Big duration cams + turbo
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hey porsche guy
Typically (read: 99% of the time) automotive designers throw turbines that are far too small for the engines in reality. They make very small turbines and housings in order to make the car spool easily and run with nice wide power bands and gobs of torque.
These small housings and tiny turbines are HUGE amounts of backpressure on the engine once you romp it and bring the rpms up to a respectable amount.
But i forgot you knew everything so i didn't need to explain that to you.
Typically (read: 99% of the time) automotive designers throw turbines that are far too small for the engines in reality. They make very small turbines and housings in order to make the car spool easily and run with nice wide power bands and gobs of torque.
These small housings and tiny turbines are HUGE amounts of backpressure on the engine once you romp it and bring the rpms up to a respectable amount.
But i forgot you knew everything so i didn't need to explain that to you.
__________________
Advocate for the People's Republic of Awesome
rest in peace tim.
Advocate for the People's Republic of Awesome
rest in peace tim.
Well, after looking around for resources unrelated to aircraft I guess we all are right within our particular areas of expertice.
In the piston aircraft engine real world that I am familiar with RPM ranges are small and turbos are perfectly matched.
In cars with wide RPM ranges I guess the best compromise when going from low to high RPMs is to have an undersized turbine at the upper RPM range.
After looking around for more automotive references here is a good reference I found on the very subject we are discussing.
There is also some good information there about cam overlap and turbo sizing.
http://www.grapeaperacing.com/GrapeA.../turbocams.cfm
In the piston aircraft engine real world that I am familiar with RPM ranges are small and turbos are perfectly matched.
In cars with wide RPM ranges I guess the best compromise when going from low to high RPMs is to have an undersized turbine at the upper RPM range.
After looking around for more automotive references here is a good reference I found on the very subject we are discussing.
There is also some good information there about cam overlap and turbo sizing.
http://www.grapeaperacing.com/GrapeA.../turbocams.cfm
With a well matched turbo / engine combo, boost pressure should be higher than exhaust gas pressure at the low side of the power band (near peak torque). As the engine nears peak hp, the pressure differential will get nearer 1:1. At some point the pressures in the intake and exhaust will be equal then crossover making the exhaust a higher pressure than the intake. At peak hp there will usually be more exhaust gas pressure than boost pressure. The ultimate goal is to have as little exhaust backpressure possible for the desired boost.
If the turbocharger is matched well to the engine combination, the camshaft selection will not need to be much different than that of a supercharged engine. The problem is that most factory turbo engines have turbo's that are sized too small and will usually have more back pressure than boost pressure over much of the useable power band. Car manufactures do this in an attempt to reduce turbo lag. When a turbocharger is too small, it will be a bigger restriction in the exhaust, causing more back pressure. A big mistake of turbo owners is to crank the boost up as high as they can thinking they are going faster, but in reality, chances are that they are just killing the efficiency of the turbo and most gains are lost. If you want to run higher boost levels and back pressure is a problem, cam timing can be altered to give respectable power increases for much cheaper than a new turbocharger. Before you go increasing boost and changing cams, remember that the oxygen content into the engine will increase power, not boost pressure. A good flowing head with a good intercooler can make a lot of power without high boost. You may not need more boost to get the power you want.
If the turbocharger is matched well to the engine combination, the camshaft selection will not need to be much different than that of a supercharged engine. The problem is that most factory turbo engines have turbo's that are sized too small and will usually have more back pressure than boost pressure over much of the useable power band. Car manufactures do this in an attempt to reduce turbo lag. When a turbocharger is too small, it will be a bigger restriction in the exhaust, causing more back pressure. A big mistake of turbo owners is to crank the boost up as high as they can thinking they are going faster, but in reality, chances are that they are just killing the efficiency of the turbo and most gains are lost. If you want to run higher boost levels and back pressure is a problem, cam timing can be altered to give respectable power increases for much cheaper than a new turbocharger. Before you go increasing boost and changing cams, remember that the oxygen content into the engine will increase power, not boost pressure. A good flowing head with a good intercooler can make a lot of power without high boost. You may not need more boost to get the power you want.
Last edited by 0HP930; 02-24-2003 at 05:08 PM.
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