Power Versus Boost Pressure
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From: Gie
Power Versus Boost Pressure
When it comes to forced induction, I think many people have a lack of understanding of boost pressure and what it means for various turbo or supercharger setups. I'm going to attempt to shed some light on the subject. Please feel free to interject any useful information as you see fit.
THE ENGINE AS A RESTRICTION
At full load, our engines act just like a restriction in a pipe. Over time, a specific amount of air flows through the engine, with the engine acting as a bottle neck. If you force more air through the restriction, a higher pressure drop will result. The amount of restriction is dependant on a number of factors: head geometry, valve sizes, valve shapes, valve configuration, port sizing, port finish, etc. The F20/F22 is a VERY free flowing engine. The 4G63 is quite restrictive in comparison.
For a given pressure delta across the engine, a specific amount of air will be able to flow through. Just like a restriction in a pipe, a higher pressure delta will result in a higher flow rate.
BACKPRESSURE
It's important to have an understanding of the pressure delta across the engine, and we'll start with the "low" pressure side, or the exhaust side.
If you push a specific amount of gas through a pipe or collection of pipes, pressure will develop in the system. Small piping, more restrictive designs (lots of elbows and bends or rough interfaces) and flow blockages will cause the magnitude of the pressure to rise.
If you flow an identical amount of air through a tubular turbo manifold and a cast log turbo manifold, the pressure magnitude will be greater in the log manifold. This is due to it's more restrictive, less free flowing design. The same holds true for small turbines versus large turbines. While the flowrate remains the same, the smaller turbine will cause higher pressure development downstream of the engine.
This "backpressure" comes as a result of many factors, but the important players are manifold design (tubular, log, etc), piping diamter (large diameter piping is less restrictive), and turbine size/design (for turbocharged applications)
BOOST PRESSURE
Boost pressure is the air pressure upstream of the engine, the "high" pressure side. A turbine-driven compressor, belt driven compressor, or belt driven positive displacement air pump, pushes air into the engine at a rate that is higher than the engine could normally "suck in". The engine, now a restriction, causes the pressure to rise in the induction system. This pressure rise is what we measure as "boost".
RELATING BOOST PRESSURE AND BACKPRESSURE
It may be confusing to see the hundreds of dyno plots and results from the many F/I members on this and other forums. Some owners are making more power than others but at 5 psi lower boost pressure. It's very easy to jump to conclusions (and usually negative ones) about various turbo setups/kits/cars.
When it all boils down to it, one needs to keep in mind the pressure differential across the engine. Two identical engines will be able to produce approximately the same about of power given the fact that the same amount of air is moving through the engine. That specific amount of moving air corresponds to a particular pressure delta across the engine. Note that a boost pressure/back pressure of 15psi/5psi results in a 10 psi pressure delta. A boost pressure/backpressure of 20psi/10psi results in the same exact 10 psi pressure delta. These two engines will be able to move about the same amount of air, and produce about the same amount of horsepower.
Anytime you reduce the amount of backpressure, you also reduce the amount of boost necessary to move the same amount of air. This is the main reason why a well designed tubular turbo manifold setup will achieve the same power numbers as a more restrictive cast log turbo manifold setup but at a lower boost pressure. It is also the reason why a free flowing exhaust is important for FI applications. Ultimately, a log manifold can achieve the same HP numbers as a tubular manfold, but due to the higher necessary boost pressure to achieve the same pressure delta, it will take longer for a compressor to spool to its operating point. Not only that, but because the pressure delta across the compressor is now higher, a different compressor size/design may be necessary.
OTHER FACTORS
Compression ratio. - A higher compression ratio will enable an engine to make more power for a given pressure delta. Period. However, problems arrise when the effective compression inside the cylinder gets too high and it becomes difficult to control detonation. It may be necessary to reduce the compression ratio if the boost pressure becomes too high. Less restrictive designs can typically get away with higher compression, as the boost pressure does not need to be as great as in a less restrictive setup.
Pressure/temperature related knock. - Higher pressures and/or temperatures in the intake charge are more likely to cause detonation. A compressor will tend to heat up the intake charge much more at 20 psi than at 10 psi. The tuning window will get smaller and smaller as the boost pressure/intake temperature rises. More restrictive setups will result in a smaller tuning threshold.
THE ENGINE AS A RESTRICTION
At full load, our engines act just like a restriction in a pipe. Over time, a specific amount of air flows through the engine, with the engine acting as a bottle neck. If you force more air through the restriction, a higher pressure drop will result. The amount of restriction is dependant on a number of factors: head geometry, valve sizes, valve shapes, valve configuration, port sizing, port finish, etc. The F20/F22 is a VERY free flowing engine. The 4G63 is quite restrictive in comparison.
For a given pressure delta across the engine, a specific amount of air will be able to flow through. Just like a restriction in a pipe, a higher pressure delta will result in a higher flow rate.
BACKPRESSURE
It's important to have an understanding of the pressure delta across the engine, and we'll start with the "low" pressure side, or the exhaust side.
If you push a specific amount of gas through a pipe or collection of pipes, pressure will develop in the system. Small piping, more restrictive designs (lots of elbows and bends or rough interfaces) and flow blockages will cause the magnitude of the pressure to rise.
If you flow an identical amount of air through a tubular turbo manifold and a cast log turbo manifold, the pressure magnitude will be greater in the log manifold. This is due to it's more restrictive, less free flowing design. The same holds true for small turbines versus large turbines. While the flowrate remains the same, the smaller turbine will cause higher pressure development downstream of the engine.
This "backpressure" comes as a result of many factors, but the important players are manifold design (tubular, log, etc), piping diamter (large diameter piping is less restrictive), and turbine size/design (for turbocharged applications)
BOOST PRESSURE
Boost pressure is the air pressure upstream of the engine, the "high" pressure side. A turbine-driven compressor, belt driven compressor, or belt driven positive displacement air pump, pushes air into the engine at a rate that is higher than the engine could normally "suck in". The engine, now a restriction, causes the pressure to rise in the induction system. This pressure rise is what we measure as "boost".
RELATING BOOST PRESSURE AND BACKPRESSURE
It may be confusing to see the hundreds of dyno plots and results from the many F/I members on this and other forums. Some owners are making more power than others but at 5 psi lower boost pressure. It's very easy to jump to conclusions (and usually negative ones) about various turbo setups/kits/cars.
When it all boils down to it, one needs to keep in mind the pressure differential across the engine. Two identical engines will be able to produce approximately the same about of power given the fact that the same amount of air is moving through the engine. That specific amount of moving air corresponds to a particular pressure delta across the engine. Note that a boost pressure/back pressure of 15psi/5psi results in a 10 psi pressure delta. A boost pressure/backpressure of 20psi/10psi results in the same exact 10 psi pressure delta. These two engines will be able to move about the same amount of air, and produce about the same amount of horsepower.
Anytime you reduce the amount of backpressure, you also reduce the amount of boost necessary to move the same amount of air. This is the main reason why a well designed tubular turbo manifold setup will achieve the same power numbers as a more restrictive cast log turbo manifold setup but at a lower boost pressure. It is also the reason why a free flowing exhaust is important for FI applications. Ultimately, a log manifold can achieve the same HP numbers as a tubular manfold, but due to the higher necessary boost pressure to achieve the same pressure delta, it will take longer for a compressor to spool to its operating point. Not only that, but because the pressure delta across the compressor is now higher, a different compressor size/design may be necessary.
OTHER FACTORS
Compression ratio. - A higher compression ratio will enable an engine to make more power for a given pressure delta. Period. However, problems arrise when the effective compression inside the cylinder gets too high and it becomes difficult to control detonation. It may be necessary to reduce the compression ratio if the boost pressure becomes too high. Less restrictive designs can typically get away with higher compression, as the boost pressure does not need to be as great as in a less restrictive setup.
Pressure/temperature related knock. - Higher pressures and/or temperatures in the intake charge are more likely to cause detonation. A compressor will tend to heat up the intake charge much more at 20 psi than at 10 psi. The tuning window will get smaller and smaller as the boost pressure/intake temperature rises. More restrictive setups will result in a smaller tuning threshold.
Joined: Mar 2004
Posts: 2,160
Likes: 35
From: Decatur, GA
A bit more info.
When you think about pressure differentials, there is a pressure differential across the motor from the intake valve to the exhaust valve. Those pressures have HUGE effects on the filling of the cylinders and the discarding of spent gas. People talk about tuned manifolds. Tuned intake and exhaust manifolds rely on adjusting the length and diameter of the tubes feeding the intake and exhaust ports to optimize pressure at the intake and exhaust valves.
There are also a lot of other pressure differentials that come into play. In general, you want to maximize the delta P across the intake and exhaust valves and minimize the delta P across any intercoolers, throttle bodies, or other similar components at WOT.
The turbo actually has a pressure ratio across the compressor and turbine sides that can serve to magnify any inefficiencies on either side of the turbine. That's part of the reason why free flowing intake and exhaust systems can make so much more power on a turbo car.
Tim
When you think about pressure differentials, there is a pressure differential across the motor from the intake valve to the exhaust valve. Those pressures have HUGE effects on the filling of the cylinders and the discarding of spent gas. People talk about tuned manifolds. Tuned intake and exhaust manifolds rely on adjusting the length and diameter of the tubes feeding the intake and exhaust ports to optimize pressure at the intake and exhaust valves.
There are also a lot of other pressure differentials that come into play. In general, you want to maximize the delta P across the intake and exhaust valves and minimize the delta P across any intercoolers, throttle bodies, or other similar components at WOT.
The turbo actually has a pressure ratio across the compressor and turbine sides that can serve to magnify any inefficiencies on either side of the turbine. That's part of the reason why free flowing intake and exhaust systems can make so much more power on a turbo car.
Tim
