Originally Posted by chevelle496
I recently put together a sbc 327 it has pro comp aluminum 2.02 heads with 64cc chambers, crowed hydraulic flat tapper cam dur. @ 050 - 234/246 lift 0.497/0.504 and lsa- 112. stock forged crank and rods with forged trw pistons. Eldelbrock Rpm air gap intake and demon 650 carb. Compression ratio- 10.35:1
I was looking for tips to what size headers and exhaust I should put on this. Itsgoing in a 69 Chevelle.
Long tubes and at least a 20 inch long collector.
1&5/8ths primary tubes will pick up the bottom through mid range, if running high over all gearing especially if combined with an automatic that has an OEM or near OEM stall this will improve on the street or out of the hole performance. These can use either a 2&1/2 inch or 3 inch collector.
1&3/4qtr primary tubes will embellish the top end power, if running low over all gearing with a manual gear box or a high stall automatic that lets you use the power of the upper RPM band these would be good. These should have a 3inch diameter collector.
The big effect with headers is on the exhaust to intake overlap. A cam with a lot of overlap and or a mixture with high lift rates whether that's arrived at with a fast lobe or 1.6 rockers is not important only the lift rate at the valve counts, not how it gets there. The difference in primary tube diameter moves the strength peaks of the tuning pulses which has the effect of moving the peak to RPM relationship. These pulses travel at the speed of sound for the gas pressure and temperature in the tube. The larger tube for a given RPM and cylinder fill density the lower the pressure and the greater surface area probably knocks the temps down faster so there are some changes to the speed of sound, but hardly anybody calculates this, so just figure that the SoS remains the same. However, the larger diameter tube spreads the signal over more area, thus it becomes weaker so the peaks go up the rev range to where density and temperature generate larger forces in the exhaust flow.
There are actually 2 major things going on with headers:
1) Improved streamlining compared to cast manifolds and a smoother internal surface reduces resistance to flow which cuts back pressure.
2) Pulse timing where the tuned length trips a negative pressure return wave. This does two things, but getting it timed to where you need it is the issue of tube length and diameter as well as gas density and temperature.
2A) When the negative pressure wave hits the valve the pressure lower than that in the cylinder draws out the remaining exhaust and pulls in fresh mixture.
2B) The wave also is reflected at this point back into the header tube taking the exhaust and some mixture with it.
2C) The undesirable byproduct can be that the wave passes into the intake tract becoming a positive pressure wave in there and pushes mixture about cylinder to cylinder or back out the carb. Large runners and plenums as well as the use of spacers dampen this undesirable effect.