I have experimented with different exhausts on my 40 Chevy/ 302 GMC. These included: a 4 into 1 header with 2 1/2" single pipe system, a larger 4 into 1 header with 3" single exhaust and finally dual 2 into 1 headers with 1 3/4" pipes. My impressions of the three systems on the road with mufflers and during 1/4 mile drag races without mufflers and pipes were as follows. The 4 into 1 with a single 2 1/2 pipe worked well one the road and at the drags. The 4 into 1 with 3" single pipe was poor on the road and while racing especially at low rpm. The dual 1 3/4 headers work well in both cases but I think the dual headers seem better than the 4 into 1 single 2 1/2" on the road especially at low rpm. Drag racing results were about equal. I went to the smaller dual system after laying under a late model Jaguar XJ6 in my local junk yard. It was a 4.2 liter (252 ci) with a 12 port head, dual OH cams, dual headers and dual 1 3/4" pipes and mufflers. I figured my old GMC will never put out the power of the Jag, so I took the whole piping system and adapted it to my car. I works great and sounds good too.
