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Porting Exhaust Manifolds
Porting the exhaust manifold on a car is a great way to optimize the performance of the engine. When most manifolds are manufactured, they are not optimized for performance, since the amount of time it would take to do this would not be cost effective. However, tuners and engine builders have been porting exhaust and intake manifolds for many years as a method of optimizing airflow and increasing an engine's horsepower.
What is Porting?
Porting is where the ports on a manifold that mate up to the motor are enlarged to optimize airflow out of the motor, thereby increasing horsepower. If the exhaust ports on the motor are larger than the ports on the intake manifold, air is not exiting the engine as efficiently as it could be.
Determining Whether You Need to Port
The easiest method to determine whether you should port the openings on your manifold is by using a manifold gasket. With the manifold off of the car, line the gasket up with the exhaust ports on the engine's head. The edges of the holes in the gasket should come right up to the edge of the intake ports. Now place the same gasket over the holes on the manifold that mate up the engine's exhaust ports. If you can see material inside the holes of the gasket, then this material can be removed to make the holes larger and match up with the holes on the gasket and the engine's exhaust ports.
How to Port
Fasten the gasket to the exhaust manifold and use the gasket as a guide for how much material needs to be removed. If you cannot fasten the gasket onto the manifold, hold the gasket against the manifold and use machinist blue dye to mark the excess material that needs to be removed. Once you have determined the area to be removed, use a high-speed, air-powered die grinder with a cutting stone to remove the excess material. Remove the material at an angle as it moves into the manifold. Once it is roughed out, use progressively finer polishing wheels to smooth out any rough edges. Try to maintain as consistent of an opening as possible. After the manifold is done, move the gasket onto the cylinder head ports and see if any material needs to be removed there as well to match the exhaust manifold ports.