrob has a dyno sheet on their website that shows a gain of 8.5 hp on a Mustang GT - 363.9 hp stock vs 372.4 hp with the axle backs - which would work out to almost exactly 10 hp at the crank. Although it isn't a huge gain, I took that with a grain of salt and didn't base my decision on it. Now that I have installed it, the car feels like it pulls very slightly stronger. I don't believe that, if it does add any power, it would be noticeable from the driver's seat. If I tell myself that enough, I'm sure I will be convinced that the car pulls just as strong as stock but for now, it's tough when it sounds that much better! If it does add about 10 hp as the dyno runs claim, great. If not, that's to be expected from just an axle back.

If you would like to look at the dyno graph, go to this link: Heartthrob 2011+ 5.0 Flowpack Axle Back Dyno. The graph looks a bit odd with a large torque spike early on in the graph. However, the graph torque range is narrow (only between 300 lb-ft and 350 lb-ft) where most dyno graphs show 0 lb-ft as a minimum. The minimum rpm shown is also high at 3,573 rpm where most graphs start close to idle, at 1,500 - 2,000 rpm. These factors make make the normal torque curve appear as if it has odd spikes.

Overall, I am very happy with it. I don't think it's for everyone because of the drone and less-than-polished looks but I would buy it again. I will have to wait and see how the aluminized steel construction fares. Check back for pictures and videos comparing this exhaust to the stock one and an installation guide!


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