Last year I installed headers on my 'Bu. I like the power increase, but The Wife
is not wild about the header noise or the heat in the passenger compartment. I could better insulate the firewall/floor for heat and noise, but I'm also considering going to a modified "factory appearing" exhaust system. What am I missing? As always constructive suggestions are appreciated
. This is what I'm thinkin':
BBC EXHAUST SYSTEMS
For Street Performance Applications
An exhaust system flows only as well as its smallest diameter or restriction. In the case of BBC Exhaust manifolds this smallest diameter will be located in the manifolds themselves.
Extrude Honed Exhaust Manifolds can increase low end performance, but won’t do much for horsepower. Exhaust Manifold Extrude Honing is expensive. An alternative is to use a flexible hone and do-it-yourself. Flexible hones are available in many sizes and grits. The manifolds must be ported and the inside diameter opened up as much as possible by honing.
BBC Exhaust Manifolds of this era incorporate small inside-the-manifold protrusions for the AIR Pump System’s injectors (this is true whether the manifolds are drilled and tapped for the system or not
) and the nubs must be ground away. The manifolds are then ported and honed (an inside diameter of 1 inch would be ideal for the inlet ports and 2.5 inches at the outlet). This process is one of the simplest and most important and can add up to 5 to 10 percent of flow.
A non-operational AIR System can be mounted, provided the AIR tubes do not intrude into the interior of the manifold. Whereas, it would be simpler to start with non-AIR manifolds and simulate the AIR Tubs’ being attached, there are zero performance gains to be had, therefore use the manifolds you already have.
If you don’t already have a pair of exhaust manifolds, consider these:
3989310 1971 - 1972 Right Hand
3989343 1971 - 1972 Left Hand
The 3989343 Left Hand Exhaust Manifold has less of a kick out angle at the outlet end than the 1969/1970 manifolds (something to keep in mind when purchasing head pipes
) and a less restrictive casting internally at the number one cylinder (making for a slightly better performing manifold when extruded
The factory Heat Riser Valve must be eliminated. If you want the look of a factory exhaust system a “Heat Riser Eliminator Block” could be used to give the external appearance of the valve. The block itself must be bored out to be circular and then honed to a 2.5 inch inside diameter.
Use 2.5 inch inside diameter mandrel bent exhaust tubing for the head and tail pipes with a 2.5 inch inside diameter H Crossover (the “H” pipe will provide better low end Torque with Exhaust Manifolds than a “X” crossover will
). Use high flow mufflers with 2.5 inch inside diameter offset inlet and 2.5 inch inside diameter offset outlet. Use 2.5 inch inside diameter Exhaust Tips. Consider heat wrapping the head pipes to keep heat in the system which provides for better flow.
Use a Camshaft which is designed to produce low end torque, not high end horsepower (i.e. less overlap to lessen exhaust gas reversion
Use an Intake Manifold which is designed to produce low end torque, not high end horsepower.