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MSR Technical Centre
 Research & Development: Exhaust System The key to good power on any turbo car is the exhaust system and its design. Over the years, I have been developing an exhaust system that allows a turbo to spool up quickly and deliver mid-range power that hits like a bomb! An exhaust will always determine the total power output of a car and how well it responds to other modifcations, based on how well it works. It isn't just about how much boost the exhaust lets the engine run, it is about how well it can run the boost it has. What dyno graphs don't show is the gains to part throttle and the reduction in turbo lag on the road.
Anyone can achieve power gains from just bolting on a whopping big turbo, but the exhaust efficiency at any RPM will affect both boost control and peak power in any case. A poorly designed exhaust will usually result in having to compensate for the inefficiency by using a larger turbo to achieve the same power (and also suffer more turbo lag) as compared to a good exhaust system with a smaller turbo. Not to mention, a good exhaust system will allow a turbo to achieve full boost up to 1000rpm earlier, thus resulting in more torque and a more drivable car in both on-boost and off-boost conditions. To get good low-range power, the turbo needs to boost as fast as it will allow.
A poorly designed exhaust will increase back pressure as engine RPM and boost pressure rises. Asking a turbo to hold a high preset boost pressure by forcing gas through the turbine or restricting wastegate flow increases the positive pressure before the turbo. This reduces scavenging on the exhaust stroke of the combustion cycle. While some exhaust systems may offer bolt-on power, this is because they force boost pressure up (ie. spiking) with restricted wastegate flow which also increases pre-turbo exhaust pressure limiting any true gains from other modifications.
If less of the old gases are expelled, there is less space for fresh charge to enter the bore meaning less potential power. This setup will also result in an engine pinging more readily as compared to an engine running the same boost pressure with a good exhaust system that lowers pre-turbo pressure and allows the turbo to expel gases more quickly and effectively. If pre-turbo pressure is high, no modification to the engine will show their true benefits unless the pressure is reduced with either a bigger turbo or ideally, a better designed exhaust system. Fitting a bigger turbo will not solve the issue on its own. You must also have a good exhaust system fitted as well.
So what does a good exhaust system do exactly? A good exhaust system is one that allows the turbo to spool up as quickly as possible (reducing lag), but does not create large amounts of unwanted turbulence in the process. Turbulence in the exhaust system reduces flow and creates back pressure at higher RPM before the turbo. This extra back pressure is one of the main causes of a turbocharged engine dropping off boost significantly as the RPM rises. Minimal back pressure at high RPM allows a turbo to hold a preset boost pressure much more readily and steadily. A good exhaust system that provides optimal scavenging on the exhaust stroke offers a greater amount of fresh charge to be available in the ignition stroke. The result: more power! So a good exhaust system does 2 main things. Firstly, it assists with optimal scavenging on the exhaust stroke, increasing volumetric efficiency whilst maintaining the same boost pressure. Secondly, is reducing back pressure at higher RPM, helping the turbo to maintain boost pressure at higher RPM. With increased turbo efficiency and lower intake temperatures as a result, a combination of more ignition advance and/or more boost can be run for even more power. This in turn gives better results when compared to just having an average exhaust and forcing more boost pressure. |
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