Turbo Cars and Exhausts
Discussion
Can anyone tell me if Back pressure affect turbo engines performance? On naturally aspirated cars I'm lead to believe that back pressure can affect the performance if you reduce the load - in other words take out the Kat and the silencer etc. Whereas on a turbo car the exhaust is used to feed the engine and this is before the Kat and the silncer - so if you remove these too will it affect the engine performance?
I think NA cars work best with little back pressure, but turbos need more. Never having had a turbo I'm not too up on the subject but I thought that the exhaust gases in a turbo engine were recirculated back into the turbo spinning the turbo up even faster. More back pressure would let less air escape & force more through the turbo?
NA cars need some back pressure (less is good up to a point) to stop fuel going straight out the exhaust ports from the intake ports, but on turbo cars there is enough pressure created by the turbo's resistance, so you want as little pressure after the turbo as possible, as less back pressure means quicker turbo spin up IIRC
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More back pressure would let less air escape & force more through the turbo?
No, all the exhaust gasses have to pass the turbo. If there is less back pressure it goes thru faster - hence turbo quicker and spools faster.
Problem with reducing back pressure however can be that the manufacturer has spec'd the whole of the engine plus exhaust system/turbos etc with a given amount of backpressure. If you reduce this, you could find problems with seals etc.
Ford increased the power to 224BHP on the 4WD Sierra Cosworth from 202BHP of the 2WD. Some say this was to compensate for the extra 2 wheels being driven others say it's to overcome the extra back pressure caused by the cat on the 4WD which isn't on the 2WD. The manual says it's for both.
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NA cars need some back pressure (less is good up to a point) to stop fuel going straight out the exhaust ports from the intake ports, but on turbo cars there is enough pressure created by the turbo's resistance, so you want as little pressure after the turbo as possible, as less back pressure means quicker turbo spin up IIRC
I absolutely agree. On turbo charged cars, a huge bore exhaust actually works in favour of the turbo, as long as it is fitted after it by allowing the gasses to evacuate from the turbo as fast as possible. This results in the turbo spinning up quicker and helps prevent turbo-lag as well. So when you see them on Nova's, that is for show. On something like an Impreza or a Mitsubishi Evo, it is functional.
Jas.
You have to be a little careful though with wide bore. Its not a simple case of the wider the better. make it too wide and the fast gases hit the slow or stationary inside the pipe causing large flow problems. The aim of good gass flow is to have a wide enough bore so that its not resticted but narrow enough so that the whole gas move as one down the pipe and doesn't have time to slow significantly down.
I encountered another problem when I stuck a big pipe and sports box on my car. The car was certainly quicker and sounded miles better and the turbo spooled out faster but cos of all this, I am now nudging the MAP sensor limit and if your car cuts the ignition as a failsafe, You will shit your pants as it usually only occurs flat out in a high gear. You can drive around it by not just leaving your foot buried but lift off to nine thenths of flat out. I think its boost creep that sends the manifold pressure over the limit. The solution is a unichip or BBR kit which remaps your fuel for the increased levels and removes the original MAP limit
I encountered another problem when I stuck a big pipe and sports box on my car. The car was certainly quicker and sounded miles better and the turbo spooled out faster. Because of this however I am now nudging the MAP sensor limit and if your car cuts the ignition by way of a failsafe, you will shit your pants as it usually only occurs flat out in a high gear. You can drive around it by not just leaving your foot buried but lift off to nine tenths of flat out. I think its boost creep that sends the manifold pressure over the limit. The solution is a unichip or BBR kit which remaps your fuel for the increased levels and removes the original MAP limit
quote:
You have to be a little careful though with wide bore. Its not a simple case of the wider the better. make it too wide and the fast gases hit the slow or stationary inside the pipe causing large flow problems. The aim of good gass flow is to have a wide enough bore so that its not resticted but narrow enough so that the whole gas move as one down the pipe and doesn't have time to slow significantly down.
Turbo engines can also be tuned for top end or bottom end performance by altering the exhaust diameters
Handy bit of info here
www.howstuffworks.com/muffler.htm
Also follow the link to more info, mentions turbos in there under the NSX link.
www.howstuffworks.com/muffler.htm
Also follow the link to more info, mentions turbos in there under the NSX link.
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