Try taking a timing light and checking your timing marks, had a toyota pickup that used to have a rich fuel mixture problem and adjusting the distributor rotor fixed it right up.

Some engine makers put an RPM limiter to prevent excessive RPMs without a load. My daughters Mercury "sputters" at 4000 RPM. Whether you engine has a problem or your friend's engine has a problem, I can't answer that. On a Ford, a P1270 code would be set if the engine speed was excessive for the current operating conditions. Also, disconnecting the exhaust will cause your O2 sensors to send erroneous signal to the computer and may affect gas mileage; may also set a DTC if done long enough.

Need to richen up air/fuel mix, dropping the exhaust leaned you out. Carb. or injected? If computer controlled, need to change chip.

That's the reason. Leave it off and you may wind up with warped exhaust valves plus a ticket when the first cop stops you.

Might be a misfire

I have a 2003 ford ranger with a 3.0 v6. I just recently unhooked the muffler to make it sound deeper. When the truck is in park idling an I rev the engine it starts to sputter kinda like a rally car. It starts doing it at 3000 rpm when I hold the gas to rev it. But when is in drive I can get it to like 5000 rpm and it doesn't sputter at all an sounds normal. And I wanted to know why it does that? My friend has the same engine in his ranger but It doesn't do that to his. Is it because I took the muffler off and it ain't getting enough back pressure? Thanks for helping!