If you keep the engine stock, removing the catalytic headpipe is only really getting rid of the heat source. Doing anything to the engine means more flow, even just (non EPA compliant) cams, the cats will start to restrict and affect hp/tq numbers. As already stated, how much effect is still an unknown until the side by side tests are done. It would take a lot of time and effort to set the record straight and I'm sure most people that own a dyno really don't care. I don't blame them for leaving it a mystery.

 

For those of you pissed about the amount of power and throttle reponse of your new $20,000+ Harley is LACKING, consider this....

I bought a brand new 2008 Heritage and did the standard Hi-flo A/C, hi-flow exhaust and V&H FuelPak. It was very responsive and powerful with the stock engine. A couple of months ago, I traded it in (after 22,000 miles) on a 2010 Ultra Classic for the better supension.

I was thoroughly disappointed in how weak it was. But I figured the same mods on it would straighten it out. I did the V&H slip ons, A?C and FuelPak. It sounded alot better and had a little more power. I played with Fuel settings and the exhaust to get as much as possible out of the bike. I was very irritated.

Talked to a couple of dealers and read through some forums and got mixed results regarding the catalytic converter (which is in a chamber between the headpipes and slip on exhaust). Some said the removal of the cat wouldn't make much difference. The dealers said that to get more power and better throttle response a race tuner and new cams would be in order. This would cost somehwere in the vicinity of $1500.

I didnt just assume everyone was telling the truth and decided I wanted to prove whether or not the cat was impacting the performance of it. I was faced with 2 choices, buy new head pipes (true duals) or cut that crappy cat out of the pipes.
Since I was already strapped for cash I decided to cut the pipe in half, air chisel the cat out, weld a divider pipe in the chamber (to separate the exhausts into true duals) and weld it all back together. I knew that once this was done there would be no going back later.

Let me say this........ That's the best Freakin thing Ive done to this bike. It's a helluva lot quicker than the previous bike (even with wife and all the other junk loaded on it). Sounds fantastic, runs cooler and I can honestly say it is the funnest bike to ride I have ever owned (even though it's a couch on 2 wheels). Unfortunately the last thing I modded made the biggest difference.

If I had the money, I would have left the stock head pipes intact and spent the extra $400 bucks for true dual headers.

My 2 cents. You can disregard or take it from someone that has spent alot of time testing different setups and results.