EXHAUST BACK-PRESSURE
I would like to know what CAR’S opinion is on exhaust back-pressure. I am in the process of tting a turbocharger to my 1988 Nissan Skyline and the idea is to have an extremely free- owing exhaust system downstream of the turbo. I think, as indicated in lots of posts on the Internet, that a certain amount of exhaust back-pressure is required and the turbine wheel of the turbo would provide this.
Interesting question, Hilton. It is bene cial to get all the spent exhaust gases out of the chamber after combustion to enable a complete ll with fresh air. Therefore, it makes sense to have the back-pressure as low as possible to promote the ow of exhaust gases from the high pressure in the combustion chamber to the low atmospheric pressure outside. However, this is not the full story.
Remember that exhaust gas has mass and the speed at which it moves out of the combustion chamber results in momentum becoming a factor. Then there is the length of the exhaust-manifold runners which affect the pressure waves running up and down the exhaust pipe. These waves can be used, together with valve timing (and valve overlap between the intake and exhaust valves) to promote cylinder lling by using the exhaust energy moving down the exhaust pipe to help “suck” in the fresh-air charge. This technique has a greater effect on naturally aspirated engines and only at certain engine speeds if the valve timing is xed.
If the exhaust diameter leaving the engine is too large with no restrictions, the back-pressure will be low but so too the exhaust-gas- ow speed, resulting in the loss of the gas momentum and pressure-wave bene ts as described (the reason why some free- ow exhausts increase the noise but do little for extra power production). A pipe diameter that is too small encourages high-speed exhaust-gas ow but the additional back-pressure would cancel any pressure-wave bene ts. The optimal exhaust pipe would have as little back-pressure as possible but still promote pressure-wave bene ts.
The turbocharger in the exhaust pipe does provide some back-pressure while the turbine extracts the required energy. The exhaust energy is also linked to the sound and, therefore, a turbo engine needs fewer silencers in the exhaust pipe to meet the legal noise requirements.