With the muflers removed; the cam plays the biggest part as far as the overall engine sound, a standard cam will allow the engine to idle very smoothly and give a restrained nice acceleration sound whereas a big cam (lumpy) will have a rougher idle and a 'full on' sound under acceleration. Fuel Injection runs smoother than carbs and affects the sound also. Turbo mufflers are generally louder than glass packs but it depends how restricted the glass packs are as they vary from different manufacturer. Header primary length/size and expansion chamber or collector length also affects the sound as does valve size, head chamber size and piston design type. Most engines with headers and open pipes will sound quite crisp and flat until under load within the cam range. deep sounds are generally due to freeflow mufflers and tailpipes. If you have your tailpipes short on mufflers that finish ahead of the rear axle then you get a very deep droning sound which shakes the neighbours windows (this may be illegal as the potential for carbon monoxide to enter the vehicle is high). Larger diameter pipe generally makes more noise too. Standard engine with headers and glass packs will be as loud as the glass packs are restricted and the tail pipe length.